Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Abstract

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed. This article is protected by copyright. All rights reserved

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Review
Materials with Tunable Optical Properties for Wearable
Epidermal Sensing in Health Monitoring
Fei Han, Tiansong Wang, Guozhen Liu, Hao Liu, Xueyong Xie, Zhao Wei, Jing Li,
Cheng Jiang, Yuan He, and Feng Xu*
F. Han, T. Wang, H. Liu, X. Xie, Z. Wei, F. Xu
The Key Laboratory of Biomedical Information
Engineering of Ministry of Education
School of Life Science and Technology
Xi’an Jiaotong University
Xi’an , P. R. China
E-mail: fengxu@mail.xjtu.edu.cn
F. Han, T. Wang, H. Liu, X. Xie, Z. Wei, F. Xu
Bioinspired Engineering and Biomechanics Center (BEBC)
Xi’an Jiaotong University
Xi’an , P. R. China
G. Liu, C. Jiang
School of Life and Health Sciences
The Chinese University of Hong Kong
Shenzhen , P. R. China
J. Li
Department of Burns and Plastic Surgery
Second Aliated Hospital of Air Force Military Medical University
Xi’an , P. R. China
C. Jiang
Department of Chemistry
University of Oxford
Oxford OX QZ, UK
Y. He
The Second Aliated Hospital
Xi’an Medical University
Xi’an , P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/./adma..
DOI: 10.1002/adma.202109055
reliable as the diagnostic information is,
the approach requires costly centralized
facilities and typically gives one piece of
data at one point in time.[] Such approach
places a burden on the healthcare system
that could be significantly alleviated if
rapid, real time diagnostic information
could be provided which inevitably means
the diagnostic device would be on the
body.[] That is, a growing interest and
awareness about the benefits of health
monitoring are transformed from current
hospital-centered diagnostics to patient-
centered diagnostics. To revolutionize
healthcare service with patient-centered
diagnostics, wearable epidermal sensors
have emerged oering new possibilities to
the analytical instruments currently used in the health moni-
toring since such devices can easily collect crucial health-related
information (e.g., breath, heart rate, pulse oximetry, photo-
plethysmogram, body motion, blood pressure, skin tempera-
ture, and potentially biochemical markers in the future) in a
real time and noninvasive manner.[]
Multifunctional wearable sensors can be found in the form
of glasses, wristwatches, fitness bands, headsets, clothes, and
belts.[] However, these devices are often made with rigid mate-
rials mounted on the skin. There is a clear benefit in having
wearable electronics with a more intimate form fit. In general,
wearable electronics consist of sensors, actuators, transistors,
power sources, wireless communication modules and links,
control and processing units, and algorithms in software for
analyzing and making decisions.[,] However, a numerous
number of cutting-edge wearable electronics remain unsat-
isfactory due to perplexing fabrication procedures, relatively
cumbersome analytical instrumentation, and the requirement
of an external power source. Accordingly, a convenient fabrica-
tion method equipped with remarkably distinct readable signal
output is highly desirable for healthcare applications.
Color provides a simple and straightforward way to transmit
information.[] Many wearable devices employ changes in color
in response to physiological signals for health monitoring.[]
These visual signals can be directly identified with naked eyes,
oering a simple and straightforward sensing/mapping route
for patients/physicians to sense stimuli. Over the last decade,
health monitoring based on the tunable optical properties has
further spearheaded the wearable devices since they exhibit
notable merits in their easy readout, fast response, revers-
ibility, low-cost, and simple fabrication procedures. Basically,
wearable devices with visible readouts are usually comprised
Advances in wearable epidermal sensors have revolutionized the way that
physiological signals are captured and measured for health monitoring.
One major challenge is to convert physiological signals to easily readable
signals in a convenient way. One possibility for wearable epidermal sensors
is based on visible readouts. There are a range of materials whose optical
properties can be tuned by parameters such as temperature, pH, light, and
electric fields. Herein, this review covers and highlights a set of materials
with tunable optical properties and their integration into wearable epidermal
sensors for health monitoring. Specifically, the recent progress, fabrication,
and applications of these materials for wearable epidermal sensors are sum-
marized and discussed. Finally, the challenges and perspectives for the next
generation wearable devices are proposed.
1. Introduction
Currently, the majority of diagnostics to guide patient treat-
ment is hospital or pathology laboratory-centric. As robust and
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of color-changing sensing elements and adhesive substrate
materials, and we only need to combine these two components
to build up a wearable device with the desirable performance
for health monitoring, which is much easier than fabricating
wearable electronics. Materials with tunable optical properties
are becoming potential alternatives as sensing elements owing
to their unique capabilities to produce visual color change
under external stimuli (e.g., mechanical force, humidity, light,
voltage).[c,] There are many types of wearable sensors using
these materials including photonic crystals (PCs), thermo-
chromic dye molecules, plasmonic materials, and liquid crystals
(LCs) as visible readouts.[] Despite the growing interest in vis-
ible readouts for wearable sensors, its development is still in the
embryonic stage, with a number of challenges to be addressed,
e.g., poor conformity, visual interference, in-continuous moni-
toring, and short durability.
Although there exist a few excellent reviews on the topic
of the wearable sensors with visible readouts and the corre-
sponding materials,[,] there is still no comprehensive review
covering the materials with tunable optical properties for
wearable epidermal sensors with visible readouts, especially
for health monitoring applications. We cover wearable sen-
sors with visible readouts for epidermal signal monitoring. A
graphical summary of these paradigms appears in Figure 1. We
outline all the key components in the wearable sensors with
visible readouts for health monitoring systems and ascertain
the demands and opportunities to expand the range of applica-
tions for patient-centered diagnostics, along with the diculties
and challenges that still need to be solved. The physiological
signals are usually divided into three parts, i.e., physical sig-
nals, electrophysiological signals, and chemical signals, which
are separately reviewed in Sections–. Three kinds of signals
are highly related to the health conditions and can also be inte-
grated into one single platform for multiphysiological signals
monitoring, which is reviewed in Section . Finally, we dis-
cuss the current challenges and provide a future perspective in
Section.
2. Materials with Tunable Optical Properties for
Wearable Epidermal Sensors in Physical Signal
Monitoring
Physical signals include temperature, humidity, light, and
mechanical force. Materials with tunable optical properties are
considered as sensing elements in wearable epidermal sensors
to transduce physical signals into visible readouts. In this sec-
tion, we review the materials with tunable optical properties for
detecting each specific physical signal and examine the strate-
gies to realize their potential applications for health monitoring.
2.1. Temperature
Temperature is an important sign for health status of our body.
An irregular rise in body temperature is a consequence of
many abnormal health conditions, including fever and hyper-
thermia.[] It occurs when the body temperature system is
aected by an infection, inflammation or other health risks.
Wearable devices may, therefore, reflect the level of body tem-
perature and facilitate the monitoring of temperature during
a fever.[a] To meet the requirements of noninvasive and con-
tinuous monitoring of body temperature, wearable temperature
devices are required to be flexible, biocompatible, light-weight,
and highly sensitive within temperatures range – °C.[]
Tremendous progress has been made in this research area.
2.1.1. Materials with Thermo-Induced Tunable Optical Properties
We categorize these materials by five distinct quantities, i.e.,
thermoresponsive polymer-based materials, thermochromic
dye, cholesteric liquid crystals (CLCs), thermochromic-conju-
gated polymers, and materials with thermoresponsive fluores-
cence emission. Table 1 summarizes these materials and their
corresponding detection range, substrates, applications.
Thermoresponsive Polymers: Thermoresponsive polymers have
received enormous attentions to date. In aqueous systems,
poly-N-isopropylacrylamide (PNIPAM) is the most prevailing
temperature-responsive polymer due to its lower critical solu-
tion temperature (LCST) of ≈ °C, which can be fabricated in
the form of polymer brushes, hydrogels, and microgels.[] Typi-
cally, PNIPAM chains are extended at low temperatures, which
undergo a hydrophilic-to-hydrophobic transition and collapse
in aqueous solution when the LCST is reached (Figure 2A(i)).[]
Owusu-Nkwantabisah et al. reported a composite hydrogel
by noncovalently incorporating PNIPAM microgel into a
benzyl methacrylate-co-octadecyl methacrylate-co-methacrylic
acid (MAA) polymer matrix with fast response and rapid
self-healing.[] Some degree of whiteness can be achieved
by heating the PNIPAM above its LCST. To boost the bright-
ness of the white of the PNIPAM upon the temperature tran-
sition, attempts have been made to incorporate channels into
the PNIPAM by forming a semi-interpenetrating network of
agarose and PNIPAM and then removing the agarose gel.[]
Regardless of the success of this approach, PNIPAM-based
polymers present only white color, which precludes the detec-
tion of subtle chromatic changes induced by small temperature
variations.
PNIPAM has also been employed within copolymer
photonic crystals or stacks of alternate films, such as polysty-
rene (PS) cores and poly(PNIPAM-bisacrylamide (BIS)–acrylic
acid (AA)) shells PCs, P(NIPAM-co-AA) microgel photonic
crystals, and stacks of PNIPAM and poly(para-methyl styrene)
(PPMS) multilayer alternate films.[] The reason to do this is
that the materials show a uniform change in structural color
upon an increase in temperature. Likewise, the incorporation
of plasmonic nanoparticles, silica nanoparticles/carbon black,
or FeO@ polyvinylpyrrolidone (PVP) colloidal nanocrys-
tals into PNIPAM gel networks also can provide reversible
color changes since the thermoresponsive hydrogels could be
recognized as a valve or actuator.[] Our group for example
designed a gold nanoparticles (AuNPs)@PNIPAM system
with reversible optical properties due to interparticle interac-
tions of surface plasmon resonance (SPR) using several dif-
ferent structured AuNPs (core–satellites, aggregates, etc.).[]
Meanwhile, Rossner et al. reported a series of well-defined
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core–satellite nanostructures linked by the multiarm revers-
ible addition-fragmentation chain transfer star polymer and
investigated its LCST-responsive optical properties.[] In addi-
tion, the Hoogenboom group described smart colloidal gold
nanoparticles clusters, demonstrating reversible thermorespon-
sive color-changing properties due to the tuning of plasmon
coupling.[] These AuNPs@PNIPAM systems could serve as
plasmonic ruler or actuators for bioimaging and sensing.[]
Similarly, based on this plasmonic particle interactions and
the change in those interactions, Ag/Au nanoparticles coupled
with the inverse opal film of P(NIPAM-co-MAA) hydrogel form
reversible full-color response triggered by temperature from
 to  °C (FigureB(i)).[] Besides hydrogels and microgels,
the array spacing of diraction grating composed of PNIPAM
brushes on super-paramagnetic iron oxide nanoparticles can
also be reversibly tuned by varying the temperature between
 and  °C.[]
Thermochromic Dye: Thermochromic dye can be embedded
in photonic crystals to create thermochromic phase change
systems. The Tang group combined the SnO inverse opals
and heat-sensitive red bisphenol A to illustrate the mutual
transformation of structural color and pigmentary color
(FigureB(ii)).[] That is, because the pigmentary color of bis-
phenol A display its original color under cooling, lactone ring
Adv. Mater. 2022, 
Figure 1. Schematic illustration depicting the wearable epidermal sensors based on visible readouts with dierent stimuli. The physiological signals
can be divided into physical: temperature. Reproduced with permission.[a] Copyright , Springer Nature. Mechanical force (scale bar:  cm). Repro-
duced with permission.[b] Copyright , American Association for the Advancement of Science (AAAS). Humidity. Reproduced with permission.[c]
Copyright , Wiley-VCH. Light. Reproduced with permission.[d] Copyright , Wiley-VCH. Chemical: Electrolytes (scale bar:  cm). Reproduced with
permission.[e] Copyright , AAAS. Lactic acid. Reproduced with permission.[f] Copyright , American Chemical Society. Glucose. Reproduced
with permission.[g] Copyright , American Chemical Society. pH. Reproduced with permission.[h] Copyright , Wiley-VCH. Electrophysiological
signals. Reproduced with permission.[i] Copyright , American Chemical Society.

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of the bisphenol A opens to form a quinone structure upon
heating, and there is a structural color of photonic crystals
appears after heating.[] Its reversible thermochromic optical
properties were exploited to unveil and hide the information for
anticounterfeiting industries.
Cholesteric Liquid Crystals: Another form of structural color-
based materials is CLCs that are spontaneously organized into
a periodic helical structure with a twist axis perpendicular to the
local director, reflecting selected wavelengths of circularly polar-
ized light.[] When temperature is below its isotropic point,
CLC can form ordered states. At temperature above its isotropic
point, it exhibits isotropic liquid nature (FigureA(ii)).[] LCs
could be dispersed in N-methyl methacrylamide (MMAA)–BIS
porous polymer, covering the whole visible region in response
to temperature.[] Liquid crystals can intersect with porous
polymer to produce uniform color when temperature reaches
its isotropic point. While decreasing temperature induces a
refractive index change in the CLCs, which results in a dierent
color display with dierent scattering intensities.[] Likewise,
Lee et al. reported uniform thermochromic microcapsules con-
sisting of a CLC core and a thin polyurethane shell layer, which
embodies the reversible color change between  and  °C
(FigureB(iii)).[]
Thermochromic-Conjugated Polymers: Polydiacetylene (PDA),
a intrinsically supramolecular-conjugated polymer, have been
massively employed in thermochromic sensors.[] For example,
PDA–wax composites display significant thermochromism
(Figure B(iv)).[] Wax molecules embedding into the crystal
lead to the growth of PDA crystal. The PDA crystals shrink
upon heating because monomers and the embedded wax mole-
cules are released from the PDA crystal and during the heating
operations. This phenomenon creates void in the PDA crystals,
therefore actuating a certain degree of CC bond isomerization
in the polymer chains, which cause the consequent blue-to-red
color transition.[]
Materials with Thermoresponsive Fluorescence Emission: A
dierent strategy to make thermoresponsive materials is to
use temperature sensitive fluorescence. For example, cyano-
substituted oligo(p-phenylenevinylene) (CN-OPV) derivatives
form a stable organogel in a cross-linking ethylene glycol
dimethacrylate (EGDMA) polymer matrix. A supramolecular
structural change of the organogel triggers a reversible thermo-
chromism.[] Another example is that Zhang et al. unveiled the
simultaneous thermochromic and thermofluorescent outputs
by encapsulating the indenoquinacridonedye (IQA) in hexade-
canol into SiO nanoparticles for smart fabrics.[] It expresses
fast and highly reversible color change upon heating and
cooling with a bright fluorescence.
According to Table  and shown in Figure C, the mate-
rial with the widest temperature detection range, between
 and  °C, is entry No.  that employs thermochromic flu-
orescence emission. However, photobleaching of fluorescence-
based materials induces instability, hindering their applications
in color displays. Compared to this, the widest temperature
detection range with nonfluorescence color-changing mate-
rials is entry No.  where there is a blue to orange change over
Adv. Mater. 2022, 
Table 1. Materials with thermoinduced tunable optical properties: detection range, substrates, and applications.
Enrty Temperature detection
range [°C]
Materials Substrates Applications Ref.
 – Silica nanoparticles/carbon black@porous
PNIPAM hydrogel
– Biochemical sensing/
electronic paper
[b]
 – P(NIPAM-co-MAA) gel – Full color displays []
 – Multilayer D photonic stack consisting of PPMS and PNIPAM Silicon D photonic sensors [c]
 – PNIPAM hydrogel inverse opal nanoparticles – Active drug loading []
 – Silica nanoparticles embedded in
P(NIPAM-co-N-methylolacrylamide (NMAM)) polymer network
– Temperature sensors []
 – Inverse opal photonic gels consisting of -acryloyl
morpholine (ACMO), and NIPAM
Glass slides – []
 – FeO@PVP colloidal nanocrytalline cluster
D photonc crystals in PNIPAM matrix
Quartz substrate – []
 – PS-b-PVP (-vinylpyridine) block copolymer photonic films – Temperature sensors []
 – MMAA-based porous polymer membranes
filled with thermosensitive LCs
– Energy-saving/multicolor
displays
[]
 – Peptide-based PDA-conjugated polymer fibers, membranes, and gels Glass slides – []
 – Layered organic composite PDA–VBA
(vinylbenzylamine) in the PNIPAM hydrogel
– Imaging/quantification
devices
[]
 – IQA/hexadecanol/SiOPolyester fabric Smart textile []
 – CN-OPV derivatives organogel in a cross-linking EGDMA polymer matrix film – – []
 – CLC core@polyurethane shell microcapsules PDMS Microwriting displays []
 – Thermochromic microparticles (Ag nanowire–SWCNT nanocomposite) PDMS Thermotherapy pad []
 – Crystal violet lactone (CVL)-encapsulated-tetradecanol
phase change materials
– Fighter protective clothing []

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Figure 2. Materials with tunable optical properties for wearable epidermal sensors in temperature monitoring. A) Mechanisms: i) Thermorespon-
sive polymer with LCST. The optical properties of poly(N-acetoxylethyl acrylamide) could be reversibly tuned by temperature since its polymer chian
undergoes a reversible and sharp phase transition with almost no hysteresis at the LCST around  °C. Reproduced with permission.[] Copyright
, Elsevier. ii) Molecule form transition in liquid crystals. When the temperature changes, the structure is reorganized and reformed from crystal,
liquid crystals, and liquid. Reproduced with permission.[b] Copyright , Royal Society of Chemistry. B) Materials: i) Thermoresponsive polymer@
AuNPs. The AuNPs on the thermoresponsive polymer hydrogels reversibly alter their assembly structures depending on temperature, demonstrating
reversible color changes (scale bar:  nm). Reproduced with permission.[a] Copyright , Springer Nature. ii) Thermochromic dye@photonic
crystals. The patterns are hidden and reproduced using the thermal response properties of these materials. At room temperature, these systems
present excellent red pigmentary colors. When these materials are heated to melt, the pigmentary colors disappear and show patterns with structural
colors (scale bars:  cm). Reproduced with permission.[] Copyright , American Chemical Society. iii) Liquid crystals. The liquid crystals coating
shows a gradual thermochromic response from orange at room temperature to greenish-blue at  °C. It becomes transparent when heating upon
 °C because of the isotropic phase formation. The thermochromic response is reversed by cooling. Reproduced with permission.[a] Copyright ,
American Chemical Society. iv) PDA–wax composites. The thermochromic behavior of these composites on paper is measured by visible absorption
spectroscopy. Photo graphs of the corresponding images are also provided. This phenomenon is due to a blue-to-red color transition at around  °C.
Reproduced with permission.[] Copyright , Wiley-VCH. C) Ashby chart: the plot of temperature detection range as a function of thermochromic
color-changing zone. D) Applications: i) Wearable elastomer hybrid fibers for temperature sensors. They can display a reversible color change based on
dierent temperatures. When taken as a temperature-sensing element, the fiber color changes from red to green after attaching on the skin. Reproduced

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the range of  to  °C. Since wearable epidermal thermo-
chromic sensors require displays to exhibit high detection sen-
sitivity within – °C, entries Nos. , –, and  are all appear
to be superior to other candidates for developing a wearable
sensor for body temperature detection as based on the distinct
and rapid color-changing over the required temperature range.
If the readers would like to acquire the information about
how to design the wearable thermochromic sensors with con-
tinuous visible color-changing depending on the certain range
of temperature, they exclusively confirmed from this figure to
initiate the preliminary experiments. It helps researchers to
choose the designated one among tons of materials shortly and
conveniently.
2.1.2. Wearable Epidermal Sensors with Visible Readouts
for Temperature Monitoring
The previous sections discussed the materials that could be
employed. In this section we look at the actual sensing devices
that have been developed. One of the simpler examples of an
actual device is the clever use of a thermochromic elastomer
composed of silicon rubber and thermochromic microcapsules
that are assembled into a flexible and wearable temperature D
fiber sensor via a wet-spinning method.[] These hybrid fibers
obtain reversible color change with environment temperatures
from – °C. For practical applications, these multicomponent
thermochromic hybrid fibers are considered as temperature
sensing elements. Its color changes from red (room tempera-
ture) to green ( °C) after contacting with the skin, which can
be potentially used for body or room temperature detection
(FigureD(i)).[] Similarly, Choe et al. reported stretchable and
wearable colorimetric patches based on thermoresponsive plas-
monic microgels.[a] It could be enclosed in a hydrogel film by
sandwiching them between thin polydimethylsioxane (PDMS)
films, and then attached conformably and bent easily on dif-
ferent parts of epidermal skin. These chromatic patches rep-
resent a noteworthy color change in a short time ( s), with a
temperature-sensing resolution of . °C. The LCST ranging
from  to  °C can be easily tuned by salt, surfactant, and
copolymerization, which enables reversible plasmonic coupling
in the raspberry-shaped plasmonic microgels (FigureD(ii)).[a]
By encapsulating the plasmonic microgel film with thin PDMS
film, it can be stretched by up to % with strain-insensitive
colors. These thermoresponsive array patches can serve as an
epidermal colorimetric thermometer for body temperature
through color changes during continuous heating and cooling
steps, which provide information about health risks such as
infections, inflammations, and antigenic reactions.
Another measure is the thermochromic liquid crystals
(TLCs) patterned into pixel arrays on flexible and stretchable
substrates, and then softly laminated on the skin surface.[]
Charaya et al. integrated TLCs in an elastomer matrix sand-
wiched by adhesive polyampholyte hydrogels. The optical
responsiveness to temperature has a precision of . °C
(FigureD(iii)).[] Another example of colorimetric stretchable
and flexible arrays is based on the PNIPAM-coated gold grid/
poly(,-ethylenedioxythiophene) PS sulfonate (PEDOT:PSS)
and thermochromic leuco dye. The sensor evokes a remarkably
high sensitivity of .%°C− at temperatures between  and
 °C,[] enabling stable spatial temperature mapping for med-
ical and health monitoring. Additionally, the applications can
also be extended to the mechanothermochromic drawable pen
by using the PDA–wax composites (FigureD(iv)).[]
2.2. Humidity
Humidity (e.g., environmental humidity, hydration of skin)
is an important physiological parameter indirectly reflecting
health status of an individual, and can potentially be harnessed
to diagnose skin diseases and measure treatment eects by non-
invasive monitoring. Detecting environmental humidity plays
an essential role for a heat stroke alarm since serious illness
such as heat exhaustion or heat stroke may occur when people
work or live in high-temperature and high-humidity environ-
ments, the body is unable to cool itself through sweating.[]
Moreover, skin can adjust its moisture by sweating to indicate
the dehydration during the physiological activities.[]
2.2.1. Materials with Humidity-Induced Tunable Optical Properties
Having looked into these materials, we now provide six
broad classes of material that exhibit humidity-induced color-
changing properties. These include hydrochromic-conjugated
polymers, hydrochromic dyes, photonic crystals, hydrochromic
materials with optical interference, liquid crystals, and perov-
skite nanocrystals. Table 2 summarizes the detection range,
substrates, and applications for these materials.
Hydrochromic-Conjugated Polymers: As temperature increases
the interchain distances also increases, generating partial dis-
tortion of the arrayed π-orbitals in the supramolecular assem-
bled polydiacetylenes (PDA).[] Similarly, humidity is an
additional stimulus to cause the blue-to-red phase change in
PDA.[] By introducing a hygroscopic element into a supramo-
lecularly assembled PDA, it creates hydrochromic-conjugated
polymer that is rapidly responsive with a short response time of
Adv. Mater. 2022, 
with permission.[] Copyright , American Chemical Society. ii) Stretchable and wearable colorimetric patches for smart actuators. The flexible
plasmonic microgel film exhibits vivid color changes between red and grayish violet during the repetitive heating and cooling between  and  °C.
When these array patches are attached on back of the hand, the dierent color patterns represent . °C on the wrist and . °C on the back of the
hand after washing (scale bar:  mm). Reproduced with permission.[a] Copyright , Springer Nature. iii) Epidermal photonic devices. It is attached
on the forearm skin because of van der Waals interactions arisen from the low eective modulus and ultra-thin thickness of the device. A pair of mag-
nified images of these devices includes interspersed color calibration pixels consisting of red, green, and blue dye. Thermochromic liquid crystals in
these devices change color in temperature between  and  °C (scale bar:  mm).Reproducedwithpermission.[]Copyright , Springer Nature.
iv) Mechanically drawable PDA pen. Manually drawing with a PDA–wax pen on paper demonstrate a blue-to-red color change at  °C and the red color
is sustained after cooling the paper to room temperature. On the contrary, the benzoic acid containing PDA–wax pen undergoes completely reversible
color change during multiple heating and cooling cycles. Reproduced with permission.[] Copyright , Wiley-VCH.

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 µs (Figure 3A(i)).[] These materials could be used as hydro-
chromic sensor for mapping human sweat pores.
Hydrochromic Dyes: Hydrochromic dyes include those con-
taining black leuco dye (ODB-)@benzyl -hydroxybenzoate
(BH), ,-naphthalimide integrated fluorophore-receptor sys-
tems, bromination of N-alkylated thiophene diketopyrrolopyr-
role, oxazolidine, oxazine, and phenyleneethynylene as all
these dyes are capable of changing color due to water-triggering
reversible molecular motions.[] They have favorable hydro-
chromic properties with satisfactory color intensity and stability
at ambient conditions. Lee et al. explored the switchable mole-
cules-oxazolidines and oxazines to fabricate the hydrochromic
rewritable papers that can be erased and rewritten dozens of
times with high color quality.[] Such dyes to have a potential
issue with photobleaching.
Photonic Crystals: Compared to leuco dye, colloidal photonic
crystals are typically made of monodispersed spherical par-
ticles, assembled in D structures periodically with dierent
refractive index.[] A variety of materials have been exploited to
formulate such structures, including inorganic materials (e.g.,
magnetic nanoparticles, PS microsphere), polymers, hydrogels,
and hybrid organic–inorganic systems.[,] A thought-provoking
system relying on the organic–inorganic systems (FeO@SiO/
polyethylene glycol acrylate PC humidity sensors) create a large
red-shift of l nm from dark green to light red at humidity
ranging from % to % (FigureB(i)), due to the refractive
indexes changing.[,]
Hydrochromic Materials with Optical Interference: Additional
approach is the layer-by-layer nanostructures assembled by the
hydrochromic materials-based systems with Bragg reflector-
induced humidity-responsive optical properties. The alter-
nating layers are comprised of two materials with dierent
refractive indexes (e.g., silk and titanate nanosheets, TiO/SiO
and phosphatoantimonic acid multilayered structures), where
the reflected color is modified by the presence of moisture.[]
Photonic crystals need to be carefully prepared to create uni-
form color over large areas, which have restricted their use.
Furthermore, the multilayer structure can make the response
time for the required swelling that reflects the humidity change
longer than other approaches.[a] To facilitate its responsivity,
the He group reported a single layer of poly(-hydroxyethyl
methacrylate (HEMA)-co-AA) hydrogel interferometers.[] This
material can be readily prepared with good homogeneity and
uniform color. The instant color change is actuated by the inter-
ference of the light. Another interesting system is uniform gra-
phene oxide (GO) films for optical humidity sensing, resulting
in a large relative humidity range (up to %) caused by the
light interference (FigureA(ii)).[]
Liquid Crystals: Liquid crystals also respond to humidity as
a stimuli.[] The Zhu group developed a cellulose nanocrys-
tals/polyacrylamide composites that manifest an evident color
change as a humidity sensors.[] Its detection range is deter-
mined by this photonic humidity composite sensor consisting
of chiral nematic cellulose nanocrystals, polyacrylamide, and
glutaraldehyde. The composite color shifts from green to red
with humidity increasing from % to %, and its entire color
change corresponds to a wavelength range of  nm. Such a
high sensitivity is attributed to the polyacrylamide swelling with
Adv. Mater. 2022, 
Table 2. Materials with humidity-induced tunable optical properties: detection range, substrates, and applications.
Entry Humidity detection range
[%]
Materials Substrates Applications Ref.
 – Silk-fibroin PCs inverse opals Silica substrate Eco-dying and multifunctional silk fabrics []
 – Multilayer film made of a silk and titanate
nanosheets
Quartz substrate Optical filters/biosensors [a]
 – FeO@C colloidal nanoparticles
in PAAm glycol gel matrix
– Visually readable colorimetric sensors [a]
 – Bragg stacks of TiO/phosphatoantimonic
acid (HSbPO) and SiO/HSbPO
Quartz substrate Smart windows [b]
 – PS microspheres in PAAm hydrogelsupraballs – Stimuli-responsive photonic devices []
 – Macroporous cross-linked PHEMA/PETPTA
(poly(ethoxylated trimethylolproane triacrylate))
PC films
Glass substrate Optical switching, Chemical/
biologicalsensing
[]
 – P(HEMA-co-AA) hydrogel interferometer Silicon wafer Breath-controlled information
encryption/humidity indicators
[]
 – Hygroscopic element in PDA supramolecular
assemblies (hydrochromic-conjugated polymer)
Polyethylene terephthalate
(PET) film
Fingerprint analysis/clinical
diagnosis of malfunctioning sweat pores
[]
 – Hydrochromic fluorescent materials (,-naph-
thalimide integrated fluorophore-receptor system)
Glass microscope slide Bioimaging [b]
 – Oxazolidines Paper Rewritable papers [e]
 – Phenyleneethynylene Paper Security labels/optically masked barcodes [f]
 – Reduced graphene oxide coupled
with bromophenol blue
PET substrate Wearable electronics []
 – Graphene oxides films – Disposable humidity sensors for
packaging/health/environment monitoring
[]

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Adv. Mater. 2022, 
Figure 3. Materials with tunable optical properties for wearable epidermal sensors in humidity monitoring. A) Mechanisms: i) CC bond rotation in
PDA. Humidity-induced increase in interchain distance results in partial distortion of the arrayed p-orbitals, leading to the phase transformation of PDA
supramolecules. The purple-colored balls represent hygroscopic elements. Reproduced with permission.[] Copyright , Springer Nature. ii) Optical
interference in GO. The humidity-induced swelling of the GO multilayers results in optical interference, leading to a visible color change. Carbon, oxygen,
and hydrogen atoms are represented by gray, red, and white spheres, respectively. Water molecules are represented by yellow stars. Reproduced with
permission.[] Copyright , American Chemical Society. iii) Reversible color-changing in CsPbBr nanocrystals. Density functional theory simulation
shows the reversible transformation process. The perovskite structure could be destroyed easily upon humidity. Once the sample is dried and the mois-
ture is removed, leading to a reversible luminescence switching. The unit cell structures represent before and after the extraction of CsBr from CsPbBr
(Purple Cs, brown Br, gray Pb, red O, white H). Reproduced with permission.[] Copyright , Wiley-VCH. iv) Thin-film interference in PSS/PDDA layer-
by-layer. The molecular structures of PDDA and PSS and the interference of the same-wavelength light across the PDDA/PSS coating are demonstrated.
The swelling and shrinking of the PDDA/PSS coating are based on the humidity change, which leads in the color change of the coating. Reproduced with
permission.[] Copyright , Royal Society of Chemistry. B) Materials: i) Magnetic photonic crystals. Magnetic photonic crystals in PEGDA–PEGMA film
shows dark green, green, yellow, and red corresponding to relatively low and high humidity environments. This PEGDA–PEGMA matrix could strongly
absorb water and therefore swells when exposed to moisture, leading to the increase of the lattice constant and color change (scale bar:  cm).Repro-
ducedwith permission.[] Copyright , Royal Society of Chemistry. ii) Cellulose nanocrystals (CNCs)/PAAm composites. The glutaraldehyde bridges
CNCs with PAAm to form the iridescent films as a function of humidity. A distinct color change is caused by polyacrylamide swelling with water and
thus enlarging the helical pitch of the chiral nematic structure. Reproduced with permission.[] Copyright , American Chemical Society. iii) CsPbBr
nanocrystals. Reversible luminescence switching between fluorescent CsPbBr nanocrystals@silica nanoparticles and nonfluorescent CsPbBr nanocrys-
tals@silica nanoparticles are demonstrated. After being exposed to humidity, the yellow powder becomes gray and the green emission disappear while
the yellow color and green emission recover when moisture in these samples is evaporated in air. Reproduced with permission.[] Copyright , Wiley-
VCH. iv) Polyelectrolyte coatings. The coatings exhibit purple, blue, green, yellow, and red with increasing relative humidity from % to %. It can be
directly employed as visible humidity-indicator strips. Reproduced with permission.[] Copyright , Royal Society of Chemistry. C) Ashby chart: the plot
of humidity detection range as a function of hydrochromic color-changing zone. D) Applications: i) D touchless sensing displays. These systems become
reversible when water is absorbed into or diused from the block copolymer. The incorporation of LiTFSI into these systems becomes more sensitive
to environmental humidity. The structural color is changed from blue, green, to orange when a finger approaches its surface. Three dierent full-colored
structural-colored images are written and erased repeatedly on these sensing displays. Reproduced with permission.[] Copyright , AAAS. ii) Flexible
nanowire cluster as wearable humidity sensors. The anisotropic structure of nanowire can be obtained by a one-step sputtering process. The preparation
of the flexible nanowire cluster film is demonstrated with star pattern on PDMS substrate. The as-obtained wearable sensing bracelet is presented under
dry and sweaty conditions with dierent color displays. Reproduced with permission.[c] Copyright , Wiley-VCH.

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water and thus enlarging the helical pitch of the chiral nematic
structure of cellulose nanocrystals. Another example is blue
phase liquid crystal networks. Its reversible humidity-driven
color-changing is induced by the hydrogen bond breaking and
conversion into a hygroscopic polymer subsequently.[]
Perovskite Nanocrystals (CsPbBr): Lastly, in one very striking
example, the CsPbBr perovskite nanocrystal located on in the
nanoconfined spaces of mesoporous silica matrix showed a
reversible hydrochromic properties (FigureA(iii),B(iii)).[] The
CsPbBr can be easily reconstructed upon water treatment, to
form CsPbBr. When water is slowly evaporated in the air, the
CsPbBr can react with Cs+ and Br− (confined in the porous
silica matrix) to reversibly form CsPbBr. Even though the
perovskite nanocrystals have widespread applications such as
anticounterfeiting, energy-saving devices and smart displays,
their high toxicity and instability prohibit their applications as
wearable devices.[]
When environmental temperature increases above  °C and
relative humidity is between % and %, people are easily
aected by transient ischemic attacks because of sweating a
lot without drinking enough fluids.[] Therefore, monitoring
environmental temperature and humidity independently at the
same time for avoiding heatstroke can be achieved by choosing
entries Nos. , , ,  or  from Table and entries Nos. ,  or
 from Table. Moreover, the level of skin humidity is an impor-
tant indicator of water loss and dehydration during exercise.
Besides, the hydration level of elderly people plays a crucial role
in reminding them of drinking water instantly especially due
to dementia since low level of skin humidity may lead to more
serious diseases.[] Accordingly, choosing materials in entries
Nos.  and  as sensing candidates for monitoring humidity
from  to % is a promising way to fabricating sweat-based
wearable devices during running, jumping, and bouncing or
wearables for monitoring the hydration level of elderly people
with forgetfulness, limited social skills, and impaired thinking
abilities. Based on FigureC and Table, other hydrochromic
materials are employed in smart windows, rewritable papers,
and bioimaging.
2.2.2. Wearable Epidermal Sensors with Visible Readouts for
Humidity Monitoring
Poly(diallyldimethylammonium) (PDDA)/poly(styrenesulfo-
nate) (PSS) polyelectrolyte homogeneous thin film coatings
have been explored as colorimetric humidity-sensitive mate-
rials for a touchless manipulating system with an extremely
short response time of ≈ ms (Figure A(iv)).[] Reversible
optical properties arising from the swelling/contracting of the
coating with humidity results in changes in the optical inter-
ference with visible light of the device (Figure B(iv)). Based
on this principle, Park and co-workers presented a thin, solid-
state block copolymer (BCP) consisting of poly(ethylene glycol
diacetate) (PEGDA) oligomers into quaternized -vinyl pyri-
dine (QVP) layers and then chemically cross-linking the pre-
cursors through ultraviolet (UV) exposure (Figure D(i)).[]
When a hygroscopic ionic liquid ink (L-ethyl--methyl-
imidazolium bis-(trifluoromethylsulfonyl)-imide/lithium
bis(trifluoromethanesulfonyl)imide, EMIMTFSI/LiTFSI) is
absorbed into the domains of the interpenetrated network,
the multiple-order photonic reflections give an even richer
color change. The color changes are reversible with humidity
changes, which provide the opportunity for a D touchless
sensing display or anticounterfeiting labels. These user-interac-
tive displays have been validated by visualizing local changes
in humidity, creating a D position of a human finger as a
function of finger-to-display distances. They also utilized these
materials for creating thermoadaptive block copolymer struc-
tural color wearable electronics with temperature and electricity
monitoring (described in part . for multisignal detection).
Wearable plasmonic devices with the high flexibility, ultrathi-
nness, and light weight are another pattern to be acted as a
promising platform to detect the humidity (Figure D(ii)).[c]
Metallic (e.g., Al, Ag, Au, and Pt) anisotropic structures of
nanowire are easily obtained by sputtering at room tempera-
ture. Metallic nanowires gradually deposit on the surface of
disordered anodic alumina oxide substrate to prepare nanowire
cluster film, and integrate these films with PDMS to fabricate
flexible devices. Due to SPR-based color changing and high
sensitivity to humidity, the flexible nanowire cluster films are
made into a bracelet to monitor sport sweating conditions with
wireless visualization, representing a sensing threshold of
. mg cm−.
2.3. Light
Sunlight reaching earth surface spectrally encompasses ≈%
UV, ≈% visible, and ≈% infrared light. Although each com-
ponent is necessary for life sustainability, UV light is the most
dominant negative influencing factor on human health. It can
be segregated into three subsets, i.e., UV-C (–nm), UV-B
(–nm), and UV-A (–nm).[] When UV light pen-
etrates and interacts with skin surface, its radiation is benefi-
cial for health outcomes by providing vitamin D requirement
for mental well-being.[] Notwithstanding this, sunlight overex-
posure is inclined to cause skin damage such as sunburn, ery-
thema, melanoma or even skin cancer.[] From the applications
standpoint, skin-mounted UV sensors are precisely designed
and personalized for monitoring of exposure at various posi-
tions across the body.
2.3.1. Materials with Light-Induced Tunable Optical Properties
Light-induced color-changing molecules present unique oppor-
tunities to detect a plethora of targets, thus oering another
intriguing pathway for light detection (e.g., UV, visible, and
near-infrared (NIR) light). Table 3 summarizes the detection
range, substrates, and applications for these materials.
Materials with Reversible Optical Properties: The reversible
photoswitchable color-changing properties can be induced by
the molecular ring opening and closing trans–cis isomerization.
Molecules that exhibit these properties includes azobenzene,
spiropyran, spirooxazine, diarylethene, or fulgide structures
(Figure 4A(i)).[] The conformational change depending on the
UV and visible light contributes to the color-changing proper-
ties. For instance, self-assembled monolayers of spiropyran
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modified with a disulfide-terminated aliphatic chain on poly-
crystalline gold surfaces can be used as sensors as when
exposed to UV light, the spiropyran converts into the merocya-
nine.[] The photochemical switching caused by both UV and
NIR light can be reversed with both visible light or heating.
Comparatively, the photochromic spirooxazine embedded in a
functionalized sol–gel hybrid matrix (phenyl-modified ormosil
matrices) obtains a deep purple color upon irradiation with
UV light (FigureB(i)).[] These colored systems can only be
restored to their original colorless state under visible light.
These reversible optical properties can be manipulated by alter-
nating irradiation with UV and visible light.[] Lastly, the trans-
to-cis state conformational change of azopolymers creates the
variations in the π–π stacking geometry, producing the promi-
nent morphology dierence with reversible crystal–liquid tran-
sitions (Figure B(ii)).[] This phenomenon is converted into
reversible optical properties, which could be controlled under
irradiation with UV and green light.
NIR Light-Responsive Materials: Unlike UV/vis light, low
energy NIR (– nm) light possesses lower phototox-
icity and higher tissue penetration depth in living systems,
laying a solid foundation for in vivo applications including cell
signal sensing, cancer diagnosis, drug delivery, visual regula-
tion, and neuromodulation.[] In particular, the Jiang group
fabricated NIR light-responsive dynamic wrinkles by using
a carbon nanotube (CNT)-containing PDMS elastomer as the
substrate for the bilayer systems, with poly(n-butylacrylate-co-
anthracene-containing styrene) functional polymers serving as
the top layer (FigureB(iii)).[] A visible pattern is obtained by
selectively exposure to  nm UV light due to the light scat-
tering of the wrinkle. It can also be easily erased by  nm
UV light because of dedimerization of the polymer and could
encode other optical information.[] The samples with the ini-
tial recorded information could temporarily be erased by NIR
irradiation and dynamically reversed to their original state. This
may find potential application in dynamic QR codes and bar-
codes anticounterfeiting.
A summary of the diverse range of materials used for devel-
oping optically based light monitoring sensors are summarized
in Table  and in FigureC, showing a plot of the wavelength
range in which dierent photochromic materials operate. Mate-
rials for a wearable UV dosimeter to avoid sunburn or even
skin cancer are exemplified by entry Nos. , , , – with
irreversible optical properties are elected as sensing elements.
Among them, Nos.  and  exhibit the most distinctive color-
changing obtained highest detection sensitivity under gradually
increasing UV light intensities. In addition, other materials
with light-induced reversible optical properties in Ashby chart
can be utilized for anticounterfeiting, smart textiles or visual
displays.
2.3.2. Wearable Epidermal Sensors with Visible Readouts for Light
Monitoring
The materials used for light monitoring can be subdivided into
two categories: reversible color-changing and irreversible color-
changing. Based on this, these materials are mainly utilized for
information encryption and UV-dosimeters.
The reversible photoswitchable color-changing properties
can be induced by photochemical reduction (e.g., Mo+→Mo+),
leading to the electron transfers from the polyacrylamide layer
to the phosphomolybdic acid layer.[] The flexible, switch-
able, and wearable light-responsive photochromic fabric is
Adv. Mater. 2022, 
Table 3. Materials with light-induced tunable optical properties: detection range, substrates, and applications.
Entry Light detection range [nm] Materials Substrates Applications Ref.
 UV nm Green  nm Azobenzene-based polymer Al/polyimide/sapphire – []
 UV-A –nm
UV-B – nm
Crystal violet lactone Congo red PDMS/acrylic adhesive
(Scapa healthcare)
Epidermal near field communication platform [d]
 UV light AgNPs Cellulose paper Wearable devices with low-resource settings []
 UV nm Visible light Bis(dithiazole)ethene – Selective gating photo-switching []
 UV nm Visible light Spirooxazine and its derivatives Glass slides Light-addressable nanoscale devices [,]
 UV nm Visible light Spiropyran molecules Polyester fabric/Gold
surface
Smart textile/biocompatible switching systems []
 UV nm Visible light Polyoxometalate-co-PAAm photochromic
hydrogel
– Flexible visual displays/wearable devices []
 NIR PAN CNT-PDMS elastomer QR codes for anticounterfeiting []
 UV-A –nm
UV-B –nm
UV-C – nm
PMA–lactic acid Transparent film filters UV dosimeters for wearable sensors []
 UV light Phosphomolybdic acid and PAAm Cotton fabrics Wearable, optical information storage and infor-
mation security encryption
[]
 UV  nm Anthraquinone dye, Remazol brilliant blue R – Food fresh indicator []
 UV light TiO, PVP, and food dye Paper Sunburn sensors []
 UV light UV-driven acid-release agent: chloral hydrate,
thymol blue, malachite green, polyvinyl
butyral, diphenyliodonium chloride
Polypropylene Indicator for preventing sunburn/erythema/skin
cancer
[]

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Figure 4. Materials with tunable optical properties for wearable epidermal sensors in light monitoring. A) Mechanisms: i) Reversible color-changing.
Photoswitchable molecules include azobenzene, spiropyran, spirooxazine, diarylethene, and fulgide. Reproduced with permission.[] Copyright ,
Wiley-VCH. ii) Photochromic activator and dye. Photochromic activator (-phenoxyphenyl)diphenylsulfonium triflate (PPDPS-TF)) and crystal violet
lactone (CVL) are provided for sensing in the UV-A band. PPDPS-TF and Congo red are provided for sensing in the UV-B band. Reproduced with
permission.[d] Copyright , Wiley-VCH. B) Materials: i) Spirooxazines. Recording–erasing process in a photochromic organic–inorganic hybrid
coating is exposed to UV or visible radiation through a mask. Reproduced with permission.[] Copyright , Royal Society of Chemistry. ii) Azopoly-
mers. Light-triggered phase transition of azopolymer. Corresponding appearances of trans-azopolymer and cis-azopolymer films under cross-polarized
optical microscopy (upper) and optical microscopy under continuous UV and green light illuminations (lower) (scale bars:  μm). Reproduced with
permission.[] Copyright , PNAS. iii) NIR-responsive dynamic wrinkles. Switchable transparency is ascribed from the reversibility of the wrinkled/
wrinkle-free surface. (The logo is underlying the sample and insets are AFM images.) Dynamic information record is collected through controlled wrinkle
formation via dierent photomasks (scale bars: mm). Reproduced with permission.[] Copyright , AAAS. C) Ashby chart: the plot of light detec-
tion range as a function of photochromic color-changing zone. D) Applications: i) Flexible and wearable image storage devices. Patterns are displayed
on the cotton fabric for multiple times by using a photomask and UV lamp. After min of UV light irradiation, the exposed area turns blue while the
unexposed area remains colorless. Reproduced with permission.[] Copyright , Elsevier. ii) Wearable nanoplasmonic patches for sun/UV exposure.
It summarizes those patches that are usually most sensitive according to each skin type. Reproduced with permission.[] Copyright , American
Chemical Society. iii) Epidermal UV dosimeters. A multimodal, colorimetric epidermal device is demonstrated with capabilities in UV light sensing.
A butterfly shaped device is exposed under exposure of UV-A and UV-B. Dierent color in the device represents dierent light intensity of UV light.
Reproduced with permission.[d] Copyright , Wiley-VCH. iv) Paper-based wristband for UV-A, B, and C dosimeters. A paper-based solar UV wrist-
band for a skin type V is demonstrated after % minimal erythemal dose exposure. Reproduced with permission.[] Copyright , Springer Nature.

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manufactured by layer-by-layer self-assembling of phosphomo-
lybdic acid and polyacrylamide.[] This prepared fabric changes
from colorless to blue rapidly (within  min) under UV light
and maintain its color intensity more than  days under
ambient conditions. It could be exploited for image informa-
tion storage (FigureD(i)).[]
The materials with irreversible optical properties are promi-
nently employed for the UV-dosimeters for protecting the
human body from sunburn. The Gooding group reported
a sun exposure sensor by using TiO as a photocatalyst to
degrade the food dyes, stimulating the gradual color-fading
of this polvinylpyrrolidone-based films.[] UV light-induced
materials, silver nanoparticles, are another route to be deco-
rated in the nanopaper for the visible color changing induced
by the size modulation of silver nanoparticles (FigureD(ii)).[]
This low-cost and colorimetric detector is able to evaluate the
dierent degrees of risk for ongoing skin cancer, which facili-
tates the healthcare in UV-alerted nanophotonic-based set-
tings. To measure the dierent types of UV light, stretchable
materials for UV colorimetric sensing combine photoactivators
((-pheno xyphenyl)diphenylsulfonium triflate, PPDP-TF) and
color changeable dye (Crystal violet lactone and Congo red) in
an elastomeric matrix of polydimethylsiloxone, which control
overall sensitivity and spectral responsivity to UV-A and UV-B
(FigureA(ii),D(iii)).[d] Its adaptable, wearable, long-term moni-
toring, and nonirritating integration with the skin at dierent
positions across the body fuels so much interests to prevent UV
exposure-caused diseases. To expand its detection range, Bansal
and co-workers spectrally dierentiated between UV A, B, and
C by adopting phosphomolybdic acid as a multiredox photo-
electrochromic polyoxometalate molecule and lactic acid as an
eective e– donor.[] Lactic acid is able to reduce phosphomo-
lybdic acid molecules to dierent redox states under dierent
types of UV radiation, resulting in the naked-eye monitoring
of blue smileys on the paper as real-time solar dosimeters
(FigureD(iv)).
2.4. Mechanical Force
Typical human motion contains large-scale motion (e.g.,
jumping, pushing squatting, bending hands, arms, legs, and
spinal) and miniscule motion (e.g., subtle muscle movements
of the face, chest, and throat during emotional expression,
breathing, and speaking). When forces are transduced to the
epidermal skin, the physiological signals derived from skin
allow us to measure heart rate, respiration rate, pulse rate,
and blood pressure.[] The mechanical forces generated from
the body movements are often distributed in pressure regimes
< kPa for healthcare applications, including hearing aid
devices (ultralow-pressure regimes < Pa), medical diagnosis
systems (< kPa), wearable bracelets for blood pressure and
pulse rate monitoring (<kPa).[] In addition, wearable sen-
sors capable of measuring various strain range have also been
exploited to measure body movements to diagnose diseases or
aid in rehabilitation, including large-scale strains (up to %),
smiles or blinking (<%), and respiratory disorder patients
while drinking or eating (<.%).[] A dominant segment
of wearable devices focuses on mechanophysiological signal
monitoring. These materials and its corresponding detection
range, substrates, applications are summarized in Table 4.
2.4.1. Materials with Mechano-Induced Tunable Optical Properties
Structural colors of materials (FeO, TiO, SiO PCs) can be
readily regulated upon mechanical stimuli by changing the
structural parameters such as interplane spacing and dirac-
tion angle as induced by mechanical strain.[] Mechanochromic
structural-colored materials with periodic ordered nanostruc-
tures interact with incident light of a certain wavelength,
producing the Bragg’s diraction.[] Structural color-based elas-
tomers and hydrogels are mainly discussed in this section. Yang
and co-workers built up a bilayer elastomeric film consisting of
silica nanoparticles/PDMS thin layer and a bulk PDMS thick
layer, which could be reversibly switched from transparent to
opaqueness and produce angle-independent reflective colors
depending on mechanical stretching (Figure 5A(i),B(i)).[] The
switchable optical property can be fully reversed during 
stretching/releasing cycles. Based on this, metastable SiO col-
loidal crystalline arrays are fixed into the hydrogel matrix of
EG and poly(ethylene glycol) methacrylate (PEGMA) through
photopolymerization.[] The invisible photonic patterns are
instantly and reversibly hidden and revealed due to the nonu-
niform change in photonic structure upon stretching/releasing.
Simultaneously, FeO@C superparamagnetic colloidal
nanocrystal clusters formed D PC chains in polyacrylamide
matrix is another avenue to convey strain-sensitive optical
properties.[]
Other architectures, including AuNPs patterned D arrays,
D aggregachromic dyes entrapped within polyurethane elas-
tomers, D silicon high-contrast metastructure, and U-shaped
aluminum nanowires on polyurethane, are described with
high strain sensitivity by stretching the flexible membrane.[]
The Sun group arranged the device containing a transparent
rigid film (polyvinyl alcohol (PVA)/laponite composite) tightly
attached on PDMS layer, which reversibly validate conspicuous
visual change based on the transparency changing mecha-
nochromism.[] The stretching and releasing of such a device
give a high strain-responsive sensitivity with a gauge factor
of ≈.. The performance of the materials is surprisingly
higher than some strain sensors based on electrical response
(FigureA(ii),B(ii)).[]
Some of the materials with mechano-induced tunable optical
properties with the detection range of strain/pressure rel-
evant for specific healthcare applications are listed in Table .
The plot of strain detection range as a function of mechano-
chromic color-changing zone is displayed in FigureC. Table
entries Nos.  and – are ideal sensing parts for large-scale
strain-induced color-changing when patients rehabilitate after
suering in serious injury. In addition, entries Nos. , , and
 could be used for subtle body movements measurement
depending on the mechanochromic properties. Another way
to demonstrate mechanochromic phenomenon is pressure-
induced tunable optical properties. For wearable healthcare sen-
sors with body movements monitoring, entry No.  with low
vibrational forces could finds utility for hearing devices, and
entry No.  with <kPa pressure would be utilized for blood
Adv. Mater. 2022, 

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pressure and pulse rate monitoring. In fact, most pressure-
induced color-changing materials are considered as human–
machine interfaces compatible as user-interactive devices.
2.4.2. Wearable Epidermal Sensors with Visible Readouts for
Mechanical Force Monitoring
Mechanochromic materials have been widely used in applica-
tions such as smart windows (FigureD(i)), displays, camou-
flages, and security.[] Generally, these materials are inserted
into the adhesive polymer to form the composites as wearable
sensors. Mechanochromic photonic fiber sensors consisting
of PDMS and polyisoprene-PS triblock copolymers maintain
repetitive stretchability of over %, and respond to compres-
sion with a predictable and reversible optical properties for
medical textiles (Figure D(ii)).[] To increase the mechano-
induced color-changing sensitivity, the Zhao group developed a
series of strain-sensitive color patches for in vitro monitoring
(Figure D(iii)).[,] The stable stretchability and brilliant
structural color of polydopamine-infiltrated PEGDA/polyure-
thane inverse opal film make the bio-inspired hydrogel-based
wearables high potential for tissue engineering and biomedical
applications. Another fascinating platform is introduced by a
Adv. Mater. 2022, 
Table 4. Materials with mechano-induced tunable optical properties: detection range, substrates, and applications.
Entry Mechanical force detection
range
Materials Substrates Applications Ref.
Strain  –% An adhesive polydopamine layer and an antiadhe-
sivePEGDA layer in an inverse opal scaold
– Tissue engineering []
 –% Nanoparticle-in-micropore architecture in porous
mechanochromic composites consisting of spiropyran,
silica nanoparticle and PDMS
PDMS User-interactive devices/
smart robotics/wearable
healthcarediagnosis
[]
 –% TiO nanoparticles layer on period linear grating
nanostructures
PDMS Tunable optical devices [b]
 –% Strain-dependent cracks and folds based on the PVA/
laponite composite on PDMS; PVA/TiO on PDMS/
rhodamine composite film; PVA/laponite composite
containing fluorescein on PDMS/YO: Eu layers; PVA/
TiO on PDMS/rhodamine composite film on PDMS/
carbon black
PDMS Mechanically controlled surface
engineering
[]
 –% SiOPDMS Smart windows []
 –% SiO/EG/PEGDA PCs PDMS Security/antifraud applications
in daily life
[]
 –% U-shaped aluminum nanowires on polyurethane PDMS Colorimetric sensors for strain
mapping
[d]
 –% Aggregachromic dyes in polyurethane elastomer – Stress sensors/Alarming
packages
[b]
 –% HCM PDMS Flexible optical devices [c]
 –% AuNPs patterned D arrays PDMS Strain sensors [a]
 –% PC fibers consisting of PS nanoparticles electrophoretic-
deposited onto continuous aligned-carbon-nanotube
sheets
PDMS Smart fabrics []
 –% PS core–polymethyl methacrylate (PMMA) interlayer–
PEA shell microspheres mechanochromic fibers
Black sandex fiber Smart wearable textiles []
 –% Photonic gel with FeO within N-hydroxymethyl acryl-
amide (NMA) and N-vinylcaprolactam (VCL)
Glass substrate High-precision displays []
Pressure  –.kPa D FeOnanoparticle photonic chains embedded in
a poly(MBA (N,N′-methylene-BIS-AAm)-EG-AAm)
hydrogel
Glass slides Mechanochromic displays []
 – kPa Dye-PDMS (thermochromic elastomer composite) PET Interactive interface devices
displays
[]
  kPa ZnS:Cu phosphor particles were mixed with the PMMA
matrix
PDMS Anticounterfeiting/security
surveillance/Illumination
[]
 .– MPa ZnS:Mn particles PET films Smart sensors/human–machine
interfaces
[]
  kPa Rare-earth (RE) codoped long-lasting lumines-
centmaterials (RELMs)
PDMS Stretchable electronics []

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Adv. Mater. 2022, 
Figure 5. Materials with tunable optical properties for wearable epidermal sensors in mechanical force monitoring. A) Mechanisms: i) Structural color
in colloidal crystallines. Brilliant color change is caused by pushing or pulling, suggesting that the elastic polymer networks could alter the crystal
structure eectively. Reproduced with permission.[] Copyright , Wiley-VCH. ii) Strain-dependent cracks and folds. The device can reversibly display
conspicuous visual change between a transparent state and an opaque state upon stretching and releasing. The opacity of the stretched state can be
ascribed to strong trapping and scattering of light because of the strain-dependent cracks and folds. Reproduced with permission.[] Copyright ,
Springer Nature. iii) Reversible ring-opening in NP-MP structures. A hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mecha-
nochromic composites includes spiropyran, PDMS, and silica nanoparticles. Spiropyran molecule undergoes a mechanical force-generated reversible
ring-opening from colorless to colored merocyanine. Reproduced with permission.[] Copyright , Wiley-VCH. iv) Mechanoluminescence ZnS:Mn.
This device are fabricated by employing wurtzite structure ZnS:Mn particles as mechanoluminescent materials in the middle, covered by PET layers.
These transparent layers are favorable to the transmission of yellow light emitted by ZnS:Mn nanoparticles under pressure. Reproduced with permis-
sion.[] Copyright , Wiley-VCH. B) Materials: i) SiO nanoparticles. Two combined semicircles are hidden in relaxed state and revealed by squeezing
or stretching. The red color is due to the expansion of crystal lattice in vertical orientation. The blue color is due to the compression of crystal lattice. The
unchanged green color is because the left semicircle is cross-linked by PEGDA and presents “hard” characteristics (scale bar:  cm). Reproduced with
permission.[] Copyright , Wiley-VCH. ii) PVA/laponites. These composites are tightly bonded to a PDMS layer to form the mechanochromic device
via strain-dependent cracks. Reproduced with permission.[] Copyright , Springer Nature. iii) TIEL composite materials. For the ZnS:Cu phosphor,
luminescence can be excited by stress, i.e., triboluminescence. When a pen-like object is used to rub the electrification layer, transient light emission
from the luminescent layer can be distinguished along the motion trajectory. A continuous trajectory showing its live luminescence image and corre-
sponding mapping of the luminescence intensity is displayed (scale bar:  cm). Reproduced with permission.[] Copyright , Wiley-VCH. iv) Thermo-
chromic dye on a pressure sensor. When the pressure is increased above kPa on the thermochromic panel, a change in the visible color switching
area is observed. It demonstrates that the external pressure can be quantified by the resistive heating of the thermochromic panel (scale bar:  mm).
Reproduced with permission.[] Copyright , Wiley-VCH. C) Ashby chart: the plot of strain detection range as a function of photochromic color-
changing zone. D) Applications: i) Smart windows. The transparent silica nanoparticles/PDMS film looks translucent at ≈% strain and is completely

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hierarchical nanoparticle-in-micropore architecture in porous
mechanochromic composites including spiropyran, poly-
dimethylsiloxane, and silica nanoparticles to enhance sensi-
tivity and stretchability (FigureA(iii),D(iv)).[] In this design,
the spiropyran mechanophore undergoes a force-induced
reversible ring-opening process from colorless spiropyran to
colored merocyanine. These porous mechanochromic com-
posites are operated as wearable E-skins and dual-mode static/
dynamic touch/stretch and audio sensors without any external
power source. In addition, Liu et al. also used spiropyran to
create mechanochromic organohydrogel as ionic skin to mimic
body-bruising for injury indication.[]
Pressure-induced-mechanical motion brings about the elec-
troluminescence light emission (FigureA(iv)).[] Primarily, the
Wang group reported a self-powered flexible pressure sensor
on ZnS:Mn wurtzite structures for a real-time, high-spatial D
pressure dynamic mapping of the handwritten signatures.[]
Piezopotential-induced light emission is due to the noncentral
symmetry of wurzite-structure piezoelectric ZnS. Also, Zhu and
co-workers portrayed triboelectrification-induced electrolumi-
nescence (TIEL) device with good stability, extremely low stress
threshold, and high responsivity (Figure B(iii)).[] The mate-
rialtransforms dynamic pressure into luminescence. As friction
from sliding across the material occurs, the tribocharging den-
sity on the moving objects accumulates rapidly, and the electric
potential is altered abruptly to excite the phosphor electrolumi-
nescence along the motion trajectory.[] It is visualized in high
resolution for position locating and motion tracking for sensing
applications. In a related study, Kim et al. developed an optoe-
lectronic-free dynamic interactive sensor based on the incorpo-
ration of a thermochromic dye on a resistive pressure sensor,
comprising thermally conductive Ag–PDMS and electrically
conductive carbon black–PDMS composite (Figure B(iv)).[]
The composite material is explored as an electrode to maxi-
mize Joule heating. The high conductivity of silver micropar-
ticles enhances the thermal conductivity of Ag–PDMS, thus
delivering heat from the electrodes to the thermochromic elas-
tomer. The color switching mechanism of the thermochromic
elastomers is based on the colored state with a lactone open
ring chain (< °C); and a transparent state with a closed ring
chain (>°C).[] A dierent strategy is a soft composite devices
“optical filters,” consisting of liquid metal wires, phosphores-
cent particles, and thermochromic pigments enclosed in an
elastomeric matrix.[] The resistance of the liquid metal wires
strongly rely on pressure, while Joule heating of the liquid metal
alters the color, intensity, and wavelength of phosphorescence
of thermochromic pigments. The composites are acknowledged
as a pressure sensor by transitioning the mechanical force to
thermal response and ultimately an optical signal.[]
Currently in literature, focus is geared toward performance
metrics such as sensitivity and reversible optical properties.
With emerging technologies, significant progress has been
made in higher accuracy for visualized sensing.[] However,
most of these systems are still in its infancy. Notwithstanding
these parameters are crucial, long-term stability is also of great
importance for practical applications.[c] These challenges
inspire us to improve personalized wearable sensor systems
with more accurate and reliable visible readouts in physical
signal monitoring. It could provide crucial information for early
disease interventions and prediction of future health issues,
especially in a globally aging community.[]
3. Materials with Tunable Optical Properties
for Wearable Epidermal Sensors in
Electrophysiological Signal Monitoring
In addition to physical signals, electrophysiological signals
monitoring can help detect abnormal vital signs since the
organ, tissue, and neuronal activities are related to electrical
potential.[] In particular, typical electrophysiological signals
include electrocardiograph (ECG), electromyogram (EMG),
electroencephalography (EEG), and electrooculogram (EOG),
which reflects the activity of its corresponding organs or tis-
sues.[] Its related diseases such as arrhythmia can be dia-
gnosed and tracked using electrophysiological monitoring
systems.[] This section summarizes materials with electrically
induced tunable optical properties in skin attachable electro-
physiological sensors for health monitoring.
3.1. Electrophysiological Signal
Bioelectrical signals emanate from the cardiac systole-diastole
cycle (ECG), brain activity (EEG), eye movement (EOG), and
muscle movement (EMG).[] ECG gives precise information on
the activity in the ventricles and atria for cardiovascular health;
EEG presents electrical activity in the brain for diseases such as
sleep apnea, epilepsy, and other neurological disorders; EMG
records the neuromuscular disorders diagnosis and muscle
pain research; and EOG is capable of monitoring the drive
fatigue and the mental disorder diagnosis in clinical.[c,,]
These signals can be tracked by materials and device designs
with electrically induced tunable optical properties.
3.1.1. Materials with Electrically Induced Tunable Optical Properties
Recent surge in display technology has spurred the develop-
ment of electric-induced tunable optical properties. Diver-
sified classes of materials are interpreted to demonstrate
Adv. Mater. 2022, 
opaque at ≈%. The changes in the optical properties could be attributed to the microroughness from wrinkles and nanovoids between PDMS and
silica nanoparticles. Reproduced with permission.[] Copyright , Wiley-VCH. ii) Medical textiles. It could be used for colorimetric sub-bandage
pressure indicators in compression therapy (upper, scale bar: µm). The color of fiber changes from red to orange, yellow, green, and finally blue as
the strain increases and the distance between layers decreases (lower, scale bar: µm). Reproduced with permission.[] Copyright , Wiley-VCH.
iii) Color patches in vitro. Bioinspired structural color patch is attached on porcine myocardium tissue. No residue or crack is observed between the
patch and tissue at stretching, distorting, bending, or immersing in water (scale bar: cm). Reproduced with permission.[b] Copyright , AAAS.
iv) Mechanochromic electronic skins. This device is used for the detection of dierent hand movements. The spatiotemporal detection of both writing
force and speed is stimulated by this dual-mode mechanochromic and triboelectric force. Reproduced with permission.[] Copyright , Wiley-VCH.

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electrochromic properties, such as transition metal oxides,
metal coordination complexes, viologens, organic molecular
dyes, and organic conducting polymers. All these materials and
its corresponding detection range, substrates, applications are
summarized in Table 5.
Transition Metal Oxides, Metal Coordination Complexes, Violo-
gens, Organic Molecular Dyes, and Organic Conducting Polymers:
The electrochromic colors of these materials originate from
intervalence charge-transfer optical transitions under oxida-
tion and redox states (Figure 6A(i,ii)).[] Prussian blue-based
nanoparticles are manufactured into a multicolor palette thin
films, indicating the colorless-blue-green-yellow color-switching
electrochromic properties.[] Meanwhile, organic conducting
polymers polyaniline (PANI) and poly(,-dimethyl-, pro-
pylenedioxythiophene) (polyProDOT-Me) are covered with
Au and Al metallic nanoslit to create a flexible display with
fast switching speed (Figure B(i)).[] The two polymers
form extremely thin covers on the metal structures with well-
controlled thicknesses. The surface plasmon polariton of the
metals maximally interacts with the electrochromic films,
and the electric field leads to the oxidation state or reduc-
tion state in polymer films, delivering the dierent colors on
areas imaged in transmission.[] In another example, the
coordination nanosheets, terpyridine-Fe(II) complex on indium
tin oxide (ITO) glasses realize distinguished electrochromism
from red at  V to orange-yellow at . V and green at . V
(Figure B(ii)).[] The device possessed outstanding stability
over  cycles. Ultimately, the transition metal oxides WO
thin layers as electrochromic device can exhibit a wide color
gamut, which are generated prior to applying voltages, with
even more subtle chromatic changing during the electro-
chromic technique (FigureB(iii)).[]
Photonic Crystals: PCs formed by crystalline colloidal arrays
are extensively used in electrochromic devices. The color would
be changed by varying the refractive index of the photonic
crystal or its periodic structure. An eective approach for cre-
ating wider photonic bandgap is introducing the materials
with high refractive index and transparency in the visible range
(e.g., TiO, ZnS, ZnO, and ZrO) to form crystalline colloidal
arrays.[] However, high density of the particles and a lack of a
strong surface charge make it challenging to self-assemble par-
ticles. To address this, polyvinylpyrrolidone has been used as
a structure-directing agent which not only eectively aids the
growth of uniform sized particles, but also prevents the agglom-
eration of the growing particles.[] Similarly, -(-chlorosulfo-
nylphenyl)ethyltrimethoxysilane has also been introduced onto
Adv. Mater. 2022, 
Table 5. Materials with electrically induced tunable optical properties: detection range, substrates, and applications.
Entry Electric field detection
range [V]
Materials Substrates Applications Ref.
 –. Hollow FeO@C suspensions ITO glass slide Dynamic photonic displays/
sensors
[]
 – Polymer-stabilized CLCs consisting of E (Merck), prepolymer
(NOA), and a chiral dopant (R, Merck)
ITO glass slide Optical elements/color
information
[]
 – LC (-cyanophenyl--(allyloxy) benzoate, -methoxyphenyl-
-(allyloxy)benzoate, and -((trimethylsilyl)oxy)phenyl--
(allyloxy)benzoate) elastomers and TiO nanoparticles
alternative layer with a top layer of a fluorescent dye (Rhoda-
mine B)
ITO glass slide/graphite
layer
Active optical devices []
− to  Electrochromic device consisting of red PHT and blue PEDOT
on cathodically coloring electrochromic (EC) layer, while
green poly(aniline-N-butylsulfonate) (PANBS) and yellow EC
poly[,-bis(′,-dihexylfluoren--yl)azulenyl]-alt-[″,″-(″,″-
dihexylfluorenyl] (PDHFA) on the opposite electrode
ITO glass slide Energy-saving displays []
−. to . Three kinds of electrochromic materials (poly(,-
ethylenedioxythiophene), poly(-methylthiophene), and
poly(,-dimethoxyaniline))
Stainless steel wire Smart color-changing fabrics []
−. to  PVA gel-based multicolor system on two viologens Glass/TCO substrates Full-color devices []
 –. Electrochromic film based onN,N,N′,N′-tetraphenyl-p-
phenylenediamine and tetraphenylbenzidine and gel electrolyte
with heptyl viologen (HV) in the supporting electrolyte
Transparent frame on the
ITO glass slide
Transparent displays/energy-
saving devices
[]
 .–. Electrochromic photonic crystal based on TiO inverse opals FTO glass slide Tunable infrared light filter/
detector
[]
 – PS-b-PVP block copolymer with a fluoropolymer spacer filled
with ,,-trifluoroethanol (TFE) electrolyte
ITO glass slide Static/dynamic displays []
 .–. Negatively charged ZnS–silica core–shell colloidal crystals ITO glass slide Full-color reflective displays []
 − to  Plasmonic cell devices consisting of Au-core/Ag-shell
nanodomes integrated in a highly ordered porous SiO films
ITO/SiO substrates Mechanical Chameleon []
 −. to . Prussian blue-based nanoparticles ITO glass Electrochromic devices []
 .– Zn–sodium vanadium oxide ITO glass Electrochromic displays []

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Adv. Mater. 2022, 
Figure 6. Materials with tunable optical properties for wearable epidermal sensors in electrical monitoring: A) Mechanisms: i) Electron transfer in
Prussian blue. Partial electrochemical oxidation of the Prussian blue chromophore yields a green color state. ii) Electron transfer in viologens. Viologen
radical cations are intensely colored due to optical charge transfer between the (formally) +-valent and zero-valent nitrogen. iii) Oblique helicoid struc-
tures of liquid crystals. Strong vertical electric field realigns the cholesteric axis perpendicularly to itself, resulting in light-scattering fingerprint texture.
The pitch P and tilt angle θ of the field-induced heliconical state both change as the electric field varies. Reproduced with permission.[] Copyright
, Wiley-VCH. B) Materials: i) PolyProDOT-Me. Full-color plasmonic electrochromic electrodes are demonstrated by polyProDOT-Me-coated Al-
nanoslit structures with dierent slit periods along with corresponding optical micrographs. Its electric field ON and OFF states of these structures
are displayed. Reproduced with permission.[] Copyright , Springer Nature. ii) Terpyridine-Fe(II). The color changes from purple red to orange
as the applied potential increases from to .V. It is attributed to the metal-to-ligand charge transfer band of the coordinated Fe(II)-terpyridine
group. When the applied potential changed from .to . V, the color further changes from orange to green, which is assigned to the oxidation/
reduction of the triphenylamine group. Reproduced with permission.[] Copyright , Royal Society of Chemistry. iii) WO thin layer. Color gallery
are obtained from our Fabry–Perot nanocavity-type electrochromic WO/FTO electrochromic electrode at dierent applied potentials. Reproduced with
permission.[] Copyright , Springer Nature. iv) FeO@SiO nanoparticles. The hydrophobic surface modified FeO@SiO-F core–shell nano-
particles move toward the oppositely charged ITO electrode sites. The reflective display cells with these materials under an applied voltage from to
V demonstrate the gradual color change. Reproduced with permission.[] Copyright , Royal Society of Chemistry. v) Cholesteric liquid crystals.
Polarizing optical microscope is employed for demonstrating electric field-induced cholesteric mixture textures in heliconical states with reflected blue,

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the surface of nanoparticles to produce electrostatic interac-
tions. This guarantees the self-assembly of nanoparticles into
long-range ordered crystalline colloidal arrays. The compact
structure brings about a reflectance shift from to  nm,
which successfully achieves a red-green-blue change when
enhancing voltages from  to .V.[]
Electric field and magnetic field may also both cause color
changes in magnetic nanoparticles based photonic crystals.
FeO@SiO core–shell nanoparticles with hydrophobic surface
have been prepared for reflective displays (Figure B(iv)).[]
F-OTS serves as a surface modifier to improve the disper-
sion stability and strengthen the color change. When applying a
magnetic field, the photonic crystals show color changes. Simi-
larly, when applying an electric field, the reflective spectrum is
shifted and various colors are displayed by changing the voltage
from to  V,dueto the finely adjusted spacing between the
charged nanoparticles.[]
Cholesteric Liquid Crystals: In addition to nanoparticles, liquid
crystals also have the ability of selective reflection of light via an
electric field (FigureA(iii)).[] A CLC is sandwiched between
two glass plates with transparent ITO electrodes.[] When
light shines on it, the cholesteric phase divides the light propa-
gating along the spiral axis into two segments. As electric field
is applied from  to  V, the molecules are rearranged along
themselves and embody dierent colors (FigureB(v)).[] With
dierent torsion forces or concentrations of chiral additives, the
color can be tuned to any spectral region of ultraviolet, visible,
and infrared.
A plot of voltage detection range as a function of electro-
chromic color-changing zone is displayed in FigureC. Basi-
cally, ECG, EEG, and EMG signals gathered from the human
body have tiny voltage variations that we need amplifier to
increase the voltage to –V. Entries Nos.  and  in Table 
and FigureC report materials that are suitable for building up
an electrochromic device with wider detection range and higher
detection sensitivity than other candidates. In addition, other
materials are usually used for smart displays and user-interac-
tive devices with instant color-changing corresponding to the
dierent voltages.
3.1.2. Wearable Epidermal Sensors with Visible Readouts
for Electrophysiological Signal Monitoring
Wearable electrophysiological sensors measure bioelectric
signals noninvasively through skins. One potential applica-
tion of electrically induced color changing materials is in
medical monitoring originated from the presence of cur-
rents in the human body. For instance, Koo et al. reported
the design of a wearable cardiac monitor based on CNT
electronics and voltage-dependent color-tunable organic
light-emitting diodes (Figure D(i)).[i] This device utilizes
the bis[-(diphenylphosphino)phenyl] ether oxide to adjust
energy level and thickness of the layers, which balances the
charges between bis[-(,-difluorophenyl)pyridinato-C,N]
(picolinato)iridium(III) and bis(-phenylquinolyl-N,C(′))-
iridium(acetylacetonate) emission layers. Therefore, the
moni tor delivers dierent colors as induced by dierent volt-
ages. This monitor gathers normal human ECG signal from the
input and then amplify the signal to –V. When detecting a
normal human ECG signal, software delivers voltages around
–V, which is reflected by the color change from red to white.
When facing abnormal signals, the program delivers voltages
around – V, which shows the color change from white to
blue.[i]
Unlike human skins, bio-inspired chameleon skin has
extraordinary tunable optical properties for camouflage, temper-
ature sensing and communications. Inspired by this, the Bao
group showed the stretchable electrochromic skin with inter-
active color changing.[] By spray-coating single-wall carbon
nanotubes (SWNTs) on pyramidal microstructured PDMS,
PHT (poly(-hexylthiophene-,-diyl)) as an electrochromic
layer is placed between these PDMS layers (FigureD(ii)).[]
This electrochromically active devices can be revealed as tac-
tile-sensing control for interactive wearable devices in health
monitoring.[]
Despite the continuous improvement of materials with elec-
trically induced tunable optical properties, they are still far from
being of practical use in wearable devices because of spatial
restrictions owing to necessary connection to an external power
source.[] Piezoelectric and triboelectric-induced light emis-
sion could be a potential solution for developing a self-powered
sensors with visible readouts in electrophysiological signals
monitoring.[c,] Besides, inferior robustness of the electrical
and physical contact to skin under daily life activities is another
issue that restricts its performance as wearable sensors.[]
Thus, future work should focus on the electrophysiological
signals monitoring systems with visible readouts along with
battery-free or self-powered capability, epidermis compatibility,
and strong skin adhesion.
4. Materials with Tunable Optical Properties for
Wearable Epidermal Sensors in Chemical Signal
Monitoring
Apart from physical and electrophysiological signals, chemical
information from the body will enable precise monitoring of
basic health and early diagnosis of various health conditions.
This section mainly focus on the materials with tunable optical
properties for detecting biomarkers such as pH, glucose,
Adv. Mater. 2022, 
green, orange, and red colors. Reproduced with permission.[] Copyright , Wiley-VCH. C) Ashby chart: the plot of voltage detection range as a
function of electrochromic color-changing zone. D) Applications: i) Wearable wearable electrocardiogram monitors. The ECG signals are measured
by a stretchable Au electrode and then amplified by a p-MOS carbon nanotube. The retrieved ECG signals are displayed via color changes of voltage-
dependent color-tunable organic light-emitting diodes. The insets indicate the region of ECG signal for respective color emission and corresponding
CIE coordinates (scale bars:  mm). Reproduced with permission.[i] Copyright , American Chemical Society. ii) Stretchable interactive wearable
devices. A chameleon-inspired electronics skin is composed of the circuit layout including pressure sensor and electrochromic device. The neutral and
oxidized states of the electrochromic polymer in poly(-hexylthiophene-,-diyl) (PHT) represents dierent color. Reproduced with permission.[]
Copyright , Springer Nature.

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electrolytes, proteins, and nuclei acids and wearable chemical
sensors with visible readouts for noninvasively biomarker anal-
ysis in biofluids for health monitoring.
4.1. Chemical Signal
With advance in skin-like wearable technologies, the analysis of
biofluids has attracted attention by oering real-time informa-
tion on chemical signals from physiological state or the envi-
ronment surrounding the body.[c] Basically, biofluids naturally
excreted by the human body include sweat, saliva, tears, or urine.
Electrolytes are one of the most notable substances to sustain
the health conditions. Electrolytes in biofluids include sodium
ion (Na+), potassium ion (K+), calcium (Ca+), magnesium
(Mg+), chloride (Cl−), bicarbonate, and phosphate. These ions
are essentially relevant with nerve firing, endocrine secretion,
and body fluid buering.[] For example, an abnormal Na+ insuf-
ficiency induces the seizures or coma, a lack of Ca+ is related to
osteoporosis, excessive Cl− in the sweat of cystic fibrosis patients
incur hyperchloremia and many symptoms related to cystic
fibrosis.[,] Besides, pH is a strong indicator of acid–base bal-
ance of skin, and monitoring glucose levels is clearly relevant for
patients with diabetes, which is also associated with Alzheimer
disease progression and cognition damage.[b] In addition, the
proteins, nucleic acids, micronutrients, metabolites, enzymes,
and hormones in sweat dynamically change in response to the
overall health status, mental stress, and diet.[c,]
4.1.1. Materials with Chemically Induced Tunable Optical Properties
For chemical sensing, we review and demonstrate all these
materials and its corresponding detection range, substrates,
and applications (Table 6).
Materials with Tunable Optical Properties for Detecting pH: The
pH-responsive polymers are combined with the fluorescent
probes or quantum dots (QDs) to occupy the tunable optical
properties. Polymerization-induced self-assembly (poly(-
(diethylamino)ethyl methacrylate)(PDEA)-polyMA-stat-
benzyl methacrylate (P(MA-stat-BzMA)) diblock copolymers are
blended with the commercial dye fluorescein and rhodamine
dye labels. Their respective amine and acid blocks contribute
to dual-color bifluorescent pH-induced color-changing systems
(Figure 7A(i)).[] In addition, two dierent pH-responsive
polymers (PAA and poly(-vinylpyridine (PVP)) are generally
chosen as linkers between the GO and the cadmium sulfide/
zinc sulfide QDs to produce a GO-based colorimetric pH sen-
sors with a wide range of pH values.[]
Meanwhile, plasmonic nanoparticles, colloidal photonic crys-
tals, natural dye, and CLCs are also candidates for measuring
pH value with colorimetric sensing.[] D photonic crystal
flakes are fabricated by inserting silver nanoparticles (AgNPs)
(– nm) multilayer gratings in ≈ µm P(HEMA-co-MAA)
hydrogel films. The narrow-band Bragg reflector in visible
region of the spectrum shows an ≈nm red-shifts as the pH
is raised from . to . (FigureB(i)).[a] Another approach is
to use a freestanding chitosan hydrogel composite films with
photonic crystal colloidal monolayers of PS spheres, where there
is a color change at pH from . to . (Figure B(ii)).[b]
The diraction peak red-shifts gradually as the pH value
decreases to .. This spectral shift is attributed to the PS
sphere interspacing distance increasing due to the ionization of
NH groups. The pH-responsive polymer and photonic crystals
can also be organized into the D templates, such as Morpho
butterfly wing template, C. rubi butterfly wing natural gyroid
structure, and Papilio paris wing template.[c,] These bio-
inspired pH sensors open new alternatives for biological detec-
tion and photonic switches. Surprisingly, polydiacetylene-based
nanocomposites are not only responsive to the temperature and
humidity, but also the pH. It is due to repulsive ionic interac-
tion between carboxylate groups. This molecular rearrange-
ment results in the color variations (FigureB(iii)).[d]
The pH of healthy skin lies between . and . If skin pH
elevates, it can be easily attacked by inflammation and red-
ness. As shown in FigureC and listed in Table, entries Nos.
, , , and – are good candidates as pH responsive optical
elements for skin pH. Entries Nos. , , and  display tunable
optical properties based on fluorescence. In such devices photo-
bleaching can have a negative eect on the durability of wear-
able devices. As such entries Nos. – show potential for the
robust monitoring optically pH changes. Besides, other bio-
chemical signals (e.g., glucose, electrolytes, nuclei acid, and
protein) also relate to the health status, inspiring researchers to
invent novel materials for multi-biosignal detection and disease
prevention.
Materials with Tunable Optical Properties for Detecting Glucose
or Electrolytes: Glucose and ions from biofluids are also impor-
tant to monitor the health status. A vast number of research
groups exploited the CLCs, colloidal photonic crystals, the chro-
mogen ,,,-tetramethylbenzidine (TMB) and glucose oxi-
dase (GOD)-peroxidase-o-dianisidine reagents to quantify and
determine the elements in the biofluids (FigureA(ii)).[] One
example of an electrolyte monitoring involves the assembly of
negatively charged latex colloids into photonic crystals. Incor-
porating the hydrogel P(acrylamide (AAm)-co-AA) into the
photonic crystal gives a photonic sensor that has the ability to
monitor electrolyte levels quickly (FigureA(iii)).[] The color
of these sensors disappear at the dried state but emerge when
its surface is soaked in water (FigureB(iv)).[]
Materials with Tunable Optical Properties for Detecting Proteins
or Nuclei Acids: Plasmonic nanoparticles have been intro-
duced into colorimetric molecular detection since proteins
and nucleic acids may influence the nanoparticle refractive
index and manipulate assembly or dispersion of nanoparti-
cles, thus inducing SPR-based color change.[,] Based on
dierent versions of intermolecular interaction (avidin–biotin,
antibody–antigen, carbohydrate–protein interactions, and DNA
hybridization), an assorted of SPR-based colorimetric sensors
have been proposed for clinical and personalized point-of-care
diagnostics.[]
4.1.2. Wearable Epidermal Sensors with Visible Readouts for
Chemical Signal Monitoring
A growing attention on biofluid detection has focused on col-
orimetric sensors because of their ability to monitor target
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analytes conveniently without power-based readout device.
Multiplexing three dierent pH indicators: bromothymol blue
(pH .–.), bromocresol green (pH .–.), and cresol red
(pH .–.) in a polyvinyl chloride (PVC) matrix on a finger-
nail platform oer fast and reversible optical properties to pH
values (FigureD(i)).[] These nail-worn colorimetric sensors
are capable of pH sensing of the surrounding environment.
To fabricate optically active wearable chemical sensors at large
scale, silk fibroin-based inks were screen-printed on textiles for
building a wearable sensing T-shirt. Its colorimetric pattern on
wearable fabrics yields a map-like distribution of pH response,
which is deployed as disease tracking across the whole body in
real-time.[h]
Sweat as one of the biofluids can be easily collected on the
epidermal skin noninvasively, providing abundant sampling
zones for wearable microfluidic sensors.[] Generally, wear-
able microfluidic sensors underscore the critical need in the
fields of personalized healthcare and medical therapy for an
expanded suite of capabilities including conformal attachment
to skin, minute sample collections (µL, nL), quick response,
real-time monitoring, and eortless operation.[c,] Xiao et al.
built a microfluidic chip-based wearable colorimetric sensor for
Adv. Mater. 2022, 
Table 6. Materials with chemo-induced tunable optical properties: detection range, substrates, and applications.
Entry Detection range Materials Substrates Applications Ref.
pH  –. -()-carboxyfluorescein Glass slide Biological sample analysis []
 .–. Polymethylacrylic acid (PMAA) Morpho butterfly wing template U-shaped pH sensors [b]
 .–. P(AA-co-AAm) PCs Papilio paris wing template Biological detectors/pH
analysis/photonic switches
[a]
 – PNIPAM with a N,N′-dimethylaminoazobenzene azo-
dye end group
– – []
 – PBI-HIPE hydrogel (pH responsive perylene bisimide-
functionalized hyperbranched polyethylenimine)
GO-PNIPAM hydrogel layer Intelligence devices []
 .–. P(NIPAM-co-fluorescein-co-rhodamine)cross-linked
with N,N′-MBA (PNFR microgels)
– Ratiometric microgel detection []
 – pH-responsive linker PAA-PVP and QDs hybrids inte-
grated on a single graphene oxide sheet (MQD-GO)
– GO-based sensors []
 – Polymer gel infiltrated monolayer PS colloidal crystals
by using a weak polyelectrolyte PVP ultrathin polymer
gel film
Silicon wafer Chemical sensing []
 .–. D PS colloidal monolayer–chitosan hydrogel composite
films
Glass slide Optical sensors [b]
 .–. D Au nanosphere array/PAA hydrogel composite
sensing films
Silicon wafer Visual sensors []
 .–. PHEMA–Ag nanoparticle hydrogel films PMMA Holographic sensors []
 – CLCs/hydrogel polymer interpenetrating network (CLC-
PAA-IPN) array
Glass slides Biosensors []
 – Roselle anthocyanins incorporated in the starch/PVA
films
– Fish freshness monitoring []
 – Curcumin incorporated in the carrageenan films – Freshness monitoring []
 .–. Universal pH indicator incorporated in the agarose film – Point of care diagnostics []
 – Ampyrone-based azo dye, -((-hydroxybenzo[d][,]
dioxol--yl)diazenyl)-,-dimethyl--phenyl-H-pyrazol-
(H)-one in MeOH:HO solution
– Fluorescent sensor []
Glucose

. × −– × −  Peroxidase TMB in GOD and ,,,-tetrakis(-
carboxyphenyl)-porphyrin (HTCPP) functionalized
FeO nanocomposites
– Medical diagnostics []
Electro-
lytes 
– ppm Negatively-charged latex colloids crystallize in the
P(AAm-co-AA) hydrogel
Glass substrates Antiforgery []
Protein

 × − – × −  DNA-protein interaction on the surface of AuNPs (thiol-
labeled DNA sequences (A, C, and T))
– Colorimetric sensors [b]
Protein
kinase

–. unit µL−Nanoceria-based assays (TMB, adenosine triphosphate,
and adenosine diphosphate)
– Peptide monitoring and drug
screening
[f]
Nuclei
acids 
 × −  – × −  DNA-functionalized AuNPs (CRISPR Casa) – Fluorescent biosensors for
point-of-care testing platforms
[c]

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detecting sweat glucose by using GOD-peroxidase-o-dianisidine
reagents.[g] This dynamic range for sweat glucose detection is
down to . × −–. × −  with a limit of detection (LOD)
of . × − . Monitoring glucose level during exercise is
decisive to avoid hypoglycemia, especially for diabetic athletes.
De la Rica and co-workers developed a colorimetric wearable
biosensors that used ,′,,′-tetramethylbenzidine combined
with glucose detection in sweat with an LOD of .× − 
and a linear range up to . × − .[] These devices are
made of filter paper, a sweat volume sensor (paper strip modi-
fied with pegylated AuNPs) and a color chart for signal correc-
tion. The lightweight and disposability makes this technology a
Adv. Mater. 2022, 
Figure 7. Materials with tunable optical properties for wearable epidermal sensors in chemical signal monitoring. A) Mechanisms for dierent chemical
signals: i) pH. The schizophrenic micellization behavior of PDEA-P(MA-stat-BzMA)y diblock copolymers in aqueous solution is demonstrated. Repro-
duced with permission.[] Copyright , American Chemical Society. ii) Glucose. Glucose detection by using GOD and HTCPP-functionalized FeO
nanocomposites-catalyzed reactions is presented. Reproduced with permission.[] Copyright , Elsevier. iii) Electrolytes. Responsive photonic
crystal film is comprised of negatively charged polystyrene nanoparticles (PS NPs) in the P(AAm-co-AA) hydrogels. The incorporation of AA confers elec-
trolyte-sensitive on these films. Reproduced with permission.[] Copyright , Royal Society of Chemistry. B) Materials: i) AgNPs in poly(HEMA-co-
MAA). Free-standing flakes are measured from pH . to . (scale bar: mm). D photonic flakes assembled on paper strips are measured from pH .
to .. The dierence in the color might be attributed to the degrees of freedom that the polymer matrix could expand. Reproduced with permission.[a]
Copyright , American Chemical Society. ii) PS NPs in chitosan hydrogels. Dierent color of the hydrogels film with D arrays on both the surfaces
is indicated at pH: ., ., ., ., and . because the swelling of the hydrogel lead to interspacing between PS spheres increase in the colloidal
monolayer array. Reproduced with permission.[b] Copyright , Royal Society of Chemistry. iii) Polydiacetylene (PDA)/zinc oxide nanocomposites.
These nanocomposites exhibit dual colorimetric response to both acid and base. The strong ionic repulsion between the carboxylate groups and the
breaking of hydrogen bonds induces segmental rearrangement of the PDA, leading to the color transition. Reproduced with permission.[d] Copyright
, Elsevier. iv) P(AAm-co-AA) in responsive photonic crystals (RPCs) films. When the concentration of salt increases, the color would change from
pale, green to blue gradually. The film would recover to its original color after being immersed in water. Reproduced with permission.[] Copyright
, Royal Society of Chemistry; C) Ashby chart: the plot of pH detection range as a function of chemochromic color-changing zone. D) Applications:
i) Wearable fingernail chemical sensing platform. The bromothymol blue, cresol red, and bromocresol green dye in PVC as nail sensors are manifested
for multiplexed colorimetric response at pH , , and . Reproduced with permission.[] Copyright , Elsevier. ii) Diabetic wound treatment. A
multifunctional zwitterionic poly-carboxybetaine hydrogel wound dressing is considered as a sensor with visible readouts to simultaneously monitor the
pH and glucose level of wound exudate. This visible readout is divided into three channels (RGB). Visible pictures are handled with analytic software
MATLAB to output the RGB data. Reproduced with permission.[] Copyright , Wiley-VCH.

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promising candidate monitoring glucose level noninvasively.[]
Similarly, the Collins group utilized lacZ β-galactosidase operon
to hydrolyze chlorophenol red-β-d-galactopyranoside to display
a yellow-to-purple color change upon exposure to nuclei acids
or metabolites.[] These wearable microfluidic optical fibers
detect the severe acute respiratory syndrome (SARS)–CoV- at
room temperature within min depending on the fluorescent
and luminescent dual-outputs. These devices can be integrated
into patient-worn coats or facial masks.[] Other applica-
tions include drug abuse tracking and athletic performance
monitoring.[]
Studies on wound healing reveal that glucose and pH varia-
tion are associated with the wound healing. A multifunctional
zwitterionic hydrogel wound dressing is designed as an optical
sensor to monitor the pH and glucose level in the diabetic
wound treatment at the same time (Figure D(ii)).[] GOD
and horseradish peroxidase are encapsulated in a hydrogel
matrix to catalyze the glucose oxidation. These phenol red flu-
orescent-labeled products are formed and gathered as pictures
by smart phone and the glucose level is defined by MATLAB
(matrix laboratory). Through experiments on mice, it is found
that these hydrogels not only express information about pH
and glucose, but also promote healing. Other wearable epi-
dermal sensors that employ visible readouts for chemical
signal monitoring are described in part . for multisignal
detection.
Traditionally, wearable chemical sensors with visible read-
outs employ colorimetric, fluorescence, and luminescence
sensing techniques to collect biomarkers information. The cor-
responding sensing elements produce color-changing phenom-
enon in the presence of a target biomarker through a certain
chemical reaction, which ensure the specificity of target analyte
detection.[c] However, wearable chemical sensors with visible
readouts for biomarkers detection only focus on monitoring
metabolites and electrolytes in body fluids. Other analytes in
body fluids, such as enzymes, peptides, hormones, proteins,
and DNAs/RNAs are rarely reported. Likewise, the extremely
low amount of analyte in body fluids is still a hurdle for long-
term robust monitoring.[] Next-generation wearable chemical
sensors must explore novel materials and detection techniques
to target a wide range biomarkers, ultimately enabling an unob-
structive, comprehensive, and noninvasive medical diagnostics
based on visible readouts.
5. Materials with Tunable Optical Properties for
Wearable Epidermal Sensors in Multiphysiological
Signal Monitoring
Aside from solely physical, electrophysiological, or chemical
signal detection principles, multifunctional integrated system
consisting of various categories of color-changing sensing ele-
ments is remarkably desirable for oering the capabilities in
monitoring multiple physiological signal simultaneously. This
section oers the recent advances in materials with tunable
optical properties for multisignal detection and their potential
applications as wearables with visible readouts.
5.1. Multiphysiological Signal
Traditionally, electrophysiological sensors detect the current,
voltage or resistance at heart (ECG), brain (EEG), and muscle
(EMG).[] Physical sensors are subjected to mechanical,
humidity, temperature, or optical stimuli.[] Chemical sen-
sors are revealed for identifying or quantifying the pH, glucose,
ions, proteins.[] For practical applications, it is highly fasci-
nating if multiple sensing modalities can be integrated into one
single sensor platform. In the foreseeable future, the developed
sensing technology can be extended to multifunctional wearable
devices by translating multisignals (e.g., physical, chemical, and
electrophysiological signals) into visual display for health mon-
itoring at the same time. This section provides a checklist of
these materials for multisignal detection along with their corre-
sponding detection range, substrates, and applications. The rest
of them are also summarized and presented in Table 7.
5.1.1. Materials with Tunable Optical Properties
for Multiphysiological Signal Detection
Materials with Dual-Stimuli-Induced Tunable Optical Properties:
Several types of p(NIPAM-co-AA) materials have been vastly
investigated for dual-stimuli responsive color-changing sys-
tems.,[,] For example, Chen et al. proposed a facile method
to crosslink PNIPAM with PAA (Figure 8A).[] These polymers
are allowed to self-assemble into highly crystalline structure
and stabilized by UV irradiation. Unlike other methods with
a low degree of swelling, this P(NIPAM-co-AA) microgel can
noticeably respond to temperature and ionic strength because
the microgel swelling induces the refractive indexes changing.
The color of these films changes reversibly ranging from red,
green, blue, to white depending on temperature, and from light
green, red, blue, to dark green depending on the concentration
of the NaCl.
Additionally, liquid crystal elastomers (LCEs) based on highly
ordered crystalline structure is reported by the Wei group
(FigureB).[] The photonic band gaps of these LCEs can be
reversibly switched by temperature due to thermal-induced
molecular orientations. When temperature elevates from  to
 °C, the structural color changes from blue-green to yellow-
green. The electric field is manipulated to orient the lattice
space of the silica opal templates at the second photopolymeri-
zation stage, which induces an obvious red-shift with a bright
orange-red structure color.[] Meanwhile, another striking
route to realize dual-signal color-changing detection based on
highly ordered crystalline structures are endowed with ther-
moresponsiveness and fully reversible strain-sensitiveness
(FigureC).[] It is the temperature-induced swelling actuating
a volume expansion of the hydrogel matrix with water while
reaching the LCST of poly(diethylene glycol methylether meth-
acrylate-co-ethyl acrylate) (P(DEGMEMA-co-EA)). This accom-
plishes higher refractive index between the PS cores and the
hydrogel matrix. Its red-shift changes from yellow (λ= nm)
to orange (λ= nm) of the Bragg peak under normal light
incidence. The blue-shift of the Bragg peak from initial yellow
(λ= nm) to green (λ= nm) under enlarging strain is
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based on the deformation of colloidal crystalline order by lat-
tice. The PS@ P(DEGMEMA-co-EA) with mechano- and ther-
moresponsiveness are determined as optical sensors with
reversible color changing properties.[]
Another fascinating example is to generate tunable optical
properties with UV light and electric field. The monochromic
conducting polymer films (PEDOT: Tosylate) are deposited on
metallic mirror via vapor phase polymerization and UV light pat-
terning (FigureD).[] Regulating the UV dose enables syner-
gistic control of both film thickness and refractive indexes, which
engenders structural colors from violet to red. In addition, elec-
trochromic properties of the conducting polymer films are fur-
ther proved by applying voltages to produce its oxidation state,
thus reducing the absorption in the visible. This simple and facile
fabrication methods, wide color gamut and dynamic optical prop-
erties make this film for dual-signal detection and displays.[]
Materials with Multi-Stimuli-Induced Tunable Optical Proper-
ties: The smart textile can sense and respond to multistimuli
such as light, temperature, electrolytes, and mechanical stress
(Figure 9A).[] The coating covalent organic polymer of
viologen (COP+) is screen-printed on the cotton fabric with
fast response to ammonia gas, ultraviolet light and heat with
great breathability, flexibility, and comfortability. These stimuli
induce charge transfer from its counteranion to the viologen
moiety to form the colored radial-cation. It is expressed as
cotton fabric with reversible optical properties triggered by
ammonia gas (exposure within  s), UV light (exposure for
 s), and temperature ( °C).[]
In addition, responsive PCs are highly ordered periodical
structures that can alter their diraction wavelength upon
chemical or physical stimuli.[] The lamellar hydrogels con-
sisting of alternating hard layers of poly(dodecylglyceryl ita-
conate) (PDGI) and soft layers of interpenetrating networks
of PAAm–PAA are organized with reversible and wide range
color switching under dierent external stimuli of temperature
(– °C), pH (.–.), and stress/strain (–%).[] The
Adv. Mater. 2022, 
Table 7. Materials with tunable optical properties for multiphysiological detection: detection range, substrates, and applications.
Stimuli Detection range Materials Substrates Applications Ref.
T
pH
–
.–.
Inverse opal hydrogel consisting of HEMA, NIPAM, AA Glass slides Sensors []
Light
pH
UV nm
.–.
P(NIPAM-co-AA) microgels-based etalons in a photo-acid
o-nitrobenzaldehyde (o-NBA) solution
Glass slides Controlled/triggered drug delivery
system
[]
pH
T
–
–
P(NIPAM-co-AA) microgels-based etalons Glass slides Color displays []
pH
NaCl
.–.
.–. 
Responsive photonic hydrogels by incorporating linearly
ordered FeO nanoparticles into the P(AAm-co-HEMA)
hydrogel
– Sensors []
pH
T
–
– °C
D photonic stacks of P(NIPAM-co-AA) and TiO layers Silicon wafers Environmental monitoring []
T
NaCl
– °C
 × −–× − 
HEMA-modified microgels by coupling of
P(NIPAM-co-AA)microgels
– Sensing and displays []
T
Electric filed
– °C
– V
Micropatterned inverse opal films based on a mixture
of a nematic diacrylate monomer and a monoacrylate
mesogenic monomer
ITO glass slide Displays/optical actuators/
Sensors
[]
Strain
T
–%
– °C
Monodisperse core-interlayer-shell (CIS) beads with non-
cross-linked hard PS cores embedded in the butanediol
diacrylate (BDDA) for matrix
– Deformation sensor []
T
Strain
.– °C
–%
Thermoresponsive P(DEGMEMA-co-EA) shells and hard
PS cores (PS@P(DEGMEMA-co-EA))
Elastomeric PET film Sensor devices/security materials []
Light
Temperature
Strain
UV/vis light
– °C
–%
Rhodamine B methacrylamide-based elastomeric
polymer opal films
– Rewritable D optical data storage []
Pressure
T
pH
–.kPa
–
.–.
Tough photonic hydrogel made of PAA network intro-
duced into the PAAm layer of a PDGI/PAAm gel
Silicone spacer/glass slides Displays/sensors []
T
Light
Humidity
– °C
UV/vis light
–
Photonic paper: PMMA and PNIPAM-co-PGMA alterna-
tively stacking thin layers
– Colorful sensors/security labels/
full-color printing displays
[]
T
Strain
Pressure
– °C
–%
kPa
HPC, PACA, and CNTs compositedliquid-crystal
hydrogel
– Multifunctional flexible E-skins []
Ammonia
Light
T
–
UV/vis
– °C
Screen-printing with COP+Cotton fabrics Stimuli-responsive chromic
textiles
[]

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hydrogen bonds in the PAAm–PAA soft layers are weakened
at high temperature, which gives rise to the thermochromic
photonic gel. Furthermore, higher pH induces swelling of the
soft layer due to the dissociation of the carboxy group of PAA.
In addition, elongation of these layer-by-layer photonic gels
disrupt the periodical ordering, which gives rise to the revers-
ible mechanochromic properties (Figure B).[] These three
phenomena endow these photonic gels with multiple tunable
optical properties. Another alluring category is the reversible
light-, thermo-, and mechanoresponsive elastomeric polymer
opal films reported by the Gallei group (FigureC).[] Rhoda-
mine B methacrylamide is locally labeled homogeneously in
the core-interlayer-shell polymer beads. The opal films convert
an ordered  lattice plane into a disordered state followed by
reorganization to an ordered  plane responding to strains.
Color of these films change from dark red, yellow to green at
elongation %, %, and %. As a result, complete revers-
ibility of light- and temperature-induced transition of opal films
could be attributed to the nonfluorescent closed form and the
fluorescent form.[]
Adv. Mater. 2022, 
Figure 8. Materials with tunable optical properties for dual-signal detection. A) T and electrolytes detection: P(NIPAM-co-AA) microgel assembled
into highly crystalline structures. The color of these freestanding films changes gradually when temperature is rising from  to  °C (middle, scale
bar: .cm). The color of these films also changes gradually when NaCl concentration increases from  to mm (lower, scale bar: .cm). These
stimuli induce the color change since the bandgap of the hydrogel can be finely tuned in the whole visible range. Reproduced with permission.[]
Copyright , Wiley-VCH. B) T and electric field detection: inverse opal films based on liquid crystal elastomers. Reflection spectra and reversible
color changes of these films are displayed as a function of temperature (upper left). The mechanism of the lattice space reversible change is induced
by temperature (lower left). The right part in this figure includes cross-sectional SEM images of these films are fabricated without (upper left) and
with the electric field (upper right), and reflection spectra and structural colors for these films fabricated without and with the electric field (lower in
right part) (scale bars:  nm). Reproduced with permission.[] Copyright , Royal Society of Chemistry. C) T and mechanical force detection: PS
cores and PDEGMEMA hydrogel matrix. These hydrogel films show the strong green reflection color and SEM images of cross-sections of these films
indicate the hexagonally ordered () plane as well as the rectangular () plane. The reversible color-change and spectral shifts are also illustrated
for mechanical force-induced deformation and T-induced volume expansion of these matrix films (scale bars:  nm). Reproduced with permis-
sion.[] Copyright , Wiley-VCH. D) UV light and electric field detection: monochromic PEDOT: Tosylate conducting polymer films deposited on
metallic mirror. The color changes in this structure are arisen from the thickness and the refractive index of the conducting polymer films induced
by the dierent doses of UV light treatment. By depositing an additional layer of electrolyte on top, the color can be further altered by adjusting the
conducting polymer redox state (upper left). Chemical structure of PEDOT: Tosylate is also demonstrated (upper right). Simulated pseudocolors
of devices with dierent UV exposure times and thicknesses are displayed (lower left). The electric field-induced color changes in these devices are
illustrated by dierent electrochemical properties (lower right, top: min UV-treated PEDOT, and bottom: min UV-treated PEDOT). Reproduced
with permission.[] Copyright , Wiley-VCH.

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5.1.2. Wearable Epidermal Sensors with Visible Readouts
in Multiphysiological Signal Monitoring
Accurate temperature sensing is crucial particularly in various
physiological detection as wearable epidermal devices, espe-
cially high fever-induced by SARS, Middle East respiratory
syndrome (MERS), and coronavirus disease  (COVID-).
The Park group described a thermoadaptive block copolymer
structural color electronics (Figure 10A).[] By employing a
PNIPAM thermoresponsive polymer/nonvolatile hygroscopic
ionic liquid and LiTFSI blend on D block copolymer (BCP,
PS-b-PVP) PC films, full visible structural colors are delivered
due to temperature-induced polymer expansion and contraction
ranging from  to  °C via ionic liquid injection and release.
The electrothermal sensing displays are precisely monitored
with the capacitance variations while visualizing the tempera-
ture. It is obtained from red, green, and blue, while the voltage
was applied from  to V, color switching cycle endurance of
 V,  s, and  times, respectively.[] Another important
aspect of wearable epidermal sensors for dual-signal detec-
tion is a bio-inspired multifunctional electronic skin based
on hydroxypropyl cellulose (HPC), poly(acrylamide-co-acrylic
acid) (PACA), and CNTs composited liquid-crystal hydrogel.
The temperature-sensitive PACA is acted as scaolds for HPC
and CNT.[] Its thermoresponsiveness endows the composite
hydrogel with reversible optical properties, which is stimulated
by the temperature and mechanical force. Thus, these elec-
tronic skins successfully map the stimulating sites and have
instant color-changing feedback to human motion and other
external stimuli. This dual-physiological signal sensing oers a
prosperous future for innovative user-interactive visible devices
with multifunctionality (FigureB).[]
Materials for multisignal detection pave the way for new
generation intelligent devices. These visual signals can be pre-
cisely discriminated with the naked eye, oering a facile and
straightforward sensing/mapping approach for people to per-
ceive the color change under various environmental condi-
tions. The Sun group reported a versatile interactive sensory
device (Figure 11A).[] These layer-by-layer structures are
equipped with thermo-/light-/mechanochromic characteris-
tics without recognizable cross-interferences. The mechano-
chromic materials include an Ecoflex/red dye and a double-
sided adhesive bilayer as stretchable substrate and polyvinyl
butyral/TiO composite/mirror chrome light shielding layer.
The mechanochromic eect is proved by the crack opening/
closing mechanism of light shielding layers, and the mechano-
chromic sensitivity can be readily adjusted via dierent degrees
of prestretching. A layer of thermochromic/photochromic
homogeneous binary mixture is fabricated on the top of the
light shielding layer. To impart a thermochromic response,
three kinds of thermochromic leuco pigment demonstrate
blue, green, red, and white at dierent temperature region. To
impart a photochromic response, a leuco pigment containing
Technocolor Purple  is chosen to express a white color under
visible light but changes to dark violet when exposed to UV at
 nm. These materials can be explored for human motion/
environmental condition monitoring and stretchable interactive
electronics.[] Nonetheless, most wearable sensors translate
dierent stimuli into the similar color-changing at the same
area (Table ). Luckily, Rogers and co-workers outlined a wear-
able device for colorimetric sensing of sweat monitoring with
pH, temperature, glucose, chloride, and lactate (FigureB).[e]
PDMS microfluidic channel is utilized as a substrate for encap-
sulating the colorimetric dye. Prominently, the dierent color
reflects the dierent stimuli at the isolated reservoirs of micro-
fluidics in a single step. The results include quantitative values
for analyzing multiple sweat biomarkers, temperature, sweat
rate/loss, and pH. A double-sided thin adhesive layer ensures
stable, strong, and seamless adhesion and compatibility of the
device to the skin without irritation. Accordingly, sweat-based
wearable devices are required for monitoring these noninvasive
health states. Notwithstanding this, the irreversibility of color-
changing is another hurdle in the context of long-term health
monitoring.
Although until now few relevant wearable devices with vis-
ible readouts in multisignal monitoring have been successfully
devised, updates on the materials sciences still lag behind pro-
gress being made in these integrated systems. Given the com-
petitive research and huge numbers of commercial opportuni-
ties in these integrated systems, we thus envision that future
wearable sensor with visible readouts will be designed into
high-performance, multifunctional, and compact skin-based
patches. Considering the complex physiological state of the
human body, fusion of multiple sensors could be allowed for
simultaneous monitoring of physical, electrophysiological, and
chemical signals.[a] By continuous or frequent detection of vital
signs, these wearable healthcare settings reduce inaccuracies
and improve prediction outcomes of health status.[c]
6. Conclusions and Future Perspectives
As mentioned above, wearable devices should preferably be
cost-eective, nonobtrusive, flexible, lightweight, easy-to-
produce, biocompatible, and multisignal sensing systems.[c]
Overall, several eminent factors are still needed to be considered
Adv. Mater. 2022, 
Figure 9. Materials with tunable optical properties for multisignal detection. A) Smart cotton fabric screen-printed with viologens responding to T,
ammonia, and light. This COP+ coated chromic textile is sensitively responsive to gas, light, and T with excellent breathability, flexibility, and comfort-
ability. The color of textile turns from yellow to green instantly when exposed to these stimuli and quickly recovers. Reproduced with permission.[]
Copyright , Springer Nature. B) PDGI/PAAm–PAA lamellar hydrogels responding to T, pH, and mechanical force. This ternary stimuli-responsive
photonic hydrogel is based on an alternating soft and hard layer structure. The hard and nonwater permeable layer includes PDGI bilayer with a thick-
ness of .nm. The soft layer is with a thickness of several hundreds of nanometers, which varies depending on stimuli such as temperature, pH,
and mechanical force (left). The combined thermo-, pH-, and mechanochromic behaviors of these hydrogels are demonstrated (right). Reproduced
with permission.[] Copyright , Wiley-VCH. C) Rhodamine B methacrylamide-based elastomeric polymer opal films responding to light, T, and
mechanical force. The color of these elastomeric films is activated by light and erased by temperature due to the close and open form of Rhodamine B
methacrylamide-labeled beads. The color of these films is also induced by mechanical strain up to % due to the lattice reorganization. Reproduced
with permission.[] Copyright , American Chemical Society.

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such as biocompatibility, adhesion, stretchability and flexibility,
long-term robust monitoring, and detection sensitivity. In this
section, we briefly introduce these factors for the realization
of next-generation wearable sensors with visible readouts and
recent works for dealing with these deficiencies. Apart from
this, future works are required to put a spotlight on the inte-
gration, perception, privacy protection, and discriminable visual
display for reshaping the next-generation wearable epidermal
sensors depending on the color-changing properties.
6.1. Biocompatibility
Wearable devices have been evolved to become a burdensome
waste for environmental protection in st-century. There-
fore, zero-waste wearable devices are profoundly appreciated
because of the degradation to the environment with minimum
impact.[] Considering any adverse eects from the device on
the human body, the use of materials with biocompatibility
and further bioresorbability endows them with safety and
Adv. Mater. 2022, 
Figure 10. Wearable epidermal sensors with visible readouts in dual-physiological signal monitoring. A) Thermoadaptive block copolymer structural
color-based flexible electronics responding to T and electric field. The bilayered block copolymer photonic crystal film is consisted of PS-b-QPVP cov-
ered with a PNIPAM/LiTFSI film. The color and reflectance spectra of the film are varied as a function of temperature (upper left, scale bars: mm). A
flexible patch is composed of this bilayered film, a PDMS layer, Tegaderm, and substrate. It could be used for temperature monitoring on wrist (lower
left, scale bars:  cm). This bilayered film is integrated with patterned Cr/Au electrothermal transducer. When voltage is applied, the transducer is
heated, giving rise to color change from red as a function of the voltage. The color and reflectance spectra of this active display are varied as a function
of voltage from  to V (upper right, scale bars: mm). When this active display is connected to Cu coil antenna pairs and portable V battery for
wireless power transfer, its color changes from green to blue after the patterned heater is on with the transmitted power. The  ×  arrays of active SC
displays are demonstrated with individually electric heaters. The selected pixels are illustrated upon heating with dierent color displays (lower right,
scale bars: mm). Reproduced with permission.[] Copyright , Wiley-VCH. B) Bioinspired cellulose liquid-crystal hydrogels as multifunctional
electronic skins responding to T and mechanical force. The color variation of the chameleon and the composition of the conductive cellulose structural
color hydrogel are illustrated (left). The red-colored multifunctional E-skins are attached on dierent fingers to detect temperature, pressure, and ten-
sion, respectively. Reproduced with permission.[] Copyright , PNAS.

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Figure 11. Wearable epidermal sensors with visible readouts in multiphysiological signal monitoring. A) Wearable motion/environmental monitoring
devices responding to light, T, and mechanical force. The sensing elements in this device is comprised of ′-(dibenzylamino)-′-(diethylamino)fluoran
(thermochromic properties), technocolor purple  (photochromic properties), and polyvinyl butyral/TiO (mechanochromic properties). Their corre-
sponding color changes are displayed in this device (scale bar: mm). Thermal mapping unit is illustrated with dierent colors at dierent tempera-
ture (upper right). Mechanochromic responses of the prestretched devices is demonstrated under (UV light, < °C), (visible light, > °C), and (UV
light, > °C) (scale bars:  μm). Reproduced with permission.[] Copyright , Royal Society of Chemistry. B) A wearable microfluidic device for
the capture, storage, and colorimetric sensing of sweat responding to pH, humidity, glucose, lactate, and chloride. This device with an enlarged image
of the integrated near-field communication system includes the top, middle, and back layers. The microfluidic channels with colorimetric assay reagents
(water, lactate, chloride, glucose, and pH) are in the middle layer. The bottom layer is a uniform layer of adhesive bonded to the bottom surfaces of
the PDMS with sweat inlets with connection to microchannels. A fabricated device is adhered on the forearm. Colorimetric detection reservoirs are
utilized for determination of water loss and concentrations of lactate, glucose, creatinine, pH, and chloride ions in sweat (upper right, scale bar:  cm).
For real applications, this device could be employed as epidermal microfluidic biosensor by injecting artificial sweat (lower right, scale bars:  cm).
Reproduced with permission.[e] Copyright , AAAS.

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long-term usability. It requires materials and devices to disap-
pear into the surroundings with nontraceable residues. Basi-
cally, the traditional wearable electronics including transistors,
diodes, capacitors, and resistors are desirably designed to dis-
solve in the water.[a] However, fabricating these dierent parts
in the wearable devices are complicated and time-consuming.
Recent reports manifest totally disintegrable flexible circuits by
incorporating with biocompatible conjugated polymer (diketo-
pyrrolopyrrole) with reversible imine chemistry.[] Wearable
sensors with visible readouts provide a bright future for bio-
compatible systems because we only need to design the color-
changing sensing element and adhesive substrates, which is
much easier for application in health monitoring. Biomate-
rials from bacteria, plants, and animals have gained so much
attention because of its superb biological properties, such as
abundant resources, biocompatible and biodegradable prop-
erties, and self-adhesive characteristics. Biomaterials, such as
collagen, silk-fibroin, polysaccharides, wood, and bone have
been immensely discovered in the wearable sensors, whereas
its conformity and sensing sensitivity needs to be escalated. It
is critical to find an appropriate balance and an essential link
between device performance and biocompatibility. Although
ineciencies remain, biomaterials are still promising can-
didates for creating the wearable sensor with outstanding
properties.[]
6.2. Adhesion
The substrates utilized for wearable sensors with visible read-
outs reported now mainly aims at the Ecoflex, tapes, bandage,
sutures, staples, and PDMS (Tables–). Its poor stretchability,
conformity, and stability severely hinder their applications for
health monitoring. Hydrogels as D hydrophilic polymeric
(natural or synthetic polymers) networks are served as prom-
ising candidates for adhesive layers in wearable sensors. The
high water content imparts hydrogels tissue-like Young’s mod-
ulus, as well as the properties of hydrogels, such as stretch-
ability, adhesion properties, ionic conductivity, and flexibility,
can be tailored during the design process. However, high water
content in hydrogels have a negative eect on adhesion since
water molecules at the boundary layer decrease the interfacial
adhesion.[] Recently, major eorts in adhesive hydrogels,
especially nanocomposite-based mussel-inspired adhesive
hydrogels, advocate on adhesion generally occurring at the
interfacial layer, arising from the covalent bonds, hydrogen
bonding, van der Waals forces, electrostatic interactions, and
hydrophobic forces.[,]
Mussel-inspired adhesive hydrogels strongly attach on all
kinds of surfaces due to noncovalent and covalent chemical
interactions with the substrates.[c] Nevertheless, the oxida-
tive agents employed for the formation of mussel-inspired
hydrogels, e.g., polydopamine, results in the short-term adhe-
siveness and single usage.[a] It poses a threat to the hydro-
gels with the excellent toughness, stretchability, and adhesion
simultaneously. Nanomaterials in mussel-inspired adhesive
hydrogels, such as carbon-based nanomaterials (graphene,
graphene oxide), inorganic nanomaterials, metal/metal oxide
nanomaterials, and polymeric nanomaterials, have attracted
significant interests in retaining the impressive mechanical
properties.[] It not only can be regarded as multifunctional
cross-linkers to reinforce the polymer networks, but also estab-
lish superior biocompatibility and biodegradability under physi-
ological conditions.[]
The quintessential feature of hydrogels for epidermal skin
monitoring requires ) adequate adhesion when in use and
peel-o easily without itch and inflammation to the skins, )
multiple adhere-to-peel-o times but still maintaining the
benign adhesion, ) excellent adhesive properties in wet envi-
ronments, such as body fluids, sweat, bloody wounds.[a]
Although many researchers have been devoted to investigate
the adhesive hydrogels with compelling toughness and stretch-
ability, self-healing and conformity, sustaining those functions
coincidently at extreme conditions (e.g., such as the high/
low temperature), sweating, and underwater is also strikingly
acceptable for epidermal health monitoring, hence remaining
an ongoing inadequacy in this field.
6.3. Flexibility and Stretchability
Attention is being paid to achieve the excellent flexibility and
stretchability, because wearable devices may be destroyed due to
the detrimental eects of scratching or mechanical breakdown
during deformation or accidental fracture in their practical
applications. Traditionally, adding flexibility and stretchability
into electronic skins weakens their electronic performance as
wearable devices.[] On the contrary, substrates in the mate-
rials for wearable sensor with visible readouts mounted on
skin is solely acted as an additional protection layer when
exposing to environmental and mechanical fractures, which do
not hamper their performance as wearable devices. Therefore,
exploring inherently flexible and stretchable materials is a vital
approach to establish flexibility and stretchability of wearable
sensors.[]
Empowering wearable sensors with flexibility and stretch-
ability calls for materials including polymers, silicone rubbers,
hydrogels, cellulose fibers, etc.[] First, the wide availability of
polymeric substrates possesses the ability to provide versatility
for larger-area process. Besides, the long chain in polymer with
weak intermolecular forces enable them to spread and slide
past each other, without disturbing the topological constraints
determined by the intertwined polymeric chains.[] These
properties give wearable sensors outstanding flexibility. Second,
many silicone elastomers, such as PDMS, Ecoflex, Silbione,
and DragonSkin, accommodate the large deformation and
have been exploited pervasively as elastomeric substrates due
to their biocompatibility and easy fabrication process.[] Third,
hydrogels possess high dissipative networks and stretchability
because of increasing free volume by reducing the entangle-
ment density.[] Apart from these above, silk, cotton fabric,
paper, and wool have also been employed as the cellulose
fiber-based substrate for wearable sensors. Notwithstanding
poor stretchability, its flexibility is enough for wearable sen-
sors applications.[] Despite their promising future, challenges
remain with both great stretchability and flexibility.[]

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6.4. Long-Term Robust Monitoring
Water is inevitably evaporated from the hydrogel, hindering
the long-term robust monitoring of the hydrogel-based sensors
with visible readouts.[] The water loss causes the hydrogels to
become dry and hard, which seriously hamper the hydrogel per-
formances, such as adhesion, flexibility, and stretchability.[a]
Thus, the real-world application of hydrogels is still restrained
by their poor stability. To achieve long-term robust monitoring,
introducing water-retaining agents (e.g., glycerin, sorbitol, eth-
ylene glycol) into hydrogels may provide a feasible solution
to prevent water loss.[] Another useful strategy is based on
high concentration salt[] or ionic liquid by solvent replace-
ment (Figure).[] However, these strategies usually decrease
mechanical properties due to the phase separation as induced
by the introduction of the second phase. Therefore, it is still
of great significance to develop a simple and versatile method
to fabricate hydrogel-based substrates for wearable devices with
visible readouts that can simultaneously achieve synergistic
performance in adhesion, flexibility, stretchability, and long-
term stability.
Besides, long-term robust monitoring of biomarkers is
still highly desirable for realizing the full potential of wear-
able chemical sensors with visible readouts. The biomarkers
detection through wearable devices gives insightful informa-
tion for screening and early diagnosis of diseases.[c] However,
body fluids including sweat, saliva, and tears only contain bio-
markers (e.g., metabolites, electrolytes, proteins, and peptides)
in trace amounts, which is the key challenge for long-term
robust monitoring.[] Establishing highly sensitive and selec-
tive sensors with proper preconcentration techniques is a fea-
sible way to deal with this issue.[] Despite this, its complicated
and time-consuming procedures are not suitable for contin-
uous monitoring. Currently, oximeter is the most implemented
wearable chemical sensors with visible readouts, which can
be utilized to measure the Oxyhemoglobin (HbO) and deoxy-
hemoglobin (Hb) noninvasively.[] Yokota et al. developed a
three-color, highly ecient polymer light-emitting diodes and
organic photodetectors to realize ultraflexible and conformable
photonic skin of thickness (µm) for continuous health moni-
toring (Figure).[] The device can unobtrusively detect the
oxygen concentration of blood when placed on a finger. Its good
repeatability in measurements with low noise enables it to be
performed for long-term robust monitoring. However, these
wearable chemical sensors with visible readouts need to be
integrated into electronics with additional adhesive substrates
(Ecoflex or tapes). Consequently, next-generation wearable
chemical sensors must explore novel materials and integration
systems to further outreach the wearable biosensing technology
for noninvasive and long-term robust monitoring.
6.5. Detection Sensitivity
The optimal wearable sensors with visible readouts should
detect the subtle changes of physiological signals in an obvious
color-changing way. Imperceptible changes in the physiological
signals may be an early warning for health conditions. For
the purpose of capturing exceedingly small changes on the
epidermal skin, materials used in skin-mounted wearables
should be surpassingly sensitive.[] For instance, thermochromic
materials for body temperature sensing should decrease down
to . °C resolution, probing minor signs of health risk.
Response time is another index to determine the detection
sensitivity. The Kim group illustrated a color change occurs
within  µs after water reaches the PDA film. An immediate
blue-to-red colorimetric response arises from the CC rota-
tions.[] Consequently, this factor is exceptionally relevant with
the color-changing mechanism in the materials. On the other
hand, the detection limit of physiological signals acquired from
epidermal skin are generally at a broad scope from extremely
low to high level, e.g., body motion (pulse, breath, and jogging)
is typically from several MPa to kPa, which confers the bench-
mark for designing wearable mechanochromic sensors.[]
Detection limit of these smart materials encapsulated in the
epidermal skin-mounted wearables is much lower compared
with implantable devices. Consequently, an adequate detec-
tion limit is particularly required by the materials for these
stimuli. The wearable sensors with visible readouts reported till
now detect the physiological signals in a limited range, which
impede their applications in health monitoring. The thickness
of substrates in the wearable sensors also have an impact on
the detection sensitivity because the thinner substrate produces
more intimate contact with epidermal skin. The excellent con-
formity could acquire the physiological signal in a short time,
which engenders the feedback to the patients.
6.6. Integration
Combining the multiple sensing elements with uniquely
designed structures such as D, D, and D configurations is
an actively thriving research area driven by the further devel-
opment of skin-attachable and wearable sensors with visible
readouts (Figure 12). It needs to be seamless and intimate with
the human body and harvesting information from users and
their surroundings. Subsequently, the fabrication of stretchable
and flexible networks hybrids with D-integrated sensors with
visible readouts are exemplified as micromazes, microfluidics,
and microgates. Based on these structural designs, the mate-
rials about sensing parts and substrates are explored for novel
wearable sensors with visible readouts. To achieve the out-
standing conformity on the epidermal skin and high detection
sensitivity by facile fabrication procedures, these smart health-
monitoring platforms will be upgraded to “one system = several
results” on an intrinsically stretchable, flexible, and adhesive
substrates since multiple physiological signals are simultane-
ously required to detect and monitor the health condition.[]
6.7. Perception
The improvement and implementation of wearable sensors
based on artificial intelligence (AI) have already been proac-
tive to help physicians and healthcare systems make decisions,
especially for elderly populations, infants and disables.[] Shin
et al. exemplified a early-stage lung cancer diagnosis through
a deep learning that was trained by using surface-enhanced

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Raman spectroscopy signals of exosomes captured from healthy
and lung cancer cells, generating a discrimination accuracy
of %.[] These simple and straightforward color-changing
sensory devices oer a promising platform for information
collection, analysis, and recognition based on AI (Figure 13).
Moreover, the integration of wearable sensors with visible read-
outs and AI could provide a reliable and aordable real-time
monitoring tool to eciently avoid misdiagnosing and econom-
ically alleviate the burden before the health conditions become
symptomatic.[]
6.8. Privacy Protection
Wearable devices can collect user data regardless of time and
place since they connect patients and hospitals with the Internet
of Things and sensing technology by uploading data to the
cloud.[] Although the system has many advantages and facili-
tates the patients and health service providers significantly, the
potential issue of privacy breaches may allow sensitive health
care information to move into the wrong hands,[] especially
for wearable devices with visible readout. Thus, secure storage,
access, and computation are necessary to be achieved without
compromising patients’ privacy.
The integration of security labels into wearable devices with
visible readout are highly desirable for data security and pri-
vacy protection of subjects, because it could be considered as
“guardian angel” for ensuring the secure storage, access, and
computation.[] When participating national governments,
industry producers, hospitals, third party participating institu-
tions, and individuals would like to access the health care infor-
mation, the security labels in the wearable optical devices are
appropriately used for determining the identity first.[] In real-
world health case scenarios, if the authentication results of secu-
rity labels on wearables do not meet a criterion, the health care
information will not be accessed by the wrong hands. However,
traditional security labels only provide limited protection to the
subjects, as they can be readily duplicated due to their predi-
cable and deterministic decoding mechanisms.[] Security
labels with physically unclonable function (PUF) have been
proven to be one of the most reliable ways to protect the data
security, which may oer a practical solution to be integrated
with wearables for avoiding the privacy infringement.[] This
is because that the PUF features within the security labels that
are impractical to duplicate can be easily converted to binary-bit
codes, enabling unbreakable encryption.[] Furthermore, We
have already illustrated two vial strategies (deep learning and
computer visions) to collect, analyze, and decode the output
data during the process of manufacturing, distribution, supply,
and end users.[] To make it more practical for encapsulating
PUF security labels into wearables, Hu etal. reported a PUF
security label with flexibility and biocompatibility by randomly
embedding microdiamonds in silk fibroin films.[] In addi-
tion, the as-prepared PUF security labels can be attached onto
the surface of polyethylene material, and human skin, and even
under chicken skin tissue. Therefore, encapsulating the PUF
security labels into the wearable devices with visible readout is
feasible to put an end to the privacy leaks.
6.9. Discriminable Visual Display for Multisignal Readouts
Wearable sensors utilize the dye or pigments as indicators for
health monitoring noninvasively, but its color irreversibility and
photochemical degradation impedes their applications. Com-
paratively, structural-color and SPR-based nanomaterials are
more suitable choices for wearable epidermal sensors because
of supremely admirable sensitivity and stability.[a,,a] Never-
theless, it is not easy to distinguish the color dierence under
dierent stimuli as a result of its range of color-changing in
the identical visible-light region. Wearable sensors with visible
readouts regularly demonstrate the similar color dierence in
the same area in response to various stimuli, which makes it
laborious to discern for health monitoring.[,] To drastically
Figure 12. Design strategies on device integrity of wearable sensors with visible readouts. It can be categorized as D fibers (segmental hybrid fibers
for temperature monitoring). Reproduced with permission.[] Copyright , American Chemical Society. Medical textiles for mechanical force moni-
toring. Reproduced with permission.[] Copyright , Wiley-VCH. D films (elastomers for mechanical force monitoring). Reproduced with permis-
sion.[] Copyright , Wiley-VCH. Papers for humidity monitoring. Reproduced with permission.[a] Copyright , Springer Nature. Hydrogels for
mechanical force monitoring. Reproduced with permission.[] Copyright , Wiley-VCH. Fabrics for light monitoring. Reproduced with permission.[]
Copyright , Elsevier. D nanostructures (wrinkles for light monitoring). Reproduced with permission.[] Copyright , AAAS. Nanodomes for elec-
tricity monitoring. Reproduced with permission.[] Copyright , American Chemical Society. Hybrids microstructures (microcracks for mechanical
force monitoring. Reproduced with permission.[] Copyright , Springer Nature. Micromazes for temperature and mechanical force monitoring.
Reproduced with permission.[] Copyright , Wiley-VCH. Suspended microgates for temperature monitoring. Reproduced with permission.[] Copy-
right , Wiley-VCH. Microfluidics for glucose monitoring. Reproduced with permission.[g] Copyright , American Chemical Society.

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improve its performance, we suggest that discriminable visual
display is a top priority while merging the dierent kinds of
materials with tunable optical properties for multisignal detec-
tion into wearable devices.
In this review, we conclude that the wearable sensors with
visible readouts have reached a certain degree achievement
in terms of adequate detection sensitivity, biocompatibility,
and multifunctionality as well as applications in health moni-
toring for smart wearable devices. The past decade has wit-
nessed encouraging progress in novel materials with tunable
optical properties. Although promising, the current generation
of wearable sensors with visible readouts is not yet capturing
the huge market opportunities available because they fail to
drive key parameter including sensitivity, specificity, reusability,
and durability for applications. We believe that consideration
of challenges and perspectives (Figure ) will push with a
purpose to facilitate the functionalities of materials with tun-
able optical properties for wearable devices, which eventually
reach the ultimate goal of enabling explicit, noninvasive, and
continuous monitoring for physiological signals depending
on the reversible optical properties. Finally, we are optimistic
that next generation wearable epidermal sensors are poised to
expand our perceptions with limitless possibilities.
Acknowledgements
The authors gratefully acknowledge the support from the National
Natural Science Foundation of China (Grant Nos.  and
), the Natural Science Foundation of Shaanxi Province, China
(Grant No. JQ-), the China Postdoctoral Science Foundation
(Grant No. M), the Fundamental Research Funds for the
Central Universities (Grant No. xjh), the Opening Project of
Figure 13. Current challenges and perspectives for the demonstration of wearable sensors with visible readouts. Current challenges: biocompatible and
biodegradable devices (scale bars:  mm). Reproduced with permission.[] Copyright , PNAS. Mussel-inspired adhesive hydrogels. Reproduced
with permission.[b] Copyright , American Chemical Society. Flexibility and stretchability of ionic hydrogels. Reproduced with permission.[g]
Copyright , American Chemical Society. Long-term usability of ionic liquid-based hydrogels. Reproduced with permission.[a] Copyright ,
Elsevier. Wearable chemical sensors with visible readouts in long-term monitoring for oxygen concentrations of blood (scale bars:  cm). Reproduced
with permission.[] Copyright , AAAS. Detection sensitivity of mechanochromic hydrogels (scale bar:  cm). Reproduced with permission.[]
Copyright , Wiley-VCH. Perspectives: integration of electrochromic interactive devices. Reproduced with permission.[] Copyright , Springer
Nature. Artificial intelligence-based perception. Reproduced with permission.[] Copyright , American Chemical Society. Physically unclonable
security labels for privacy protection. Reproduced with permission.[a] Copyright , Royal Society of Chemistry. Discriminational visual display for
sweat analysis (scale bars:  cm). Reproduced with permission.[e] Copyright , AAAS.

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Key Laboratory of Shaanxi Province for Craniofacial Precision Medicine
Research, College of Stomatology, Xi’an Jiaotong University (Grant
No. LHM-KFKT), and the Postdoctoral International Exchange
Talent-Introducing Program (Grant No. YJ). The authors also
thank for the contribution from Prof. J. Justin Gooding.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
health monitoring, materials with tunable optical properties,
physiological signals, visible readouts, wearable sensors
Received: November , 
Revised: February , 
Published online:
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Fei Han studied in School of Chemistry at Lanzhou University from  to , where he
obtained M.S. degree. Afterward, he earned a Ph.D. degree under the supervision of Scientia Prof.
J. Justin Gooding in School of Chemistry at University of New South Wales (UNSW). He is cur-
rently an assistant professor in Bioinspired Engineering and Biomechanics Center (BEBC) at Xi’an
Jiaotong University (XJTU), where his research focuses on wearable devices for health monitoring,
materials with tunable optical properties, stimuli-responsive polymer, and anticounterfeiting
labels.
Feng Xu received his Ph.D. from Cambridge University and worked as a research fellow at Harvard
Medical School and Harvard-MIT HST. Prof. Xu found Bioinspired Engineering and Biomechanics
Center (BEBC) in XJTU. His research aims at advancing human health through academic excel-
lence in education and research that integrates engineering, science biology and medicine. He
has been recognized in the Top % Scientists Worldwide (/) and also Clarivate Highly
Cited Researchers ().
Adv. Mater. 2022, 