An Overview of the Development of Flexible Sensors

Abstract

Flexible sensors that efficiently detect various stimuli relevant to specific environmental or biological species have been extensively studied due to their great potential for the Internet of Things and wearable electronics applications. The application of flexible and stretchable electronics to device-engineering technologies has enabled the fabrication of slender, lightweight, stretchable, and foldable sensors. Here, recent studies on flexible sensors for biological analytes, ions, light, and pH are outlined. In addition, contemporary studies on device structure, materials, and fabrication methods for flexible sensors are discussed, and a market overview is provided. The conclusion presents challenges and perspectives in this field.

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An Overview of the Development of Flexible Sensors
Su-Ting Han, Haiyan Peng, Qijun Sun, Shishir Venkatesh, Kam-Sing Chung,
Siu Chuen Lau, Ye Zhou, and V. A. L. Roy*
Dr. S.-T. Han
College of Electronic Science and Technology
Shenzhen University
Shenzhen 518060, P. R. China
Dr. H. Peng
Key Laboratory for Material Chemistry of Energy Conversion and Storage
Ministry of Education
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074, China
Q. Sun, S. Venkatesh, K.-S. Chung, S. C. Lau, Dr. V. A. L. Roy
Department of Physics and Materials Science
City University of Hong Kong
Hong Kong SAR
E-mail: val.roy@cityu.edu.hk
Dr. Y. Zhou
Institute for Advanced Study
Shenzhen University
Shenzhen 518060, P. R. China
DOI: 10.1002/adma.201700375
control were identified as future market
growth drivers in the report.[1]
The emerging field of flexible elec-
tronics, a technology that is compatible
with movable parts and arbitrarily curved
surfaces, is promising for a new applica-
tion paradigm in large-area electronics.
Rapid developments in the design of
ultrathin sensors and actuators, elec-
tronic and optoelectronic devices, and soft
biocompatible encapsulating layers are
expected to markedly expand the possibili-
ties of flexible electronics from the afore-
mentioned curved panels and foldable
displays to soft systems with interfaces
that offer curved surfaces and complex geometries.[2–8] Figure 1
summarizes flexible electronics for the application paradigm in
various kinds of electronics.
Conventional sensors, hindered by their rigidity from cap-
turing analytes, suffer from poor-quality signal transduction.
Flexible sensors, by contrast, can capture target analytes more
efficiently and generate higher quality signals. Production of
flexible sensors requires novel approaches in materials design,
including active materials and conductors, as well as the selec-
tion or synthesis of flexible substrates. Bulk rigid materials
become deformable once they are thinned and oriented into
nanostructures.[9] For active materials,
π
-conjugated low-cost,
print-compatible, solution-processable and lightweight organic
semiconductors have been used.[3] Non-transition-metal oxides
that exhibit high sensitivity and favorable conductivity, such
as ZnO, SnO2, In2O3, and Ga2O3, have also been employed as
active materials.[10,11] Among emerging active material candi-
dates for flexible sensors, one-dimensional nanostructure-based
materials such as nanowires[12,13] and two-dimensional (2D)
nanostructure-based semiconductors including graphene,[14]
transition-metal dichalcogenides (TMDs),[15] and black phos-
phorus[16] all offer distinctive optical and electrical performance
characteristics.
Various flexible substrates including polyimide (PI), poly-
etheretherketone, polyethersulfone (PES), polycarbonate,
poly(ethylene napthalate) (PEN), and polyester resins such as
polyethylene terephthalate (PET) are often selected for sensing
applications. The thermal resistance, chemical resistance,
transparency, and flexibility of a material should be consid-
ered for practical sensing applications.[17] Conducting materials
allowing connection between various components of elec-
tronic devices are critical to the functioning of those electronic
devices. Nanoparticles, nanotubes, nanowires, and thin films
can be employed as core materials for particular processing
conditions.[18,19]
Flexible sensors that efficiently detect various stimuli relevant to specific
environmental or biological species have been extensively studied due to
their great potential for the Internet of Things and wearable electronics
applications. The application of flexible and stretchable electronics to device-
engineering technologies has enabled the fabrication of slender, lightweight,
stretchable, and foldable sensors. Here, recent studies on flexible sensors
for biological analytes, ions, light, and pH are outlined. In addition, con-
temporary studies on device structure, materials, and fabrication methods
for flexible sensors are discussed, and a market overview is provided. The
conclusion presents challenges and perspectives in this field.
Flexible Sensors
1. Introduction
According to IC Insights’ new 2016 O-S-D Report—A Market
Analysis and Forecast for Optoelectronics, Sensors/Actuators,
and Discretes,[1] in 2015, sales of sensors and actuators achieved
a new all-time annual peak of $10.2 billion. Increasing numbers
of sensors and actuators are integrated into new high-volume
applications such as the millions of internetworked devices in
the Internet of Things and in other wearable systems; there-
fore, the selling prices of sensors and actuators have declined.
The report predicted that the worldwide sales of sensors and
actuators would expand by a compound annual growth rate
(CAGR) of approximately 6% through 2020 with the help of
the Internet of Things. Novel applications in semiautonomous
cars, Internet of Things products, and intelligent embedded
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Here, we aim to illustrate various types of flexible sensors.
Rather than summarizing all relevant previous work, this paper
selects noteworthy work that may suggest crucial future trends
of flexible sensors. The discussion presented herein is expected
to induce a more fluent exchange of ideas and more intense
interest in this emerging field. We summarize the recent state-
of-the-art flexible electronics currently employed as flexible
light sensors, flexible pH sensors, flexible ion sensors, and flex-
ible biosensors. The selections of materials and the fabrications
of devices are included in every part. We also provide a detailed
description of engineering technologies with an emphasis on
flexible sensor fabrication. We then present market analysis on
the world sensor market, printed sensors, and wearable sen-
sors. Finally, a conclusion is given and perspectives for this
emerging field are addressed.
2. Flexible Sensors for Ultraviolet and Other
Photonic-Based Sensing
Light provides energy to the earth and enables our eyes to see
the world. Light sensors or photodetectors, which can convert
light signals into electrical pulses, are indispensable for most
contemporary optical and optoelectronic applications,[20] such
as imaging, spectroscopy, fiber-optic communication, and time-
gated distance measurements.[21] Fabrication on plastic sub-
strates using large-area mass-production compatible processes
such as roll-roll printing facilitates flexible, lightweight, and
transparent light sensors.[22] Because of these developments,
extensive research interest has been expressed worldwide.
The detection of light typically involves the following four
processes: i) light absorption, ii) exciton diffusion, iii) exciton
dissociation, iv) charge transfer and collection.[23] A light sensor
is required to possess a specific spectral response range, short
response time, high quantum efficiency, high responsivity,
normalized detectivity, and wide linear dynamic range.[23] It is
believed that the sensed spectral range of a photodetector is
of vital importance for a myriad of fundamental and practical
applications including but not limited to environmental moni-
toring, communications, and surveillance; thus, this necessi-
tates the development of light sensors working independently
in the ultraviolet (UV, <400 nm), visible (400–700 nm) or
infrared (IR) (>700 nm) spectral ranges.[23] Fast response is usu-
ally required for a light sensor to detect incident light in a short
time. The response time is the time required to reach 63.2%
of the steady-state or final value,[23] or the curve-rise/decay
time constant.[24] Deeper traps are expected to require longer
times to release charges, thus resulting in a slow response.[25]
Quantum efficiency, the parameter applied to characterize the
exciton generation process, is the ratio of electron–hole pairs
or photoelectrons generated through light to the number of
incident photons. High external quantum efficiency is a pre-
requisite for high photodetector performance levels.[26] Respon-
sivity (measured in units of A W−1), directly reflecting the
photodetector sensitivity to light radiation, is defined as the
product of generated photocurrent (Jph) divided by the inci-
dent optical power (Llight), R = Jph/Llight.[23] However, the nor-
malized detectivity (in Jones units; 1 Jones = 1 cm Hz1/2 W−1),
D* = R/(2qJd)1/2 = (Jph/Llight)/(2qJd)1/2, is usually employed as
the figure of merit to evaluate the detector sensitivity, where Jd
Su-Ting Han is an associate
professor at Shenzhen
University. She received her
M.Sc. degree in analytical
chemistry from Hong Kong
Baptist University (2010) and
her Ph.D. degree in physics
and materials science from
City University of Hong Kong
(2014). After graduation, she
worked at the City University
of Hong Kong as a postdoc-
toral fellow and then joined Shenzhen University in 2016.
Her research interests include functional electronic devices
and flexible, stretchable, and wearable electronics.
Vellaisamy, A. L. Roy is cur-
rently an associate professor
at the Department of Physics
and Materials Science, City
University of Hong Kong. His
research focuses on materials
and device engineering for
electronic devices, such as
transistors, flash memory,
photovoltaic devices, and sen-
sors. His research is interdis-
ciplinary and he leads a team
of physicists, chemists, and engineers.
Adv. Mater. 2017, 1700375
Figure 1. Schematic illustration of flexible electronics in development
today across a broad range of applications.

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is the dark current, and q is 1.6 × 10−19 Coulombs representing
the elementary charge.[21,23] The normalized detectivity is useful
because it not only shows how large a photocurrent can be gen-
erated by a given light irradiation but also can reflect another
vital parameter known as the photocurrent/dark ratio, thereby
normalizing for variations in device area and response speed.
This allows for comparison between different devices.[21] Not
only high photocurrents but also low dark currents are required
to achieve high-performance light sensors.[23,27,28] Both large
photocurrents and high photocurrent/dark ratios are indispen-
sable when driving and switching displays.[29]
From a structural perspective, two types of light sensors
are usually used, namely the photodiode type and the photo-
transistor type. A photodiode is a semiconductor device with
two-terminal electrodes. The simplicity of this structure facili-
tates fabrication and operation. Photodiodes typically have fast
response times, which, in the case of direct-gap semiconduc-
tors, can be measured in microseconds or less.[21] Phototransis-
tors, pioneered by William Shockley in 1951, are three-terminal
devices; a phototransistor uses a stream of photons as an addi-
tional terminal. They are able to realize functions of light detec-
tion, photomodulation, electric-field-controlled switching, and
magnification of signals in a single device.[23,30] Particularly
large gains are achievable through transistor action because the
application of a gate bias causes a depletion zone where effi-
cient separation of light-induced excitons into electrons and
holes can occur. Subsequently, these charge carriers are accele-
rated towards source and drain electrodes by a source–drain
voltage. The noise of phototransistors can be depressed to a
value much lower than the noise values of other light detec-
tors.[23,31] Apart from high photosensitivity and responsivity
values, phototransistors are highly suitable for integration into
conventional electronic circuits due to the unique structure
of field-effect transistors (FETs).[31] Compatibility with CMOS
technologies allows for miniaturization. Thus, phototransistors
can be applied in Internet of Things devices and comply with
Moore’s Law.[32,33]
Sensed photons represent some point on a spectrum; nar-
rowband applications such as full-color imaging or visible-
blind near-infrared (NIR) detection necessitate spectral (color)
discrimination.[34] UV photodetectors have drawn tremen-
dous interest for various scientific and industrial endeavours,
including environmental monitoring, space communication,
fire alarms, chemical sensing, biological analysis, and military
defence.[22,25,35] Visible light sensors also serve noteworthy pur-
poses in daily life. IR detectors exhibit special characteristics
with potential for medical, industrial, communication, night-
vision imaging, military, and security applications, because
IR detectors have the remarkable capability of detecting light
transmitted through atmospheric, biological, and other mate-
rials.[34,36] Some typical applications in the IR spectral range
are telecommunications (1300–1600 nm), thermal imaging
(>1500 nm), biological imaging (800–1100 nm), thermal photo-
voltaics (>1900 nm), and solar cells (800–2000 nm).[37]
Semiconductors can be produced from materials other than
silicon to deliver semiconductor bandgaps that are appropriate
for specific application requirements.[38] For example, inorganic
metal oxides have been widely used for UV photodetectors.
Conventional metal-oxide semiconductor photodetectors are
based on various metals including tin oxide (SnO2), titanium
dioxide (TiO2), zinc oxide (ZnO), indium oxide (In2O3).
Research on light sensors using ZnO semiconductors
is an extremely active subfield of sensor research. ZnO
without dopants is an n-type semiconductor. ZnO is attrac-
tive in photo detectors because its exciton binding energy is
as high as 60 meV, its electron mobility is as high as nearly
100 cm2 V−1 s−1, and its remarkably wide bandgap energy has
a value of 3.37 eV under ambient conditions.[22,39] Devices
constructed with rational structural designs can have high
surface-to-volume ratios for harvesting maximal light energy
and accelerating efficient exciton dissociation.[40] Bai et al. con-
structed a UV sensor with a photocurrent/dark ratio as high
as 1.2 × 105 by integrating multiple ZnO nanowires connected
in parallel through a contact printing method on a large-scale
PET substrate, and they demonstrated the linear scaling of the
photo response current with the number of nanowires.[39]
Doped ZnO can be a p-type semiconductor. Hsu et al. formed
a UV sensor with a La-doped ZnO nanowire on a flexible PI
substrate (Figure 2).[22] For most n-type metal-oxide semicon-
ductors, humidity is expected to depress the electron transfer
Adv. Mater. 2017, 1700375
Figure 2. I–V characteristics of a p-type ZnO:La detector. Reproduced with permission.[22] Copyright 2013, American Chemical Society.

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and the consequent photocurrent responsivity. However, this is
not the case for p-type metal-oxide semiconductors. Hsu and
his collaborators found that water and humidity were able to
improve the photocurrent/dark current ratio up to 212. The
authors attributed this performance enhancement to simulta-
neous dark conductivity decreases and photocurrent enhance-
ments. When under dark conditions, the water replaced the
O2− ions absorbed on the p-type ZnO nanowire surface, thus
reducing the hole concentration and the consequent dark cur-
rent. However, due to the high photocatalytic activities of the
water that covered the ZnO nanowires, more electron and
hole exchanges with the water molecules were expected, which
dramatically increased the photocurrent by generating a con-
ductive path in the photocatalytic layer.
Two-dimensional graphene has emerged in recent years and
has been utilized for a flexible channel material in phototran-
sistors due to its high mobility, which is as high as approxi-
mately 106 cm−2 V−1 s−1 at room temperature,[41] along with
its high conductivity, favorable chemical stability, and ease of
microfabrication and nanofabrication. Graphene’s inherent
ultrasensitivity presents great opportunities for this material
to be explored for new sensors and optoelectronic devices.[42]
Lee’s group successfully fabricated a flexible UV photodetector
composed of ZnO nanorods and graphene on a PI substrate,
with a 400-nm-thick poly(vinylpyrrolidone) (PVP) layer as a
dielectric sandwiched between bottom and top layers of 20-nm-
thick Al2O3. A 60-nm-thick Ni layer was used as the gate elec-
trode (Figure 3). This hybrid structure afforded a photo current
responsivity as high as 2.5 × 106 A W−1, which was also main-
tained after 10000 cycles of bending. The high light sensitivity
was attributed to the efficient electron transfer from ZnO
nanorods to graphene, which was confirmed by a negative shift
of the Dirac point. The transferred electron concentration per
unit of UV intensity was calculated to be as high as 6 × 1012
electrons per mW cm−2.[41] Lee’s group also used a simple
spin-coating procedure to fabricate a transparent and flexible
array of reduced-graphene-oxide/P(VDF-TrFE) composites as
a channel with IR-responsive capability on a PES substrate,
which is transparent in visible light but can respond to IR irra-
diation from human bodies.[43]
Carbon nanotubes have also been employed to improve light
sensing. Liu et al.[44] presented a flexible light sensor in the
form of a PET sheet that had been coated with single-walled
nanotubes (SWNTs) using Cu2O/ZnO hybrid nanofilms. As
shown in Figure 4, 50-nm ZnO nanofilms and 50-nm Cu2O
layers were sequentially deposited on top of the SWNTs, pro-
ducing a photodetector with a short response time (<100 ms),
excellent repeatability, robust stability, and wide broadband
range from 365 to 625 nm without any need to apply an
external bias. One noteworthy phenomenon is that once the
deposition sequence of ZnO and Cu2O had been changed, the
photocurrent dramatically decreased to one-quarter of that for
the Cu2O/ZnO nanofilm photodetector. The authors speculated
that whispering-gallery mode resonances could be formed in
the Cu2O/ZnO hybrid nanofilms because of the higher refrac-
tive index of Cu2O outside, leading to the enhanced light
absorption and trapping.[44]
Sawyer and co-workers reported a hybrid paper device incor-
porating thorn-like, flexible ZnO-multiwalled carbon nanotubes
(MWCNTs); because of its high surface-to-volume ratio and
high mobility for efficient carrier transport as well as collection,
that paper device exhibited excellent UV sensing.[45] Its maxi mal
photoresponsivity to UV light was 45.1 A W−1 at 375 nm, indi-
cating that the external quantum efficiency reached 14 927%.
Additionally, the device exhibited fast transient response char-
acteristics with a rise time of 29 ms and a decay time of 33 ms
(Figure 5).
SnS2 and MoO3 have also been used to fabricate flexible
light sensors. For example, Wu and co-workers reported a
flexi ble photodetector using a self-assembled SnS2 microsphere
nanosheet on a transparent polypropylene (PP) film to detect
light ranging from UV to NIR (300–830 nm).[46] Zheng et al.
reported a flexible light sensor with 2D MoO3 single-crystalline
belts, with a bandgap of 3.28 eV, on a PET substrate, which
exhibited a responsivity of 183 mA W−1, a photocurrent/dark
ratio of over two orders, and a response time shorter than 1 s.[35]
Quantum dots of materials such as CdSe, CdS,Fe3O4, CdTe,
TiO2, NiO, Co3O4, and Mn3O4 have recently attracted particular
attention in light sensors. Quantum dots decorate graphene to
form compatible heterostructures and achieve size-dependent
semiconductor optical properties. Multicarrier effects in nano-
particles are envisioned to provide optical gain and performance
enhancement through carrier multiplication.[21] For example,
the bandgap of PbS nanoparticles with a diameter of 3 nm or
less is three times larger than that of bulk PbS, which is able
to generate strong quantum confinement for high-responsivity
light sensing in the visible spectral range.[38] Quantum dots can
also realize the IR sensitivity that is not accessible by pristine
Adv. Mater. 2017, 1700375
Figure 3. a) Schematic illustration of a photodetector composed of ZnO nanorods and graphene on a flexible PI substrate. b) Cross-sectional field
emission scanning electron microscope (inset: top-view) image of ZnO nanorods on a flexible PI substrate. Reproduced with permission.[41] Copyright
2015, American Chemical Society.

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graphene or ZnO. Liu et al. fabricated a high-performance
heterostructure on a flexible PET substrate by integrating syn-
thesized germanium quantum dots, monolayer graphene elec-
trodes, and n-type ZnO, which improved the IR responsivity to
approximately 9.7 A W−1 at 1400 nm without any sacrifice of
the response speed (approximately 40 and 90 µs for rising and
decay, respectively). These excellent characteristics along with
the high photocurrent/dark ratio of approximately 103 could
be guaranteed by the restricted dark current (approximately
1.4 nA, −3 V) due to the effective barrier formed between the
graphene and the ZnO interface.[47] Qi and co-workers pio-
neered symmetric mirror-imaging photoswitching effects in
PbS-decorated graphene-based phototransistors; the device
responsivities were calculated to be as high as approxi mately
2.8 × 103 A W−1 at a negative gate bias and approximately
1.7 × 103 A W−1 at a positive gate bias.[42] Quantum dot sur-
face modification is an efficient strategy to improve the photo-
responsivity. Sargent and co-workers created films of densely
packed colloidal quantum dots with stable benzenedithiol sur-
face passivation, resulting in a 1000-fold decrease of the dark
current owing to the elimination of the interaction between pri-
mary butylamine ligands and the aluminum contact.[36]
Compared with their inorganic counterparts, organic elec-
tronic devices or circuits integrated on lightweight, foldable
plastic substrates have numerous fundamental advantages and
show great promise for applications in electronic displays, elec-
tronic smart cards, or biomedical systems, which are difficult to
implement using conventional inorganic electronics.[48] In addi-
tion, the manufacturing processes of organic electronics should
be much cheaper and more environmentally friendly when
fabricated with low-temperature procedures such as a printing
technique or roll-to-roll processing.[49–51] The most essential and
distinguishing attribute of organic devices is that their chem-
ical versatility allows for easy molecular functionalization and
chemical engineering of optoelectronic performance character-
istics through artificial molecular designs to afford high sen-
sitivity to optical and electrical stimuli,[52,53] thus allowing for
the integration of energy conversion, light detection, and signal
magnification in one device.[23] Although conjugated polymers
can be easily processed at low cost, with physical flexibility, and
at large area coverage,[37] small molecular semiconductors and
their composites are more readily available and tend to form
more ordered domains for higher charge carrier transport,
resulting in greatly improved device reproducibility.[23]
High mobility is critical for light sensors; it determines the
photocurrent because the conductivity (
σ
) is the product of the
number of carriers (n) times the mobility (
µ
) and electronic
charge (e).[52] Conjugated organic molecules that have strong
Adv. Mater. 2017, 1700375
Figure 4. a) Scheme to show a photodetector fabrication process involving coating Cu2O/ZnO hybrid nanofilms on a SWNT-coated PET. b) I–V curves
of the Cu2O/ZnO photodetector with an ITO/Cu2O/ZnO/SWNT structure under dark conditions. c) Time-resolved photoresponse. d) Light-intensity-
dependent time-resolved photoresponse. e) Enlarged portion of one photocurrent rising and reset under white light-emitting diode illumination.
Reproduced with permission.[44] Copyright 2014, Royal Society of Chemistry.

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intermolecular interactions through
π
–
π
overlap stacking are
expected to offer high mobility.[53] Li and co-workers fabricated
phototransistors using single-crystalline sub-micrometer and
nanoscale ribbons of n-type F16CuPc, and they achieved a photo-
current/dark ratio as high as 4.5 × 104 at a gate bias of −6 V.
They also observed that a bias voltage significantly increased
the photoresponse, probably because the applied gate bias on
the transistors provided an effective method for the separation
of the light-induced excitons, benefiting the formation of the
transistor conducting channel.[52] As expected, nanocoils of an
anthracene derivative in J-aggregates that was
driven by weak
π
–
π
overlap did not show any
photoresponse.[53,54] Apart from the crystal
morphology, the crystal orientation also sig-
nificantly influenced the light response.[55]
However, some defects can enhance the
photocurrent responsivity. Kim and co-
workers employed plasma and ion irradia-
tion on graphene to induce the formation
of different types of surface defects such as
vacancies, graphene islands, doping, and
impurities, which disrupted the honeycomb
lattice. These defects strongly scattered the
incident light to offer more effective light
absorption than pristine graphene, while
simultaneously reducing the carrier recom-
bination and facilitating continuous channels
for charge carriers, leading to a substantially
enhanced photocurrent.[56]
The heterojunction is another critical factor to be considered
for organic light sensors. For example, quinacridone thin films
prepared through vacuum sublimation only showed respon-
sivity levels lower than 1 mA W−1, but a solution-processed film
could offer greater responsivity values due to the formation of
bulk heterojunctions.[51] Heiss and co-workers prepared a paper
photodetector by painting riboflavin-functionalized quinacri-
done nanocrystals on paper using a paint brush; they sputtered
gold through a shadow mask over the nanocrystals to create
four electrodes (Figure 6a). Although the sample surface was
Adv. Mater. 2017, 1700375
Figure 5. a) Scheme to illustrate a ZnO-MWCNT hybrid paper UV photodetector. b) Typical I–V curves of the UV photodetector. c) Transient response
and d) photoresponsivity spectra of the UV photodetector. Reproduced with permission.[45] Copyright 2014, Royal Society of Chemistry.
Figure 6. Photoconductivity in films of quinacridone micro–nanocrystals with riboflavin
myristate ligands. a) Device structure was painted on paper. b) Photoresponsivity spectrum
measured on the device. The inset shows the used micro–nanocrystals (scale bar, 200 nm).
Reproduced with permission.[51] Copyright 2014, American Chemical Society.

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substantially rough, they easily measured a photoconductivity
spectrum with a favorable signal-to-noise ratio (Figure 6b), and
the responsivity reached 1 A W−1. The riboflavin-coated quina-
cridone device was found to exhibit not only a high responsivity
but also a high dark resistance of 100 GΩ, leading to a high
normalized detectivity of 3 × 1013 Jones.[51]
Heterojunctions with different multilayer structures have
also been developed for photoresponsivity improvement. The
formed multilayer can create blocking electrodes for sup-
pressing a thermally induced dark current, but the bulk hetero-
junction still allows the transport of photogenerated electrons
to opposite electrodes.[28]
Despite extensive research on photodiodes and phototransis-
tors, the photocurrent/dark ratio is still lower than six orders
of magnitude.[24] Hu, Liu and co-workers made a breakthrough
with organic light-dependent resistors (OLDRs); each was
composed of an organic resistor (OR) as a load resistor, with
an organic field-effect transistor (OFET) as a readout element.
This type of photosensor is of the transconductance type rather
than the conductance type; thus, this type can provide a photo-
current/dark ratio up to eight orders of magnitude higher than
the other type can (Figure 7).[29]
We do not intend to list all relevant work in this miniature
review. For clarity, we compare recently reported flexible photo-
sensors in Table 1. Because some studies did not report the
normalized detectivity, to compare the performance levels of
different devices would be challenging. Although apparent
progress has been made, further research is required for opti-
mizing performance.
3. Flexible Biosensors
OFETs are attracting increasing attention as reliable devices
because of their desirable characteristics that include their
applicability in organic circuits such as inverters, oscillators,
Adv. Mater. 2017, 1700375
Figure 7. a) Cross-sectional diagram of a photosensor consisting of three components: an OFET, OLDR, and OR. b) Flexible sensor arrays on the
human finger. Scale bar, 1 cm. Reproduced with permission.[29] Copyright 2015, Wiley-VCH.
Table 1. Typical flexible light sensors reported recently.
Channel material Structure Substrate Bias
[V]
Wavelength
[nm]
Photocurrent/
dark ratio
Maximum responsivity
[A W−1]
Rise time
[s]
Decay time
[s]
Ref.
ZnO nanorods/graphene phototransistor PI 5 365 1.2 2.5 × 106/ / [41]
La-doped ZnO nanowires photodiode PI 10 365 212.1 / / / [22]
ZnO nanowires photodiode PET 3 UV 1.2 × 105/ / / [39]
ZnO/Ge quantum dot
decorated graphene
phototransistor PET −31400 103≈9.7 4 × 10−59 × 10−5[47]
ZnO-multiwalled carbon
nanotube hybrid paper
photodiode None 5 375 / 45.1 0.029 0.033 [45]
Graphene-oxide-decorated
ZnO nanostructures
photodiode PDMS 10 UV 116 / 6 3.5 [57]
Cu2O/ZnO photodiode PET 0 365–625 520 / 0.1 0.1 [44]
ZnO tetrapods photodiode PET 20 365 45 / 0.9 0.9 [58]
ZnO nanorods photodiode PET 0.5 365 / / 100 120 [59]
SnS2 nanosheet photodiode PP 5 300–830 <3 2.1 × 10−4≈0.8 ≈0.6 [46]
MoO3 nanosheet photodiode PET 10 380 369 ≈0.18 <1 <1 [35]
InGaZnO photodiode PET 2 310 3.1 × 1030.45 0.8 33.8 [60]
Si phototransistor PET <0.5 400–700 nm 10552 5 × 10−51 × 10−4[20]
Riboflavin-functionalized
quinacridone nanocrystals
photodiode paper 0 550 190% 1 / / [51]
PDI/Pc OLDRs PET −22 visible 108/ / / [29]

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and logic circuits without the necessity of advanced pat-
terning techniques.[61,62] Moreover, as the development
of industrial printing technology has advanced, the cost
of fabricating OFETs on flexible substrates has decreased.
Sensors based on OFETs with organic small molecules and
polymers have been successfully demonstrated in detecting
a wide range of analytes in vapor, ions, and light systems.
In addition, biological sensors based on organic electronics
are expected to be widely used for detecting various chem-
ical analytes for environmental monitoring, food security,
and prophylactic physical examination.[63] In this section,
we focus on the recent development of flexible OFET-
based biosensors. The device fabrication, operation, and
working principles of detecting analytes in various media is
explained.
3.1. Organic Field-Effect Transistors
OFETs, which can be applied as fundamental components
in price tags, smartcards, and sensors, are desirable for the
manufacturing of low-cost, large-area devices with plastic sub-
strates. An OFET device contains three different components:
a thin semiconductor layer, a gate insulator layer, and three
metal electrodes. The semiconductor layer is usually fabricated
with small molecules or polymer materials. For small mole-
cules, the semiconductor film is usually fabricated by thermal
deposition. Regarding the conjugated polymers, the film is
formed by spin-coating or printing methods. Research is still
ongoing about how to improve the quality of semiconductor
films by tuning fabrication conditions. The insulator layer sep-
arates the gate electrode and semiconductor film. For the fab-
rication of insulator films, polymeric dielectric materials (e.g.,
poly(4-vinylphenol), poly(methyl methacrylate), and polysty-
rene) and inorganic oxide materials (such as SiO2 and Al2O3)
are usually used. The role of the insulator film is paramount
for achieving fast and low-power devices, as well as high-sta-
bility devices, in various complex environments. Electrodes
can be composed of metal and conductive materials such as
doped conjugated polymers and metallic nanoparticles. The
source and drain electrodes inject charges into the semicon-
ductor film for transport. High-work-function metals such
as gold, palladium, platinum, and silver are usually chosen.
In the operating state of an OFET, the current flows between
the source and drain electrodes when voltages are applied on
the gate (Vg) and drain (Vd) electrodes and the voltage on the
source (Vs) is 0. Charges are injected from the source elec-
trode into the active layer, and the number of charge carriers
accumulated in the active layer is determined by the mag-
nitude of potential difference between the gate and source
electrodes (Vgs). The mobility (
µ
) is used to gauge an OFET’s
performance; it shows how easily the charges are trans-
ported in the active layer under a given electric field. Other
device parameters include the on–off ratio (the current ratio
of conductive current “on” state to the current closed “off”
state), and the threshold voltage (a voltage which turns on the
device to allow the current to flow through the active layer).
More introductory information about OFETs can be found in
a related review.[64–66]
3.2. General OTFT Fabrication Approaches
For OFET fabrication, one of the most essential advantages is
the adaptability of various fabrication processes to the charac-
teristics of organic materials. Materials’ fabrication conditions
such as solubility, melting point, and deposition temperature
can be directly regulated by structural design and function-
ality, contributing a wide range of high-performance, low-cost,
flexible, and stable organic materials. One method of semi-
conductor film fabrication is thermal evaporation. Small mole-
cules are evaporated to form a film at a high vapor pressure
and high-temperature state. The film quality can be controlled
by changing the deposition speed and substrate temperature.
Solution spin coating is another procedure commonly used to
form a uniform and flat film. The film thickness can be regu-
lated by changing the rotation speed and solution concentra-
tion. The volatile solvent can be removed by a postannealing
process. Furthermore, the temperature condition is vital to con-
trolling the film quality. However, the aforementioned methods
are usually used in the laboratory. In industrial fields, screen
printing or ink-jet printing approaches, which are suitable for
roll-to-roll processing, are employed.[67–70] They lead to low-cost,
large-scale device production on flexible substrates, showing
obvious advantages in device commercialization.
3.3. Material Considerations for Biosensors
Among various biosensors, DNA is one of most popular bio-
materials for detection processes. Lin et al. fabricated label-free
DNA sensors by combining organic electrochemical transistors
with flexible microfluidic systems.[71] The device performance
levels showed little difference at various bending states. In
these systems, single-stranded (ss) DNA samples are modified
on top of gold gate electrodes as DNA probes to detect com-
plementary DNA targets. The DNA sensor detection limit can
be set to 10 pM through applying an electric pulse. Lee and
co-workers reported DNA sensors using low-temperature solu-
tion-processed flexible In-Zn-O (IZO) TFTs.[72] They employed
a dry–wet method to modify DNA nanostructures on the IZO
surface, and the sensing limit was estimated to be 0.5 µL of
50 nM DNA target solution. The sensing mechanism for cur-
rent changes was based on the oxidation of DNA. Serre et al.
fabricated novel DNA sensors using a silicon nanowire through
a filtration method.[73] Device networks demonstrated favorable
reproducibility and homogeneity and were compatible with
different types of substrates. Karnaushenko et al. fabricated a
flexible diagnostic platform for early detection of avian influ-
enza virus subtype H1N1 DNA sequences, which was based
on Si nanowire field-effect transistors (SiNW-FETs).[74] The sen-
sors showed excellent mechanical properties in bending tests
down to a 7.5-mm radius after 1000 consecutive bending cycles.
The detection limit of the diagnostic platform could reach
40 × 10−12 M within 30 min, which would be very suitable for
early stage disease diagnosis. Another relevant research field is
glucose. Kwak et al. reported a type of flexible glucose sensor
with graphene-based ambipolar FETs using FETs as substrates,
which could detect glucose levels in a range of approximately
3.3–10.9 mM.[75] The device could perform high-resolution,
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real-time measurement, showing high advantages of low-cost,
wearable, and implantable glucose level monitoring applica-
tions. Lee et al. reported a type of pH/glucose sensor based
on flexible ion-sensitive field-effect transistors (ISFETs) on
PET substrates.[76] In the devices, single-walled carbon nano-
tubes (SWCNTs) and poly(diallyldimethyammonium chloride)
are deposited between two patterned metal electrodes. The
electronic conductance changes of the SWCNT nanocom-
posite were measured to characterize the pH levels and then
to detect the glucose levels. For flexible substrates, silk protein
was used as both a flexible substrate and enzyme fixed mate-
rial by You et al.[77] Enzymatic biosensors based on graphene
FETs showed a detection range of 0.1–10 mM.[77] Minami et al.
studied biosensors for lactate detection based on extended-gate
type OFETs.[78] In the sensors, two functional layers were modi-
fied on a flexible substrate for an enzymatic redox reaction of
lactate. The reported detection limit was approximately 66 nM
and the quantification limit was 220 nM. The advantage of the
extended-gate structure was to avoid the influence of semicon-
ductor films in the aqueous media. Other types of biosensors
for detecting trimethylamine,[79] urea,[80] and multisaccharide[81]
have also been fabricated and studied.
4. Flexible pH Sensors and Ion Sensors
4.1. Flexible pH Sensors
pH is a measure of proton activity and is defined as:
pH logH
a=− +
(1)
where
H
a
+
defines the proton activity. The strategies for pH
sensing can be categorized into electrochemical sensing and
spectrometric sensing. This review focuses on some develop-
ments in flexible electrochemical pH sensors. Electrochemical
pH sensing is achieved through the selective bonding of H+
ion to an immobilized receptor. The bonding induces a charge
that is subsequently transduced to a readable electrical signal.
Electrochemical pH sensing can be further categorized on the
basis of sensor structure into sensitive electrode- and ISFET-
based sensing. The most common method of measuring pH is
through an ion-sensitive glass electrode. Conventional ISFETs
are fabricated on silicon substrates. The obvious inadequacy of
these methods is their incompatibility with flexible or wearable
technologies. Consequently, considerable research effort has
been dedicated to the development of flexible pH sensors over
the last 25 years.
4.1.1. Ion-Sensitive Electrodes
Ion-sensitive electrodes work by measuring the potential of a
working electrode (in this context, the working electrode has
the sensing layer) against that of a reference electrode. In the
early years, the driving force behind research was the motiva-
tion to study the in vivo ion concentrations in blood in the coro-
nary arteries.[82,83] To accomplish this, flexible ion sensors were
fabricated as sensing layers on PI substrates utilizing hydrogen
ionophore embedded in a plasticized PVC membrane. Ulti-
mately, the researchers were successful in finding a correlation
between artery occlusion and changes in ion concentrations
in the extracellular regions within a pig’s heart. However, the
plasticized PVC membrane was plagued by problems associ-
ated with membrane adhesion, reversibility, and sub-Nern-
stian sensitivity. One of the standout works during the early
years involved the screen printing of a H+-sensitive ruthenium
oxide film on a flexible polyester film.[84] These films exhibited
superior stability, repeatability, and sensitivity compared with
the plasticized PVC membranes. Subsequently, a flexible pH
sensor with an iridium oxide sensing film on a PI substrate
was developed,[85] and sensors of this type have been applied as
pH monitors within a live pig’s oesophagus,[86] a rabbit’s heart,
and a human heart.[87] pH sensors based on carbon nanotubes,
with surface-functionalized H+ receptors (PANI/PPy), that are
spin coated on flexible plastic substrates show enhanced pH
sensitivity that is close to a Nernstian response.[88] Flexi ble
nanostructured oxide films, which do not require surface-
functionalized H+ receptors, such as WO3 nanoparticles, also
exhibit behavior close to the Nernstian.[89] A highly noteworthy
development in terms of engineering and application was
reported in which a flexible sensor for the measurement of epi-
dermal pH was constructed in a tattoo format (see Figure 8).[90]
Another intriguing possibility was revealed in which a flexible
piezoelectric self-powering unit was used in measuring the
pH across a rigid ZnO microwire.[91] A recent study also pre-
sented a unique flexible sensor fabrication procedure in which
microporous and nanoporous laser-carbonized patterns were
transferred to PDMS substrates and subsequently coated with
a H+ receptor (PANI).[92]
Adv. Mater. 2017, 1700375
Figure 8. Influence of repeated mechanical strain (stretching) upon the
response of the tattoo ISE: (a) pH-responsive behavior of the ISE tattoo
sensor prior to stretching (black) and following the 40th (red) stretch on
GORE-TEX; one unit pH decrement per addition. (b) Images of the tattoo
applied to the forearm in the normal state, during stretching, and after
the 10th stretch. Reproduced with permission.[90] Copyright 2013, Royal
Society of Chemistry.

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4.1.2. Ion-Sensitive Field-Effect Transistors
In ISFETs, the sensing layer can either form the conducting
channel (resistance-modulated sensor) or the gate (capacitance-
modulated sensor). Flexible resistance-modulated ISFETs using
carbon nanotubes[76,93] and a nanostructured oxide film (ZnO
nanowall)[94] as the channel sensing layer have been realized. In
the study by Maiolo et al.,[94] a Nernstian response was observed.
Flexible capacitance-modulated sensors can have their sensing
layers deposited directly on top of their dielectric layers[95] or as
extended gates. Flexible extended-gate, capacitance-modulated
sensors have been demonstrated by making use of pH-sensitive
Parylene-C,[96] amorphous IGZO,[97] and roll-roll RF sputtered
ITO films[98] deposited on plastic substrates. Notably, in such
devices, only the sensing extended gate is flexible, whereas the
rest of the device is not. In a recent study, a flexible dual-gate
transistor was fabricated in which both gates are capacitatively
coupled to a common floating gate.[99] The device structure
is shown in Figure 9. The advantage of this design is that it
can “scale up” the sensitivity of the device to a super-Nernstian
response. This device structure is the latest development in
flexible ISFETs and holds promise for numerous applications
in the biosensing field, where the range of measurement is
small.
4.2. Flexible Ion Sensor
Ion sensors are functionally similar to pH sensors. Ion sensors
are designed to detect specific target ions, usually in aqueous
media. The conventional method of selectively detecting ions
is through ion-selective electrodes (ISEs) in which glass or
glassy carbon electrode tips are covered by an ion-selective
membrane. However, from the perspective of wearable tech-
nology, they offer little value due to their rigidity, bulkiness,
and the requirement of internal filling solutions. Conversely,
flexible solid-state ion-selective electrodes (SSISEs) offer attrac-
tive properties that can be employed rather trivially in wearable
Adv. Mater. 2017, 1700375
Figure 9. pH sensitivities of IZO-based neuromorphic transistor measured in dual-gate synergic modulation mode. Reproduced with permission.[99]
Copyright 2015, Macmillan Publishers Limited.
Figure 10. Carbon-paper-based potentiometric sensor for Na+, K+, and pH. Reproduced with permission.[105] Copyright 2012, American Chemical
Society.

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technology applications. The general structure of an SSISE is as
follows: i) an ion-selective top layer (in contact with the target
analyte), ii) an ion–electron transduction middle layer, and iii)
a conducting bottom layer (although in some studies, the ion–
electron transduction layer and the conducting layer are com-
bined into one layer). SSISEs also have the added benefit of not
requiring any internal filling solution (An internal filling solu-
tion causes reverse transmembrane fluxes that raise the lower
detection limit).[100] This section focuses on the latest develop-
ments in flexible SSISEs. In addition, novel flexible ion-sensing
platforms are discussed.
The simplest form of an ion sensor is the so-called chemore-
sistor, in which conductivity of an active material is modulated
by its interaction with the target ion.[101] These can be applied
on flexible substrates through simple fabrication processes,
and this makes them attractive options for low-cost sensing.
Adv. Mater. 2017, 1700375
Figure 11. Screen-printed electrode (back–back) for Pb(II) anodic strip-
ping voltammetry. Reproduced with permission.[114] Copyright 2015,
Royal Society of Chemistry.
Figure 12. Epidermal tattoo sweat sensor for NH4+ detection. Reproduced with permission.[118] Copyright 2013, Royal Society of Chemistry.

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However, they suffer from several drawbacks such as narrow
working ranges, high detection limits, and poor stability. The
obvious alternative is an SSISE, for the reasons discussed pre-
viously. Most commonly, SSISEs are employed as working
electrodes in potentiometric sensors. In a pioneering study, a
freestanding strip of a conducting polymer (PEDOT:PSS) was
coated with an ion-selective membrane for the selective detec-
tion of K+ and Ca2+ ions.[102] The performance of these sensors
was similar to that of their rigid ISE counterparts. Neverthe-
less, due to the relatively complex fabrication process and low
mechanical strength of freestanding conducting polymer films,
research has moved into replacing glassy carbon electrodes with
flexible carbon-based substrates for potentiometric SSISEs.
Flexible carbon-based sensors are categorized as carbon–
epoxy-resin composite substrates and freestanding carbon-coated
cellulose filter papers (filter papers coated with carbon nanotube
ink or graphene oxide). H2O2[103] and Cu(II)[104] potentiometric
working electrodes have been demonstrated using carbon–
epoxy-resin composite substrates. K+,[105,106] NH4+,[105] pH
(see Figure 10),[105,106] Ca2+,[106] NO2−,[107] and H2O2[108] SSISEs
have been built using freestanding carbon paper substrates.
The NO2− and H2O2 SSISEs were used in amperometry sensors
whereas the rest were used in potentiometric sensors.
Flexible electrodes have also been realized on plastic sub-
strates. Flexible working electrodes on plastic substrates for the
detection of sodium dodecylsulfate,[109] NO3−,[110] Cl−,[111] methyl
parathion,[112] H2O2,[113] and Pb (II)[114] (see Figure 11) have all
been demonstrated.
The primary motivation driving research into flexible ion
sensors and electrodes is to develop epidermal “sweat sensors”
for real-time ion monitoring of sweat. Several strategies have
been used to develop these sensors: using a nonflexible ISE,[115]
flexible electrodes with internal filling solutions,[116] chemo-
resistors,[117] and tattoo-based sensors (see Figure 12).[118,119] A
sweat sensor array with sensing capabilities for Na+, K+, glu-
cose, and lactose was fabricated on a PET substrate in a wrist
band format (see Figure 13).[120] Epidermal sensors designed
for ion monitoring in wounds have also been demonstrated
(see Figure 14).[121–123] This review contains further information
on wearable chemical sensors.[124] A developing trend in the
field of epidermal sensors is the integration of the sensor with
wireless transducers.[119,120,125]
Flexible ion sensors have also been built on FET platforms. A
flexible graphene (functionalized with aptamer) FET designed
for the detection of Hg2+ in mussels was demonstrated with
picomolar sensitivity.[126] The emerging field of MoS2 transistors
has led to the development of nanomolar sensitive and selec-
tive mercury sensors using unfunctionalized few-layer MoS2
structures as the active sensing FET channels.[127] Although this
was demonstrated on nonflexible silicon substrates, it could be
developed on flexible substrates to produce cheap and sensi-
tive mercury sensors. An offshoot of the FET sensor design is
Adv. Mater. 2017, 1700375
Figure 13. Wrist band sweat sensor with integrated wireless transducer. Reproduced with permission.[120] Copyright 2016, Macmillan Publishers
Limited.

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the organic electrochemical junction transistor, which has been
used for the detection of K+, Ca2+, and Ag+.[128]
Ion exchange across living cell membranes, because of
millions of years of evolution, is very selective. The struc-
ture of ion channels (which are responsible for ion exchange)
has inspired the design of a very appealing ion sensor. Ion-
tracked PET substrates are etched to produce nanopores with
conical profiles. The walls of these nanopores are then func-
tionalized with selective receptors for specific target ions (see
Figure 15). These functionalized conical nanopores have been
used to detect Na+, K+, and Cr(III).[129] The advantage of these
sensors is their low detection limit (nanomolar) and wide
linear range.
To achieve general and high-performance multi-analyte
discriminant testing, Huang et al. designed and fabricated a
multi-stopband photonic crystal microchip based on hydro-
philic-hydrophobic patterned substrate. It is remarkable
that recognition and analysis of 12 different metal ions and
8-hydroxy-quinoline are realized in one simple sensor (see
Figure 16).[130]
Real-time measurement of water profiles on flexible sub-
strates using electromagnetic waves is an emerging field. The
principle of sensing lies in measuring the shifts in the resonant
amplitude and frequency in the microwave to gigahertz range
due to the influences of ion concentrations on the dielectric
constant of water.[131]
5. Approaches to Fabricate Flexible Sensors
In this section, we describe the approaches that are commonly
used to fabricate flexible sensors, particularly the different
methods used for the preparation of the sensing components,
which have been demonstrated previously.[132–151] Here, we list
several commonly used methods and discuss the advantages as
well as the disadvantages of each approach.
5.1. Flexible Sensors Fabricated by Thermal Evaporation
For the preparation of FETs and FET-based sensors, the
active layer is usually fabricated by thermal evaporation
methods. Pentacene is one of the commonly used thermally
evaporated semiconductors for electronics and sensors due
Adv. Mater. 2017, 1700375
Figure 14. Band-aid-integrated K+ cotton/CNT ink sensor. Reproduced with permission.[121] Copyright 2013, Royal Society of Chemistry.
Figure 15. Conical nanopore ion channels for the detection of K+ and Na+. Reproduced with permission.[129a] Copyright 2015, American Chemical
Society.

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to its relatively high mobility as well as its stability.[152–154]
Yakuphanoglu et al. demon strated a flexible photodetector by
employing thermally evaporated pentacene as the sensor’s
active material.[155] Figure 17a shows the device structure, indi-
cating that this device comprises a PES flexible substrate, an
ITO gate electrode, a PVP dielectric layer, a thermally evapo-
rated pentacene active layer, and thermally evaporated source
and drain electrodes. The sensitivity of this pentacene photo-
detector was measured with and without UV radiance, and
Figure 17b shows the corresponding output curves. As shown
in Figure 17b, the flexible pentacene transistor demon strated
very favorable sensitivity for detecting UV light. Sensing
films prepared by thermal evaporation are relatively uni-
form; however, there are also some disadvantages, such as the
relatively expensive equipment, low throughput, and high time
consumption, all of which render the device unsuitable for
industrial production.
5.2. Flexible Sensors Fabricated by Chemical Vapor Deposition
In recent years, nanowires, nanorods, and nanotubes have
attracted much attention from researchers due to their unique
properties for sensors compared with related thin-film struc-
tures.[156–160] Nanostructures can be realized by various routes;
the chemical vapor deposition (CVD) method is one of the com-
monly used approaches. Liu et al. demon strated a CVD-grown
ZnO nanowire array on top of flexi ble polymer substrate, as
Adv. Mater. 2017, 1700375
Figure 16. a) Photograph of a multi-stopband PC microchip. b) SEM image of the PCs. c) Fluorescence spectra of 8-HQ (c) and 8-HQ-Al3+ (a), and
transmittance spectra of the PCs. The stopbands of the PCs overlap the different position at three peaks of emission spectra of 8-HQ and 8-HQ-Al3+,
respectively. d) The Al3+ concentration dependence of the fluorescence at 506 nm on PC 5 (*) and on a blank surface. Copyright 2013, Wiley-VCH.
Figure 17. a) Schematic of a pentacene photodetector. b) Output curves measured in the dark and under illumination. Reproduced with permission.[155]
Copyright 2011, Elsevier B.V.

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shown in Figure 18a.[161] Current curves derived with UV light
on and off are shown in Figure 18b, and current curves derived
under different UV intensities are shown in Figure 18c. The
CVD-grown ZnO nanowire array showed a favorable on–off
ratio as well as notable sensitivity.
5.3. Flexible Sensors Fabricated by Spin Coating
Spin-coating processes have been widely used in the fabri-
cation of organic electronics and sensors; such processes
can reduce fabrication costs greatly compared with CVD
Figure 18. a) Schematic diagram of a UV sensor. b) Current curves with UV light on and off. c) Current curves with different UV intensities. d) The
curve of response current with increased UV intensity. Reproduced with permission.[161] Copyright 2011, American Chemical Society.
Figure 19. Schematic of a spin-coating fabrication process for a flexible gas sensor array. Reproduced with permission.[151] Copyright 2012, Wiley-VCH.

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or thermal evaporation methods. Additionally, spin-coating
methods are compatible with low-cost plastic substrates.[162,163]
As shown in Figure 19, He et al. described a spin-coated
single-layer MoS2 gas sensor on a plastic substrate.[151] First,
a GO film was patterned on the PET substrate to serve as
both source and drain electrodes. Then, a MoS2 active layer
was deposited on top by spin coating. The flexible sensor array
exhibited high sensitivity to detect toxic gases and its sensi-
tivity was improved further by deposition of PtNPs, as shown
in Figure 19.
Zirkl et al. demonstrated optothermal sensors based on
spin-coated pyroelectric poly(vinylidenetrifluoroethylene)
[P(VDF-TrFE)] copolymer film, as shown in Figure 20.[164]
The sensing principle is related to the pyroelectric effect in
a ferroelectric P(VDF-TrFE) copolymer. These flexible sen-
sors were observed to exhibit favorable sensitivity, as shown
in Figure 20d.
5.4. Flexible Sensors Fabricated by Printing
Printing technologies have been demonstrated as cost-effective
approaches for realizing high-performance electronic compo-
nents and related sensors based on flexible substrates (PET,
PEN, PI, etc.) at proper processing temperatures.[136,165–167]
Compared with the other methods, printing routes have some
advantages such as simple processing technique, reduced mate-
rial wastage, and low cost, which make printing methods a very
attractive way to fabricate cost-effective sensors. Printed pres-
sure sensors could be wrapped around the arms of robots to
sense different pressures.
Contemporary printing approaches could be divided into two
categories: noncontact printing and contact printing. With non-
contact printing, the patterned structures do not contact with
the substrate directly. However, for contact printing, the pat-
terned structures contact with the substrate directly. Compared
Figure 20. a) Images of a flexible optothermal sensor element. b) Schematic of a fully flexible sensor circuit. c) Its operation as a light activated
switch. d) On–off cycles and the voltage responsibility versus the frequency for the sensor circuit. Reproduced with permission.[164] Copyright 2007,
Wiley-VCH.

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with contact printing,[168] the noncontact
approach has the merits of simple fabrica-
tion steps, low cost, and reduced material
wastage, which makes noncontact methods
much more attractive. Here, we mainly dis-
cuss the applications of noncontact printing
technologies for flexible sensors. Khan et al.
described contact printing for flexible elec-
tronics and sensors in detail.[167] Among the
noncontact approaches, screen printing is
the most popular route for flexible sensors.
Chang et al. demonstrated flexible large-
area pressure sensors by employing screen
printing; in their work, the fabrication of
the pressure sensors was carried out in two
parts.[168] First, holes were drilled and Cu was
plated on one side of the substrate followed
by 5-µm Au deposition on top of Cu to pre-
vent the oxidation of Cu. Then, a thixotropic
material was printed on the other side of the
substrate following annealing at 150 °C to
form bump structures. For the bottom part,
the film fabrication resembled that of the
top substrate. A cover layer was then formed
on the bottom film using hot pressing to
produce post structures. Subsequently, a
resistive material was printed on the sensing
electrode area. Finally, the two PI substrates
were aligned and assembled together to form
the pressure sensor.
Another commonly used route to fabri-
cate flexible sensors is ink-jet printing. Ink-
jet printing is another solution approach
that can form flexible electronics as well
as related flexible sensors. As with screen
printing, ink-jet printing is cost effective, but
some of its drawbacks include low printing
speed, poor uniformity of films on the sub-
strates, low pattern resolution, and low
throughput due to the low printing speed,
which is a challenge for industrial produc-
tion. Noguchi et al. demonstrated a 33-cm
diagonal flexible sensor array on a flexible PI
substrate with a dielectric layer fabricated by
ink-jet printing.[136] The cross-sectional view of the device struc-
ture and the images of the flexible large-area pressure sensor
are presented in Figure 21a and 21b, respectively. The single
pressure sensor and the equivalent circuit diagram are shown
in Figure 21c and 21d, respectively.
6. Market Analysis
Flexible sensors can be made using inherently flexible mate-
rials such as polymer substrates and organic semiconductors.
Because a flexible sensor has a high degree of design freedom,
it can be trimmed down to different sizes or folded into dif-
ferent shapes. This design parameter is crucial for consumer
electronics such as wearable electronics, because wearable
electronics must be incorporated into clothing and accessories,
which require the sensor to be as tiny as possible. A report
from INTECHNO[169] predicted that approximately 2.7% of the
sensor market share, approximately €4.97 billion ($5.4 billion),
will come from household appliances and consumer elec-
tronics, as shown in Figure 22.
As mentioned, one major application for flexible sensors in
consumer electronics is wearable electronics. A report from
IDTechEx predicted that the market for wearable sensors
would be worth $5.5 billion by 2025.[170] The market for wear-
able sensors may seem unremunerative when compared with
the market for consumer electronics, because the market for
consumer electronics was $5.4 billion in 2016; nevertheless,
the market for wearable sensors may only reach that value
10 years later. The cause may be that wearable electronics have
Figure 21. a) Cross-sectional view of a flexible pressure sensor; the nanoparticles and the
dielectric layer were fabricated by ink-jet printing. b) Microscopy image of the flexible pressure
sensor array. c) A single sensor cell. d) Circuit diagram of a pressure cell. Reproduced with
permission.[136] Copyright 2006, AIP Publishing LLC.

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just emerged recently and are starting from a small base. With
proliferating investment from commercial companies such as
Google, Microsoft, and Apple, the actual market value of wear-
able sensors may be higher than previously predicted.
Although one of the major applications for flexible sensors
is wearable sensors, flexible sensors can be applied in other
areas. Therefore, the market for flexible sensors is expected
to be larger than the market for wearable sensors. According
to IDTech Ex, the market for printed and flexible sensors is
expected to reach $8 billion by 2025.[171]
In the following decade, the top five fastest-growing flexi ble
sensor types, according to IDTechEx,[171] are expected to be
humidity sensors, temperature sensors, photodetectors,
biosensors, and gas sensors, as shown in Figure 23. Humidity
sensors are expected to have the fastest growth for the next
decade because they have started from a low base.[171] If a firm
can develop a chemical sensor by detecting different types of
gases (gas sensor) or biological molecules (biosensor), that firm
might able to catch the trend of a fast-growing market because
IDTechEx forecasts that chemical sensors will have the largest
wearable sensor market share, as shown in Figure 24.[170]
Both IDTechEx and INTECHNO claim that the sensor market
will be worth billions of dollars. Wearable chemical sensors
are expected to have the largest share of the wearable sensor
Figure 22. Predicted sensor market share of 2016 from different industries. Reproduced with permission.[169] Copyright 2012, INTECHNO.
Figure 23. Printed sensor CAGR 2015–2025. Reproduced with permission.[171] Copyright 2015, IDTechEx.

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market in the next decade, and humidity sensors are expected
to have the highest growth rate for the next decade.
7. Conclusion and Perspectives
Considering that most wearable systems, healthcare electronics,
and laboratory-on-a-chip testing tools can be expected to come
into contact with arbitrarily curved interfaces, the flexibility of
sensors is essential for improving their interactions with target
systems and improving the reliability and stability of the tests.
Hence, flexible sensors hold great promise for various innova-
tive applications in fields such as medicine, healthcare, envi-
ronment, and biology. Over the past decade, the development of
flexible and stretchable sensors for various functions has been
accelerated by rapid advances in materials, processing methods,
and platforms. For practical applications, new expectations are
arising in the pursuit of highly economical, multifunctional,
biocompatible flexible sensors.
Numerous opportunities and challenges remain in the future
research and development of high-performance flexible sen-
sors. For flexible sensors attached to the human body or its
organs, the biocompatibility of the active materials and flexible
substrates, including long-term toxicity analyses, is a crucial
research area, especially for invasive applications. Innovative
utilization of device designs, materials, assembly methods,
and surface engineering, as well as interface engineering, can
address these challenges. The development of new materials for
active layers, substrates, and conductive layers can give rise to
soft, stretchable sensors; this emerging paradigm can extend the
scope of current technologies for different sensing functions.
Improvement of both flexibility and sensitivity is another
challenge for state-of-the-art flexible sensors. The development
of novel elastic materials for flexible and stretchable substrates,
geometrical electrode designs, combinations of molecular
designs in organic materials, and utilization of conceptually
novel materials could optimize the trade-off between sensitivity
and flexibility.
High-density sensor arrays with multiple functions must
be integrated for enabling a high spatiotemporal resolution to
realize fully functional flexible electronics. Nevertheless, raising
the density of sensors leads to increased crosstalk. Reducing
the size of sensors diminishes the amplitude of signals. These
problems can be addressed by connecting each sensor with
active devices such as transistors to enable local signal ampli-
fication and transduction. Designing effective circuits not only
facilitates multiplexing but also saves substantial amounts of
power. We believe that these challenges can be principally over-
come with the aforementioned methods.
The development of sensors is the enabling technology for
Internet of Things. A surge in Internet of Things provides plen-
tiful opportunities for spreading out of flexible sensors with
reconfigurable shape and size. Thanks to their light weight,
thinness, and robustness, flexible sensors can be seamlessly
integrated onto any surface to provide users more improved
avenues, which is difficult to realize in conventional electro-
mechanical sensors. With the development of polymers, oxides,
printing technologies, and CMOS technologies, flexible sen-
sors will unlock a completely novel set of Internet of Things
products. The achievement of these fantastic sensing applica-
tions will bring us closer to the new electronic era promised by
flexi ble sensors.
Figure 24. Relative market size by wearable sensor type by 2020. Reproduced with permission.[170] Copyright 2015, IDTechEx.

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Acknowledgements
The authors acknowledge the grants from RGC of Hong Kong under
Grant No. C7045-14E, the National Science Foundation of China
under Grant Nos. 61601305 and 61604097, the Shenzhen Science and
Technology Projects under Grant No. JCYJ20150625102943103, the
Young Innovative Talents Project of the Department of Education of
Guangdong Province (No. 2015KQNCX141) and the Natural Science
Foundation of SZU (STH) and City University of Hong Kong project
number 7004378.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
biosensors, flexible electronics, pH and ion sensors, sensors, UV and
light sensors
Received: January 19, 2017
Revised: February 28, 2017
Published online:
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