Environmentally stable, mechanically flexible, self-adhesive, and electrically conductive Ti3C2TX MXene hydrogels for wide-temperature strain sensing

Abstract

Conductive hydrogels are promising in the flexible wearable electronic applications due to their unique feature of intrinsic stretchability, reversible flexibility, and high electrical conductivity. However, severely poor adaptability under cold or hot environmental conditions along with inferior adhesiveness to various substrates greatly hinder the potential applications in such emerging field. Herein, we describe a mechanically flexible and electrically conductive nanocomposite hydrogel composed of polyacrylamide-co-acrylic acid/chitosan covalent-network reinforced by Ti3C2TX MXene nanosheets within water-glycerol binary solvent via a simple one-pot free radical polymerization. Notably, incorporation of a low content (0.1-0.3 wt.%) of MXene promotes the rapid gelation of the polymer molecules in only 10 min. The optimized hydrogel containing 0.2 wt.% MXene not only possesses excellent mechanical performance (e.g., tensile elongation of ~1000%) and improved electrical conductivity (~1.34 S/m), but also shows stable temperature tolerance from -20 to 80 oC and self-adhesion with various substrates (e.g., steel, glass, rubber, plastics and skin) as well as a rapid self-healable feature (~1.3 s). Further, such hybrid MXene hydrogel exhibits dual sensations under different strain (1-600%) and stress (80-3200 Pa) ranges, good applicability for various deformation conditions (tension/bend/compression), and wide temperature adoptability with stable repeatability. Clearly, this versatile MXene nanocomposite hydrogel developed may provide a new route for the rational design and development of advanced skin-like sensor for complex environmental application.

Full text

Nano Energy 90 (2021) 106502
Available online 11 September 2021
2211-2855/© 2021 Elsevier Ltd. All rights reserved.
Environmentally stable, mechanically exible, self-adhesive, and
electrically conductive Ti
3
C
2
T
X
MXene hydrogels for wide-temperature
strain sensing
Shi-Neng Li
a
,
b
,
*
,
1
, Zhi-Ran Yu
a
,
1
, Bi-Fan Guo
a
,
1
, Kun-Yu Guo
a
, Yang Li
d
, Li-Xiu Gong
a
,
Li Zhao
a
, Joonho Bae
d
, Long-Cheng Tang
a
,
c
,
*
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, College of Material, Chemistry and Chemical Engineering, Hangzhou
Normal University, Hangzhou 311121, PR China
b
College of Chemistry and Materials Engineering, Zhejiang A & F University, Hangzhou 311300, PR China
c
Key Laboratory of Silicone Materials Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121, PR China
d
Department of Nano-physics, Gachon University, Seongnam-si, Gyeonggi-do, South Korea
ARTICLE INFO
Keywords:
MXene-based hydrogel
Temperature tolerance
Mechanical exibility
Self-adhesion
Strain sensor
ABSTRACT
Conductive hydrogels are promising in the exible wearable electronic applications due to their unique feature of
intrinsic stretchability, reversible exibility, and high electrical conductivity. However, severely poor adapt-
ability under cold or hot environmental conditions along with inferior adhesiveness to various substrates greatly
hinders the potential applications in such emerging eld. Herein, we describe a mechanically exible and
electrically conductive nanocomposite hydrogel composed of polyacrylamide-co-acrylic acid/chitosan covalent-
network reinforced by Ti
3
C
2
T
x
MXene nanosheets within water-glycerol binary solvent via a simple one-pot free
radical polymerization. Notably, incorporation of a low content (0.1–0.3 wt%) of MXene promotes the rapid
gelation of the polymer molecules in only 10 min. The optimized hydrogel containing 0.2 wt% MXene not only
possesses excellent mechanical performance (e.g., tensile elongation of ~1000%) and improved electrical con-
ductivity (~1.34 S/m), but also shows stable temperature tolerance from −20 to 80 ◦C and self-adhesion with
various substrates (e.g., steel, glass, rubber, plastics and skin) as well as a rapid self-healable feature (~1.3 s).
Further, such hybrid MXene hydrogel exhibits dual sensations under different strain (1–600%) and stress
(80–3200 Pa) ranges, good applicability for various deformation conditions (tension/bend/compression), and
wide temperature adoptability with stable repeatability. Clearly, this versatile MXene nanocomposite hydrogel
developed may provide a new route for the rational design and development of advanced skin-like sensor for
complex environmental application.
1. Introduction
As a typical soft and wet material with high water content, hydrogel
has been ubiquitously used in a wide range of existing and emerging
areas, including tissue engineering [1,2], wearable electronics [3,4],
articial skin [5,6], smart switch [7,8], solar desalination & water pu-
rication [9,10] and supercapacitor [11,12]. In addition to poor me-
chanical performance, for conventional polymeric hydrogel with high
water content, another intrinsic feature is the terrible anti-freezing
performance at subzero temperatures and the seriously dehydrated
phenomenon even in ambient or high temperature. These shortcomings
greatly hinder widespread applications of hydrogels. Thereby, it is
highly urgent, but also challenging to develop a novel class of composite
hydrogels with a combination of excellent conductivity and extremely
temperature tolerance.
To address the above issue, inspired by the antifreezing behaviors of
natural organisms such as frogs that possess antifreezing agents in their
body fluids [13], some freezing-tolerant hydrogels were developed
through introducing salts [14–16], polyols [17–19] and ionic liquid [20]
into the hydrogel in recent periods. Thanks to the outstanding tolerance
* Corresponding authors at: Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, College of Material, Chemistry and
Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, PR China.
E-mail addresses: lisn@zafu.edu.cn (S.-N. Li), lctang@hznu.edu.cn (L.-C. Tang).
1
These authors contributed equally to this work.
Contents lists available at ScienceDirect
Nano Energy
journal homepage: www.elsevier.com/locate/nanoen
https://doi.org/10.1016/j.nanoen.2021.106502
Received 24 May 2021; Received in revised form 21 August 2021; Accepted 6 September 2021

Nano Energy 90 (2021) 106502
2
against to the complex environment including high (>60 ◦C) or low
temperature (<0 ◦C) for a long time, this new-class hydrogel has been
shown a promising candidate for battery [21], biomimetic skin [22] and
other high-end applications [23] under a complex environment. How-
ever, the lack of conductive path in traditional hydrogels leads to a low
conductivity due to the short of appropriate medium such conductive
polymer (polypyrrole) [24] or nanoparticles (carbon nanotube) [25],
which seriously constraints the scope of practical application, e.g. ex-
ible batteries [26].
Owing to its high aspect ratio morphology and excellent solution
processability along with rich surface chemistry [27–30], MXene sheets
have garnered considerable attention for various applications. Recently,
MXene-based hydrogels are enabled with engaging and versatile prop-
erties including photoredox catalysis, photothermal behavior and
sensing [31–34]. Especially, the exceptional conductivity for the resul-
tant hydrogels can be achieved by assembling MXene sheets into poly-
mer architectures due to their exceptional metallic conductivity. For
example, Alshareef et al. [35] developed a highly stretchable strain
sensor based on poly(vinyl alcohol)/MXene hydrogel, which can
monitor the complex motions composed of different direction and speed
expect detecting various strains. Additionally, conductive hydrogel
composed of MXene and poly(acrylic acid) demonstrated highly sensi-
tive deformation responses along with excellent
electromagnetic-interference shielding [36]. Unfortunately, these
MXene-based hydrogels cannot endure a long-term environmental
temperature mainly due to the lack of indispensable temperature
tolerance. Learning from the hydrogels with long-term environmental
stability, polyols (e.g., ethylene glycol and glycerol) [37] have been
introduced into the hydrogel for depressing the freezing point. However,
inuenced by the evolution of hydrogen bonding between the cryo-
protectants and water molecules, the mechanical performance tend to be
declined with the increasing content (over 50 vol%) of cryoprotectants,
which hardly meet the strain sensing application in complicated
environments.
It is well established that the hydrogel materials for strain sensing
applications should comply with the following criteria: (i) offer a rapid
and reliably real-time signals response to the variation of strain (high
and stability conductivity), (ii) work efciently under both various
conditions (e.g., subzero temperature, ambient temperature and dehy-
drated phenomenon), (iii) be able to keep structural stability during
mechanical deformation, i.e., high mechanical performance, (iv) possess
strong adhesivity with different substrate without extra adhesives, and
(v) be suitable for quick scaling-up and easily manipulated. Unfortu-
nately, many current hydrogel-based sensors (e.g., ionic gel, organogel
or composite hydrogel with conductive nanomaterials) cannot well meet
all the above requirements. Some shortcomings in previous hydrogel
systems, including poor structural stability (low mechanical strength
and exibility), high water content (weak anti-freezing and anti-
dehydration) and lack of self-adhesivity, greatly limit the potential
strain monitoring applications in complicated environmental condi-
tions. Therefore, it is imperative, but also challenging, to develop wide-
temperature applicable, mechanically reliable, self-adhesive, and elec-
trically conductive hydrogel materials via a facile strategy.
Herein, we report a poly(acrylamide-acrylic acid)/chitosan/MXene
hydrogel (PACG-M) containing water-glycerol hybrid solvents via an
extremely simple one-pot free radical polymerization approach. The
introduction of low-content MXene promoted the quick sol-gel process
of the PACG hydrogel in only 10 min, which cannot be achieved in the
pure system. The optimized PACG-M possessed an enhanced mechanical
performance with temperature tolerance, electrical conductivity and
self-healing feature. The incorporation of MXene empowered the
resultant hydrogel with improved mechanical and self-healing perfor-
mance except engaging conductivity owing to the construction of rein-
forced polymer architecture with conductive path. Additionally, such
MXene-based hydrogel showed good structural stability and reliability
under a wide temperature range of −20–80 ◦C by simply tuning the
glycerol/water ratio. According to the results of a series of standard tests
combined with the evolution of chemical structure and micro-
morphology, the effects of various factors (e.g., MXene content, glyc-
erol content) on the macro-behaviors and micro-structure were inves-
tigated. The optimized PACG-M hydrogel displayed excellent sensing
property response to complex deformations along with a wide detection
range (up to 600%) and remained the splendid reliability even in low
temperature or arid environment. These unique characteristics enable
PACG-M hydrogel with tremendous potential as an alternative material
for strain sensing applications.
2. Experimental section
2.1. Materials
Ti
3
AlC
2
powders were purchased from Jilin Technology Co., Ltd.,
China. Acrylamide (AAm), acrylic acid (AAc), ammonium persulfate
(APS), hydrochloric acid (HCl, 35 wt%), and acetic acid were purchased
from Sinopharm Chemical Reagent Co., Ltd., China. γ-methacryloxy-
propyltrimethoxylsilane (MPTMS), γ-glycidyloxypropyltrimethoxy
silane (GPTMS), lithium uoride (LiF), glycerol (Gly) and chitosan (CS)
were purchased from Aladdin Co., Ltd., China. All the materials and
chemicals were used as received.
2.2. Fabrication of a solution of MXene nanosheets
Ti
3
C
2
T
x
MXene nanosheets were fabricated according to the reported
works [38]. First, Ti
3
AlC
2
was added to 30 mL solution of 9 M HCl that
contained 3 g of LiF and the solution was stirred at 40 ◦C for 36 h. Af-
terwards, the product was washed and centrifuged with deionized water
until the pH of the decantate reached 7. Finally, a dark-green and
exfoliated supernatant solution with a certain concentration could be
obtained that the resultant sediment was dispersed in 100 mL deionized
water with the aid of sonication.
2.3. Synthesis of PACG-M hydrogels
PACG-M hydrogel was fabricated by thermally-induced polymeri-
zation in which hyperbranched polysiloxane with dual groups (vinyl and
epoxy) (HSi) as cross-linker according to our previous works [39,40],
using a typical condensation reaction of silane molecules [41–46].
Firstly, CS was dissolved in acetic acid/glycerol solution containing
AAm and AAc under vigorously stirring. After that, APS and HSi were
added into the mixture. Under a vigorous stirring, MXene solution was
dropwisely added and then was poured into mould tubes. After
degassing oxygen and pre-gelling in the ambient temperature,
as-prepared hydrogel could be achieved through the free-radical poly-
merization at 45 ◦C for 1 h. The detailed formulations of various
hydrogels were shown in Table S1. The obtained hydrogels were
denoted as PAxCyGz-Mn, with PA for poly(acrylamide-co-acrylic acid),
C for chitosan, G for glycerol and M for MXene, respectively. x is for
molar fraction of acrylamide, y is for weight percentage of chitosan in
solution, z is for the volume fraction of glycerol in total solvent, and n is
for the weight percentage of MXene, which is respect to the weight of
monomer. The concentration of initiator (APS) and crosslinker (HSi)
were xed at 0.05 wt% and 1.0 vol%, if there were no special
instructions.
2.4. Characterizations
The chemical structure of MXene and nancomposite hydrogel was
explored by Fourier transform infrared spectroscopy (FTIR) (Bruker,
Alpha-T) with a range from 400 to 4000 cm
−1
and X-ray Photoelectron
Spectroscopy (XPS) (VG Scientific, ESCALab 220I-XL) equipped with
MgKa X-ray source. The crystal structure of Ti
3
C
2
T
x
and MXene was
analyzed by X-ray diffraction (XRD) measurements using an X-ray
S.-N. Li et al.

Nano Energy 90 (2021) 106502
3
diffractometer (Rigaku, D/Max 2550 V) from 5 to 80◦with a scan rate of
5◦/min. The micro-morphology of MXene nanosheets and composite
hydrogels was performed by scanning electron microscopy (SEM)
(ZEISS, Sigma-500) and transmission electron microscopy (TEM)
(HITACHI, H-7650).
Mechanical tests were conducted by a mechanical testing machine
(Ametek, Ls100plus) equipped with 100 N load cell at ambient tem-
perature. To guarantee the dependability, all the parameters are calcu-
lated by stress-strain curves with three repeated times. As for tensile
tests, the cross-head speed was xed at 100 mm⋅min
−1
. The tensile
strength was estimated based on the formula:
σ
=F/A
0
, where F is
corresponding to load force and A
0
is for original cross-sectional area.
The tensile strain (
ε
) was determined as
ε
=(Δl/l
0
) x 100%, where ∆l is
the change in length relative to the initial length (l
0
). The work of
fracture was calculated by the area under the stress-strain curve.
Compressive tests were measured using the same machine at a speed of
1 mm⋅min
−1
with cylindrical samples (diameter: 20 mm and height: 10
mm) and cyclic measurements (ten times) were carried out with the
same speed. Additionally, all the specimens were coated with a silicon
oil layer for preventing the evaporation of water during the mechanical
measurement.
To fabricate the strain sensor, the PACG-M hydrogel was incubated
into a cube with a size of 15 mm (length) x 10 mm (width) x 2 mm
(thickness). The electrical resistance values of the sensors during the
mechanical deformation were measured by a multimeter (ESCORT
3146A). The sensing performance was evaluated by the relative resis-
tance change that was according to the equation: ∆R/R
0
=(R-R
0
)/R
0
,
where R
0
is corresponding to the resistance at the initial state, and R is
assigned to the real-time resistance under a certain strain. Meanwhile,
the gauge factor (GF) is defined as GF =((R-R
0
)/R
0
)/
ε
, where
ε
for the
applied strain.
Fig. 1. Schematic illustration of fabrication and interaction of PACG-M hydrogels and their gel process. (a) Preparation process and formation mechanism of the
hydrogels via a simple method (inset is the image of precursor, pre-hydrogel and PACG-M hydrogel); (b) Schematic of multiple hydrogen bonds in the hybrid MXene
hydrogel. (c) Comparison of the sol-gel process of the PACG and PACG-M and (d) the corresponding thermograms images, demonstrating the quick gel process after
incorporation of low-content MXene.
S.-N. Li et al.

Nano Energy 90 (2021) 106502
4
3. Results and discussion
3.1. Design, synthesis and structural characterisation of the PACG-M
hydrogels
The PACG-M hydrogels were fabricated via a facile in-situ free
radical polymerization, as depicted in Fig. 1. Acrylamide, acrylic acid,
HSi and ammonium persulfate were introduced into a water-glycerol
hybrid solvent that could be regulated by simply adjusting the ratio of
glycerol in total solvent (Fig. 1a), which were served as monomer,
crosslinker and initiator, respectively. After that, MXene, a novel 2D
transition metal carbides and carbonitrides with lamellar structure
(Fig. S1) that contains considerable uorine and hydroxyl groups, was
introduced into the above mixture [47]. This favoured them to construct
the hydrogen bonding with amine and hydroxyl groups of chitosan
chains and thus facilitate the formation of 3D molecular network
(Fig. 1b i). Besides, the presence of MXene strengthened the stress
transferring and energy consumption of polymer network through other
interactions (Fig. 1b ii and iii), but also laid the basic of stable
conductive path. In fact, compared with the PACG remaining a solution
state even after 24 h, a homogeneous and stable precursor could trans-
form from a uid to gel state for the PACG-M after only 10 min at R.T.
(Fig. 1c). Meanwhile, the corresponding thermographic images dis-
played an obvious temperature increase (up to ~50 ◦C) and dissipation
of intensive heat throughout the whole PACG-M sample, whereas an
unchanged temperature (~27.5 ◦C) was found in the PACG sample
(Fig. 1d and Fig. S2). It may be attributed to the generation of free-
radicals stemmed from APS decomposition, benetting from the help of
the self-heating behavior owing to the multiple interactions between the
functional groups of nanosheet and the polarized groups of various
Fig. 2. Mechanical exibility and structure analysis of PACG-M hydrogels. The optimized hydrogel can be (a) knotted and stretched with a high strain, (b) com-
pressed and recovered under a large strain of 90% and (c) sustain a weight of 200 g without any mechanical fracture (Scale bars are 1 cm in (a, b) and 2 cm in (c). (d)
FTIR spectra of MXene nanosheet, PACG and PACG-M hydrogels. (e) C1s XPS spectrum, (f) SEM image and (g) corresponding element mapping of the PACG-
M hydrogels.
S.-N. Li et al.

Nano Energy 90 (2021) 106502
5
molecules [31,48–50]. Then, the pre-hydrogel was further incubated at
45 ◦C to realize the effective formation of highly cross-linked structure,
further reecting by the lower mechanical performance of untreated one
(Fig. S3). Additionally, the incorporation of glycerol empowered the
obtained hydrogels with anti-freezing and drying performance along
with improved mechanical properties due to the multiple interactions
between chitosan and glycerol (Fig. 1b iv and v). Benet from these
advantages, the resultant hydrogels exhibited intriguing integrated
properties, i.e., high mechanical ductility, impressive wide temperature
tolerance, good electrical conductivity, outstanding self-adhesion, and
rapid healable ability (discussed later).
In the view of the introduction of strengthened effect induced by
MXene nanosheets in AAm/AAc/CS hybrid polymer network, the
resultant hydrogels displayed impressive mechanical performance in
rigorous conditions, e.g., high stretchability of ~1000% even with
several knots (Fig. 2a and Fig. S4), outstanding elasticity and recover-
ability after 90% compressive strain (Fig. 2b) and excellent weight-
carrying ability (e.g., 200 g weight for the 10 g sample with ~10 cm
diameter shown in Fig. 2c). More importantly, no obviously fracture was
happened in any deformation process, further revealing extraordinary
mechanical performance. This may be attributed to the formation of
robust and homogeneous polymer network composed of multiple in-
teractions, including covalent bond, hydrogen bond and physical
entanglement among polymer chains and/or MXene sheets.
To clarify above assumption, FTIR and XPS spectra of the hydrogels
were conducted and analyzed. As expected, two characteristic peaks
corresponding to
–
OH and
–
NH groups for the nanocomposite
hydrogel shifted from 3425 cm
−1
(blue line) to 3421 cm
−1
(red line) in
Fig. 3. Mechanical optimization of PACG-M hydrogels. (a) Tensile stress-strain curves of the hydrogels with different compositions. (b) Tensile loading-unloading
curves for PA
80
C
3
G
50
-M
0.2
hydrogel at varied strain from 100% to 900%. (c) Twenty successive loading-unloading cycles for PA
80
C
3
G
50
-M
0.2
hydrogel at a strain
value of 500%. (d) Compressive loading-unloading curves for PA
80
C
3
G
50
-M
0.2
hydrogel with a strain ranged from 20% to 90%. (e) Cyclic compressive loading-
unloading and (f) compressive stress-strain curves with varying rest intervals at the strain of 90% for the PA
80
C
3
G
50
-M
0.2
hydrogel.
S.-N. Li et al.

Nano Energy 90 (2021) 106502
6
FTIR spectra (Fig. 2d), powerfully indicating the occurrence of strong
hydrogen bonding [41,51]. This was mainly due to the fact that a vast of
-OH and -F groups on the MXene’s surface could greatly facilitate the
formation of hydrogen bonding between polymer chains and nano-
sheets, which was also found in the previous other nanocomposite sys-
tems [40,52–54]. Notably, in the C1s XPS spectrum (Fig. 2e), the
increased content of signal assigning to N
–
C
–
–
O (288.0 eV) in PACG-M
provided a solid evidence for the successful incorporation of MXene
nanosheets when compared to that of PACG (Fig. S5 and Table S2).
Meanwhile, two characteristic peaks at 286.5 and 288.7 eV corre-
sponding to and C
–
N and
–
C
–
–
O showed an obvious increase, sug-
gesting that amino and carboxylic groups in the polymer skeleton indeed
involved in the formation of hydrogen bonding with nanosheets. This is
well consistent with the FTIR result. Further, as shown in Fig. 2f, SEM
images of the PACG-M hydrogels exhibited a rough and dense fracture
surface with mountain-like structure, implying a typical sign of ductile
fracture process. The homogeneous distribution of Ti element in the
selected area was also clearly observed, declaring the excellent distri-
bution of nanosheets in matrix. Consequently, a robust, compact, and
homogeneous cross-linked network was successfully constructed via
multiple hydrogen and covalent interactions that were integrated into
the polymer network along with MXene nanosheets.
3.2. Mechanical properties
To further explore the role of the composition in determining the
mechanical properties, a series of tensile tests were carried out by using
a mechanical testing machine (Fig. 3). As displayed in Fig. 3a, compared
with original one (PA
80
C
3
), the hydrogels combined with MXene
nanosheet or glycerol alone exhibited two different mechanical behav-
iours. Typically, incorporation of MXene nanosheet dramatically
strengthened the mechanical strength due to the efcient nano-
reinforcement but induced an obvious decrease in the ductility. This
phenomenon was attributed to the additionally physical crosslinking
points through interactions between high activated groups of polymer
chains (i.e., hydroxyl, carboxylic and amido groups) and MXene nano-
sheets (i.e., hydroxyl and uorine containing groups), which has been
well proved by the evolution of chemical structure. In case of glycerol,
the obtained hydrogels showed an appealing enhancement on tensile
strain (~1500%) along with lightly improved tensile strength on ac-
count of the improved mobility and slippage of the polymer molecular
chains, greatly contributing to the weaken hydrogen bonds formed be-
tween polymer molecular chain and water molecules by competing with
them [55].
Surprisingly, the combination of MXene nanosheet and glycerol in
the hydrogel produced signicant improvements in the tensile strength
and strain simultaneously, which was increased up to 100.0 kPa and
896%, respectively. This can be explained by the fact that the well
combination of nanoparticle and polyols produced a balanced
strengthening effect on the mechanical performance. After analysing
well-organized and systematic regulation (see the details in Fig. S6), the
optimized PACG-M hydrogels contained 80 mol% AAm, 3.0 wt% CS,
50 vol% glyceriol and 0.2 wt% MXene; and the crosslinker (HSi) and
initiator (APS) were xed at 0.05 wt% and 1.0 vol%, respectively.
Consequently, the trade-off among various mechanical parameters could
be well achieved, i.e., excellent tensile strength of 120.4 kPa, high
tensile strain of 918% and outstanding work of fracture of 590.0 kJ⋅m
−3
for PA
80
C
3
G
50
-M
0.2
hydrogel. These results demonstrated that the me-
chanical properties could be easily tailored by simply adjusting the
material composition. Additionally, for the PA
80
C
3
G
50
-M
0.2
hydrogel,
the appearance of hysteresis loops was observed with increasing the
applied strain from 100% to 900% in Fig. 3b; meanwhile, an obvious
decrease in tensile stress value was also visible during the loading-
unloading cyclic tests (Fig. 3c). These results suggested the occurrence
of Mullins effect and demonstrated the excellent energy dissipation,
further revealing the intriguing tensile mechanical performance of the
PACG-M hydrogel. Similar phenomenon was always drawn in mechan-
ically robustness hydrogels [56,57], especially in double-network
hydrogels [58].
In addition to the remarkable tensile mechanical properties, the
optimized hydrogels also display good compressive performance and
stable compressive cycles. Various compressive measurements of
PA
80
C
3
G
50
-M
0.2
hydrogel were conducted and analysed (Fig. 3d-f). As
mentioned in Fig. 2b, the PACG-M hydrogel can recover its original
shape during the mechanical compress-release process in a very short
time (several seconds) even at a large strain of 90%. This is well in
accordance with the result of the compressive loading-unloading curves
for the PA
80
C
3
G
50
-M
0.2
hydrogel with a strain ranged from 20% to 90%
(Fig. 3d). Further, for consecutive compressive loading-unloading tests,
the compressive strength decreased with increasing the cyclic time and
remained over 1.95 MPa (~66.3% of the virgin one) after ve cycles
(Fig. 3e). Additionally, the cyclic compressive tests in Fig. 3f disclosed
that a smaller hysteresis loop in the following loading-unloading curves
without any rest interval (orange line) along with a decreased maximum
stress (2.48 MPa) can be obviously observed in comparation with rst
cycle, which is mainly stemmed from the time dependency of physical
interaction in most hydrogel materials [59,60]. Evidently, the me-
chanical deformation can be gradually recovered (including the area of
hysteresis loop and the maximum stress) and is close to that of the
original one with increasing the rest time. When the rest interval was
xed at 45 min (see the red line), the corresponding curve nearly
overlapped with the pristine one and the recovery ratio of strength and
dissipated energy is 93.5% and 95.5%, respectively. The rapid recon-
guration of dissociated or unzipped physical interactions such as
hydrogen bonds and chain entanglement was responsible for the
remarkable recoverability of PACG-M hydrogel.
3.3. Self-healing and adhesive performance
As the design concept described in Fig. 1, the abundant hydrogen
bonds formed between MXene nanosheets and polymer chains in the
PACG-M hydrogels, which would enable the nanocomposite hydrogel
with good self-healing behavior. For such purpose, the self-healing ex-
periments were conducted. As demonstrated in the inset of Fig. 4a, the
hybrid MXnene nanocomposite hydrogel could be self-healed from four
cut pieces without any external stimulation (such as chemical or thermal
treatment [61]), although the healing efciency based on mechanical
performance is 45.3% (Fig. S7). Due to the existence of MXene nano-
sheets, the resultant nanocomposite hydrogel exhibited excellent con-
ductivity compared to the untreated one (Fig. S8). Additionally, during
the electrical healing process, the corresponding resistance got back to
the initial value in a very short time of 1.32 s when the fracture surface
of two pieces contacted together, which further conrms the effective
reconguration of the molecular or electrical path networks. Further,
the conductivity of the hydrogels exhibited similar value (remain
~1.30 S/m) in several cutting-healing cycles as the original one, which
also revealed the intriguing self-healing property (Fig. 4b).
To further conrm the effective healable ability, a demon process of
the hydrogel was performed. When the hydrogel was served as
conductor in a circuit comprising a light-emitting diode (LED) lamp and
1.5 V constant voltage, the LED lamp was successfully lit in a closed
circuit (Fig. 4c i). Once the appearance of open circuit after cutting the
PACG-M hydrogel, the light of lamp was immediately switched off
(Fig. 4c ii). As expected, when the fracture parts recontact by manual
operation, the LED lamp could be lit again and brightness barely
changed due to the reconstitution of polymer networks via strong
hydrogen bonding, along with the restoration of electrical channel
(Fig. 4c iii). This is also well coincided with the results of real-time
resistance and conductivity (Fig. 4a and b). Interestingly, the healed
hydrogels can still be stretched to be ~50%, although the LED lamp
became weaker (see Fig. 4c iv), indicating its good healable ability for
potential strain sensing capability [62]. The corresponding schematic
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Nano Energy 90 (2021) 106502
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diagrams of the circuit during the healing process is presented in Fig. 4c
v. Additionally, owing to the considerable hydrogen bonds in the
PACG-M hydrogel, the sample exhibits impressively good adhesion to
many different substrates, such as skin, metal, glass, synthetic rubber,
and plastic (Fig. 4d i-iv). More importantly, the adhesive performance
can be remained at a high level even after undergoing low or high
temperature and a relatively long-time exposure of 14 days, as displayed
in Fig. S9 and S10. This means that PACG-M hydrogel possessed
outstanding and stable adhesion in the range of all temperature without
any extra adhesives. Based on the above results, it is envisioned that the
PACG-M hydrogels exhibit promising applications for exible wearable
sensors with multiple characteristics, i.e., good electrical conductivity,
tunable exibility, self-healing and self-adhesive performance.
3.4. Temperature tolerance
For the potential applications, the MXene nanocomposite hydrogels
inevitably encounter different environmental temperatures. Notably,
owing to the utilization of water-glycerol binary solution system, the
optimized PACG-M hydrogel materials exhibited splendid temperature
tolerance i.e., anti-freezing and anti-drying performance, compared
with conventional conductive hydrogels. To study the temperature-
tolerant performance of PACG-M hydrogels, the samples with or
without glycerol were stored at −20 ◦C and 80 ◦C for 1 day, respec-
tively. As demonstrated in the top insets of Fig. 5a, the PACG-M hydrogel
could maintain good exibility under severely mechanical deformation,
such as twisting at −20 ◦C and stretching at 80 ◦C. In stark contrast, the
PAC-M hydrogel almost lost elasticity whatever at low or high temper-
ature (bottom insets of Fig. 5a), and it showed a typical brittle fracture
which was derived from the formation of ice crystal or the evaporation
of water molecule [63,64]. As for the PACG-M hydrogel, this
outstanding temperature tolerant performance could be interpreted by
the fact of the formation of strong hydrogen bonding among the poly-
ol/water molecule, MXene sheets and polymer chains [65], further
producing the long-lasting water retention performance in ambient
environment (Fig. 5b).
Notably, the impressive anti-freezing and anti-heating performance
were greatly affected by the volume percentage of glycerol in the binary
solution system, which can be reected by the digital photos of the
hydrogels with different volume percentage of glycerol. Visually
Fig. 4. Self-healable and self-adhesion performance of PACG-M hydrogels. (a) The self-healing behavior of the PACG-M hydrogel after cutting four pieces and time
evolution of the healable process by the real-time resistance measurements. (b) The conductivity of the PACG-M hydrogel after each cutting/healing process. (c)
Digital images of the luminance variations of LED lamp responding to different status: (i) original, (ii) completely cutted, (iii) self-healed, (iv) stretched at 50% strain
and (v) the corresponding schematic diagrams of the circuit during the healing process. (d) Adhesive performance of the PACG-M hydrogel to various substrate
surfaces at room temperature: (i) metal, (ii) glass, (iii) rubber, and (iv) plastic.
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Nano Energy 90 (2021) 106502
8
displayed in Fig. S11, the hydrogel state could be well kept when the
volume percentage of glycerol was xed in the range of 50–66.7%, but
the hydrogels with relative low volume percentage (≤33.3%) were still
froze or dehydrated. The evident difference on the water-retention of
PACG-M hydrogel with various glycerol content solidly conrmed the
above phenomenon (Fig. 5c). Comparatively, in a certain time (6–9
days), there was a dramatical decrease in the weight (reduces to 52.5 wt
%) for low glycerol content (16.7 vol%) and retained a stable value
(50.0 wt%) in further prolonged storage (up to 15 days). Note that the
equilibrate time was also highly dependent on the glycerol content,
which was clearly proved by the curve of weight variation, e.g., 12 days,
9 days and 1 day for 16.7 vol%, 33.3 vol% and 50.0 vol%, respectively.
Clearly, the positive impediment of ice crystallization and evaporation
of water after introducing adequate glycerol into the hydrogel capaci-
tated PACG-M hydrogel with splendid anti-freezing and anti-drying
performance.
To gain further insights into temperature tolerance of PACG-M
hydrogel, the mechanical properties were systematically investigated
before and after treating in different conditions via a series of tensile
tests. The samples were pre-placed in low, ambient and high tempera-
ture conditions (i.e. −20, 25, and 80 ◦C) for 1 day and immediately
measured. As shown in Fig. 5d-f, the values for tensile strength, tensile
strain and work of fracture of the PACG-M hydrogel can be almost
remained after undergoing the harsh temperature environments. In
contrast, after the same storage process, the PAC-M hydrogel damaged
due to the severely loss of intrinsic elastic characterization originated
from the poor freezing resistance or water-retaining ability, thus
restricting the promising applications in wide temperature range. It is
clear that adjusting volume percentage of glycerol in the hydrogel
benets for achieving mechanical exible hydrogel with excellent
temperature tolerant performance, dramatically prolonging the service
life of hydrogel materials in practice.
3.5. Electromechanical performance
Based on the combination of excellent mechanical exibility,
extraordinary self-adhesiveness and good conductivity, the PACG-M
hydrogel are promising for detecting the mechanical deformation via
monitoring the variation of electronic signals. Fig. 6a and b showed the
relative resistance change (∆R/R
0
) of the hydrogel-based sensor against
the variation of tensile strain during the consecutive cyclic tests. Clearly,
the distinguished and stable intensity of the response signal peak
assigned to the pre-set strain from small (1–10%) to large (200–600%)
could be recognized, indicating a wide work window and a stable
detection signal even after 50 tensile cycles and 14 days exposure in air
(Fig. S12 and S13). In addition, the sensor could also monitor and
distinguish the characteristic speed of measured machine, which was
well reecting by the interval time of adjacent waveforms (Fig. S14).
The above results demonstrate that this hydrogel is an ideal candidate
for tracking the gentle deformation of human limbs, e.g., nger motion,
Fig. 5. Anti-freezing and anti-drying performance. (a) The digital photos of the PACG-M and PAC-M hydrogels after being frozen at −20
◦C or heated at 80 ◦C,
exhibiting the outstanding all temperature-tolerant performance of the PACG-M hydrogels. (b) Photos of volume change of the PACG-M and PAC-M hydrogels after
being stored in 25 ◦C for 15 days. (c) Water-retaining ability versus lasting time of the PACG-M hydrogels with various glycerol content under 25 ◦C and 50%
humidity. (d) Tensile strength, (e) tensile strain and (f) work of fracture of the PACG-M and PAC-M hydrogels under different conditions: −20 ◦C, 25 ◦C and 80 ◦C
(the cross sign indicates the sample damage).
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Nano Energy 90 (2021) 106502
9
as displayed in Fig. 6c and d. Notably, the hydrogel could be easily
attached on the nger without the aid of additional tape owing to the
excellent adhesive performance. Once the nger bent a certain angle,
the ∆R/R
0
of hydrogel-based sensor changed promptly and showed a
rapid response to the different bending angle, thus exhibiting a stepwise
shape of the electrical signal. Meanwhile, the clear and stable signal
assigned to the nger behavior “click”, i.e., cyclic touching the surface of
the hydrogel (Fig. 6d), and the routine motion of shake with high fre-
quency could be tracked clearly and timely (Fig. 6e), further indicating
the high sensitivity.
Astonishingly, our sensor based on the PACG-M hydrogel evidently
detected the change of relative resistance induced by a tiny light object
such as a peanut with a weight of 0.5 g (inset in the Fig. 6f) and the
resistance returned to original value in only ~0.5 s after removing the
weight, suggesting a relatively low detection limit. Further conducting
the measurements by a large range of compressive stress (from 80 to
3200 Pa), it can be seen that the resistance values of the hydrogel were
clearly measured under the different stress values (Fig. 6g). Gauge factor
(GF), dened as the ratio of ∆R/R
0
to the applied strain (
ε
), is a crucial
index corresponding to the sensitivity of hydrogel-based sensor. Based
on the presented temperature tolerance of PACG-M hydrogel, the GFs of
the corresponding sensor were also investigated in various environ-
ments, as shown in Fig. 6h. With increasing of tensile strain, the ∆R/R
0
of the samples at different temperature conditions exhibited a monoto-
nously increasing tendency until the occurrence of mechanical fracture.
Typically, the GF values for low, ambient and high temperature of −20,
25 and 80 ◦C are 3.15, 3.93 and 2.55, respectively, revealing the
outstanding excellent sensitivity in a wide temperature range. Mean-
while, the highly overlap curves of signals against applied strain
demonstrated in Fig. 6i elucidated the prominent stability of our
hydrogel-based sensor in the cyclic tests. The above-mentioned elec-
tromechanical performance of the PACG-M hydrogel may lay a solid
foundation of exible wearable strain sensor in human daily life with a
wide temperature range.
Based on the above results and analysis, the electrical conductivity,
work temperature range, mechanical ductility, self-adhesion perfor-
mance of our PACG-M hydrogels and other hydrogel materials were
compared and summarized in Table S3. The hydrogel systems chosen in
the table have similar multi-functionalities for strain sensing applica-
tions, which are similar as the PACG-M hydrogels prepared in this work.
It can be found that our PACG-M hydrogel materials display a good
balance among the electrical conductivity, work temperature range,
elongation at break, self-adhesion performance and detecting stability at
large strain. Although we do know that the excellent multi-functional
properties depend on not only the type and property of nano-ller, but
also other factors (such as molecular network, process method, water
content and composition), the signicance of these results consists in the
fact that the introduce of low-content MXene aqueous solution is indeed
an effective and extremely simple method to efciently fabricate envi-
ronmentally stable, mechanically exible, self-adhesive, and electrically
conductive PACG-M hydrogel for wide-temperature strain sensing
applications.
4. Conclusion
In summary, a temperature-tolerant, electrically conductive, self-
adhesive, self-healable, and mechanical robust nanocomposite hydro-
gel was fabricated via incorporating MXene into polymer network
Fig. 6. Sensing behavior of PACG-M hydrogels under different stress and temperature conditions. (a, b) Real-time relative resistance change (∆R/R
0
) responding to
different tensile strain levels from 1% to 600%. Relative change in resistance response to nger motion, including (c) different angles, (d) click and (e) shake, (f) a
small peanut with weights of 0.5 g and (g) different pressure levels of 80–3200 Pa. (h) Gauge factor versus consecutive applied strains and (i) cyclic tensile tests at a
xed strain (200%) of the PACG-M hydrogel-based sensor against to different temperature conditions (−20 ◦C, R.T. and 80 ◦C).
S.-N. Li et al.

Nano Energy 90 (2021) 106502
10
composed of water-glycerol binary solvent system. The addition of a low
content (0.1-0.3 wt%) of MXene sheets produced signicantly enhanced
mechanical and electrical performance of the hydrogel. Additionally,
owing to the formation of strong hydrogen bonds between water mol-
ecules and glycerol, the formation of ice crystal lattices (at low tem-
perature) and the evaporation of water (at high temperature) were
restrained effectively, thus resulting in reliable temperature-tolerant
performance (from −20–80 ◦C). The optimized nanocomposite hydro-
gel displayed excellent mechanical performance (e.g., tensile elongation
of ~1000%) and improved electrical conductivity (~1.34 S/m), but also
present good adhesion with various substrates (e.g., steel, glass, rubber,
plastics and skin) and a rapid healable feature. Such MXene-based
hybrid hydrogel as a exible strain sensor exhibited dual sensations
toward wide strain (1–600%) and stress (80–3200 Pa), high sensitivity
(GF =3.93) and broad detection range (up to 600% at cyclic tests), and
good adaptability in a wide temperature range. Therefore, the strategy
mentioned in present work may provide a newfound avenue for the
fabrication of versatile hydrogel materials, which are promising mate-
rials in electronic device such as personalized healthcare.
CRediT authorship contribution statement
Shi-Neng Li: Investigation, Writing – original draft. Zhi-Ran Yu:
Investigation, Methodology. Bi-Fan Guo: Investigation, Validation.
Kun-Yu Guo: Visualization, Resources. Yang Li: Visualization, Valida-
tion. Li-Xiu Gong: Visualization. Li Zhao: Data curation. Joonho Bae:
Writing – review & editing. Long-Cheng Tang: Supervision, Concep-
tualization, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
The authors thank the funding support from the National Science
Foundation of China (51973047 and 51203038), the Natural Science
Foundation of Zhejiang Province (China) (LY18E030005 and
LY15E030015), and the Science and Technology Program of Hangzhou
(China) (20191203B16).
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.nanoen.2021.106502.
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Shi-Neng Li received his M.S. degree from Soochow University
in 2012 and his Ph.D. degree from Harbin Institute of Tech-
nology (China) in 2020. He is now a lecturer in College of
Chemistry and Materials Engineering at Zhejiang A & F Uni-
versity. His current research interests mainly focus on the
design and fabrication of tough and multifunctional hydrogel
materials and their multifunctional applications, such as smart
devices and sensors.
Zhi-Ran Yu received her M.S. degree from Hangzhou Normal
University of Chemistry in 2020. She is currently pursuing her
Ph.D. degree in Materials Science at the University of Lille in
France. Her current research focuses on the functional nano-
composites, and conductive and tough hydrogel materials.
Bi-Fan Guo was born in 1998. He is currently pursuing his M.S.
degree under the supervision of Prof. Long-Cheng Tang at
Hangzhou Normal University. His current research focuses on
design, synthesis, and applications of multifunctional silicone-
based nanocomposite materials.
S.-N. Li et al.

Nano Energy 90 (2021) 106502
12
Kun-Yu Guo was born in 1994. He received his M.S. degree
from Hangzhou Normal University in 2021. His research
mainly focuses on the design and fabrication of conductive
silicone composites for multifunctional applications.
Yang Li received her M.S. degree from Hangzhou Normal
University in 2019. She is currently a Ph.D. candidate in the
department of Nano- Physics, Gachon University, Korea, under
the supervision of Prof. Joonho Bae. Her research interests
include fabrication and development of 2D transition metal
carbides (MXenes)-based composites for energy storage
application.
Li-Xiu Gong received her M.S. degree from Soochow Univer-
sity in 2013. She is working as a research assistant at Hangzhou
Normal University. Her research interests focus on the syn-
thesis of advanced silicone composites and structure-
relationship of functional polymer composite materials.
Li Zhao received her Ph.D. degree in materials processing at
Zhejiang University (China) in 2011. She is currently working
as an assistant professor at Hangzhou Normal University. Her
main research interests focus on the eld of multifunctional
polymer nanocomposites and smart strain/gas sensors.
Prof. Joonho Bae received his B.S. and M. S. degrees from
Seoul National University in 1996 and 1998, respectively. He
was awarded Ph. D. by the University of Texas at Austin in
2007 under supervision of Prof. C. K. Shih, and carried out
postdoctoral research at Georgia Institute of Technology under
supervision of Prof. Z. L. Wang until 2011. After the post-
doctoral research, he worked in Samsung electromechanics as a
senior researcher. He joined department of nano-physics at
Gachon University in 2013. His recent research interests
include synthesis of nanomaterials for energy harvestings and
storage devices including supercapacitors and lithium-ion
batteries.
Prof. Long-Cheng Tang received his Ph.D. degree in Solid
Mechanics from University of Science and Technology of China
and in Materials Science from National Center for Nanoscience
and Technology (China) in 2011, respectively. He then joined
Key Laboratory of Organosilicon Chemistry and Material
Technology of Ministry of Education at Hangzhou Normal
University and became a full professor since 2019. For more
than 10 years, he has been engaged in fundamental research
and product development of advanced silicone composite ma-
terials (such as silicone foam/aerogel/coating), functional
polymer nanocomposites, and smart strain/gas monitoring and
re warning sensors.
S.-N. Li et al.