Extremely Stretchable Strain Sensors Based on Conductive Self-Healing Dynamic Cross-Links Hydrogels for Human-Motion Detection.

TLDR: Extremely stretchable self‐healing strain sensors based on conductive hydrogels are successfully fabricated and have good response, signal stability, and repeatability under various human motion detections.

Abstract: Extremely stretchable self-healing strain sensors based on conductive hydrogels are successfully fabricated. The strain sensor can achieve autonomic self-heal electrically and mechanically under ambient conditions, and can sustain extreme elastic strain (1000%) with high gauge factor of 1.51. Furthermore, the strain sensors have good response, signal stability, and repeatability under various human motion detections.

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ThisdocumentisdownloadedfromDR‑NTU(https://dr.ntu.edu.sg)
NanyangTechnologicalUniversity,Singapore.
Extremelystretchablestrainsensorsbasedon
conductiveself‑healingdynamiccross‑links
hydrogelsforhuman‑motiondetection
Cai,Guofa;Wang,Jiangxin;Qian,Kai;Chen,Jingwei;Li,Shaohui;Lee,PooiSee
2017
Cai,G.,Wang,J.,Qian,K.,Chen,J.,Li,S.,&Lee,P.S.(2017).ExtremelyStretchableStrain
SensorsBasedonConductiveSelf‑HealingDynamicCross‑LinksHydrogelsfor
Human‑MotionDetection.AdvancedScience,4(2),1600190‑.
https://hdl.handle.net/10356/87229
https://doi.org/10.1002/advs.201600190
©2017TheAuthors.PublishedbyWILEY‑VCHVerlagGmbH&Co.KGaA,Weinheim.Thisis
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Extremely Stretchable Strain Sensors Based on Conductive
Self-Healing Dynamic Cross-Links Hydrogels for
Human-Motion Detection
Guofa Cai, Jiangxin Wang, Kai Qian, Jingwei Chen, Shaohui Li, and Pooi See Lee*
Dr. G. F. Cai, Dr. J. X. Wang, K. Qian, J. W. Chen,
Dr. S. H. Li, Prof. P. S. Lee
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798
E-mail: pslee@ntu.edu.sg
DOI: 10.1002/advs.201600190
100%. Recently, there are intense research on highly stretch-
able hydrogels, which are mainly focusing on ionic conductors
due to their excellent transparency and small resistance varia-
tion under high stretching states.
[25–29]
In particular, conductive
hydrogels are promising materials for the fabrication of ionic
skin, bioelectrodes, and biosensors because many hydrogels
with high water concentration have biocompatibility proper-
ties.
[24,30–32]
Therefore, it is of great interest to fabricate highly
stretchable self-healing strain sensor by combining the advan-
tages of both biocompatible hydrogels and electronic conduc-
tors for applications in robotics, human motion detection,
entertainment, medical monitoring, and treatment etc.
Herein, we introduce a new type of extremely stretch-
able self-healing piezoresistive strain sensor using different
electronic conductors comprised of single wall carbon nano-
tube (SWCNT), graphene, and silver nanowire in self-healing
hydrogel (SWCNT, graphene, and silver nanowire/hydrogel) as
the conductive sensing channel built on a commercial trans-
parent elastic substrate. The conductive hydrogel exhibits a fast
self-healing capability which can restore 98 ± 0.8% of its initial
conductivity within 3.2 s healing time. Moreover, no external
stimuli (such as heat, pH, light, or catalyst) are required. The
fast self-healing process of the SWCNT/hydrogel ensures rapid
recovery of the electrical property of the sensor after being
released to the relaxed state and avoids the degradation of the
device performance during the large deformation. The self-
healing strain sensor is capable of monitoring strain, flexion,
and twist forces. Moreover, it can measure and withstand strain
up to 1000%, with high gauge factor and excellent cycling sta-
bility. Based on these key features, the self-healing strain sensor
can be used to accurately detect large-scale human motion by
embedding it in gloves, garments, or directly attaching it on
skin. The present methodology developed paves the way for
practical applications of highly stretchable self-healing strain
electronic devices.
The fabrication process of conductive hydrogel is illustrated
in Figure 1a (see the Experimental Section in the Supporting
Information for details). Figure 1b illustrates the key reaction
in forming crosslinked hydrogel. Borax, the salt of a strong base
and a weak acid, is hydrolyzed in aqueous solution, yielding
a boric acid/tetrafunctional borate ion. In the gelation experi-
ments, trigonal planar B(OH)
3
and tetrahedral B(OH)
4
−
exist
as monomeric species due to the low concentration of borax
employed (0.02
M). B(OH)
3
is capable of complexing polyvinyl
alcohol (PVA), however, it cannot produce polyol gels. The
main reason is that the complexation reaction occurs through
the attachment of boron to adjacent alcohol groups of the
same polymeric chain and this prevents cross-linking from
This is an open access article under the terms of the Creative Commons
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any medium, provided the original work is properly cited.
Stretchable, wearable, flexible, and human friendly soft elec-
tronic devices are of significance to meet the escalating require-
ments of increasing complexity and multifunctionality of
modern electronics.
[1–6]
Strain sensors can generate repeatable
electrical changes upon mechanical deformations. They have
found particular interest and broad applications in robotics,
sports, health monitor, and therapeutics, etc. To date, several
representative strain sensors using carbon nanotubes,
[7–9]
metal/
semiconductor,
[10–12]
graphene,
[13–15]
conductive polymer,
[16,17]
and microfluidic
[18,19]
as conductive materials combining with
elastomeric substrates have been successfully fabricated. How-
ever, most of these devices can only be stretched to a very lim-
ited extent (usually less than 200%). Lewis and co-workers
[20]
have developed a capacitive soft strain sensor using an ionically
conductive fluid and silicone elastomer as the conductor and
dielectric/encapsulant respectively, which can be stretched up
to 700%, but the gauge factor is small (0.348 ± 0.11). We can
define the gauge factor as (∆R/R
0
)/
ε
, where ∆R/R
0
is relative
resistance change, R
0
is the resistance at 0% strain, R is the
resistance under stretch, and
ε
is the applied strain.
[21]
In addi-
tion, introducing self-healing properties to these soft electronic
devices that can repeatably recover mechanical and electrical
performance under room temperature, even at the same dam-
aged location or under extremely stretchable situation, is of
high importance to avoid the degradation of the device perfor-
mance during the deformation.
Nowadays, self-healing materials have attracted increasing
attention, especially in soft electronics field. Haick and co-
workers
[22]
have reported a self-healing flexible sensing plat-
form by dispersing metal particles in polyurethane diol as self-
healing electrode. Bao and co-workers
[23]
have demonstrated
a self-healing electronic sensor skin based on nanostructured
µNi particles-supramolecular organic composite. Park and co-
workers
[24]
have developed self-healing conductive hydrogel
by polymerizing pyrrole in agarose solution. However, none
of these self-healing electronic devices can be stretched over
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taking place. Therefore, the crosslinked hydrogel is formed
via tetrafunctional borate ion interaction with –OH group of
PVA. The process is particularly effective in forming 3D gel
networks. The hydrogen-bonding between tetrafunctional
borate ion and –OH group provides the self-healing function
because the cross-link is so weak that it is neither resemblance
of covalent bond character nor esterification involved.
[33–35]
The
hydrogen-bonding can be easily broken and reformed, allowing
the hydrogel to self-heal and reform. The cross-links are
dynamically associated and dissociated readily under room tem-
perature. The network cross-linked by weak hydrogen-bonding
is easily disrupted by a mechanical deformation, however, it
is relatively facile for the bonds to reform due to proximity of
plenty –OH groups and borate ions, hence allowing self-healing
at room temperature. In addition, the PVA-borax hydrogel
exhibits non-Newtonian behavior, resulting in flow under low
stress and limited dimensional stability.
[35,36]
Therefore, the
sufficient mobility of polymer chain and free tetrafunctional
borate ions enables the hydrogen bond across broken inter-
faces to trigger the self-healing process rapidly and without
the need of external stimuli. Although the PVA itself can form
hydrogel and autonomously self-healing property according
to the previous reports, the concentration of the PVA used is
very high and the stretchability is limited.
[37–39]
Before forming
the hydrogel, SWCNT and 4 wt% PVA solution were homo-
geneously mixed under the surfactant assistance of BYK 348
which is a polyether modified siloxane (purchased from BYK-
Chemie GmbH). The SWCNT and water are wrapped in the
3D networks during the hydrogel crosslinking process, thereby
the conductive self-healing hydrogel was formed as shown in
Figure 1c. It is worth noting that most of the free volume (or
pores) within the hydrogel is taken up by water. The hydrogel is
composed of water with weight percentage more than 95 wt%.
The scanning electron microscope (SEM) micrographs taken
from freeze-dried SWCNT/hydrogel are shown in Figure 1d,e.
The microstructure of the freeze-dried SWCNT/hydrogel is
3D porous networks cross-linked by the SWCNT and some
immobilized polymer. The porous structure of the SWCNT
and polymeric network inside the SWCNT/hydrogel is highly
beneficial to the stretchability and facilitating rapid response of
hydrogels. In order to reveal the interactions between the PVA
and tetrafunctional borate ion, Fourier transform infrared spec-
troscopy (FTIR) experiment was conducted on Spectrum GX
FTIR Spectrometer. As shown in Figure S1 (Supporting Infor-
mation), the broad and strong peak around 3400 cm
−1
is attrib-
uted to the symmetrical stretching vibration of –OH groups.
The –OH stretching peak is sensitive to hydrogen bonding.
Compared with pure PVA, the –OH stretching peak shifts to a
higher wavenumber and the peak is enhanced after formation
of the hydrogel, indicating the presence of hydrogen bonding
interactions between the hydroxyl groups on the PVA molecular
chains and tetrafunctional borate ion.
[40,41]
In addition, dynamic
mechanical measurements of the pure hydrogel and SWCNT/
hydrogel were carried out to investigate their rheological prop-
erties. Figure S2 (Supporting Information) shows the changes
in the storage (G′, solid symbols) and loss modulus (G″, hollow
symbols) as a function of angular frequency for hydrogel and
SWCNT/hydrogel. It can be seen that the presence of SWCNT
raises the moduli and enhances the elastic response of the
hydrogel. In addition, both hydrogels have a solid behavior with
the storage modulus exceeding the loss modulus over the entire
frequency range.
Figure 2a
1
–a
3
shows the representative optical microscope
images of how the SWCNT/hydrogel was healed after being
completely separated by a scapel. The two fractured surfaces
rapidly contact each other after the scapel was removed. The
cutting groove was partially healed after 30 s and totally restored
to normal after 60 s at room temperature without any external
assistance (such as heat, light, and force). To further investigate
the healing property of the SWCNT/hydrogel and recovery of
the conductivity, the SWCNT/hydrogel was completely bifur-
cated and then the two furcated parts were rapidly brought
together. Figure 2b presents the resistance changes over time
of the SWCNT/hydrogel during the cutting and healing pro-
cess. Once the conductive hydrogel was completely cut off, an
open circuit was formed and the resistance changed to infinity.
As the two furcated parts were brought together, the resist-
ance dropped quickly and the resistance reached a constant
value within 3.2 s. In addition, the self-healing efficiency of the
SWCNT/hydrogel was calculated by Rr/Ri (Rr is the recovered
conductivity and Ri is the initial conductivity). Rr/Ri is 98.6%
after healing for 3.2 s. It is worth noting that the resistance is
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Figure 1. a) The fabrication process of conductive hydrogel. b) Crosslinking
reaction between PVA and tetrafunctional borate ion. c) Photo image of
SWCNT/hydrogel. d,e) SEM images of the freeze-dried SWCNT/hydrogel.

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lower than that of original value at the moment the two fur-
cated parts get in contact, which is due to the transfer of free
ions in the hydrogel, similar phenomenon was observed in the
reduced graphene oxide based hydrogel.
[42]
Figure 2c shows the
repetitive cutting-healing processes with five cycles at the same
location. The resistance of the sample is relatively stable during
the cycling. The high self-healing efficiency was observed in
each cutting-healing process (Figure S3, Supporting Informa-
tion). The average efficiencies are 98 ± 0.8% for the five self-
healing cycles within about 3.2 s, indicating the SWCNT/
hydrogel possesses significant and repeatable electrical restora-
tion performance.
The self-healing property of SWCNT/hydrogel was also dem-
onstrated on a complete circuit composed of a LED indicator with
SWCNT/hydrogel as the conductor, as shown in Figure 2d
1
–d
3
.
The LED indicator was successfully lighted when a driving
voltage of 5 V was applied. The LED indicator was extinguished
when the SWCNT/hydrogel was completely bifurcated and the
circuit became open-circuit state. Once the two furcated parts
were partially brought together, the circuit was restored and the
LED indicator could be lighted up again. The demonstration
here illustrates that the SWCNT/hydrogel has great potential in
applications of self-healing electronic device such as biosensors,
electronic skin, wearable electronics, and so on.
To evaluate the performance of the strain sensor, we realized
the self-healing piezoresistive strain sensor using the SWCNT/
hydrogel as conductor, and Scotch permanent clear mounting
tape (VHB 4010, 3
M) as elastomeric substrates and encapsu-
lant, as shown in Figure 3a. The high stretchability of both
SWCNT/hydrogel and VHB tape allowed the self-healing strain
sensor to remain intact up to 1000% strain,
the highest value for electronic strain sensor
so far, to the best of our knowledge.
[43–45]
The excellent performances of the device
are derived from all parts of the device or
their coordination with each other. Although
the SWCNT/hydrogel itself could not be
recovered to the initial state under extreme
strain conditions, it can work well when
attached to the VHB tape. A strain sensor
using the hydrogel without the electronic
conducting component (SWCNT) was also
prepared on the elastic substrate with the
same parameters for comparison. Relative
resistance changes versus strains are shown
in Figure 3b. The relative resistance change
increases with increasing tensile strain.
A relative resistance change [(R −R
0
)/R
0
=
∆R/R
0
, R
0
is the resistance at 0% strain, R
is the resistance under stretch] of 1514%
was observed at 1000% strain for SWCNT/
hydrogel, the sensitivity is nearly three times
higher than that of the hydrogel without elec-
tronic conductor (533%). The large resistance
change is highly desired for strain sensing
applications, which is prerequisite for high
sensitivity. Moreover, the SWCNT/hydrogel
based strain sensor also showed reproduc-
ible and reliable responses to the small strain
from 2% to 100% (Figure S4, Supporting Information). These
results indicate that the SWCNT/hydrogel based strain sensor
can work well from small strain to extreme strain. There are
two aspects leading to the piezoresistive effects of SWCNT/
hydrogel: one is the intrinsic piezoresistivity of the hydrogel,
the other is the change of the contact conditions of SWCNT for
electron conduction, such as contact area, loss of contacts and
spacing variations upon stretching, and so on. The electrical
conductivity of hydrogel without electronic conductor comes
from ions conductivity (such as Na
+
, H
+
in the hydrogel).
[26,32]
In addition, the relative resistance changes versus strains of the
strain sensor can fit into a parabolic equation y = A
ε
2
+ B
ε
+ C,
where y is the relative resistance changes and
ε
is the tensile
strain.
[23,46]
There is no relative resistance change when there
is no strain applied to the sensor, so C is zero in the equation.
Moreover, the value A can be defined as the sensitivity factor.
Larger A value leads to more relative resistance changes, cor-
responding to higher sensitivity in the sensor. Hence, the sen-
sitivity of the strain sensor can be quantified by the equation in
Figure 3b. It can be seen that the SWCNT/hydrogel based strain
sensor possesses a larger A than that of the hydrogel without
electronic conductor. Therefore, the sensitivity of the SWCNT/
hydrogel based strain sensor is higher than that of the hydrogel
without electronic conductor, which is consistent with the
experimental data. Figure S5 (Supporting Information) shows
the photographs of the SWCNT/hydrogel based self-healing
strain sensor stretched to different strains.
Gauge factor represents the sensitivity of the sensors. Usu-
ally, brittle or poorly stretchable conductive materials have
higher gauge factor. However, these materials do not possess
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Figure 2. a) In situ self-healing optical images of the SWCNT/hydrogel at room temperature,
the healing time of (a
1
–a
3
) are 0, 30, 60 s, respectively. b) Time evolution of the electrical
healing process by resistance measurements under ambient conditions. c) cycling of the cut-
ting-healing processes at the same location. d) Circuit comprises self-healing SWCNT/hydrogel
in series with an LED indicator, d
1
) undamaged, d
2
) completely bifurcated, and d
3
) electrical
healing.

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or only sustain small stretchability. A relatively small strain
could result in an irreversible fracture and lead to an infinite
gauge factor. In the cases that high stretchability is required,
the gauge factor of SWCNT/hydrogel was 0.24 at 100% strain
and increased to 1.51 at 1000% strain (Figure S6, Supporting
Information). The gauge factor is higher than that of the
hydrogel without electronic conductor (from 0.09 at 100%
strain to 0.53 at 1000% strain), and other piezoresistive elec-
tronic strain sensor (0.06 at 200% strain)
[8]
and capacitive soft
strain sensors based on ionic conductor (0.348 ± 0.11 at 700%
strain).
[20]
Although some strain sensors exhibit much higher
gauge factors, the poor stretchability and lack of self-healing
capability restrict their applications under rigorous mechanical
deformations.
[47–49]
The responses of the SWCNT/hydrogel based strain sensor for
other types of deformation such as flexion and twist which are
related to human body movements were also tested. Figure 3c
displays the resistance change as a function of flexion angle
for the SWCNT/hydrogel based strain sensor. When the sensor
was flexed, tensile stress built up at the outer curvature and
compressive force built at the inner curvature. The conductive
SWCNTs are separated from one another at the outer curvature
and approached closer to one another at the inner curvature.
However, the separated SWCNT played a dominant role for the
device, thereby the resistance increased with increasing flexion
angle. It can be seen that the resistance increased to 118% from
its original value with bending angle increasing from 0° to 150°.
When the strain sensor was twisted, the resistance changes
versus twist angle still obeys the flexion parabolic equation
within the twist angle of less than 540° as shown in the Figure 3d.
However, the conductive SWCNT will be separated from one
another around the twist point under larger twist angle (more
than 540°), hence, the resistance of the device rapidly increases
with increasing twist angle that detaches the SWCNT contacts
or entanglements. The sensor responds to the twist angle with a
good sensitivity, the resistance increases to 457% from the value
of the untwisted state after two revolutions (720°) as shown in
Figure 3d. The stability of the sensor was also investigated by
repeatedly applying 700% stretching strain to the sensor and
the resistance was measured at the released state (Figure S7,
Supporting Information). The resistance of the sensor remains
almost constant with minor fluctuations within 10% within the
first 700 cycles strain test (between 0% to 700% strain). How-
ever, the largest resistance fluctuations were observed after
700 cycles due to partial water evaporation of the hydrogel during
the long-term cycles. The loss of water from hydrogels might
become significant in long-term cycles, which can be reduced by
an appropriate encapsulation. Finding ideal packaging materials
and technology for long lifetimes is a large undertaking beyond
the scope of this paper. We suggest that SWCNT/hydrogel with
1000% stretchability is a superior candidate for strain sensing
applications, considering the fast self-healing property and sig-
nificantly improved piezoresistive responses compared to the
hydrogel without electronic conductor counterparts.
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Figure 3. a) A piezoresistive strain sensor was fabricated by sandwiching a layer of conductive hydrogel between two layers of commercial tape
(VHB 4010, 3
M)), which were then connected to two metallic electrodes, the piezoresistive strain sensor exhibited high extensibility up to 1000%.
b) Plot of relative resistance change versus strain for SWCNT/hydrogel and hydrogel without electronic-conductor-based strain sensors. The equa-
tion represents a parabolic equation y = A
ε
2
+ B
ε
+ C, where y is the relative resistance changes and
ε
is the tensile strain. c) Variation of normalized
resistances as a function of flexion angle from 0° to 150°, inset presents the definition of flexion angle with bending radius of 2 cm. d) Variation of
normalized resistances as a function of twist angle up to two revolutions. Equations in (c) and (d) represent a parabolic equation y = A
θ
2
+ B
θ
+ C,
where y is the resistance changes and
θ
is the flexion or twist angle. Error bars indicate standard deviation based on measurements of three devices.