Self-repairing flexible strain sensors based on nanocomposite hydrogels for whole-body monitoring

Journal Pre-proof Self-repairing flexible strain sensors based on nanocomposite hydrogels for whole-body monitoring Hongwei Zhou (Supervision) (Writing - original draft) (Conceptualization), Zhaoyang Jin (Investigation) (Data curation), Ying Yuan (Data curation), Gai Zhang (Writing - review and editing), Weifeng Zhao (Writing - review and editing), Xilang Jin (Methodology) (Writing - review and editing), Aijie Ma (Writing review and editing), Hanbin Liu (Conceptualization) (Methodology) (Writing - review and editing), Weixing Chen (Writing - review and editing)

PII:

S0927-7757(20)30180-1

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124587

Reference:

COLSUA 124587

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

17 December 2019

Revised Date:

1 February 2020

Accepted Date:

12 February 2020

Please cite this article as: Zhou H, Jin Z, Yuan Y, Zhang G, Zhao W, Jin X, Ma A, Liu H, Chen W, Self-repairing flexible strain sensors based on nanocomposite hydrogels for whole-body monitoring, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124587

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Self-repairing flexible strain sensors based on nanocomposite hydrogels for whole-body monitoring

Hongwei Zhou,*,a Zhaoyang Jin,a Ying Yuan,a Gai Zhang,a Weifeng Zhao,a Xilang Jin,*,a Aijie Ma,a Hanbin Liu,*,b Weixing Chena

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Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School

of Materials and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, P. R. China. E-mail: [email protected], [email protected]

College of Bioresources Chemical and Materials Engineering, Shaanxi University of

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Science & Technology, Xi'an, 710021, P. R. China, E-mail: [email protected]

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Graphical abstract

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Highlights 

Carboxylate modified cellulose nanofibril (c-CNF) efficiently improved the strength and resilience of fully supramolecularly cross-linked poly(AA-co-SMA)/c-CNF/Fe3+ nanocomposite hydrogels.



Self-repairing flexible strain sensors possessing large sensing range, low

response time, high sensitivity and excellent durability were fabricated with the nanocomposite hydrogels. 

Whole-body motion and physiological activity monitoring were achieved utilizing the flexible strain sensors.

Abstract:

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Flexible strain sensors based on extensible hydrogels are extensively investigated for human motion monitoring, but the performances of such strain sensors are limited by

failure or high residual strain of hydrogels. Herein, self-repairing flexible strain

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sensors are fabricated with poly(AA-co-SMA)/c-CNF/Fe3+ nanocomposite hydrogels (AA: acrylic acid, SMA: stearyl methacrylate, c-CNF: carboxylate modified cellulose

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nanofibril). Because the nanocomposite hydrogels are fully supramolecularly cross-linked by hydrophobic association and ionic interaction, the flexible strain

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sensors can repair themselves upon damage and recover their sensing ability. Compared with other hydrogel-based self-repairing sensors, the flexible strain sensors

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in this work have intrinsically reversible sensing ability based on the improved resilience of the nanocomposite hydrogel by introducing c-CNF. In addition, such

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sensors also have integrated large sensing range (0~800%), low response time (0.25 s), high sensitivity (strain 0~300%, gauge factor=1.9; strain 400%~800%, gauge

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factor=4.4) and excellent durability (~1000 cycles) for guaranteeing their potential applications in whole-body monitoring. Keywords: strain sensor; hydrogel; crosslinking; composite; CNF; self-repairing

1. Introduction Flexible strain sensor, an important electronic device that responds timely and reversibly to mechanical deformations by electronic signals, is increasingly required in various fields, including soft robots, health monitoring, wearable devices and human-machine interfaces.1-6 Resistive-type strain sensor is often assembled by electrodes

to

conductive

materials

that

have

strain-dependent

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connecting

conductivity.7-11 For most of the flexible strain sensors that detect large deformation,

the sensing mechanism is on the basis of resistance variation induced by the

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cross-sectional area change of the conductive materials according to R=ρL/S, in

which R, ρ, L and S are the resistance, resistivity, length and cross-sectional area of 12-13

To fabricate such strain sensors, it is

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conductive materials, respectively2,

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significant to explore strain sensitive flexible materials.

Hydrogels with integrated self-healing ability, high extensibility and electronic

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conductivity are extensively investigated for flexible strain sensors due to their superior advantages over water-free conductive elastomers in terms of health 4, 13-21

So far, various conductive components, including metal

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monitoring.2,

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nanoparticles/wires,22-23 polymers7-8,

18

carbon

and ions3-4,

14, 26

nanomaterials,24-25

intrinsically

conductive

have been introduced to construct hydrogels for

flexible sensors. To obtain hydrogels with high extensibility and self-healing ability, Ran and coworkers27 developed a dually physically crosslinking strategy on the basis of hydrophobic association and ionic interaction. In other examples, extremely extensible and self-healing hydrogels have been developed by introducing carbon

nanotube or 2D early-transition metal carbides/carbonitrides (MXenes) into dynamically cross-linked poly(vinyl alcohol) (PVA) network and further utilized in assembling flexible strain sensors for detecting large strains.2, 13 However, mostly reported self-healing and highly extensible hydrogels suffer from large residual strain and low elasticity, which limits their application in cyclic strain sensing. Moreover, the recovery of strain sensors is often assisted by commercial tapes, which hinders the

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convenient use of such hydrogels in assembling strain sensors. Therefore, it is meaningful to develop resilient hydrogels with integrated self-healing ability and high extensibility.

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Cellulose nanofiber (CNF) is a typical biomass-derived natural polysaccharide

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with nano-structured morphology. Compared with inorganic fillers, cellulose nanofiber has distinguished advantages in the following aspects. First, cellulose

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nanofiber has a large number of hydroxyl groups which can be further converted to

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carboxyl group or sulfonic acid group by surface modification.28-29 Thus, cellulose nanofiber has superior compatibility with organic matrix and can be tightly fixed in

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polymer matrix via various interactions between polar groups to reduce possible aggregation of fillers and fatigue of polymer composites. Second, cellulose nanofiber

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has excellent mechanical properties on the basis of its rigid structure. In polymer composite, cellulose nanofiber may work as rigid sacrificial framework to improve the elasticity of polymer matrix. So far, cellulose nanofiber has been widely utilized as key component to construct rigid network,3 to disperse carbon nanofiller13 and to improve the properties of hydrogels.30-32 These investigations have provided a

provoking clue to construct functional hydrogels by introducing cellulose nanofiber as functional filler. Herein, self-repairing flexible strain sensors are fabricated based on poly(AA-co-SMA)/c-CNF/Fe3+ nanocomposite hydrogel (AA: acrylic acid, SMA: stearyl methacrylate, c-CNF: carboxylate modified cellulose nanofibril), as illustrated in Scheme 1. Hydrophobic association and ionic interaction are introduced as

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supramolecular crosslinking points to realize self-repairing functionality of strain

sensor. c-CNF works as rigid component to lower the residual strain and enhance the

mechanical strength of nanocomposite hydrogels. By modulating the network

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structure of nanocomposite hydrogels, flexible strain sensors possessing large sensing

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applied in whole-body monitoring.

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range, low response time, high sensitivity and excellent durability are fabricated and

Scheme 1. (a) Illustration of sensing mechanism of flexible strain sensors based-on nanocomposite hydrogels. (b) Schematic preparation and network structure of

nanocomposite hydrogels.

2. Material and methods 2.1. Materials

Acrylic acid (AA, 99.5%), Sodium dodecyl benzene sulfonate (SDBS, 99.5%), Ferric chloride hexahydrate (FeCl3·6H2O, 99.5%) and Potassium persulfate (KPS,

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98%) were purchased from Tianjin Yongsheng Fine Chemical Co. Ltd. Stearyl

methacrylate (SMA, 96%) was purchased from Energy Chemical. Carboxylate

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modified cellulose nanofibril (c-CNF, 1 wt%) was purchased from Guilin Qihong Technology Co. Ltd. Deionized water was used in all experiments.

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2.2. Fabrication of flexible strain sensors with nanocomposite hydrogels

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Nanocomposite hydrogels were first prepared. In a typical experiment, a mixture solution of H2O (10 mL)，AA (3 mL), SMA (0.4 g), SDBS (0.4 g), aqueous c-CNF

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dispersion (0.5 g, 1 wt%), FeCl3·6H2O (0.05 g) was firstly prepared with the

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assistance of magnetic stirring and ultrasound. After the above solution being bubbled with N2 for 20 min, KPS (0.12 g) was dissolved. The resulting solution was then

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injected into a mold (two Teflon sheets with a carved silicone rubber spacer) with a syringe and the gelation process was conducted at 50 oC for 24 h. Different nanocomposite hydrogels were prepared by varying the concentration of Fe3+ and c-CNF.

Flexible strain sensors were fabricated by connecting wires to both sides of a nanocomposite hydrogel strip (40 mm×12 mm×1.8 mm).

2.3. Characterization

The fracture surface morphology of freeze-dried hydrogels was analyzed on a scanning electron microscope (SEM, TESCAN VEGA 2 XMU). The freeze-dried

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hydrogel samples were fractured in liquid nitrogen and then sputtered with gold under vacuum before observation. Tensile stress-strain tests and loading-unloading tests

were conducted on a universal testing machine. The samples were 40 mm × 12 mm ×

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1.8 mm in dimension and the moving speed of clamp was 20 mm/min. Dissipated

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energy during one loading-unloading cycle was calculated from the integral area under loading-unloading curves. Storage moduli (G’) and loss moduli (G”) of

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hydrogels were studied on a TA rheometer (DHR 2) system. The strain was 1% and the temperature was 25 oC. Resistance response of the sensors to various strains was

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computer.

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investigated on a digital electric bridge (LCR, UC2858A) and recorded by a

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3. Results and discussion 3.1. Mechanical properties of nanocomposite hydrogels

The resilience of hydrogels was enhanced by introducing c-CNF. As displayed in

Figure 1a and Video S1, a poly(AA-co-SMA)/c-CNF/Fe3+ hydrogel strip is highly extended (980%) and almost completely recovers to its initial state by releasing the

tensile force, indicating the excellent resilience. To further evaluate the resilience of the nanocomposite hydrogel, the ratio of recovered length to initial length of hydrogel strip was recorded during tensile loading-unloading test (Figure S1). As summarized in Figure 1b, as the loading-unloading cycle increased, the residual strain increased and L2 /L0 became higher than 1. Comparatively, L2 /L0 for nanocomposite hydrogel is lower than that of poly(AA-co-SMA)/Fe3+ hydrogel. For instance, in the case of a

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c-CNF content of 37 mg/mL, the residual strain after ten loading-unloading cycles is about 20%. These results indicate that c-CNF bonded by ionic interactions works as

rigid component and improves the resilience of fully physically cross-linked

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poly(AA-co-SMA)/Fe3+ hydrogel, similar to previously reported ones.3

Figure 1. Mechanical properties of nanocomposite hydrogels. (a) Photographs of a

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hydrogel strip in original state, stretched state and recovered state. (b) The resilience of hydrogels prepared under different c-CNF content after being subjected to different loading-unloading cycles (strain: 200%). (c) Photographs of a knotted hydrogel strip and a twisted hydrogel strip. (d) Stress-strain curves of nanocomposite hydrogels prepared under different c-CNF content. (e) Photographs of a hydrogel film stretched

on pen point and blown to a balloon shape. The nanocomposite hydrogels are flexible and their mechanical properties are further regulated by c-CNF content and Fe3+ concentration. As displayed in Figure 1c, the hydrogel strips can be freely knotted and twisted without breakage, indicating their well flexibility. From the stress-strain curves shown in Figure 1d, the breaking strength and elongation at break were both enhanced by increasing c-CNF content. As

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the c-CNF content increased from 0 to 37 mg/mL, the breaking strength increased

from 60.6 kPa to 98.6 kPa and elongation at break increased from 863% to 902%. The strengthening effect might be originated from tightly cross-linked network structure of

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hydrogels, as indicated by the fracture surface morphology of freeze dried hydrogels

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(Figure S2) and swelling behaviours of hydrogels (Figure S3). Because Fe3+ forms ionic interactions with carboxyl groups in polymer chains and c-CNF as additional

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crosslinking points, increase of Fe3+ concentration led to an increase of breaking

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strength and a decrease of elongation at break (Figure S4). Due to their fully supramolecularly cross-linked structure, nanocomposite hydrogels also have high

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energy dissipation ability (Figure S5). As seen from Figure 1e, a hydrogel film stretched on pen point was not punctured and could be further blown to a balloon,

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indicating the excellent toughness of the nanocomposite hydrogels. The excellent and tunable mechanical properties of nanocomposite hydrogels provide an opportunity to fabricate high-performance flexible strain sensors.

3.2. Performances of flexible strain sensors

Before assembling flexible strain sensors, the strain-dependent conductivity of nanocomposite hydrogels was investigated. When a hydrogel strip was connected in a circuit as wires, the LED shines immediately (Figure S6). This indicates the conductivity of the nanocomposite hydrogel. The conductivity of nanocomposite hydrogels derived from charge carriers of various ions. Interestingly, when the

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hydrogel strip was reversibly stretched, the brightness of the LED changed

accordingly (Video S2). Stretching process induced darkening of LED while releasing

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process induced brightening of LED. This means that the hydrogel has strain-dependent conductivity. The conversion of mechanical deformation into

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such nanocomposite hydrogels.

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electrical signal provides an opportunity to fabricate flexible strain sensors utilizing

Flexible strain sensors were assembled by connecting wires to nanocomposite

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hydrogel strips and their sensing ability were investigated by recording resistance variation upon applied strains in various modes. Figure 2a shows the reversible

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resistance response of the sensor to periodical deformation. As seen, when the sensor

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was stretched to a strain of 100%, the relative resistance variation ΔR/R0 (ΔR=Ri-R0, Ri and R0 are the resistance of the sensor in original state and deformed state, respectively) increased accordingly. Holding of the deformation leads to a platform of the ΔR/R0-time curve, indicating the stability of the sensor. When the sensor was released and recovered, ΔR/R0 changes back to almost zero. Upon cyclic deformation, ΔR/R0 changes periodically. When increasing strain (0~800%) was applied to the

sensor, ΔR/R0 increased gradually (Figure 2b). Similarly, step-like curve was obtained when the deformation was held. Linear fitting of the relationship between ΔR/R0 and strain gives two gauge factors (GF), GF1=1.9 in the strain range of 0~300% and GF2=4.4 in the strain range of 400%~800% (Figure 2b inset). These gauge factors are comparable to that of some previously reported hydrogel-based strain sensors.8, 14, 33 Further investigations were conducted to evaluate the durability and response

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speed of the sensor. As seen, the strain sensor has the ability to work cyclically more

than 1000 times (Figure 2c). This means that the sensor has excellent durability and works repeatedly. In addition, the sensor also responds timely to various deformations

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and minimum response time of 0.25 s was obtained (Figure 2d). This guarantees the

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immediate sensing of strains for on-line detection. Figure 2e shows the resistance response of the sensor to periodical and reversible strain of 100% with different

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stretching frequency. The deformation conducted at high speed, medium speed and

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low speed was obviously distinguished by the changing speed of ΔR/R0. More complicatedly, the sensor also responded efficiently when the amplitude and changing

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speed of applied strain were synchronously changed (Figure 2f).

Figure

2.

Performances

of

flexible

strain

sensors

assembled

from

poly(AA-co-SMA)/c-CNF/Fe3+ hydrogel prepared under a Fe3+ concentration of 14 mmol/L and a c-CNF content of 37 mg/mL. (a) Relative resistance (ΔR/R0) variation of strain sensor under cyclic strain of 100%. (b) ΔR/R0 variation of the sensor upon gradually increasing strain from 0 to 800%. The inset shows the linear fitting of ΔR/R0-strain curve. (c) ΔR/R0 variation of the sensor upon cyclic strain of 200% for

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more than 1000 times. (d) ΔR/R0 variation of the sensor upon cyclic instantaneous

tensile strain. (e) ΔR/R0 variation of the sensor upon tensile deformation of 100%

with different stretching frequency. (f) ΔR/R0 variation of the sensor upon different

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applied strains with rapid stretching and slow releasing.

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3.3. Self-repairing ability of flexible strain sensors

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The flexible strain sensors have interesting self-repairing ability due to the fully supramolecularly cross-linked structure of nanocomposite hydrogels. As displayed in

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Figure 3a, the nanocomposite hydrogel colored with ink gradually grown together upon a sequential cutting, contacting and self-healing procedure. The self-healing

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process is on the basis of re-formation of dynamic interactions, such as ionic

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interaction and hydrophobic association.27 After 48 h, the contact area becomes obscure and the crack between two pieces almost disappeared, indicating the matter exchange across the interfaces. The self-healed hydrogel was freely stretched to a strain of 933% of its original length (Figure 3b), indicating the efficient recovery of

mechanical properties. The stress and strain gradually increased over healing time and a healing efficiency of 94.4% was obtained after 48 h (Figure 3c). In addition to mechanical properties, the conductivity and sensing ability of flexible sensors were also recovered by self-repairing. Figure S7 displays the repairing of a sensor connected to a circuit. As seen, the LED shines after contacting the two cut hydrogel pieces, indicating the recovered conductivity. Further

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quantitative investigation also revealed that the resistance almost changed back to its original value. On the basis of their recovered mechanical stretchability and

conductivity, strain sensing was conducted by self-repaired flexible sensor. As seen in

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Figure 3d, the sensor responded periodically for more than 1000 cycles to reversible

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deformation at a strain of 100%, indicating that the sensor still has excellent durability. Moreover, distinguishing of strains with different speeds was also realized by the

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self-repaired sensor (Figure 3e). These results indicate that the flexible sensor have

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the ability to repair themselves efficiently to expand the life span of sensor.

Figure 3. Self-repairing performance of flexible sensors. (a) Self-healing process of nanocomposite hydrogels and the microscopic images of healing points. (b) Photographs display the gradually stretching of self-healed hydrogel strip to 933% of its original length. (c) Stress-strain curves of the hydrogels with different self-healing time. (d) ΔR/R0 variation of a self-repaired strain sensor upon reversible deformation

deformation of 200% with different frequency.

3.4. Whole-body monitoring by flexible strain sensors

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for more 1000 cycles. (e) ΔR/R0 variation of a self-repaired strain sensor under tensile

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Based on the excellent performances, the flexible strain sensors are attempted in

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whole-body monitoring. Generally, any human motion that induces detectable electric signals of sensors will be detected, such as bending of joints and subtle physiological

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activities. As examples, cyclic bending motions of finger joint, elbow joint, knee joint and ankle joint were detected by the flexible sensor. When such sensor was attached

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on the joint of finger and bending motion was reversibly conducted between 0o to 90o, the resistance of sensor varied according. As seen in Figure 4a, bending of joint

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induced an elongation effect of sensor and the resistance increased (from state A to

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state B). Unbending of joint released the sensor and induced a decrease of resistance (from state B to state A). With periodical bending-unbending, the resistance signal also changed simultaneously. In similar ways, bending of elbow joint and knee joint were also detected, as respectively shown in Figure 4b and Figure 4c. In addition, the

flexible sensor fixed on the ankle joint has the ability to detect the ankle bending during walking and running (Figure 4d). Interestingly, subtle physiological activities were also detected efficiently by the flexible strain sensors. Rapid eye movement (REM) stage during sleeping is a known phenomenon and closely related to deep sleep. Herein, oculogyration was detected to mimic the monitoring of REM stage. Figure 4e shows the resistance variation of a

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sensor attached on the eyelid of a volunteer when oculogyration was conducted. In the

initial state ○ 1 , the sensor was right above the eyeball and slightly tensile deformation was induced. The resistance of sensor was relatively high. When eyeball moved to the

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left (from state ○ 1 to state ○ 2 ), the sensor relaxed and a resistance decrease was

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recorded. Returning of eyeball to normal state (from state ○ 2 to state ○ 3 ) induced an increase of resistance. From state ○ 3 to state ○ 4 , eyeball gradually moved to the right.

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A relaxation of sensor was induced and the resistance decreased. Such eye movement

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could be periodically detected for many cycles. This provides an opportunity to record

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the REM stage during sleeping utilizing such strain sensors.

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Figure 4. Whole-body monitoring by flexible strain sensors based on nanocomposite

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hydrogels. Resistance response of flexible strain sensor during detection of (a) cyclic

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finger joint bending, (b) cyclic elbow joint bending, (c) cyclic knee joint bending, (d) ankle joint bending induced walking and running, (e) eyeball movement, (f) muscle motion of ala nasi during inhaling via nose, (g) upper lip muscle motion during exhaling via mouth, (h) muscle motions induced by neck twisting and nodding. Moreover, muscle motions induced by deep inhaling via nose and exhaling via mouth were also detected. Inhaling via nose induced an expansion of ala nasi and the

resistance of attached sensor increased (Figure 4f, from state A to state B). When exhaling via mouth was conducted, muscle motion of upper lip was induced and resistance of sensor attached on the upper lip increased (Figure 4g, from state A to state B). On the basis of a similar mechanism, muscle motion induced by neck twisting and nodding was also detected by the strain sensor attached on the back of volunteer’s neck (Figure 4h). From the above examples, it is reasonable to infer that

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other kinds of human motions should also be detected in a similar way by utilizing such strain sensors. Moreover, human motions could also be detected utilizing

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self-repaired strain sensors in a similar way (Figure S8 and Figure S9).

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4. Conclusion

In conclusion, flexible strain sensors are fabricated with nanocomposite hydrogels.

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poly(AA-co-SMA)/c-CNF/Fe3+

Introduction

of

c-CNF

efficiently

improves the strength and resilience of nanocomposite hydrogels. Flexible strain

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sensors assembled from nanocomposite hydrogels possess integrated high

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performances, interesting self-repairing ability and can be applied in monitoring whole-body motions and physiological activities. Expectedly, not only self-repairing

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flexible strain sensors, but also other kinds of flexible electronic devices, such as supercapacitors, pressure sensors and display devices, may also be fabricated utilizing the nanocomposite hydrogels.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Credit Author Statement Hongwei Zhou: Supervision, eriting-original draft, conceptualization

Ying Yuan: Data curation Gai Zhang: Writing-review & editing Weifeng Zhao: Writing-review & editing

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Xilang Jin: Methodology, writing-review & editing

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Zhaoyang Jin: Investigation, data curation

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Aijie Ma: Writing-review & editing

Hanbin Liu: Conceptualization, methodology, writing-review & editing

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Weixing Chen: Writing-review & editing

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Acknowledgments

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This work was financially supported by the National Natural Science Foundation of China (Nos. 51603164, 21807085 and 21805178), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2019JM-124), the Young Talent Fund of University Association for Science and Technology in Shaanxi, China (No. 20170706), Special Natural Science Project of Shaanxi Provincial Education

Department (No. 17JK0380) and Technology Foundation for Selected Oversea

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Chinese Scholar in Shaanxi Province (No. 2017016).

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