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Highly stretchable, self-healing, and strain-sensitive based on double-crosslinked nanocomposite hydrogel
Journal Pre-proof Highly stretchable, self-healing, and strain-sensitive based on double-crosslinked nanocomposite hydrogel Jie Mao, Chunxia Zhao, Yuntao Li, Dong Xiang, Zhixuan Wang PII:
S2452-2139(19)30141-X
DOI:
https://doi.org/10.1016/j.coco.2019.10.007
Reference:
COCO 269
To appear in:
Composites Communications
Received Date: 28 August 2019 Revised Date:
30 October 2019
Accepted Date: 31 October 2019
Please cite this article as: J. Mao, C. Zhao, Y. Li, D. Xiang, Z. Wang, Highly stretchable, selfhealing, and strain-sensitive based on double-crosslinked nanocomposite hydrogel, Composites Communications (2019), doi: https://doi.org/10.1016/j.coco.2019.10.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Highly Stretchable, Self-Healing, and Strain-Sensitive Based on Double-Crosslinked Nanocomposite Hydrogel Jie Maoa, Chunxia Zhaoa*, Yuntao Lia,b**, Dong Xianga, Zhixuan Wanga a b
School of Materials and Engineering, Southwest Petroleum University, Chengdu 610500, China
State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu
610050, China
Abstract: In recent years, flexible hydrogel strain sensors have shown potential applications in artificial intelligence, such as medical monitoring, human motion detection, and intelligent robotics. It is a challenge for flexible strain sensors with stretchable and efficient healing to ensure stable sensing under repeated deformations or damage. In this study, a highly stretchable, self-healing, and strain-sensitive hydrogel was prepared from acrylic acid (AA), graphene oxide (GO), iron ions (Fe3+), and ammonium persulfate (APS) via one-step in-situ polymerization without a chemical crosslinker. The polyacrylic acid (PAA)-GO hydrogel showed dual crosslinking effect: (i) ionic coordination bonding between Fe3+ ions and the carboxylic functional groups of PAA and GO and (ii) hydrogen bonding between the polar functional groups of PAA and the oxygen-containing functional groups of PAA and GO. Because of dynamic double-crosslinked networks, the hydrogel exhibited superior stretchability (1185.53% elongation at break) and self-healing property (88.64% healing efficiency) as well as electrical self-healing performance. Moreover, strain-sensitive conductive hydrogels can be used as flexible sensors to monitor body motions (e.g., bending of fingers, wrists, and elbows) by detecting change in electrical signal and can be used as wearable sensors and for personal health monitoring. Keywords: strain sensor, self-healing, strain-sensitive, double crosslinking 1. Introduction
Wearable and flexible strain sensors transform dynamic deformation into electrical signals [1–3]; they have been used in many fields such as human motion detection, speech recognition, and healthcare [4–7]. Strain sensors must have excellent mechanical properties so that they can be stretched and recovered quickly with repeated deformation [8,9]. Self-healing ability can help sensors to rebuild their structure after damage and maintain normal applications. However, self-healing hydrogels usually exhibit inherent contradictions between the mechanical strength of a tough structure and the self-healing ability of a dynamically crosslinked structure, thus limiting its applications [10,11]. More recently, nanofillers such as carbon nanotubes, silica nanoparticles and cellulose nanocomposite have been reported to improve the mechanical properties of gels [12–17]. Recently, graphene oxide (GO) has been evaluated as a reinforcing agent by adding to a gel to obtain a nanocomposite hydrogel with both self-healing and mechanical properties [18–20]. However, studies only focused on the self-healing ability of hydrogel, while ignoring the electrical conductivity, it can be applied to flexible strain sensors. In this study, a highly stretchable, self-healing, and strain-sensitive sensor was prepared from a hydrogel with a dual network structure, consisting of polyacrylic acid (PAA), GO, and Fe3+ ions. The composite hydrogel was prepared via one-pot in-situ polymerization without a chemical crosslinker to form metal ion coordination bonds by chelation between Fe3+ ions and the carboxylic groups of PAA and GO, and hydrogen-bonding interactions between the oxygen-containing groups of GO and the polar functional groups of PAA. Hence, owing to dynamic crosslinked networks, the PAA-GO hydrogel exhibited enhanced mechanical strength as well as excellent self-healing property. The conductive gels also exhibited significant strain sensitivity and electrical self-healing property; thus, the gel can be used in wearable strain sensors to monitor human joint motion.
2. Materials and methods 2.1 Materials Acrylic acid (AA), ammonium persulfate (APS), and ferric chloride hexahydrate (FeCl3·6H2O) were purchased from Chengdu Kelong Chemical. GO was prepared in our laboratory following the previous study [21]. 2.2 Preparation of hydrogels A suspension of GO was prepared following previous studies using a modified Hummers’ method. The GO suspension was sonicated for 10 min before usage. PAA-GO hydrogel was prepared using a one-step method. AA (2 mL) and FeCl3·6H2O (0.056 g) were initially dissolved in the GO suspension, stirred for 1 h and then 1.5 mg APS was added to the mixture, and the mixture was stirred in an ice-water bath for 10 min. Then, the reaction mixture was transferred to cylindrical glass bottles with an internal diameter of 8 mm and sealed for storage. The mixture was polymerized at 30 for 8 h. 2.3 Characterization 2.3.1 Morphological observation and spectroscopic analyses The morphology of GO was characterized using a Nanoscope IIIa atomic force microscope (AFM, Asylum research). The morphology of crosslinked hydrogel was observed by scanning electron microscopy (SEM, ZEISS EV0 MA15). FT-IR spectra were recorded using a Nicolet 6700 spectrophotometer from 4500 to 500 cm−1. 2.3.2 Mechanical test Mechanical tests were performed using a microcomputer-controlled electronic universal testing machine (CMT4104) with a 20 N load cell. (The hydrogel sample size is 8 mm inside diameter × 25
mm in length) 2.3.3 Self-healing experiments The cylindrical hydrogel samples were cut into halves and then freshly cut surfaces were placed together. After self-healing, hydrogels were subjected to tensile tests. 2.3.4 Electrical test The strain sensors were prepared using conductive hydrogels equipped with electrodes at each end. The real-time electrical signals of strain sensors were recorded using Tektronix PWS4323 and Keithley 6485 instruments. 3. Results and discussion PAA-GO hydrogel was prepared by in-situ free-radical polymerization in Fig. 1a. Fig. 1b shows a schematic diagram of the three-dimensional (3D) network structure of PAA-GO hydrogel. GO nanosheets and Fe3+ ions acted as cross-linkers, forming double-crosslinked networks. Fe3+ ions chelated with the carboxylic groups of PAA chains and GO to generate a coordination network. A hydrogen-bonding network was formed by the oxygen-containing functional groups of GO and the polar functional groups of PAA. Fig. 1c shows the FT-IR spectra of GO, PAA, and PAA-GO. The peaks of PAA-GO hydrogel were observed at 3410 cm−1, 1630 cm−1, and 596 cm−1, corresponding to O–H stretching vibration, C=C stretching vibration, and C–O stretching vibration, including the characteristic peaks of GO and PAA. The SEM images show the morphology of nanocomposite hydrogel at different magnifications. Fig. 1d shows the sheet structure of GO after intercalation stripping. The size of GO sheet is a few microns, and the sheet thickness is ∼1 nm, consistent with previous study [22]. As shown in Fig. 1e, the PAA/GO hydrogel has a porous structure after freeze-drying because the iron ions form synergistic crosslinking with GO, and the PAA/GO
hydrogel forms a 3D network structure.
(a)
Add monomer
Add initiator
and crosslinker
Polymerization 30 ℃ for 8 h
GO suspension
(b)
PAA-GO hydrogel
(c)
(d)
(e)
Fig. 1. Schematic diagram of (a) the preparation process of hydrogels and (b) network structure of PAA-GO hydrogel. (c) FTIR spectra of PAA, GO, and PAA-GO. (d) AFM image of GO. (e) SEM image of PAA-GO.
The PAA-GO nanocomposite hydrogel exhibited outstanding mechanical properties: The
hydrogel can be stretched, compressed, and bent as shown in Fig. 2a, and the shape of hydrogel can be recovered quickly to the original shape after the hydrogel was compressed as shown in Fig. 2b. A series of tensile tests were designed to specifically evaluate the mechanical properties of hydrogels (Fig. 2c). Fig. 2d shows the effect of GO content on mechanical properties. According to the stress– strain curves, as the GO content increased, the elongation at break decreased (from 1185.53% to 612.13%), and the tensile strength increased (from 303.83 KPa to 391.21 KPa). Thus, the crosslinking density of gel increased, requiring more force to deform the gel under stretching and showing a decrease in elasticity. As shown in Fig. 2e, to demonstrate the self-healing properties of gel, the cylindrical samples were cut in half with a blade, and then the two fresh-cut surfaces of the halves were immediately combined together. After healing, the hydrogel still exhibited a high elongation during stretching, indicating that the gel has efficient self-healing ability. Therefore, the healed hydrogels were subjected to tensile tests with different healing times to further evaluate the self-healing ability of gels. Fig.2f shows as increasing healing time, the tendency of gel healing efficiency, which is defined as the ratio of elongation at break between the healed gel and the original gel, including two stages. In the first stage, the healing efficiency rapidly increased with increasing healing time. Then, the healing efficiency increased at a slower rate until it gradually reached equilibrium in the second stage. Finally, the healing efficiency reached 88.64%, indicating that the PAA-GO gel has excellent self-healing ability based on reversible dynamic crosslinking reconstruction. (a)
(b)
(c)
(e)
(d)
(f)
Fig. 2. Photographs of PAA-GO (a) that can be stretched, compressed, and bent, (b) that can be recovered quickly after pressing deformation. (c) Tensile test images and (d) stress–strain curves of PAA-GO. (e) The cut gel can be stretched after self-healing, and (f) healing efficiency of hydrogel.
As shown in Fig. 3a, an electric circuit connecting a light-emitting diode (LED) bulb and the hydrogel sensor was designed to test strain sensitivity. Clearly, the LED darkened when the sensor was stretched instantly. In the circuit, the LED bulb was extinguished immediately after the gel was cut in half. When the two halves were brought close together, the circuit was restored, and the LED
was illuminated again (Fig. 3b), indicating that the hydrogel sensor has excellent electrical self-healing property to maintain a circuit path and stable sensing after being destroyed. The strain sensitivity (S) of the hydrogel sensor can be further evaluated by the ratio of resistance change (Δ R/R0) to the strain (ε), S = (ΔR/R0)/ε. Fig. 3c shows the relative resistance variation in the strain sensor under different deformation conditions of 5–50%. And the inset image exhibited an approximately linear response with strain sensitivity of 0.46. This is because of tension: An effective electrical pathway was broken, and the contact conditions for electron conduction in the hydrogel changed, such as a break of contacts, contact area, and spacing variations. Fig. 3d shows the relative resistance change in gel sensor tested under different GO contents at 10% strain. An increase in the content of GO sheet led to an increase in the relative resistance change at the same strain. This is because upon stretching, the degree of separation of stacked GO sheets became different. (a)
(b)
(c)
(d)
Fig. 3. Response of LED bulbs when gels were (a) stretched and (b) brought close together after cut. Relative resistance changes in gel sensor under (c) different deformations and (d) different GO contents
To demonstrate the application of hydrogel strain sensor as a wearable device, it was attached to the human body to detect different joint bending motions and obtain real-time resistance changes. As shown in Fig. 4a, the sensor was attached to the forefinger with different angles of 0°, 60°, and 90°. The results show that the relative resistance change increased with the increase in finger bending angle. When the forefinger was bent at a certain angle, the resistance of sensor remained stable because of excellent electrical stability. Fig. 4b shows that the sensor attached to the wrist was lengthened when the joint was bent continuously or intermittently. The resistance increased when the wrist was bent; the resistance recovered to the original state immediately when the wrist was straightened. Similarly, the gel sensor was attached to elbow joint to monitor the motion of the arm as shown in Fig. 4c. A maximum resistance was obtained when the arm was bent to 90°, and a minimum resistance was obtained when the arm was straightened and relaxed. The relative resistance change of sensor remained stable after 500 cycles of 30% strain repeatedly as shown in Fig. 4d. The results indicate that the response behavior of strain sensor is repeatable, stable, and rapid, and the
sensor can be used for behavior detection and personal health monitoring as a wearable sensor. (a)
(c)
(b)
(d)
Fig. 4. Relative resistance changes in gel sensor with (a) forefinger, (b) wrist, and (c) elbow joint bending. (d) Stability of gel sensor by repeatedly apply a strain of 30% for 500 cycles. 4. Conclusions A conductive nanocomposite hydrogel with a dual network structure was prepared by one-step in-situ polymerization. The PAA-GO hydrogel thus prepared exhibited excellent mechanical properties, high stretchability (strain up to 1185.53 %), and high tensile strength of 391.21 kPa. This can be attributed to the effect of dual crosslinking including ionic coordination crosslinking and hydrogen-bonding interactions. The nanocomposite hydrogels also exhibited a high healing efficiency (up to 88.64%) owing to reversible dynamic crosslinking. Moreover, the hydrogels
exhibited strain sensitivity and repeatability that can be used in flexible sensors to detect electrical signals under dynamic deformation. The gel sensor exhibited an approximately linear response with strain sensitivity of 0.46 under different deformations of 5–50%. Moreover, the flexible sensor can be applied to monitor body motions involving the bending of finger, wrist, and elbow. Thus, the PAA-GO nanocomposite hydrogel can be used as a flexible strain sensor with potential applications in artificial intelligence such as wearable sensing, healthcare monitoring, and intelligent robotics.
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Highlights 1. The hydrogel shows dual crosslinking effect including metal ion coordination bonds and hydrogen-bonding interactions. 2. The hydrogel exhibits high stretchability as well as excellent self-healing property. 3. Conductive hydrogel has strain-sensitive that can be used in wearable strain sensors to monitor human joint motion.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
S2452-2139(19)30141-X
DOI:
https://doi.org/10.1016/j.coco.2019.10.007
Reference:
COCO 269
To appear in:
Composites Communications
Received Date: 28 August 2019 Revised Date:
30 October 2019
Accepted Date: 31 October 2019
Please cite this article as: J. Mao, C. Zhao, Y. Li, D. Xiang, Z. Wang, Highly stretchable, selfhealing, and strain-sensitive based on double-crosslinked nanocomposite hydrogel, Composites Communications (2019), doi: https://doi.org/10.1016/j.coco.2019.10.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Highly Stretchable, Self-Healing, and Strain-Sensitive Based on Double-Crosslinked Nanocomposite Hydrogel Jie Maoa, Chunxia Zhaoa*, Yuntao Lia,b**, Dong Xianga, Zhixuan Wanga a b
School of Materials and Engineering, Southwest Petroleum University, Chengdu 610500, China
State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu
610050, China
Abstract: In recent years, flexible hydrogel strain sensors have shown potential applications in artificial intelligence, such as medical monitoring, human motion detection, and intelligent robotics. It is a challenge for flexible strain sensors with stretchable and efficient healing to ensure stable sensing under repeated deformations or damage. In this study, a highly stretchable, self-healing, and strain-sensitive hydrogel was prepared from acrylic acid (AA), graphene oxide (GO), iron ions (Fe3+), and ammonium persulfate (APS) via one-step in-situ polymerization without a chemical crosslinker. The polyacrylic acid (PAA)-GO hydrogel showed dual crosslinking effect: (i) ionic coordination bonding between Fe3+ ions and the carboxylic functional groups of PAA and GO and (ii) hydrogen bonding between the polar functional groups of PAA and the oxygen-containing functional groups of PAA and GO. Because of dynamic double-crosslinked networks, the hydrogel exhibited superior stretchability (1185.53% elongation at break) and self-healing property (88.64% healing efficiency) as well as electrical self-healing performance. Moreover, strain-sensitive conductive hydrogels can be used as flexible sensors to monitor body motions (e.g., bending of fingers, wrists, and elbows) by detecting change in electrical signal and can be used as wearable sensors and for personal health monitoring. Keywords: strain sensor, self-healing, strain-sensitive, double crosslinking 1. Introduction
Wearable and flexible strain sensors transform dynamic deformation into electrical signals [1–3]; they have been used in many fields such as human motion detection, speech recognition, and healthcare [4–7]. Strain sensors must have excellent mechanical properties so that they can be stretched and recovered quickly with repeated deformation [8,9]. Self-healing ability can help sensors to rebuild their structure after damage and maintain normal applications. However, self-healing hydrogels usually exhibit inherent contradictions between the mechanical strength of a tough structure and the self-healing ability of a dynamically crosslinked structure, thus limiting its applications [10,11]. More recently, nanofillers such as carbon nanotubes, silica nanoparticles and cellulose nanocomposite have been reported to improve the mechanical properties of gels [12–17]. Recently, graphene oxide (GO) has been evaluated as a reinforcing agent by adding to a gel to obtain a nanocomposite hydrogel with both self-healing and mechanical properties [18–20]. However, studies only focused on the self-healing ability of hydrogel, while ignoring the electrical conductivity, it can be applied to flexible strain sensors. In this study, a highly stretchable, self-healing, and strain-sensitive sensor was prepared from a hydrogel with a dual network structure, consisting of polyacrylic acid (PAA), GO, and Fe3+ ions. The composite hydrogel was prepared via one-pot in-situ polymerization without a chemical crosslinker to form metal ion coordination bonds by chelation between Fe3+ ions and the carboxylic groups of PAA and GO, and hydrogen-bonding interactions between the oxygen-containing groups of GO and the polar functional groups of PAA. Hence, owing to dynamic crosslinked networks, the PAA-GO hydrogel exhibited enhanced mechanical strength as well as excellent self-healing property. The conductive gels also exhibited significant strain sensitivity and electrical self-healing property; thus, the gel can be used in wearable strain sensors to monitor human joint motion.
2. Materials and methods 2.1 Materials Acrylic acid (AA), ammonium persulfate (APS), and ferric chloride hexahydrate (FeCl3·6H2O) were purchased from Chengdu Kelong Chemical. GO was prepared in our laboratory following the previous study [21]. 2.2 Preparation of hydrogels A suspension of GO was prepared following previous studies using a modified Hummers’ method. The GO suspension was sonicated for 10 min before usage. PAA-GO hydrogel was prepared using a one-step method. AA (2 mL) and FeCl3·6H2O (0.056 g) were initially dissolved in the GO suspension, stirred for 1 h and then 1.5 mg APS was added to the mixture, and the mixture was stirred in an ice-water bath for 10 min. Then, the reaction mixture was transferred to cylindrical glass bottles with an internal diameter of 8 mm and sealed for storage. The mixture was polymerized at 30 for 8 h. 2.3 Characterization 2.3.1 Morphological observation and spectroscopic analyses The morphology of GO was characterized using a Nanoscope IIIa atomic force microscope (AFM, Asylum research). The morphology of crosslinked hydrogel was observed by scanning electron microscopy (SEM, ZEISS EV0 MA15). FT-IR spectra were recorded using a Nicolet 6700 spectrophotometer from 4500 to 500 cm−1. 2.3.2 Mechanical test Mechanical tests were performed using a microcomputer-controlled electronic universal testing machine (CMT4104) with a 20 N load cell. (The hydrogel sample size is 8 mm inside diameter × 25
mm in length) 2.3.3 Self-healing experiments The cylindrical hydrogel samples were cut into halves and then freshly cut surfaces were placed together. After self-healing, hydrogels were subjected to tensile tests. 2.3.4 Electrical test The strain sensors were prepared using conductive hydrogels equipped with electrodes at each end. The real-time electrical signals of strain sensors were recorded using Tektronix PWS4323 and Keithley 6485 instruments. 3. Results and discussion PAA-GO hydrogel was prepared by in-situ free-radical polymerization in Fig. 1a. Fig. 1b shows a schematic diagram of the three-dimensional (3D) network structure of PAA-GO hydrogel. GO nanosheets and Fe3+ ions acted as cross-linkers, forming double-crosslinked networks. Fe3+ ions chelated with the carboxylic groups of PAA chains and GO to generate a coordination network. A hydrogen-bonding network was formed by the oxygen-containing functional groups of GO and the polar functional groups of PAA. Fig. 1c shows the FT-IR spectra of GO, PAA, and PAA-GO. The peaks of PAA-GO hydrogel were observed at 3410 cm−1, 1630 cm−1, and 596 cm−1, corresponding to O–H stretching vibration, C=C stretching vibration, and C–O stretching vibration, including the characteristic peaks of GO and PAA. The SEM images show the morphology of nanocomposite hydrogel at different magnifications. Fig. 1d shows the sheet structure of GO after intercalation stripping. The size of GO sheet is a few microns, and the sheet thickness is ∼1 nm, consistent with previous study [22]. As shown in Fig. 1e, the PAA/GO hydrogel has a porous structure after freeze-drying because the iron ions form synergistic crosslinking with GO, and the PAA/GO
hydrogel forms a 3D network structure.
(a)
Add monomer
Add initiator
and crosslinker
Polymerization 30 ℃ for 8 h
GO suspension
(b)
PAA-GO hydrogel
(c)
(d)
(e)
Fig. 1. Schematic diagram of (a) the preparation process of hydrogels and (b) network structure of PAA-GO hydrogel. (c) FTIR spectra of PAA, GO, and PAA-GO. (d) AFM image of GO. (e) SEM image of PAA-GO.
The PAA-GO nanocomposite hydrogel exhibited outstanding mechanical properties: The
hydrogel can be stretched, compressed, and bent as shown in Fig. 2a, and the shape of hydrogel can be recovered quickly to the original shape after the hydrogel was compressed as shown in Fig. 2b. A series of tensile tests were designed to specifically evaluate the mechanical properties of hydrogels (Fig. 2c). Fig. 2d shows the effect of GO content on mechanical properties. According to the stress– strain curves, as the GO content increased, the elongation at break decreased (from 1185.53% to 612.13%), and the tensile strength increased (from 303.83 KPa to 391.21 KPa). Thus, the crosslinking density of gel increased, requiring more force to deform the gel under stretching and showing a decrease in elasticity. As shown in Fig. 2e, to demonstrate the self-healing properties of gel, the cylindrical samples were cut in half with a blade, and then the two fresh-cut surfaces of the halves were immediately combined together. After healing, the hydrogel still exhibited a high elongation during stretching, indicating that the gel has efficient self-healing ability. Therefore, the healed hydrogels were subjected to tensile tests with different healing times to further evaluate the self-healing ability of gels. Fig.2f shows as increasing healing time, the tendency of gel healing efficiency, which is defined as the ratio of elongation at break between the healed gel and the original gel, including two stages. In the first stage, the healing efficiency rapidly increased with increasing healing time. Then, the healing efficiency increased at a slower rate until it gradually reached equilibrium in the second stage. Finally, the healing efficiency reached 88.64%, indicating that the PAA-GO gel has excellent self-healing ability based on reversible dynamic crosslinking reconstruction. (a)
(b)
(c)
(e)
(d)
(f)
Fig. 2. Photographs of PAA-GO (a) that can be stretched, compressed, and bent, (b) that can be recovered quickly after pressing deformation. (c) Tensile test images and (d) stress–strain curves of PAA-GO. (e) The cut gel can be stretched after self-healing, and (f) healing efficiency of hydrogel.
As shown in Fig. 3a, an electric circuit connecting a light-emitting diode (LED) bulb and the hydrogel sensor was designed to test strain sensitivity. Clearly, the LED darkened when the sensor was stretched instantly. In the circuit, the LED bulb was extinguished immediately after the gel was cut in half. When the two halves were brought close together, the circuit was restored, and the LED
was illuminated again (Fig. 3b), indicating that the hydrogel sensor has excellent electrical self-healing property to maintain a circuit path and stable sensing after being destroyed. The strain sensitivity (S) of the hydrogel sensor can be further evaluated by the ratio of resistance change (Δ R/R0) to the strain (ε), S = (ΔR/R0)/ε. Fig. 3c shows the relative resistance variation in the strain sensor under different deformation conditions of 5–50%. And the inset image exhibited an approximately linear response with strain sensitivity of 0.46. This is because of tension: An effective electrical pathway was broken, and the contact conditions for electron conduction in the hydrogel changed, such as a break of contacts, contact area, and spacing variations. Fig. 3d shows the relative resistance change in gel sensor tested under different GO contents at 10% strain. An increase in the content of GO sheet led to an increase in the relative resistance change at the same strain. This is because upon stretching, the degree of separation of stacked GO sheets became different. (a)
(b)
(c)
(d)
Fig. 3. Response of LED bulbs when gels were (a) stretched and (b) brought close together after cut. Relative resistance changes in gel sensor under (c) different deformations and (d) different GO contents
To demonstrate the application of hydrogel strain sensor as a wearable device, it was attached to the human body to detect different joint bending motions and obtain real-time resistance changes. As shown in Fig. 4a, the sensor was attached to the forefinger with different angles of 0°, 60°, and 90°. The results show that the relative resistance change increased with the increase in finger bending angle. When the forefinger was bent at a certain angle, the resistance of sensor remained stable because of excellent electrical stability. Fig. 4b shows that the sensor attached to the wrist was lengthened when the joint was bent continuously or intermittently. The resistance increased when the wrist was bent; the resistance recovered to the original state immediately when the wrist was straightened. Similarly, the gel sensor was attached to elbow joint to monitor the motion of the arm as shown in Fig. 4c. A maximum resistance was obtained when the arm was bent to 90°, and a minimum resistance was obtained when the arm was straightened and relaxed. The relative resistance change of sensor remained stable after 500 cycles of 30% strain repeatedly as shown in Fig. 4d. The results indicate that the response behavior of strain sensor is repeatable, stable, and rapid, and the
sensor can be used for behavior detection and personal health monitoring as a wearable sensor. (a)
(c)
(b)
(d)
Fig. 4. Relative resistance changes in gel sensor with (a) forefinger, (b) wrist, and (c) elbow joint bending. (d) Stability of gel sensor by repeatedly apply a strain of 30% for 500 cycles. 4. Conclusions A conductive nanocomposite hydrogel with a dual network structure was prepared by one-step in-situ polymerization. The PAA-GO hydrogel thus prepared exhibited excellent mechanical properties, high stretchability (strain up to 1185.53 %), and high tensile strength of 391.21 kPa. This can be attributed to the effect of dual crosslinking including ionic coordination crosslinking and hydrogen-bonding interactions. The nanocomposite hydrogels also exhibited a high healing efficiency (up to 88.64%) owing to reversible dynamic crosslinking. Moreover, the hydrogels
exhibited strain sensitivity and repeatability that can be used in flexible sensors to detect electrical signals under dynamic deformation. The gel sensor exhibited an approximately linear response with strain sensitivity of 0.46 under different deformations of 5–50%. Moreover, the flexible sensor can be applied to monitor body motions involving the bending of finger, wrist, and elbow. Thus, the PAA-GO nanocomposite hydrogel can be used as a flexible strain sensor with potential applications in artificial intelligence such as wearable sensing, healthcare monitoring, and intelligent robotics.
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Highlights 1. The hydrogel shows dual crosslinking effect including metal ion coordination bonds and hydrogen-bonding interactions. 2. The hydrogel exhibits high stretchability as well as excellent self-healing property. 3. Conductive hydrogel has strain-sensitive that can be used in wearable strain sensors to monitor human joint motion.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.