Adhesive conductive hydrogels with wrinkled Janus surface and ultra-high sensitivity used as strain sensors

doi:10.21203/rs.3.rs-1607290/v1

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Adhesive conductive hydrogels with wrinkled Janus surface and ultra-high sensitivity used as strain sensors

https://doi.org/10.21203/rs.3.rs-1607290/v1

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Conductive hydrogels have attracted enormous attention in wearable electronic devices due to considerable flexibility and similarity with human skin. However, the majority of hydrogels were fabricated by one-pot process and were completely homogeneous, which limited the response to electrical signals. In this paper, we propose a simple strategy to obtain asymmetrical polyacrylic acid/chitosan@silver nanowires (PAA/CS@AgNWs) hydrogels, namely, depositing AgNWs layer by dropping AgNWs dispersions in ethanol on PAA/CS hydrogel prepared via one-pot process. Ethanol diffused into the hydrogel and endowed the upper surface with wrinkle structure. Besides, the inevitable conductivity difference between hydrogel itself and AgNWs further enhanced the sensitivity, leading to the ultra-high gauge factor (GF = 191.2, 7413, 18720). The adhesion of the hydrogel not only improved the adaptability to skin, but also the fastness of AgNWs. Reliable application performance (flexibility, self-healing, dry resistance and adhesion) and excellent electrical signal response (ultra-high sensitivity, identification, repetition and stability) indicated the potential applications in motion detection, gesture recognition and health monitoring.

Conductive hydrogels

wrinkled structure

adhesion

high sensitivity

strain sensors

Recently, in order to meet the demand of personalized monitoring of physiological signals (heart-beating rate (Ye et al, 2020), movement (Wu et al, 2019) and vocalization (Pan et al, 2019)), abundant wearable devices have emerged in the market, such as smart glasses, smart watches and medical smart wristbands. However, the problems of uncomfortable wearing and poor adaptability to complex surfaces have not been solved yet. Nowadays, flexible wearable electronic devices (flexible sensors (Bao et al, 2021; Chen et al, 2020), implantable medical devices (Li et al, 2019; Zheng et al, 2019), bionic skin (Ma et al, 2019; Zheng et al, 2021), etc.) have attracted extensive attention due to their wearability, real-time feedback, remote control and other advantages (Huang et al, 2020). Acting as the indispensable unit of flexible wearable devices, flexible sensors, the component that can accept external stimuli (strain (Mao et al, 2020; Zha et al, 2020), pressure (Liu et al, 2020; Wang et al, 2021), temperature (Chen et al, 2021; Feng et al, 2021), humidity (Pan et al, 2020; Wu et al, 2019b) and gas (Wu et al, 2020; Wu et al, 2021)) and convert them into detectable electrical signals, overcome the rigidity of traditional metal-based or semiconductor-based sensors and exhibit higher flexibility, ductility and adaptability. Defined as flexible materials with three-dimensional porous structure and high-water content, hydrogels are similar to human skin in many aspects, hence they are widely application in the fields of drug carriers (Mandal et al, 2017) and tissue engineering (Henn et al, 2022). Additionally, conductive hydrogels are supposed to be desirable choice for sensors because of their excellent stretchability and biocompatibility, and yet the sensitivity of most hydrogel-based sensors is lower than that of others (fiber-based (Pan et al, 2020), elastomer-based (Gong et al, 2017; Zhang et al, 2020), mixed (Han et al, 2019)). Therefore, exploiting highly sensitive conductive hydrogels without sacrificing physical properties is of great significance to the promotion of flexible wearable devices.

Since all external stimuli are ultimately reflected in the form of electrical signals, the selection and distribution of conductive materials is the top priority of wearable sensors. Based on different conductive principles, the selectable conductive materials include ionic conductors (electrolyte (Chen et al, 2021; Liu et al, 2021; Lu et al, 2021), ionic liquid (Izawa et al, 2009; Lee et al, 2008)) and electronic conductors (intrinsic conductive polymer (Jiao et al, 2021; Zhou et al, 2021), carbon-based materials (Liao et al, 2019; Sun et al, 2021) and metal-based materials (Wang et al, 2019; Zhang et al, 2022)). Among them, AgNWs have attracted extensive attention because of their good conductivity and bactericidal properties (Pan et al, 2021). At present, conductive hydrogels are usually prepared by a simple one-pot process to obtain the homogeneous dispersion in the hydrogel (Chen et al, 2018; Hu et al, 2022; You et al, 2021). But it is not economical since a large number of conductive fillers are needed, and the cost problem is intensified when the expensive conductors like AgNWs and MXene are used. Moreover, electronic conductors tend to aggregate with the movement of water molecules, resulting in the deteriorated conductivity during long-term storage. The effect of conductive fillers on the mechanical properties and biocompatibility of hydrogels is also a latent question worth considering as well. The Janus structure (Zhao et al, 2017) can be obtained by depositing conductive materials on the upper surface of hydrogels, which not only can decrease the quantity of fillers, but also can enhance the sensitivity through the conductivity difference between the surface layer and the hydrogel itself. In view of strain sensors, the conductivity network varies with the overall stretch of the hydrogel, and the surface structure hardly affects the sensitivity for homogeneous hydrogels. For instance, Wang et al. (Wang et al, 2019) prepared a poly (3, 4-ethylenedioxythiophene): sulfonated lignin/PAA hydrogel and then succeed in constructing self-wrinkled structure by solvent replacement, but the improvement of GF was not obvious enough. However, the change of the surface structure for the stretched hydrogel with Janus structure will directly affect the conductive network, thereby improving the sensitivity of strain sensing.

As an all-in-one conductive hydrogel, the adhesion of conductive material and hydrogel body is important to the service life of the sensor. The conventional method is encapsulating to prevent the surface conductors from falling off, but the commonly used packaging materials such as PDMS (Gong et al, 2017) will greatly influence the sensitivity and extensibility of the hydrogels. To address the issue of good adhesion, the combination of fillers and polymer network by coordination bonds, hydrogen bonds or others interactions needs designing. Inspired by mussels, researchers found that substances with catechol structure (polydopamine (Rahim et al, 2016; Teixeira et al, 2012; Wu et al, 2020), tannic acid (He et al, 2021; Sun et al, 2021)) combine matrix via π-π interactions, metal coordination, covalent crosslinks and hydrogen bonds (Xie et al, 2020). The use of adhesive matrix materials is an easier method to make adhesive hydrogels, though the adhesion of these materials is relatively weak (Xia et al, 2019). Fu et al (Fu et al, 2021) and Lu et al. (Lu et al, 2020) prepared PAA/PEDOT hydrogels and PAA/CNS hydrogels respectively, and proved their adhesion. In addition, good adhesion is also conducive to tight fit between hydrogels and skins, so that their deformations are precisely matched and the accuracy of sensors reflecting physiological signal is improved (Pan et al, 2021; Sun et al, 2021).

Herein, we demonstrated a simple two-step method to fabricate hydrogel-based strain sensors with self-healing, water retention, and ultra-high sensitivity as flexible wearable devices. Briefly speaking, the hydrogel body is comprised of PAA and CS via a one-pot process, and the wrinkled surface of AgNWs layer is obtained by simple dripping-drying (Fig. 1). The Janus structure and the conductivity difference between AgNWs and PAA/CS hydrogels endows the outstanding sensing sensitivity (GF = 191.2, 7413, 18720). In addition, the good self-adhesive of the PAA/CS hydrogel ensures the adhesive strength of the AgNWs and the precise matching of the sensor to external strain stimuli. Good physical properties and excellent sensing performance show the potential application in electronic skins, soft robots and interactive devices.

2.1 Materials and chemicals

Acrylic acid (AA), ammonium persulfate (APS), acetic acid, glycerol, ethanol and FeCl₃·6H₂O were purchased from Sinopharm Chemical Reagent Co. Ltd. Chitosan (CS) was obtained from Shanghai San yuan Co. Ltd. AgNWs dispersion in ethanol (10 mg/ml) was commercially available from Tanfeng Tech. (Suzhou). Deionized (DI) water was used throughout the process unless otherwise specified.

2.2 Fabrication of PAA/CS@AgNWs hydrogels

PAA/CS hydrogels were prepared via a simple one-pot process. Specifically, CS (160 mg) and AA (16 ml) were added to a binary solvent consisting of DI water and glycerol. A trace of acetic acid was added to facilitate the dissolution and the mixed solution was magnetic stirred at 70 ℃ until completely dissolved. Unequal amounts of FeCl₃ (0.02, 0.04, 0.06, 0.08, 0.10 mol) and 1 mL APS solution (0.4 M) were mixed into above mixture to obtain uniform solutions. After removing bubbles in vacuum oven, 1.5 ml solution was poured into a mould sized 40 mm × 10 mm. Finally, thermally initiated radical polymerization led to the gelation of solution at 70 ℃ for 4 h and the PAA/CS hydrogel was obtained. Low concentrations (2, 4, 6, 8 mg/ml) AgNWs dispersions in ethanol were obtained by diluting 10 mg/ml AgNWs. All AgNWs dispersions needed ultrasonic treatment to make them disperse evenly. A total of 1 ml AgNWs dispersion was added dropwise to the PAA/CS hydrogel and dried to obtain the AgNWs/PAA/CS hydrogel with wrinkled Janus structure.

2.3 Measurements and Characterizations

The AgNWs/PAA/CS hydrogel was put in water for 72 h to exchange glycerol, and the water was dislodged by Vacuum Freeze Drier (FD-1F-50, China). The surface topography of the AgNWs/PAA/CS hydrogel was exhibited by scanning electron microscopy (SEM, TM-1000, Hitachi, Japan). The mechanical properties of hydrogels were tested by Universal Testing Machine (ZQ-990B, China). The square resistance was measured by Four-Point Probes (RTS-9, China). Measure the mass of hydrogels for different storage-time and the weight retention rate was calculated. The electrical signals varied with strain were gauged by Digital Multimeter (DMM6500, Keithley, USA). For further explanation, the relative resistance was defined as (R – R₀)/R₀, where R₀ and R represent the initial resistance and the real-time resistance.

3.1 Formation and micromorphology of the hydrogel

The preparation mechanism of the target PAA/CS@AgNWs hydrogel was shown in Fig. 1b. Acrylic acid (AA) monomers were polymerized by thermal initiation with oxidant APS to obtain PAA chains that were twined via coordination and electrostatic interaction with Fe³⁺ to form a three-dimensional (3D) network structure, which assured the shape stability of hydrogels. As a well-known biocompatible material, CS chains contain numerous -OH and -NH₂ groups, providing binding sites for hydrogen bonding with -COOH in PAA and further enhancing physical and mechanical properties of hydrogels. Afterwards, the AgNWs dispersion in ethanol was added in the upper surface of the PAA/CS hydrogels, and the AgNWs layer was deposited. As partial ethanol diffused into PAA/CS hydrogel, causing uneven swelling on the top surface and wrinkled structure that could be observed by naked eyes (Fig. 2a).

The optical image of the PAA/CS@AgNWs hydrogel prepared was showed in Fig. 2b. The PAA/CS hydrogel acted as a support for the AgNWs layer that was obvious observed in Fig. 2c. As shown in Fig. 2d and 2e, the PAA/CS hydrogel itself contained dense pores to form network structure, which proved the successful preparation of the PAA/CS@AgNWs hydrogel. Noticeably, the surface wrinkled structure was unable to be observed in the SEM images because the hydrogel is treated by solvent exchange before freezing and drying.

3.2 Mechanical properties

The reliable mechanical properties are the basis of normal work as a wearable device. Fe³⁺ was the crosslinking agent of PAA network that was the major component of the hydrogel, which mean the concentration of FeCl₃ had a significant effect on the mechanical properties of the PAA/CS@AgNWs hydrogel. A series of hydrogel samples with different FeCl₃ contents were prepared and used for investigation on tensile properties (Fig. 3a). Because of the low concentration of crosslinking agent, the solution with 0.01 M FeCl₃ could not be effectively shaped for stress-strain test. With increasing concentration of FeCl₃, more crosslinking points were obtained inside the hydrogel, resulting in a more compact cross-linking network and higher breaking strength. However, high crosslinking density limited the free slip of PAA chains, resulting in the breakage of hydrogel due to stress concentration. Therefore, 0.03 M FeCl₃ was chosen to further research. At this concentration, the Young's modulus of the hydrogel was 73.6 KPa, matching with that of the human skin (25–220 KPa) (Pan et al, 2020), which allowed the human body to move without feeling restricted and uncomfortable.

The PAA/CS@AgNWs hydrogel could be exposed to various complex working situations, including stretching (Fig. 3c), bending (Fig. 3d), twisting (Fig. 3e), and recovered after external force removal, which proved the excellent softness and elasticity. Otherwise, the hydrogel possessed considerable self-healing performance (Fig. 3f). After cutting and putting the two fracture surfaces of hydrogels in contact at 25 ℃ for 30 minutes, two pieces of hydrogel were reintegrated into one piece and could be stretched, although the crack could still be observed. Since there was no irreversible covalent crosslinking in the hydrogel, the coordination, electrostatic and hydrogen bonds reformed when the cross section of the hydrogel was placed together, endowing the hydrogel excellent self-healing property.

3.3 Water retention and adhesion properties

As we know, high water content is an important feature of hydrogels. It is significant to retain water for a long-term storage to guarantee comfort and working stability. For the PAA/CS@AgNWs hydrogel, all ingredients except water would not lead to weight change at room temperature. Therefore, the weight retention rate could be used to represent water retention. The PAA/CS@AgNWs hydrogel was stored for 10 days and the weight was recorded at different days (Fig. 4a). After 10 days, the hydrogel maintained more than 87% weight, which indicated outstanding water retention. In the preparation process, part of the water had evaporated at 70 ℃, working like pre-drying effect. From the perspective of composition, glycerol is an excellent humectant and hygroscopic agent. The hydrogen bond between glycerol and water is stronger than that between water molecules, resulting in the decrease of the saturated vapor pressure and hard evaporation of water.

Adhesion properties are beneficial to the close bonding between hydrogel and skin, so that the deformation of them is accurately matched and correspondence between sensing signal and actual motion is guaranteed. The PAA/CS hydrogel was bonded to the surface of different materials, including rubber gloves (Fig. 4d), glass sheet (Fig. 4e), stainless steel ruler (Fig. 4f) and plastic ruler (Fig. 4g). The -COOH groups on PAA macromolecular chain were capable to form ionic bond, coordination bond or hydrogen bond with different surfaces and the -OH and -NH₂ groups on molecule acted as binding sites for hydrogen bonding, further improving the adhesion. Additionally, good flexibility maximized the contact area between hydrogel and complex interface. For the PAA/CS@AgNWs hydrogel, adhesion enhanced the fastness of the AgNWs layer on the upper surface, extending the service life as a wearable device.

3.4 Electrical properties

Because all external stimulation showed via electrical signals, electrical properties are crucial for strain sensors. Different concentration of AgNWs dispersions were added to the PAA/CS hydrogels, and the square resistance was recorded (Fig. 4b). Interestingly, the conductivity of PAA/CS hydrogel added 2 mg/ml AgNWs dispersions was poorer than PAA/CS hydrogel. Because the conductive pathway depended on the hydrogel itself instead of AgNWs on the surface which was too few to build a complete conductive network. However, the partial dispersant ethanol entered the PAA/CS hydrogel and inhibited the migration of electrolyte, resulting in the enlargement of square resistance. With the increasing concentration of AgNWs dispersions, the conductive pathway composed of AgNWs tended to be intact and the contact sites increased, which resulted in better conductivity. The 10 mg/ml AgNWs was chosen for further research because of the minimum resistance. As shown in Fig. 4c, the PAA/CS@AgNWs hydrogel, DC power and the light-emitting diode (LED) were connected to form a series circuit. When the hydrogel was stretched, the brightness of LED decreased significantly, intuitively reflecting the damage of conductive path in the tensile direction. When the hydrogel was restored to its original length, the brightness of LED also recovered, which indicated the sensing performance and fatigue resistance.

3.5 Sensing performance and practical application

Undoubtedly, the sensing performance is of great significance to a wearable sensor. Defined as (R- R₀)/R₀/ε (where R₀, R and ε mean the initial resistance, the real-time resistance and the strain respectively), GF is used to measure the sensitivity of the sensor. The dependent tendency of the relative resistance with strain of the PAA/CS@AgNWs hydrogel was shown in Fig. 5a, and the GF could be divided into three parts (= 191.2, 7413, 18720) and calculated by this graph. In the first stage, electrons transferred through AgNWs layer, and the resistance change resulted from the slip and crack of AgNWs. In the second stage, the crack in AgNWs layer was so large that the integrity of AgNWs could not be maintained. The transition of the conductive pathway from the silver nanowire layer on the upper surface to the hydrogel body remarkably. In the third stage, the AgNWs network was completely destroyed, and the hydrogel body was the conductive path, which meant that the stretching of hydrogel itself led to the transformation of electrical signal. By comparison, our work was much higher than the GF of the hydrogel-based sensors in recent years (as shown in Table 1). On the one hand, the wrinkled structure on the upper surface engendered more cracks of the AgNWs layer under the same strain, obtaining more obvious change in relative resistance and higher GF. On the other hand, the enormous discrepancy in conductivity between AgNWs layer and hydrogel itself played an important role to display the excellent sensitivity. The PAA/CS@AgNWs hydrogel also produced a certain response when bending deformation occurs. Analogous to the strain in tensile deformation, (L₀-L)/L was used as the independent variable, where L represented the distance between the ends of the hydrogel and L₀ represented the original length. Obviously, the influence of electrical signal caused by bending was much lower than that by stretching.

Table 1

GF of the hydrogel-based sensors in recent years
Sample	Strain range	GF	Ref.
PAA/GO	5–50%	0.46	(Mao et al, 2020)
PVA/AgNWs	5–500%	0.58	(Azadi et al, 2020)
PVA/SNF/CN	0–100%	0.74	(Bao et al, 2021)
PVA/Gly/CNTs/CB	0–600%	2.07	(Gu et al, 2020)
PVA/rGO/GO	0–600%	2.10	(Chen et al, 2021)
PAM/gelatin	0–100%	2.5	(He et al, 2021)
PVA/P(AM-AA)	0–500%	2.74	(Huang et al, 2021)
PVA/P(AM-AA)	500–1000%	6.04	(Huang et al, 2021)
PAA/GE/TA	0–600%	4.37	(Lu et al, 2020)
PAA/GE/TA	600–1100%	7.6	(Lu et al, 2020)
PAA/PEDOT:SL	0–100%	7	(Wang et al, 2019)
TOCNF/PAA/PANI	0–900%	8.0	(Jiao et al, 2021)
PAM/MMT/CNTs	0–1000%	2.77	(Sun et al, 2021)
	1000–4000%	5.17
	4000– 4196%	8.5
PVA/TA/PAM/AgNWs	0–700%	68.64	(Zhang et al, 2022)
PAA/carrageenan@CNTs	0–110%	343	(Han et al, 2020)
PAA/CS@AgNWs	0–40%	191.2	This work
	40–60%	7413
	60–100%	18720

Figure 5c revealed the relationship between relative resistance and small strain (1%, 2%, 5% and 10%). Figure 5d showed the electrical signal response at normal strain (10%, 20% and 30%). This meant that the PAA/CS@AgNWs hydrogel could guarantee the identification, repetition and stability of electrical signals facing with complex work conditions. Significantly, no perceptible wastage of electrical signal was discovered with the continuous loading-unloading cycle (5% for more than 200 times) as shown in Fig. 5e, indicating the reliable service life. Excellent sensitivity, signal repeatability and stability of the PAA/CS@AgNWs hydrogel were the basis of practical application in which stretching and bending always occurred at the same time. The PAA/CS@AgNWs hydrogel was placed on the index finger and wrist for practical application test. The resistance was the minimum in the initial state, and reached the maximum when bending. When the index finger bent at different angles, the hydrogel presented distinguishable electrical signal responses, demonstrating the its potential application in motion tracking, health monitoring and other fields.

In summary, we designed the flexible, anti-drying and self-healing PAA/CS@AgNWs hydrogel based on non-covalent crosslinking. By dropping AgNWs dispersion in ethanol and drying, the AgNWs layer was successfully deposited and ethanol induced uneven swelling, causing the wrinkled structure on the upper surface. The wrinkled Janus structure and the electrical conductivity distinguishment between AgNWs and hydrogel itself led to ultra-high GF (= 191.2, 7413, 18720) compared with the recently reported hydrogels. Reliable adhesion ensured the seamless fit with the skin to accurately sense and promoted the fastness of AgNWs to extend the service life. Additionally, the PAA/CS@AgNWs hydrogel possessed identifiable, repeatable and stable electrical signal when strain occurred, which proclaimed the application prospect in wearable strain sensors.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Yu Jiang, Wanqi Feng and Yixiang Chen. The first draft of the manuscript was written by Yu Jiang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Editorial decision: Major revisions
18 Jun, 2022
Reviewers agreed at journal
23 May, 2022
Reviewers invited by journal
23 May, 2022
Editor invited by journal
16 May, 2022
Editor assigned by journal
29 Apr, 2022
First submitted to journal
28 Apr, 2022

You are reading this latest preprint version

Adhesive conductive hydrogels with wrinkled Janus surface and ultra-high sensitivity used as strain sensors

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Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials and chemicals

2.2 Fabrication of PAA/CS@AgNWs hydrogels

2.3 Measurements and Characterizations

3. Results And Discussions

3.1 Formation and micromorphology of the hydrogel

3.2 Mechanical properties

3.3 Water retention and adhesion properties

3.4 Electrical properties

3.5 Sensing performance and practical application

4. Conclusions

Declarations

References

Status:

Version 1