Adhesive and tough hydrogels promoted by quaternary chitosan for strain sensor

Journal Pre-proof Adhesive and tough hydrogels promoted by quaternary chitosan for strain sensor Te Wang (Conceptualization) (Methodology) (Data curation) (Formal analysis) (Writing - original draft), Xiuyan Ren (Supervision) (Writing - review and editing), Yu Bai (Investigation) (Formal analysis) (Supervision), Li Liu (Conceptualization) (Methodology) (Data curation) (Formal analysis) (Writing - review and editing), Guangfeng Wu (Conceptualization) (Methodology) (Resources) (Writing - review and editing) (Funding acquisition)

PII:

S0144-8617(20)31471-5

DOI:

https://doi.org/10.1016/j.carbpol.2020.117298

Reference:

CARP 117298

To appear in:

Carbohydrate Polymers

Received Date:

5 May 2020

Revised Date:

9 October 2020

Accepted Date:

19 October 2020

Please cite this article as: { doi: https://doi.org/

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. © 2020 Published by Elsevier.

Adhesive and Tough Hydrogels Promoted by Quaternary Chitosan for Strain Sensor Te Wang, Xiuyan Ren, Yu Bai, Li Liu*, Guangfeng Wu* Engineering Research Center of Synthetic Resin and Special Fiber, Ministry of Education, Changchun University of Technology, Changchun 130012, China Corresponding to Li Liu, Guangfeng Wu E-mail: [email protected]

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Graphical abstract

Highlights

Hydrogels were prepared by adding HACC to the PAA/Fe3+ cross-linking system.

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The hydrogel exhibited excellent self-healing and anti-swelling properties.

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The hydrogel could adhere to various materials through physical interactions.

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The hydrogel exhibited skin-like sensory capability for monitoring various human

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motions.

The hydrogel had a performance similar to skin intelligence.

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

Abstract

As a flexible material, hydrogels have attracted considerable attention in the exploration of various wearable sensor devices. However, the performance of the existing hydrogels is often too single, which limits its further application. Here, a 1

conductive hydrogel with adhesiveness, toughness, self-healing and anti-swelling properties was successfully prepared by adding 2-hydroxypropyltrimethyl ammonium chloride chitosan (HACC) to the polyacrylic acid/ferric ionic (PAA/Fe3+) cross-linking system. Based on the existence of three types of non-covalent interactions in the hydrogel system, including electrostatic interaction, coordination interaction and hydrogen bonds, the hydrogel possessed excellent mechanical properties (tensile stress and strain were 827 kPa and 1652%, respectively), self-healing properties (self-healing efficiency reached 83.3% at room temperature) and anti-swelling properties. In addition,

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the introduction of HACC also successfully gave the hydrogel outstanding adhesiveness. Moreover, the existence of iron ions provided sensitive conductivity to the hydrogel, which could be used as a flexible sensor for directly monitoring various

motions. Therefore, this simple strategy for preparation of multifunctional hydrogels

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would expand the application of a new generation of hydrogel-based sensors.

Keywords: Tough; adhesive; self-healing; conductive; anti-swelling; hydrogel-based

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sensors

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1 Introduction

With the progress of flexible electronic technology, flexible sensors with good mechanical properties and conductivity have attracted more attention in wearable

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electronic devices, human movement detection, health monitoring, and intelligent robots (Fiori et al., 2014; Nathan et al., 2012; Stoppa & Chiolerio, 2014; Zeng et al.,

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2014). They can monitor external stimuli and convert them into electric signals ( K. Ren et al., 2019; Sui et al., 2019; Hu et al., 2019; Xia, Song, Jia, & Gao, 2019; Zhu,

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Liu, Shi, Han, &Liang, 2018). To achieve flexibility and conductivity at the same time, some flexible sensors are made of a soft polymer matrix and conductive components including graphene, carbon nanotube and conductive polymer (Li, Wu, Wang, Luo, & Cao, 2019; Liu et al., 2017). However, these sensors will lose their sensitivity under large deformation due to the deterioration of conductivity, which limits the application range of sensors. Therefore, it is vital to obtain a material with good extensibility, toughness and highly sensitive conductivity, which can maintain the properties under 2

complex deformation (such as human movement), for the preparation of stable and versatile flexible sensors. As a soft material containing a large amount of water, the hydrogels can achieve good flexibility, high stretchability, biocompatibility and conductivity by structure design, and show great potential in the fabrication of flexible sensors. So far, the researchers designed and fabricated a large number of hydrogel-based sensors, which can respond to external stimuli (Han et al., 2020; J. Wang et al., 2019; Wu et al., 2018; Xia, Zhang, Song, Duan, & Gao, 2019; Y. Zhang et al., 2019; Yang et al., 2019; Shao, Meng, Cui,

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& Yang, 2019; Shin et al., 2019). However, the existing hydrogel-based sensors have more or less defects in performance or preparation. For example, Yang et al. reports a

poly (N-hydroxymethyl acrylamide) conductive hydrogel sensor with dual physically cross-linked network, which has good mechanical properties and conductive sensing

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capabilities, but the healing efficiency is not ideal. Shao et al. reports a dynamic self-

adhesive and self-healing conductive sensing hydrogel. Although the prepared hydrogel

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has superior mechanical performance, reliable self-healing capability and strong adhesion, its preparation method is complex. In a word, these hydrogels lack versatility

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and hardly become the best choice for flexible sensors. Therefore, it is urgent to explore a conductive hydrogel material with multiple functions.

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As we all know, PAA/Fe3+ interaction is often used to prepare hydrogels with good performance and conductivity ( Ren, A. Zhang, Zhang, Li, & Yang, 2019; Zhang, Cheng, Hou, Yang, & Guo, 2018; Zhou et al., 2016 ). However, the network formed

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only by single non-covalent bond is unstable. Thus, the integration of cross-linked networks and macromolecular chains with active functional groups into the

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PAA/Fe3+network has become a common method for fabricating multifunctional hydrogels with excellent properties (Wang et al., 2018; Fan, Liu, Wang, &Tang, 2019; Liu et al., 2018; Argun, Can, Altun, & Okay, 2014). Quaternary chitosan is an advanced derivative of chitosan prepared by chemical modification (Lin et al., 2019). Because of its biodegradability, biocompatibility, biological activity, non-toxic, hemostatic, good water solubility and low cost, it is widely used in tissue engineering and biomedical 3

fields. (Bagheri et al., 2019; Dekina, 2020; Godfred, Tran, Sewu, & Woo, 2019; Peers, Alcouffe, Montembault, & Ladavière, 2019; Rao et al., 2020; Viezzer et al., 2019.; J. Yang et al., 2019; Yang, Wang, Rossia, & Logana, 2020). As a result, it can be predicted that the addition of quaternary chitosan to hydrogels can give the hydrogel the characteristics of adhesion, antibacterial, and biomimetic properties, etc. (Sochilina et al., 2019; Zhang et al., 2019). In this work, a multifunctional hydrogel with excellent toughness, adhesiveness, selfhealing, anti-swelling and conductivity was prepared by simple one-step

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polymerization. The hydrogel was designed by adding 2-hydroxypropyltrimethyl ammonium chloride chitosan (HACC) into PAA/Fe3+ ionic crosslinking network. The addition of HACC improved the mechanical strength, swelling resistance and selfhealing properties of the hydrogel, and at the same time effectively imparted excellent

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adhesion properties to the hydrogel, which can bond various materials such as plastic,

glass, metal, rubber and so on. In addition, the presence of Fe3+ and Cl- ions in the

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hydrogel system made it have outstanding conductivity and strain sensitivity. Therefore, the hydrogel would be an ideal material as a flexible ionic sensor for smart wearable

2.1 Materials

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2 Experimental Section

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devices and other artificial intelligence.

Acrylic acid (AA ≥ 99%) was supplied by Xilong Science Co. Ltd. 2-

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hydroxypropyltrimethyl ammonium chloride chitosan (HACC) was supplied by Wuhan Lanabai Pharmaceutical Chemical Co. Ltd (degree of substitution ≥ 90%, degree of

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deacetylation ≥ 85%, Brookfield viscosity of the chitosan was 146 mPa·s at 25 °C at 1% w/w chitosan in 1% w/w acetic acid aqueous solution). Ferric chloride (FeCl3 ≥ 98%), potassium persulfate (KPS, 99%) were supplied by Wuhan Lanabai Pharmaceutical Chemical Co. Ltd. 2.2 Preparation of hydrogels The PAA-HACC hydrogels were prepared by radical polymerization, the steps were 4

as follows. Initially, HACC and AA with determined proportions were dissolved in deionized water under constant stirring at 50 °C. After uniform dissolution, a certain amount of FeCl3 solution (0.15 M) was added into the solution. The solution was continuously stirred and cooled to room temperature. Then, KPS (0.015 g) was added to the above solution and stirred to a homogeneous solution. After purging with nitrogen for 10 min, the mixture solution was put into a reaction mold composed of a pair of glass (25 × 25 × 0.7 cm) and a 2 mm thick silica gel pad. Finally, the precursor solution was heated at 50 °C for 12 hours to form PAA-HACC hydrogel. In the experiment, the

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total mass of AA and HACC was fixed to 5.0 g, the solid content of the hydrogel was fixed to 25% (excluding the self-healing experiment). The detailed formulation of hydrogels was shown in Table 1. The codes and meanings of hydrogels were as follows:

Hx, Fy, Ax, HSj, Sj, Ck, among of them, x is the mass percentage of HACC in the sum of

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AA and HACC; y is the mass percentage of FeCl3 in the sum of AA and HACC;

j is the solid content of hydrogels; k is the component of hydrogels, 1-4 represent PAA,

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PAA+HACC, PAA+FeCl3 and PAA+HACC+FeCl3, respectively.

HACC (g)

FeCl3 (g)

KPS (g)

H2O (mL)

0.0

0.050

0.015

15.2

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AA (g)

H0%

5.0

H2%

4.9

0.1

0.050

0.015

15.2

H6%

4.7

0.3

0.050

0.015

15.2

H10%

4.5

0.5

0.050

0.015

15.2

4.3

0.7

0.050

0.015

15.2

4.1

0.9

0.050

0.015

15.2

F0.5%

4.7

0.3

0.025

0.015

15.0

F0.75%

4.7

0.3

0.038

0.015

15.0

F1%

4.7

0.3

0.050

0.015

15.2

F1.25%

4.7

0.3

0.062

0.015

15.2

A0%

5.0

0.0

0.025

0.015

15.1

H14%

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H18%

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Hydrogels

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Table 1 The formulation of the hydrogels

5

4.9

0.1

0.025

0.015

15.1

A6%

4.7

0.3

0.025

0.015

15.1

A10%

4.5

0.5

0.025

0.015

15.1

A14%

4.3

0.7

0.025

0.015

15.1

A18%

4.1

0.9

0.025

0.015

15.1

HS15%

4.7

0.3

0.050

0.015

28.2

HS17.5%

4.7

0.3

0.050

0.015

23.9

HS20%

4.7

0.3

0.050

0.015

20.3

HS22.5%

4.7

0.3

0.050

0.015

17.4

HS25%

4.7

0.3

0.050

S15%

4.7

0.0

0.050

S17.5%

4.7

0.0

0.050

S20%

4.7

0.0

0.050

S22.5%

4.7

0.0

S25%

4.7

0.0

C1

5.0

0.0

C2

4.7

0.3

C3

5.0

C4

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A2%

15.2

0.015

27.0

0.015

22.5

0.015

19.1

0.050

0.015

16.4

0.050

0.015

14.3

0

0.015

15.0

0

0.015

15.0

0.0

0.05

0.015

15.2

0.3

0.05

0.015

15.2

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0.015

2.3 ATR-FTIR measurement

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The chemical structure of PAA-HACC hydrogel (H6%) was analyzed using an FTIR

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spectrometer (IS50, Nicolet, USA) in attenuated total reflectance (ATR) mode at room temperature. Before measurement, the hydrogel was soaked in deionized water to purify and freeze-dried under vacuum in a freeze (FDU-2110, EYELA). 2.4 Tensile measurement The tensile properties of PAA-HACC hydrogels were measured by using a tensile tester (QX-W5502, 100N, USA) at room temperature. The hydrogels were cut into 4mm dumbbell shapes. Stretching speed was set to 100 mm/min. The toughness of the 6

hydrogels was evaluated by a deformation work calculated from the area integral under the stress-strain curve. In addition, the loading-unloading test of hydrogel samples was performed at the maximum elongation of 600%. The area of hysteresis loop was defined as hysteresis energy. 2.5 Rheological measurement A modular compact rheometer (Brookfield R/S, USA) was used at room temperature for dynamic rheological testing using a circular spline with a diameter of 25 mm. After fixing the spline, apply a layer of silicone oil around the fixing plate to prevent the

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evaporation of water in the hydrogel. The linear viscoelastic region of the hydrogel was determined by an oscillating strain sweep experiment. Fixed angular frequency is ω = 10 rad/s, then changed the dynamic strain scan test range from 0.1% to 100%. The hydrogel spline was then subjected to a frequency sweep test under constant conditions

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of γ = 0.5%, the frequency ranged from 0.01 rad/s to 100 rad/s. 2.6 Peeling measurement

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The adhesive strength of the hydrogels on the surface of the aluminum plate was measured by a peel strength tester (SG-305A, made in the Suzhou). The length, width

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and thickness of the sample were 100 mm, 20 mm and 6 mm, respectively. As a hard backing of the hydrogel, a cyanoacrylate adhesive was used to adhere the hydrogel to

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the PET film. The peeling test speed of the sample was set to 5 mm/s. For each sample, repeat the test at least five times. 2.7 Conductive property

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Electrochemical property of hydrogels was measured by Autolab (AUT86925). In

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the frequency range of 1-106 Hz, the conductivity of hydrogels was calculated as folows: σ = L/(R × S)

where σ was the conductivity of the hydrogel (S/cm), L was the length between any two electrodes (cm), R was the resistance of the hydrogel (Ω), and S was the cross-sectional area of the hydrogel (cm2). In order to detect the strain responsiveness of the hydrogel, the relative change of resistance of the hydrogel was measured by the above Autolab, and the formula was as 7

follows: Δ𝑅/𝑅0 = (𝑅 − 𝑅0 )/𝑅0 × 100%, Where R0 and R are the initial resistance and strain resistance respectively. The gauge factor (GF) is also defined as GF = (ΔR/R0)/ε, where ε is the strain. 2.8 Self-healing performance The hydrogel samples were cut from the middle. Then, the cut samples were put in the plastic bag for 24 hours at room temperature without any irritation. Calculate the self-healing efficiency of the hydrogel by the ratio of the tensile strength of healing

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hydrogel to the original one. 2.9 Swelling test

We make the hydrogel into cylinder (25 mm in diameter and 2 mm in thickness)，

soak it in excess deionized water for 15 days at room temperature and replace with

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deionized water every day. The swelling behavior of hydrogels was evaluated by the

2.10 UV-Vis Spectrum analysis The

ultraviolet-visible

spectrum

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volume change before and after swelling.

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studied

by

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ultraviolet-visible

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spectrophotometer (UV752). The sample was precursor solution. The recorded scanning wavelength was in the range of 200 nm to 800 nm. The scanning speed was

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960 nm·min-1.

2.11 Morphology observation of hydrogels Under the conditions of 15 kV acceleration voltage and 15 mm working distance, the

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morphology of the hydrogel was observed by FESEM (JEOL JSM-7610F, Japan). The samples used was swollen, freeze-dried, quenched by liquid nitrogen and coated with

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platinum.

2.12 Human living experiment All human-related experiments were conducted under the approval of the

Institutional Animal Care and Use Committee (JPCH-IRB-20200313) and strictly comply with live animals or human use rules and ethics policies. In addition, all humanrelated experiments were performed with the knowledge and consent of the human 8

subjects.

3 Results and Discussions 3.1 Formation mechanism A multifunctional PAA-HACC hydrogel was fabricated by a one-step method of solution polymerization. To prove the formation of the PAA-HACC hydrogel, the FTIR curve of hydrogel (H6%) was shown in Figure S1. The structure and formation mechanism of hydrogels were shown in Figure 1. After the polymerization initiated by free radicals, the hydrogel network contains negatively charged PAA chains, positively

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charged HACC chains, Fe3+ ions and Cl- ions. There were three kinds of non-covalent

dynamic bonds in the network, including the electrostatic interaction of PAA and

HACC chains, the coordination interaction of –COOH, –NH–, –NH2 and Fe3+ ions, and

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the hydrogen bond interaction of PAA and HACC chains. As the result of various dynamic crosslink, the PAA-HACC hydrogels possessed good mechanical properties,

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self-healing and anti-swelling properties. Meanwhile, the introduction of HACC containing abundant hydroxyl and amino groups also promoted the adhesion of the

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hydrogels. In addition, the presence of Fe3+ ions and Cl- ions endowed the hydrogel

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with excellent conductivity.

Figure 1. The structure and formation mechanism of the PAA-HACC hydrogels 3.2 Mechanical performance In order to study the influence of HACC on the mechanical property of hydrogels, 9

the tensile tests and single-cycle loading-unloading experiments were carried out on the PAA hydrogel and PAA-HACC hydrogel (F1.25%). Figure 2(a, b) showed the stressstrain curves and toughness of the PAA hydrogel and PAA-HACC hydrogel. These results indicated that the tensile strength and toughness of the PAA hydrogels were 435 kPa and 2.5 MJ/m3, but the PAA-HACC hydrogels were 827 kPa and 5.06 MJ/m3 respectively. Obviously, both the tensile strength and toughness of the PAA-HACC hydrogels had nearly double growth, which fully demonstrated that the introduction of HACC could effectively improve the mechanical strength of the hydrogel. Besides, the

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effective energy dissipation mechanism of hydrogels could be evaluated by dissipative energy. Figure 2(c, d) were the single loading-unloading curves and the dissipated

energy of PAA and PAA-HACC hydrogels. The results showed the dissipative energy of PAA-HACC hydrogel was larger, indicating their effective dissipation energy and

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excellent toughness compared with others. In short, the addition of HACC promoted

the improvement of mechanical properties by successfully participating in the

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construction of multiple dynamic cross-linking networks, in which the hydrogel had a

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more excellent energy dissipation mechanism.

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Figure2. (a) Tensile stress-strain curves and (b) corresponding stress and toughness of PAA hydrogel and PAA-HACC hydrogel; (c) Single loading-unloading curves and (d)

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dissipated energy of PAA hydrogels and PAA-HACC hydrogel. The influence of different HACC and FeCl3 content on the mechanical properties

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of hydrogels was also explored. Figure 3 (a, b) showed the stress-strain curves and corresponding fracture strength and toughness for the PAA-HACC hydrogels with different mass radio of HACC/(AA+HACC), respectively. With the increase of the

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mass radio of HACC/(AA+HACC), the tensile strength and toughness of the PAAHACC hydrogels first increased and then decreased. When HACC mass fraction was

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6%, the tensile strength was the highest, up to 643 kPa, and the toughness was 3.9 MJ/m3. This was because the appropriate addition of HACC could increase the number of dynamics cross-linking points and make the cross-linked network more compact. However, excessive HACC would increase the viscosity of the prepolymer, hinder the diffusion of free radicals and the movement of macromolecular chains in the polymerization process, and then form an uneven hydrogel network, leading to a 11

decrease in its tensile strength. Besides, Figure 3 (c, d) showed the stress-strain curves and corresponding fracture strength and toughness of PAA-HACC hydrogels with different contents of FeCl3, respectively. It was found that the PAA-HACC hydrogels had better mechanical properties with the increase of FeCl3 content. And the tensile strength and toughness of the PAA-HACC hydrogels at FeCl3 content of 1.25%, could reach 824 kPa and 5.1 MJ/m3, respectively. This was because the increasing of Fe3+ could increase the number of coordination crosslinking point, thereby improving the

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mechanical properties of the hydrogel.

Figure 3. (a) Tensile stress-strain curves and (b) corresponding stress and toughness of

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PAA-HACC hydrogels with the different mass ratio of HACC/(AA+HACC); (c) Tensile stress-strain curves and (d) corresponding stress and toughness of PAA-HACC hydrogels with the different content of FeCl3. Besides, the excellent mechanical properties of PAA-HACC hydrogels were also demonstrated through loading, knotted stretching and puncture test. Figure 4 (a, b) exhibited the loading capacity of the PAA-HACC hydrogel. The weight of 400 g and 1 12

kg were pulled up, hydrogel samples with different initial lengths were stretched to the same shape variable. At the same time, the knotted stretching experiment of hydrogel was carried out, and no fracture was observed in Figure 4 (c). In addition, as shown in Figure 4 (d), the PAA-HACC hydrogel was barely damaged when it was substantially

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punctured.

Figure 4. (a, b) The loading exhibition of the PAA-HACC hydrogels (H6%); (c) Knotted

recovery of the PAA-HACC hydrogel (H6%).

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stretching of the PAA-HACC hydrogel (H6%), (d) Stabbing and the corresponding

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As we all know, the mechanical properties of hydrogels are determined by their microstructure. Therefore, the internal structure of hydrogels was analyzed by

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rheological measurement. Figure 5 (a) exhibited the storage modulus (G') of C1, C2, C3 and C4 hydrogels at different frequency scans. Among the four systems, G' of hydrogels containing HACC were always higher than those without HACC. and the G' of C4

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hydrogel cross-linked by Fe3+ was the highest. Meanwhile, with the increasing of angular frequency (ω), G ' of C1 and C2 hydrogels gradually increased while C3 and C4

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hydrogels remained unchanged. It was revealed that the addition of HACC and Fe3+ was contribute to the formation of more stable networks. Figure 5 (b) showed G' and

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G'' of the above four hydrogels under different shear strains. Obviously, G' is much higher than G'' in the linear region, which is consistent with the solid elastic properties of hydrogel. Moreover, a clear strain-dependent viscoelastic response could be also observed. In the linear viscoelastic region of σ (from 0.01% to 10%), the variation trend of the storage modulus of four hydrogels were basically consistent with that in Figure 5 (a). In the range of 10% to 1000%, the hydrogels exhibited nonlinear viscoelastic behavior. Moreover, it could be clearly seen that the storage modulus of C4 hydrogel 13

dropped the slowest in the nonlinear deformation region, which indicated that its network structure was the most stable and not easily damaged. Even under the action of shear stress, the network structure was not easily deformed, showing excellent mechanical properties. Meanwhile, the tan δ of C1, C2, C3 and C4 hydrogels were shown in Figure 5 (c). In the linear viscoelastic region, for PAA hydrogel (C1), the internal friction was higher due to weak entanglement of macromolecular chains. When HACC was added to the PAA hydrogel (C2), the entanglement between PAA molecular chains was slightly weakened, and the internal friction factor was only slightly

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increased. When FeCl3 was added to the PAA hydrogel system (C3), the carboxyl group in PAA was coordinated with Fe3+ to form a strong complexation, resulting in a significant reduction in internal friction. However, when HACC was added to the above

system (C4), HACC weakly reduced the coordination with PAA and Fe3+, so the internal

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friction factor was slightly increased compared to the above system.

To further prove the interaction between HACC and Fe3+, the measurements of UV-

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Vis absorption spectrum of HACC and HACC-Fe3+ complexes were carried out. As shown in Figure 5 (d), HACC had no peaks in the range of 200-400 nm. However, an

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obvious peak of the HACC-Fe3+ complexes appeared at 262 nm, which was attributed to the transfer of charge from the ligand to the metal, indicating the presence of HACC-

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Fe3+ coordination interactions.

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Figure 5. Storage modulus (G′) and loss modulus (G″) of C1, C2, C3 and C4 hydrogels as a function: (a) frequency, (b) strain; (c) Tan δ of C1, C2, C3 and C4 hydrogels as a

3.3 Swelling behavior

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strain; (d) UV-Vis spectra of HACC and HACC-Fe3+ complexes.

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The various interactions between PAA and HACC chains can promote the increase of the crosslink density of the hydrogel, which further affect the swelling behavior of

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the hydrogel. Anti-swelling property is also a desirable attribute for hydrogels. In Hx hydrogels, the swelling behavior of PAA hydrogel (H 0%) and PAA-HACC hydrogel

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with the best tensile strength (H6%) was compared. As shown in Figure 6 (a), PAA hydrogel (H0%) and PAA-HACC hydrogel (H6%) were immersed in deionized water for 15 days respectively. Obviously, PAA-HACC hydrogel still showed excellent antiswelling performance after being soaked for 15 days, with almost no swelling. Meanwhile, scanning electron microscopy (SEM) was also utilized to analyze its internal structure of the soaked PAA and PAA-HACC hydrogels, and the quenched 15

section of freeze-dried samples soaked for 5 days were observed in Figure 6 (b-d). By contrast, it was obvious that PAA hydrogel showed porous structure, while PAA-HACC hydrogel showed dense and non-porous structure, proving the PAA-HACC hydrogels hardly swelled in water. To explain the appearance, the dynamic modulus of the two hydrogels as a function shear strain was tested. As shown in Figure 6 (e), in the linear viscoelastic region，the storage modulus G' of H6% was higher than that of H0%, indicating that H6% had a greater network crosslink density than H0%. In addition, the

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intersection of the storage modulus G' and the loss modulus G'' was referred to as a gel point. It was found that the gel point of H6% and H0% appeared at shear strain γ of 500% and 259%. The larger γ at the time of the gel point appearance proved that the network

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hydrogel containing HACC hardly swelled.

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structure of the H6% hydrogel was denser and not easily damaged. Therefore, the H6%

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Figure 6. (a) Swelling behavior of the PAA hydrogel (H0%) and PAA-HACC hydrogel

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(H6%); (b and c) SEM photos of H0% and H6% (×500); (d) SEM photo of H6% (× 5000); (e) Storage modulus (G′) and loss modulus (G″) of H0% and H6% as a function strain.

3.4 Adhesive performance The addition of chitosan to the hydrogel system helped to regulate the balance of cohesion and adhesion and endow the hydrogel with adhesion. Here, we tried to improve the adhesion of hydrogels by adding water-soluble HACC and investigated the 17

influence of different components on adhesion. As shown in Figure 7 (a), when the mass fraction of HACC was 2%, the PAA-HACC hydrogel exhibited a maximum adhesive strength of 110 N/m, almost four times the adhesion of the PAA hydrogel (30N/m). This fully demonstrated that the addition of HACC could effectively improve the adhesion of hydrogel system. However, with the increase of HACC content, the adhesion of PAA-HACC hydrogel gradually decreased, mainly because excessive addition of HACC would lead to restricted dense network and mesh chain, leading to the increasing of cohesive and the decreasing of adhesion. Meanwhile, the effect of

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FeCl3 content on the adhesion of hydrogel was also displayed in Figure 7 (b). With the increase of FeCl3 content, the adhesion of the hydrogel decreased gradually, which was

attributed to the increase of hydrogel network density. In a word, the PAA-HACC hydrogel with the appropriate composition could exhibit excellent adhesion property.

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In addition, the adhesive behaviors and durability of the PAA-HACC hydrogel for various materials were explored. As seen in Figure 7 (c, d), the PAA-HACC hydrogels

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exhibited excellent adhesion properties for various materials, such as aluminum, silica rubber, glass, titanium, plastic, etc., and even displayed repeatable adhesive ability. This

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was due to the existence of abundant active functional groups in polysaccharide molecules, such as –OH, -NH-, and -NH2, which enabled them to form the multiple

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physical interactions with various materials, including hydrogen bond metal complexation or electrostatic interaction, thus achieving excellent and long-lasting adhesion performance. Therefore, it could be expected the adhesive hydrogels would

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be widely used in wound dressing, surgical band-aid and other fields.

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Figure 7. (a, b) Peeling curves of PAA-HACC hydrogels containing different content

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of HACC and FeCl3; (c) Peeling curves of PAA-HACC hydrogels (H2 %) adhering to various materials; (d) Peeling curves of PAA-HACC hydrogels (H2 %) during 5 repeated peeling tests on silica rubber substrate.

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3.5 Conductivity and strain sensitivity

The existence of Fe3+ and Cl- endowed PAA-HACC hydrogel with excellent

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conductivity. In Figure 8 (a), the conductivity of the hydrogel gradually decreased from 12.2 mS/cm to 5.5 mS/cm with the increase of HACC content. In Figure 8 (b), the

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conductivity gradually increased to a maximum of 8.73 mS/cm with the growth in FeCl3 content. First, HACC had coordination effect with Fe3+, which would reduce the free Fe3+concentration and lead to the decrease of conductivity; Second, the increase of HACC content would appropriately increase the viscosity of the hydrogel system, which would hinder the effective movement of Fe3+ and further reduce the conductivity.

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Figure 8. The conductivity of PAA-HACC hydrogels with (a) different HACC contents

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and (b) different FeCl3 contents.

Additionally, the PAA-HACC hydrogels also possessed high electrical sensitivity,

fast responsiveness, and excellent stability. Figure 9 (a-f) showed the relative resistance

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changes of hydrogels (H6%) under different bending conditions at the neck, wrist, elbow,

finger, knee and throat. The hydrogels train sensor showed excellent strain response to

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all kinds of motions. After multiple joint movements, the corresponding resistance signal changed almost remain unchanged. And the hydrogel could accurately detect

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voice signals when directly adhering to the throat. This further confirmed the feasibility of the PAA-HACC hydrogel as strain sensors, making it ideal candidates for monitoring human health and movement. In order to better evaluate the sensing capability of PAA-

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HACC hydrogels, as shown in Figure 9 (g), the real-time resistance change of the hydrogels upon stretching to different tensile strains (0–900%) was recorded and the

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corresponding GFs were also calculated. The strain response curves displayed four linear regions with various slopes, corresponding to GF of 2.25 in the strain range of

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0–100%, GF of 5.34 in the strain range of 100–500%, and GF of 11.65 in the strain range of 500–900%, showing excellent strain sensitivity. Meanwhile, the loading– unloading process of the hydrogel was also examined to explore the responsiveness (Figure 9 (h)). It was surprisingly found that the PAA-HACC hydrogel exhibited a short response time (240 ms) and a short recovery time (180 ms), fully indicating that the hydrogels possess a rapid responsiveness to external forces. In addition, in Figure 9 (i), the hydrogel produced stable output signals, when they were loaded for 50 loading20

unloading cycles under a tensile strain of 20 %. A slight upward drift of the signal curve

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might be caused of water loss of the hydrogel.

Figure 9. (a-f) The relative resistance changes for the PAA-HACC hydrogel under various human motion detection: neck, wrist, elbow, finger, knee and throat; (g) The

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relative resistance changes for the PAA-HACC hydrogel under various tensile strains (0–900%); (h) The resistance variation curves of the PAA-HACC hydrogel during

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loading–unloading process (i) 50 loading-unloading cycles under a tensile stain of 20 %. 3.6 Self-healing Performance

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The self-healing properties of the hydrogels were quantitatively analyzed by tensile

tests, as shown in Figure 10 (a, b, c). It was obvious that the self-healing efficiency of the PAA and PAA-HACC hydrogels increased with the decrease of solid content. Regardless of high or low solid content, the self-healing efficiency of the PAA-HACC hydrogel was higher than PAA hydrogel, up to 83.3%. The reason was the non-covalent interactions of PAA-HACC hydrogel networks were richer than PAA hydrogels. Moreover, the effect of different FeCl3 contents on the self-healing properties of PAA21

HACC hydrogels was also explored. As shown in Figure 10 (d, e), with the increase of FeCl3 content, the healing efficiency of PAA-HACC hydrogel first increased and then decreased, because the increase of FeCl3 would increase the number of coordination crosslinking points, thus improving the self-healing ability. However, the excessive cross-linking would make hydrogels rigid, which hindered the restoration of network chains, so the self-healing ability was reduced. In order to better demonstrate the selfhealing properties of PAA-HACC hydrogels, the changes of bulb brightness before and after self-healing was explored. As illustrated in Figure 10 (f), the brightness of the bulb

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remained almost unchanged before and after healing, confirming the good self-healing

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property of the PAA-HACC hydrogel.

Figure 10. (a)The tensile curves of PAA hydrogels (Sj) and (b) PAA-HACC hydrogels (HSj) with different solid contents before and after healing; (c) The healing efficiency of PAA and PAA-HACC hydrogels; (d) The tensile curves of PAA-HACC hydrogels (Fy) with FeCl3 contents before and after healing; (e) The healing efficiency of PAAHACC hydrogels (Fy); (f) Comparison of bulb brightness before and after self-healing. 22

4 Conclusion In this study, a series of conductive hydrogels with toughness, self-healing property, adhesion and anti-swelling property base on PAA, HACC and FeCl3, in different proportions, were successfully prepared by one-step solution polymerization. The introduction of HACC facilitates a multiple noncovalent cross-linking network (hydrogen bond, coordination interaction and electrostatic interaction) with the dynamic and reversible nature. Based on this structure, the hydrogel had a surprising

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anti-swelling ability，a fracture stress of a maximum of 827 kPa and a toughness of 5.06 MJ/m3 when the contents of HACC and FeCl3 were 6 wt% and 1.25 wt%,

respectively. At 15% solid content, it also had an excellent self-healing efficiency up to 83.3%. At the same time, HACC containing multiple active groups also contributed to

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the hydrogel adherence to rubber, plastic, glass, metal and other materials. The

maximum adhesion strength to silica rubber is 166 N/m and the hydrogel (H2%)

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displayed repeatable adhesive ability. In addition, the existence of Fe3+ and Cl- also provided excellent electrical conductivity for PAA-HACC hydrogels (maximum

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conductivity up to 12.2 mS/cm). And the hydrogel could sensitively perceive electrical signal changes under different joint movements of human body. Therefore, the

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multifunctional hydrogels could be applied in flexible robot, health detection, human motion detection and other flexible functional sensors.

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CRediT authorship contribution statement Te Wang: Conceptualization, Methodology, Data curation, Formal analysis, Writing original draft. Xiuyan Ren: Supervision, Writing review & editing. Yu Bai: Investigation, Formal analysis, Supervision. Li Liu: Conceptualization, Methodology, Data curation, Formal analysis, Writing review & editing, Guangfeng Wu: Conceptualization, Methodology, Resources, Writing - review & editing, Funding acquisition Acknowledgements This work was financially supported by Jilin Scientific and Technological Development Program (No. 20180201075SF) and Jilin Province Development and 23

Reform Commission (No. 2019C043-8)

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