A multi-functional reversible hydrogel adhesive

Journal Pre-proof A Multi-Functional Reversible Hydrogel Adhesive Yonggan Yan, Shulei Xu, Huanxi Liu, Xin Cui, Jinlong Shao, Peng Yao, Jun Huang, Xiaoyong Qiu, Chuanzhen Huang

PII:

S0927-7757(20)30215-6

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124622

Reference:

COLSUA 124622

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

14 January 2020

Revised Date:

23 February 2020

Accepted Date:

24 February 2020

Please cite this article as: Yan Y, Xu S, Liu H, Cui X, Shao J, Yao P, Huang J, Qiu X, Huang C, A Multi-Functional Reversible Hydrogel Adhesive, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124622

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A Multi-Functional Reversible Hydrogel Adhesive Yonggan Yan1, Shulei Xu1, Huanxi Liu1, Xin Cui4, Jinlong Shao5, Peng Yao1, Jun Huang 1,2,3*[email protected], Xiaoyong Qiu6* [email protected], Chuanzhen Huang1

1

Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of

High efficiency and Clean Mechanical Manufacture of Ministry of Education, School

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of Mechanical Engineering, Shandong University, Jinan, Shandong, 250061, China 2

State Key Laboratory of Mineral Processing, Beijing, 102628, China

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Advanced Medical Research Institute, Shandong University, Jinan, Shandong,

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250012, China

Advanced Interdisciplinary Technology Research Center, National Innovation

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Institute of Defense Technology, Beijing 100071, China

Department of Periodontology, School and Hospital of Stomatology, Shandong

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University & Shandong Provincial Key Laboratory of Oral Tissue Regeneration & Shandong Engineering Laboratory for Dental Materials and Oral Tissue Regeneration, Jinan, Shandong, 250012, China Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, School

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of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong

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250100, China

Graphical abstract

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Highlights

The fabricated hydrogel could be stretched 55 times its initial length.

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The hydrogel can heal within 30 seconds.

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The hydrogel exhibits reversible wet adhesion.

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Abstract

Designing and fabricating self-healing hydrogel adhesive is essential in many

physical and biological applications such as wound closure and cartilage repair. In this work, a new type of hydrogel adhesive has been developed through ultraviolet (UV) curing acrylamide (AM) monomer and hyper-branched polyethylenimine (PEI) polymer. The synthesized PAM-PEI hydrogel (weight ratio for PEI: PAM = 1: 9) Page 2

shows high stretchability (1D extension ratio > 5500 %) and good plasticity (it can be extended into 2D hydrogel film), quick self-healing property (within 30 seconds) at ambient enviornment and good adhesiveness to both inorganic (23 kPa ~ 90 kPa for metal and glass) and organic substrates (~ 8kPa for polytetrafluoroethylene, 1 ~ 7 kPa for skin and liver). More importantly, the adhesion-strip-cycle test suggested that the adhesion measured was reversible and did not attenuate for at least 5 cycles on various surfaces. Rheology tests have demonstrated the repeatability of self-healing behavior of the hydrogel between small (1%) and large (1000%) strain. In addition,

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the self-healed hydrogel fragments show good injectability, which can be potentially used for 3D-printing. This work provides a facile way of preparing multifunctional

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hydrogel adhesive with potential engineering applications.

1. Introduction

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Keywords: Hydrogel adhesive, self-healing material, ultra-stretchable, UV curing

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Hydrogels are three-dimensional polymer networks trapping a large amount of water,[1, 2] which have been widely used in many fields such as tissue

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engineering,[3-5] biological medicine,[6, 7] and flexible electronics.[8-10] Hydrogel adhesive is one type of special hydrogel that can adhere both inorganic and organic

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materials, especially under wet conditions. Previous research demonstrated that hydrogel adhesives can be used as hemostatic materials,[11-13] wound closure substrates,[14, 15] and cartilage repairing matrixes.[16-18] Understanding the formation and structure of hydrogel adhesive is of great significance for fundamental study and engineering applications such as biomedicine,[11, 12, 14, 17] flexible supercapacitors[19] and wearable devices.[20-22] Therefore, great efforts were dedicated to developing hydrogel adhesives by using different approaches such as Page 3

casting,[19, 22, 23] thermal polymerization[17, 19] and ultraviolet (UV) curing.[11, 18, 23] For example, polydopamine has been introduced into polyacrylamide (PAM) network to obtain hydrogels with good self-healing ability and tissue adhesiveness, but the healing process of the hydrogel generally took a long time (~ 2 h).[14] Alternatively, a reversible underwater adhesive was obtained by using charge-balanced polyampholyte hydrogels consisting of both positively and negatively charged function groups.[23] Besides, a tissue adhesive hydrogel that can polymerize and form covalent bonds to wet biological-tissue surface within seconds

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after UV irradiation was developed by using methacrylated gelatin, N-(2-aminoethyl)−4-(4-(hydroxymethyl)−2-methoxy-5-nitrosophe-noxy) butanamide and glycosaminoglycan hyaluronic acid.[11] Alginate and adipic acid dihydrazide were employed to achieve covalent topological adhesion.[24] In spite of the great

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progress that has been achieved in developing multifunctional hydrogels, it is still

challenging to prepare a hydrogel adhesive that combines high stretchability, quick

adhesion and biological applications.

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self-healing ability and reversible tissue adhesiveness, which is critical for wet

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PAM hydrogel is widely used for gel electrophoresis[25, 26] in biomedical research. However, pure PAM hydrogel is fragile and prone to fracture. A general

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strategy to improve the performance (e.g. stretchability, self-healing, toughness and tissue adhesiveness) of PAM-based hydrogels is to incorporate PAM with other polymers and forming composite hydrogels. For example, alginate (a natural

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polysaccharide polymer) has been introduced into PAM hydrogel network to create a double-network hydrogel exhibiting enhanced stretchability and toughness.[27]

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Moreover, phenylboronic acid grafted alginate and poly(vinylalcohol) (PVA) were combined with PAM to develop a stretchable supramolecular hydrogel possessing triple shape memory effects.[28] Recently, acrylamide (AM), acrylic acid (AA), allylamine and 1-vinylimidazole have been used to fabricate PAM-based hydrogel interferometry sensor,[29] which could be used for ultrasensitive chemical detection. A bioinspired lubricating hydrogel system was developed by combining a supramolecular fluorenylmethoxycarbonyl-L-tryptophan network and PAM/PVA Page 4

double network for achieving reduced friction force during loading process.[4] Polyethylenimine (PEI) is a positively charged aliphatic polymer that can be used for gene delivery and therapy.[30-32] The linear PEI only contains secondary amines, while the branched PEI contains primary, secondary and tertiary amino groups. The chemical structure of PEI (linear or branched) has a strong effect on its chemical property.[30] And the branched PEI exhibiting good crosslinking property was commonly used for developing functional hydrogels.[33-35] Some PEI-incorporated hydrogels have been fabricated and used for islet transplantation

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therapy,[36] stretchable sensors,[37] water release[38] and removing of heavy metal ions (e.g. Chromium) from wastewater.[39] Previous research based on PAM-PEI composite hydrogels were mostly focused on the gelation kinetics and dynamic

rheological behavior for oil drilling applications.[34, 35, 40-44] Experiment results

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revealed that the gelation time of fly-ash-reinforced PAM-PEI gel increased with increasing the content of fly ash.[34, 42] Additionally, differential scanning

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calorimeter (DSC) analysis indicated that the crosslinking of PAM/PEI gel system is associated with endothermic as well as exothermic processes.[35, 41, 44] However,

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PAM-PEI-based composite hydrogels possessing high stretchability (e.g., extension ratio > 5500 %), self-healing (within 30 seconds) ability as well as reversible

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adhesiveness on different substrates have not been reported. In this work, a multi-functional composite hydrogel adhesive was developed by a facile one-pot synthesis method. By simply polymerizing acrylamide (AM)

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monomer and branched PEI polymer through UV curing method, the fabricated hydrogel exhibits high stretchability, self-healing property and reversible substrate

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adhesion. Systematical studies were carried out to understand the stretchability, self-healing, viscoelastic properties and adhesiveness of the hydrogels with different weight ratios of PEI: PAM, BAM/AM and water content. The objective of this work was to obtain a PAM-PEI based multi-functional hydrogel adhesive that can be potentially used for different engineering applications.

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2. Materials and Methods 2.1 Materials Acrylamide (AM, 98.0%) was purchased from Tokyo Chemical Industry Inc. (Shanghai, China). Branched polyethylenimine (PEI, 50% (w/w) aqueous solution, Mw ~ 70000) and N, N-methylenebis(acrylamide) (BAM, 99%) were purchased from Aladdin Inc. (Shanghai), Photoinitiator-2960 (2-hydroxy-1-(3-(hydroxymethyl)phenyl)-2-methylpropan-1-one) was purchased from Shanghai Yinchang New Materials Inc. (Shanghai, China). All reagents were

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used as received without further purification. Deionized water was used for solution preparation during the experiment.

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2.2 PAM-PEI Hydrogel Preparation

A series of PAM-PEI hydrogels were fabricated by using a solution-casting

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method, followed by UV curing. Typically, AM (3.6 g), PEI (0.4 g), BAM (0.06wt% of AM) and photoinitiator-2960 (2wt% of AM) were dissolved in deionized water to

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obtain a homogeneous hydrogel precursor solution (unless otherwise specified, the water content was fixed at 85wt%). Then the prepared solution was stirred for 50 min and degassed at reduced pressure for 30 min. Then the solution was put into syringes

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with different capacities (1 mL, 2 mL, and 10 mL) and polymerized by UV irradiation (365 nm, 10 W) for 2.5h. A series of hydrogels with different weight ratios of PEI to

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AM (0: 10, 0.5: 9.5, 1: 9, 1.5: 8.5, 2: 7) at fixed BAM ratio (0.06wt% of AM) and water content (85wt%) were obtained according to the method mentioned above.

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Hydrogels with different water contents (65wt% ~ 85wt%) and BAM/AM weight ratios (0.06%, 0.6%, 1.5%, 3%) were also prepared for understanding the effect of water content and crosslinker (BAM) on hydrogel properties.

2.3 Characterization Methods Scanning Electron Microscopy (SEM) Test The internal polymer network of the hydrogel was investigated by using a field emission SEM (JEOL, JSM-7800F) operated at 2 kV acceleration voltage. The Page 6

hydrogels were first freeze-dried at -56 °C for 48 h and then fractured and coated with thin layers of gold film (< 5 nm) before SEM characterization. FTIR Spectra Test FTIR spectra was performed on a Tensor II infrared spectrometer (Bruker Company, Germany) to record the function groups of the hydrogel. The hydrogel samples were first freeze-dried at -56 °C for 48 h and ground into powders. And KBr disks were prepared by careful grinding 1 g KBr with 10 mg hydrogel powder, followed by drying with an infrared lamp for 5 min. The disks were then pressed at 10

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tonne load and immediately placed into the FTIR instrument for recording the infrared spectra. Rheology Analysis

Rheology analysis of the hydrogel was performed at room temperature (25℃)

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using a Thermo Scientific HAAKE RheoStress RS300 (Karlsruhe, Germany) with a

35 mm, 1° cone configuration plate. Silicone oil was used to seal the edge of the plate

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for preventing water loss during experiments. In order to determine the storage (G’) and loss modulus (G”) of the hydrogels, oscillatory frequency sweep measurements

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were performed at a strain of 1% with the shear frequency of 0.1 to 40 Hz (viz., 0.628 rad/s to 251.2 rad/s) at 25 °C. Strain amplitude sweep measurements were conducted

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with strain increasing from 1% to 10000% at a frequency of 1Hz. G’ and G” versus time was tested at a fixed strain (γ = 1%) and frequency (f = 1Hz). The continuous step strain measurements were performed at a frequency of 1 Hz, and amplitude

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oscillatory strains were switched from small strain (γ = 1%) to large strain (γ = 1000%) with a strain interval of 100 s.

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Tensile Tests

The mechanical properties of the hydrogel were characterized using a universal

testing machine (ZLC-2D, Jinan XLC Testing Machine Co., Ltd, China) at a loading rate of 50 mm/min with a 100 N load cell in air. The cylindrical specimens were prepared by using 2 ml syringes with a dimension of 9.0 mm (diameter) × 20.0 mm (length). Adhesion Tests Page 7

Adhesion tests were performed to evaluate the adhesion strength of the PAM-PEI hydrogel with different substrates including metal, glass, polytetrafluoroethylene (PTFE), pigskin and liver using the same tensile testing machine at a loading rate of 10 mm/min with a load cell of 100 N. The adhesion tests were conducted according to previously reported procedure.[45, 46] Briefly, the hydrogel sample was uniformly dispersed onto a polished-iron-rod surface (diameter 15.0 mm). Then the iron rod coated with hydrogel adhesive was pressed against the tested substrate (e.g., iron, glass, polymer and etc.) at a fixed normal load of 50 N for

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2 min followed by separation. The adhesion strength (σ) of the hydrogel can be evaluated by Equation 1,

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Fmax A

(1)

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where Fmax is the maximum force measured during the adhesion test and A is the contact area. In order to check the reproducibility of the measured adhesion, the

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adhesion test was repeated five times on different samples with the same polymer

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ratio.

3. Results and Discussion

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3.1 Preparation of PAM-PEI Hydrogel

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Figure 1 Scheme of the fabrication process of the PAM-PEI hydrogel. (a) The

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PAM-PEI hydrogel was formed after UV irradiation of the precursor solution (i, ii), and the hydrogel firmly adhered two pieces of pigskin (iii); (b) Proposed molecular structure and interactions of the formed PAM-PEI hydrogel.

As shown in Figure 1, the hydrogel was prepared by a facile one-pot synthesis method. The PAM-PEI hydrogel was formed after UV irradiation of the precursor solution for 2.5 h (Figure 1a(i, ii)). The fabricated hydrogel could tightly adhere two Page 9

pieces of pork, exhibiting tissue-adhesion property (see Figure 1a(iii)). Figure 1b shows the proposed molecular structure of the PAM-PEI hydrogel. After UV irradiation, the PAM backbone was covalently linked by the crosslinker (BAM), serving as the primary network, and those PEI chains were both covalently and non-covalently linked to the PAM network through –NH– group or hydrogen bonds.[34, 35, 43] It should be noted that covalent bonds were formed by the substitution reaction between the nucleophilic amine nitrogen on PEI and the acrylamide pendant groups (–NH2 in this work) on PAM[34, 35, 40, 43] (see Figure

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S1). Additionally, multiple hydrogen bonds were formed between the amino groups of PEI and the PAM polymer network. The dynamic nature of the hydrogen bonds

between the PEI chain and PAM network could efficiently dissipate energy through the breakage of those non-covalent bonds to prevent crack propagation during

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stretching, therefore enhance the stretchability. Besides, the reversible nature of hydrogen bonds endowed the hydrogel with good self-healing ability at room

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temperature as well as the reversible adhesion to both inorganic and organic materials. The morphologies of the obtained hydrogels were examined by SEM and shown in

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Figure S2. All the hydrogels displayed similar 3D porous networks because of the loss of water during the freeze-drying process. However, the SEM images show that

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the pore size of the hydrogels and the thickness of the pore walls gradually diminished when the amount of PEI increased during gel formation. This result suggests that the content of PEI was critical in determining the microstructure of the obtained

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hydrogels.

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3.2 FTIR Spectra Analysis

Figure 2 FTIR spectra of the PAM-PEI hydrogel with different weight ratio of PEI to

PAM (PEI: PAM = 0: 10, 1: 9, 1.5: 8.5). The weight ratio of BAM over AM was fixed

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at 0.06% and the water content was fixed at 85wt%.

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Figure 2 shows the FTIR spectra of the PAM-PEI hydrogels with the PEI: PAM ratio being 0: 10, 1: 9 and 1.5: 8.5, respectively. The FTIR spectra of the three

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hydrogel samples show five features. (1) The presence of a broad absorption bond at 3419 cm-1 is due to –OH and –NH2 stretching.[47-50] (2) There were large amounts of amine groups with typical C=O (amine) stretching at 1640 cm-1 and C–N (amine)

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stretching at 1114 cm-1.[48, 49, 51, 52] (3) Other characteristic bonds were assigned to the stretching of –CH2 at 2930 cm-1 to 2853 cm-1, the scissoring and twisting of –

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CH2 at 1450 cm-1 and 1400 cm-1, and the bending of –OH at 1320 cm-1;[47, 50] (4) Compared with the hydrogel at PEI: PAM = 0: 10, the absorption peak of the

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hydrogels at PEI: PAM = 1: 9 and 1.5: 8.5 was shown at 3192 cm-1 due to the stretching vibration of N–H.[49, 53] Meanwhile, the intensity of the absorption peak at 3192 cm-1 increased significantly when the concentration of PEI increased from 0:10 to 1.5: 8.5. The peak is likely caused by the generation of N–H groups due to the substitution reaction between the nucleophilic amine nitrogen on PEI and the acrylamide pendant groups on PAM.[35, 40, 44] (5) Besides, the peak at 1619 cm-1 of the hydrogel at PEI: PAM = 1: 9 and 1.5: 8.5 was resulted from the –NH2 of the PEI Page 11

chains.[49, 54] The FTIR results support that the PEI chains were covalently linked to the PAM network through –NH– group after UV curing.

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3.3 Rheology Test

Figure 3 Rheology analyses of the PAM-PEI hydrogel. (a) Storage modulus (G’) and loss modulus (G”) of the hydrogel with different weight ratios of PEI: PAM (PEI:

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PAM = 0.5: 9.5, 1: 9, 1.5: 8.5) on strain amplitude sweep (γ = 1% ~ 10000%) at fixed

frequency (f = 1 Hz). (b) G’ and G” of the hydrogel with different weight ratio of PEI

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to PAM (PEI: PAM = 0: 10, 0.5: 9.5, 1: 9, 1.5: 8.5, 2: 8) at small strain (1% ~ 100%) at fixed frequency (f = 1 Hz). The weight ratio of BAM over AM was fixed at 0.06%

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and the water content was fixed at 85wt%.

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Rheology tests were conducted to understand the gel formation mechanism and the self-healing property of the fabricated hydrogels. Figure 3 illustrates the change of storage modulus (G’) and loss modulus (G”) of the PAM-PEI hydrogels at different

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weight ratios of PEI: PAM during strain-amplitude-sweep tests. As shown in Figure 3a, G’ and G” remained unchanged on small strain (< 100%), while they decreased

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rapidly at large strain of 100% ~ 10000%, indicating the hydrogel network started to rupture over 100% strain. When the weight ratio of PEI: PAM is low (PEI: AM ≤ 1: 9), the G’ curve intersected the G” curve at a critical strain (290% for PEI: PAM = 0.5: 9.5, 170% for PEI: PAM = 1: 9). And G’ was lower than G’’ when the strain was larger than this critical value, indicating the hydrogel transformed from solid to fluid state because of the collapse of the polymer network. However, when larger amount of PEI (e.g. PEI: PAM = 1.5: 8.5) was introduced into the hydrogel network, G’ was Page 12

lower than G” for the whole strain-amplitude-sweep process and no intersection point was observed (see Figure 3a), suggesting the PAM-PEI composite exhibited fluid like property. Figure 3b shows the trend change of G’ and G” at small strain (1% ~ 100%) with increased weight ratio of PEI to PAM. It is observed that G’ decreased more quickly than G” when increasing the amount of PEI, and the two curves intersected at the point where the weight ratio of PEI: PAM = 1.5: 8.5. And the PAM-PEI network is not stable enough to form hydrogel when the PEI content is larger than the critical weight ratio (PEI: PAM = 1.5: 8.5, see the gel-formation tests results in Figure S3).

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Therefore, the hydrogel at a weight ratio of PEI: PAM = 1: 9 exhibiting reasonable

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mechanical strength and flexibility was selected for further analysis.

Figure 4 Rheology analyses of the PAM-PEI hydrogel (the weight ratio of PEI: PAM = 1: 9) (a) Storage modulus (G’) and loss modulus (G”) of the hydrogel on strain amplitude sweep (γ = 1% ~ 10000%) at fixed frequency (f = 1 Hz). (b) G’ and G” on frequency sweep (f = 0.1 ~ 40 Hz) at fixed strain (γ = 1%). (c) G’ and G” versus time at fixed strain (γ = 1%) and fixed frequency (f = 1 Hz). (d) G’ and G” when alternate step strain switched from small strain (γ = 1%) to large strain (γ = 1000%) in Page 13

continuous step strain measurements at a fixed frequency of f = 1 Hz. The weight ratio of BAM over AM was fixed at 0.06% and the water content was fixed at 85wt%.

Figure 4 shows the rheological properties of the hydrogel with the weight ratio of PEI: PAM = 1: 9. The G’ curve intersected the G” curve at the strain of 170% (Figure 4a). The G’ was lower than G” when the strain was larger than this critical value, indicating the transformation from solid to fluid state caused by the collapse of the gel network. This phenomenon can be attributed to the dissociation of hydrogen

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bond on large shear strain. Figure 4b illustrates the frequency sweep measurements of the hydrogel at 1% strain, where G’ and G” as a function of frequency was measured.

Both G’ and G” were sharply increased with increasing shear frequency, implying that a temporary mechanical reinforcement resulted from the hydrogen bonds between

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amino groups of PAM and PEI.[20] The frequency dependence of G’ and G’’ revealed

the dynamic nature of the hydrogen bonding in the hydrogel network.[20] It should be

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noted that the G’ curve was above the G” curve over the whole measurement range in Figure 4c, indicating that the covalent bonding and hydrogen bonding served as

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cross-linkers of the network and contributed to the elastic modulus. Moreover, upon 1% strain, the G’ and G” remained unchanged with the time increasing, indicating the

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hydrogel network was stable at small shear strain (Figure 4c). The repeatability of self-healing behavior of the hydrogel was demonstrated by continuous step strain measurement between small and large amplitude as shown in Figure 4d. As the

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oscillatory shear strain took steps from 1% to 1000%, both the G’ and G” values suddenly decreased due to the dissociation of the hydrogen bonds in the polymer

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network. However, the hydrogel exhibited total recovery of the G’ and G” in a few seconds after decreasing the amplitude, indicating the quick recovery of the inner network. This recovery process could be repeated several times without any loss in mechanical strength during continuous step strain measurement. Intriguingly, both G’ and G” increased slightly after continuous strain sweep, suggesting the obtained hydrogel exhibited self-strengthening behavior.[55] The self-strengthening behavior is mainly caused by an increased density of bonding sites of the hydrogel network due to Page 14

repeated destruction and reconstruction of the hydrogen bonds in the polymer

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network.

Figure 5 (a) G’ and G” of the hydrogel with different weight ratio of BAM to AM

(BAM/AM = 0.06%, 0.6%, 1.5%, 3%) at small strain and fixed frequency (f = 1 Hz). The samples were at fixed PEI: PAM ratio (PEI: PAM = 1: 9) and water content

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(85wt%). (b) G’ and G” of the hydrogel with different water content (65wt%, 70wt%,

75wt%, 80wt%, 85wt% and 90wt%) at small strain (1% ~ 100%) at fixed frequency (f

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= 1 Hz). The samples were at a fixed PEI: PAM weight ratio of 1: 9 and BAM/AM

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weight ratio of 0.06%.

The effect of crosslinker (BAM) concentration on the rheological properties of the hydrogel was investigated by using the hydrogels with fixed PEI: PAM ratio (PEI:

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PAM = 1: 9) and water content of 85wt%. Figure S4 (a-d) shows the change of G’ and G” of the hydrogel samples at different weight ratios of BAM to AM (0 ~ 3%)

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during strain-amplitude sweep. It can be observed that the values of G’ and G” are highly depend on the weight ratio of BAM to AM (Figure 5a). G’ increased

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monotonously when the weight ratio of BAM increased, while G” first decreased at the weight ratio of 0.06% - 0.6% and then quickly increased at the weight ratio of 0.6% - 3%. The increasing of G’ is most likely caused by the increased density of the covalent cross-linkers in the polymer network. The water content also had a strong effect on the rheological properties of the hydrogels. The viscoelastic behavior of the hydrogels with different water content (65wt%, 75wt%, 85wt% and 90wt%) was shown in Figure S5. For all samples, G’ and G” remained unchanged at small strain Page 15

and decreased when the strain was larger than a certain value. The values of G’ and G” at small strain (1% ~ 100%) were plotted in the Figure 5b. It can be observed that both G’ and G’’ decreased with increasing the water content, which is most likely caused by the decreasing density of cross-linkers. However, the decreasing rate of G’ was larger than that of G’’, and when the water content was larger than a critical value, the G’’ would higher than G’, indicating that the PAM-PEI network was incapable to form a stable hydrogel in this condition.

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3.3 The Self-Healing Property of the Hydrogel

Figure 6 The demonstration of self-healing and injectable properties of the hydrogel samples (PEI: PAM = 1: 9). (a) The self-healing process of two hydrogel pieces after cutting. (i-iii) The hydrogel was cut into two parts; (iv, v) The separated pieces were brought into contact for 30 seconds; (vi) The healed hydrogel could be stretched immediately after healing. (b) The self-healing process of hydrogel fragments. (i-iii) Pieces of hydrogel healed together after contacting; (iv) The healed hydrogel could be Page 16

extended in two directions. (c) The injection process of the healed hydrogel. (i-ii) The gel fragments were collected in a syringe; (iii) The healed hydrogel could be injected out of a syringe. Bulk self-healing tests were conducted to demonstrate the self-healing property of the hydrogel. The hydrogen-bond network in the hydrogel could undergo reversible association/dissociation at room temperature,[1, 56, 57] which endowed the PAM-PEI hydrogel with rapid self-healing ability. As shown in Figure 6a, the hydrogel was first cut into two separate pieces, and then brought into contact. The two separate pieces

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could rapidly merge into a single piece after healing for 30 seconds in ambient enviorment. Besides, the healed hydrogel could withstand large stretaching strain

without discernible interface being observed (Figure 6a(vi)). Interestingly, numbers of hydrogel pieces could heal together after simple contacting, and the healed

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hydrogel could be expanded into thin film immediatedly in ambient enviorment without any discernible interface (see Figure 6b and Video S1 in supporting

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information). Figure 6c indicated that the healed hydrogel is also injectable. In a typical experiment, the specimens were cut into gel fragments, followed by collecting

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in a syringe. After that, the hydrogel fragments were healed and injected out of the

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syringe, showing great potential for 3D-printing.

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3.4 Mechanical Properties of the PAM-PEI Hydrogel

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Figure 7 Tensile test results of the obtained PAM-PEI hydrogel samples. (a) The PAM-PEI hydrogel (the weight ratios of PEI: PAM = 1: 9) was stretched 53 times of

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its initial length. (b) The tensile stress-strain curve and (c) The loading-unloading curves at diverse maximum strains (500%, 1500%, 2500% and 3500%) of the

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hydrogel (the weight ratios of PEI: PAM = 1: 9). (d) Typical tensile stress-strain curves of the hydrogel with different PEI: PAM ratios (PEI: PAM = 0: 10, 0.5: 9.5, 1: 9). (e) Fracture strain and tensile stress of the hydrogel different PEI: PAM ratios (PEI: PAM = 0: 10, 0.25: 9.75, 0.5: 9.5, 0.75: 9.25, 1: 9). The weight of BAM was fixed at 0.06% that of AM with fixed water content (85wt%). The fabricated PAM-PEI hydrogel (the weight ratio of PEI: PAM = 1: 9) exhibited super flexibility (see Figure 7a and Video S2 in supporting information), Page 18

which could be stretched more than 53 times its initial length without breaking. To further understand the mechanical properties of the hydrogel, a universal testing machine was used to obtain the tensile stress-strain curves (see Figure S6). Figure 7b and 7c illustrate the typical tensile stress-strain curves and loading-unloading curves at diverse maximum strains of the hydrogel. It can be observed that the hydrogel was broken at a large fracture strain of 5600% with a relatively low tensile stress (2.76 kPa) (see Figure 7b). Importantly, the obtained hydrogel can be extended into a 2D-hydrogel film (see Figure S7a-b and Video S3 in supporting information),

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which could be poked with a Buchner funnel without breaking (see Figure S7c-d)), demonstrating excellent stretchability at multiple directions. Moreover, the hydrogel

could retain its outstanding stretchability after diverse large strain sweep (see Figure 7c). The loading-unloading curves suggest that the hydrogel has a large mechnical

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hysteresis, which is similar to previous reported results.[58-60] It can be observed that the subsequent loading curve is higher than the previous unloading curve, which is

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consistent with the exhibition of a slight recovery of the hydrogel hysteresis. The excellent stretchability can be attributed to the automatic breaking and recombining of

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the hydrogen bonds between amino groups of PAM and PEI in the hydrogel network during large strain extension. Besides, the reversible association/dissociation of

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dynamic hydrogen bonds facilitated energy dissipation[56] when the hydrogel netwrok was under strain testing.

Figure 7d shows the stress-strain curves of the hydrogel with different weight

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ratios of PEI: PAM. It can be observed that introducing PEI into PAM network could significantly improve the stretchability but decrease the tensile stress. As shown in

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Figure 7e, the increasing of PEI ratio would improve the flexibility of the hydrogel, which is likely caused by the increased density of hydrogen bonds in the hydrogel network. However, a high PEI content tended to impair the fracture stress due to the decreasing density of covalent bonds. Besides, the PAM-PEI network becomes instable and it is incapable to form the gel when the PEI content is larger than a critical weight ratio (PEI: PAM = 1.5: 8.5). And this result is consistent with the rheology analyses (see Figure 3b and Figure S3). Page 19

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Figure 8 (a) Typical tensile stress-strain curves with the weight ratio of BAM/AM of

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0.06%, 0.6% and 3%, respectively. (b) Fracture strain and tensile stress of the hydrogel with different BAM/AM ratios at fixed PEI: PAM ratio (PEI: PAM = 1: 9) and water content (85wt%). (c) Typical tensile stress-strain curves with different

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water content (65wt%, 75wt%, 85wt%) at a fixed PEI: PAM ratio (PEI: PAM = 1: 9) and BAM (0.06wt% of AM). (d) Fracture strain and tensile stress of the hydrogel with

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different water content (65 ~ 85wt%) at a fixed PEI: PAM ratio (PEI: PAM = 1: 9) and

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BAM (0.06wt% of AM).

To further understand the effect of crosslinker (BAM) on the strength of

PAM-PEI hydrogel, the hydrogels with various concentrations of BAM were prepared at a fixed PEI: PAM weight ratio (PAM: PEI = 1: 9) and water content of 85wt%. It should be mentioned that increasing the concentration of BAM would increase the covalent crosslink density of the hydrogel network.[27] Therefore, increasing the weight ratio of BAM would increase the fracture tensile stress but decrease the Page 20

fracture strain (see Figure 8a-b). This result suggests that a high density of crosslinker tended to reduce the stretchability. In addition, the water content also had a significant effect on the mechanical strength of the hydrogel (see Figure 8c-d). It can be observed that the fracture strain increased but the fracture stress decreased with the increasing of water content. The increasing of water content would significantly decrease the density of the crosslinker for the hydrogel network. Therefore, the hydrogel became “softer” when increasing water content.

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3.5 Adhesive Property of the Hydrogel

Figure 9 The adhesion property of the hydrogels. (a) The adhesion strength of the

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hydrogel (PEI: PAM = 1: 9) on different substrates. (b) The adhesion strength of the hydrogels on iron substrate as a function of PEI: PAM ratio. (c) The adhesion strength of the PAM-PEI hydrogel (PEI: PAM = 1: 9) to pigskin and liver surfaces over multiple adhesion-strip cycles. The weight of BAM was fixed at 0.06% that of AM with fixed water content (85wt%).

Importantly, the obtained PAM-PEI hydrogel showed good adhesion to both Page 21

inorganic and organic substrates such as iron, glass, PTFE, pigskin and liver (see Figure 9a). The adhesion strength between the hydrogel and the iron substrate was the highest (~ 90 kPa) among the tested substrates, which is due to the synergetic interactions of van der Waals interaction, hydrogen bonds and metal complexation.[56, 61] For example, the –NH2 and C=O groups can easily form metal complexation with iron ions, [61, 62] and the –NH2 and –NH function groups in PAM-PEI network can quickly form hydrogen bonds with the oxide layer on iron substrate. Besides, the omnipresent van der Waals interaction could also contribute to the measured adhesion.

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For the case of glass and PTFE, van der Waals forces and hydrogen bonding[56] may play dominant role between hydrogel and substrates. The measured adhesion strength for glass (~ 23 kPa) was larger than that for PTFE (~ 8 kPa), which is caused by the low surface energy of PTFE substrate. Additionally, the adhesion strength between

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hydrogel and the wet organic substrates such as pigskin and porcine liver were ~ 7

kPa and ~ 1 kPa, respectively. The adhesion strength with pig liver was lower than

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that with pigskin, which was likely caused by the smoother and higher water content of pig liver than that of pigskin. Figure 9b shows that the content of PEI could

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significantly affect the adhesion strength of the PAM-PEI hydrogel. Taking iron as an example, increasing the weight ratio of PEI: PAM from 0: 10 to 1: 9 would enhance

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the adhesion strength from ~ 25 kPa to ~ 90 kPa. And this enhancement is likely caused by the stronger polar interaction (vdW force) and increased number of hydrogen bonds between PAM and PEI chains. However, the adhesion strength

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started to decrease when the ratio of PEI: PAM was above a critical value (e.g. PEI: PAM = 1: 9). The decreased adhesion is likely caused by the low cross-linking degree

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of the gel network, as demonstrated by the rheology test (Figure 3, where G’ and G’’ become close to each other) and the bulk gel formation test (Figure S3). Figure 9c illustrated the change of adhesion strength between the PAM-PEI hydrogel and pigskin/liver surfaces over multiple adhesion-strip cycles. The results demonstrated that the measured adhesion is fully reversible. The dynamic non-covalent interactions (e.g. hydrogen bonds, metal complexation, hydrophobic interaction and van der Waals interaction) between the substrate surface and the hydrogel could mainly contribute to Page 22

this reversible adhesion (see Figure 10b). Interestingly, the measured adhesion strength would slightly increase during the adhesion-strip cycles tests. The enhanced adhesion is likely caused by the increased density of hydrogen bonds of the hydrogel network during the destruction-reconstruction cycles of the hydrogel network, which is similar to recently reported “mechano-responsive, self-growing hydrogels”.[55]

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3.6 Adhesion Mechanism of the Hydrogel Adhesive

Figure 10 (a) Adhesive performance of the PAM-PEI hydrogel (the weight ratios of PEI: PAM = 1:9 at a fixed BAM (0.06wt% of AM) and water content (85wt%)). The Page 23

hydrogel can adhere on the surfaces of tissue (e.g. (i) pigskin), glass (e.g. (ii) glass bottle), metal (e.g. (iii) iron), paper (e.g. (iv) paper box), rubber (e.g. (v) rubber stopper) and plastic (e.g. (vi) polytetrafluoroethylene (PTFE), (vii) polypropylene (PP) syringe, (viii) high density polyethylene (HDPE) and (ix) polyethylene terephthalate (PET)). (x) The hydrogel can form a strong bong between a polymethyl methacrylate (PMMA) chip and a 200g weight as well as (xi) two steel weights of 200g and 500g. (b) Schematic illustration of possible adhesion mechanisms.

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Figure 10a further demonstrated that the hydrogel exhibited excellent adhesiveness to various substrates include pigskin, glass, metal, paper, rubber, PTFE, PP, PMMA, HDPE and PET. Besides, the hydrogel can firmly adhere a PMMA chip

and a 200g weight as well as two steel weights of 200g and 500g (see Figure 10a(x,

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xi)). As show in Figure 10b, the reversible adhesiveness of the hydrogel is caused by the quick establishment of multiple physical interactions with the substrate at the

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interface. Breaking the interface would destroy those interactions, but the physical interactions would be rebuilt when bringing the surface into contact again (Figure

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10b(i)). It should be noted that the adhesion between hydrogel and substrates can be attributed to different interaction mechanisms.[61, 63] The –NH2, –NH and C=O

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function groups in PAM-PEI network can quickly form hydrogen bonds with –OH groups (or some other function groups containing N, O, F) on substrates.[61, 62, 64] For some metal substrates, the –NH2 and C=O groups can easily form metal

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complexation with metal ions, exhibiting enhanced adhesion.[61, 62] Additionally, hydrophobic interaction between the carbon skeleton (hydrogel) and hydrophobic

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substrate, and the omnipresent van der Waals interaction could also contribute to the measured adhesion.[61, 65] Therefore, the measured adhesion between hydrogel and different substrates is a combination of all those physical interactions as illustrated in Figure 10b(ii). The reversible nature of these physical interactions endows the hydrogel with reversible adhesion property.

Page 24

4. Conclusions In summary, this work has provided a facile method of obtaining highly-stretchable, self-healing, injectable and reversible adhesive hydrogel that showed adhesiveness to a variety of surfaces such as metal, glass, polymer, pigskin and pig liver. Systematically studies were carried to understand the gel formation mechanism. The obtained hydrogel adhesive could be stretched to 55 times its initial length, resulting from the automatic breaking and recombining of those physical interactions such as hydrogen bond, metal complexation, hydrophobic interaction and

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van der Waals force. Moreover, the hydrogel could achieve self-healing within 30 seconds at ambient enviorment due to the dynamic nature of those physical

interactions in the polymer network. Our study provides a facile way of fabricating

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highly stretchable, self-healing hydrogel adhesive with potential engineering

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applications.

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CRediT Author Statement

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Yonggan Yan, Jun Huang and Xiaoyong Qiu: Conceptualization, Methodology. Yonggan Yan, Shulei Xu and Huanxi Liu: Data collection,

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Yonggan Yan, Shulei Xu and Jinlong Shao: Data analysis and interpretation.

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Yonggan Yan, Jun Huang and Xiaoyong Qiu: Writing- Manuscript draft preparation. Xin Cui, Peng Yao and Chuanzhen Huang: Critical revision of the article. Jun Huang, Xiaoyong Qiu: Supervision. All authors discussed the results and approved the submission of the final version of the manuscript.

Declaration of interests Page 25

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

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51905305, No. 21902183, No. 81901009), the Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2020-10), the Open

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Foundation of Advanced Medical Research Institute of Shandong University (Grant

No. 22480089398408, J. Huang), China Postdoctoral Science Foundation (Grant No. 2019M662326, No. 2019M652409 and No. 2019TQ0187), Beijing Natural Science

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Foundation (2204103).

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