Strong Wet Adhesion of Tough Transparent Nanocomposite

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Applications of Polymer, Composite, and Coating Materials

Strong wet adhesion of tough transparent nanocomposite hydrogels for fast tunable-focus lenses Feibo Li, Gongzheng Zhang, Zhaoshuo Wang, Haoyang Jiang, Shuang Yan, Li Zhang, and Huan-Jun Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02556 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Materials & Interfaces

Strong Wet Adhesion of Tough Transparent Nanocomposite Hydrogels for Fast TunableFocus Lenses Feibo Li, Gongzheng Zhang, Zhaoshuo Wang, Haoyang Jiang, Shuang Yan, Li Zhang and Huanjun Li* School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China *Corresponding author. [email protected]

ABSTRACT Tough hydrogel adhesives that can bond strongly to wet surfaces have shown great potential in various applications. However, it still remains a challenge to develop the adhered hydrogels integrated with strong wet adhesion, high transparency, exceptional mechanical properties and fast self-recovery. Herein, tough nanocomposite hydrogels demonstrating high tensile strength, high transparency and fast self-recovery are reported. The strong wet strong adhesion between two tough hydrogel films can be realized by introducing chemical bridging across the hydrogel-hydrogel interface, while the interfacial adhesion energy and shearing adhesion strength are up to 2216 J m-2 and 385 N/m, respectively. The strong adhesion and superior toughness of our hydrogels enable their reassembly capability to produce stretchable sealed balloons that can endure high air pressure without leakage. Most interestingly, the combination of excellent sealability and high transparency also allows our hydrogel balloons to turn into hydraulically driven fast tunable-focus convex lenses, which is first reported here for hydrogel lenses. The hydrogel adhesives may open up the door to develop soft sealed containers and intelligent optical devices. KEYWORDS: transparent nanocomposite hydrogel, strong wet adhesion, exceptional mechanical properties, superior sealing property, fast tunable-focus lenses INTRODUCTION Achieving the strong adhesion of hydrogels to various materials has received huge attention due to their great potential in diverse fields such as electronic devices,1,

2

tissues repair,3-5 wound

healing6-8 and actuators.9-11 Conventional hydrogels suffer from weak adhesion to themselves or

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other materials, limiting their practical applications. To address the above issues, several strategies have been developed for the wet adhesion of hydrogels. For example, nanoparticles as the adhesives could be used for realization of rapid adhesion of synthetic hydrogels to themselves or biologic tissues.12-14 Ionic bonds were also able to serve as interfacial connection points to bond the hydrogels with special charged groups together.15-18 Additionally, other approaches such as host–guest recognition19, 20 and mussel-inspired catechol interactions21-23 were also employed to carry out the adhesion of hydrogels to wet materials. However, these adhered hydrogel-hydrogel interfaces were susceptible to debonding under external load due to the weak bridging across the hydrogel substrates. Thus, an effective strategy to develop strong interfacial bridging across the hydrogel-hydrogel contact region is urgently needed. Recent studies show that introducing covalent bonds between the tough hydrogels will be a useful approach for improving interfacial interactions.24 For example, Zhao et al fixed the polymer network of tough (polyacrylamide-alginate) hydrogels to diverse non-porous solids or elastomer by covalent bonds to achieve strong adhesive interfaces with the interfacial toughness of over 1500 J m−2.25, 26 Suo et al developed strong hydrogel adhesives (adhesion energy of over 1000 J m-2) by using the synergistic effect of the strong interfacial bridging and high hysteresis of hydrogels (polyacrylamide-alginate hydrogels), which could be capable of tightly adhering to diverse wet surfaces.27 However, the tensile strength of these polyacrylamide-alginate hydrogels was less than 200 kPa,28, 29 implying that these adhered hydrogels could not withstand strong external forces. In fact, several practical applications require these hydrogel substrates which can sustain the strong mechanical loads.30, 31 In addition, high transparency and good self-recovery are also needed for some special applications of adhered hydrogels such as stretchable touch screen32 and optical devices33, 34. However, the high-performance hydrogels integrated with strong wet adhesion, high transparency, excellent mechanical properties and fast self-recovery are still not available. Here, we designed transparent tough aluminum hydroxide nanocomposite hydrogels that exhibited high transparency (over 95%) in visible light region. These tough hydrogels showed high tensile strength of up to 1 MPa and fast self-recovery that the stretched hydrogel could recover to 90% of their original state after relaxing for 1 h. Subsequently, these transparent hydrogels could be stitched together by chitosans that were bonded covalently to hydrogel networks. This covalent bridging enabled two hydrogels to form a complete network, generating a strong adhesion interface.


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Furthermore, the effect of interfacial bridging interactions and mechanical properties of hydrogels on the adhesion energy was also systematically examined. Both interfacial adhesion energy and shear adhesion strength were affected by the toughness of hydrogels. Notably, our hydrogels could be assembled into various high stretchable balloons that showed excellent sealing and pressureresistant properties. Most importantly, the combination of superior sealability, high transparency, exceptional mechanical properties and fast self-recovery enabled the hydrogel balloons to turn into fast tunable-focus convex lenses. These transparent tough hydrogels will hold great potential in optical lens and flexible stretchable sealed devices. RESULTS AND DISCUSSION The tough aluminum hydroxide nanocomposite hydrogels (Al-NC gels) were synthesized by in-situ polymerization of acrylic acid (AA) and N,N-dimethylacrylicamide (DMAA) in the present of Al(OH)3 nanoparticles solution, referring to our previous work.35 The Al-NC gels were defined as AD10Alx hydrogels, where x represented the weight ratio of Al(OH)3 to water (Al(OH)3/H2O = x %) and the ratio of AA to total monomers was fixed at 10 mol %. The FTIR spectra demonstrated that the

polymer chains of the Al-NC gel were attached to nanoparticle surfaces by ion coordinations (Figure S1). The scanning electron microscope (SEM) revealed that the surface of the Al-NC gel possesses abundant porous structures in micrometer range (Figure 1b), which was beneficial for diffusion of bridging polymer solution into the hydrogel substrate. Subsequently, the wet adhesion of hydrogel-hydrogel interface was carried out, as illustrated in Figure 1a. The bridging polymer solution containing chitosan and coupling reagents was spread on the hydrogel surface and then covered by other hydrogel film. During compression, the bridging polymers could gradually diffuse into hydrogel bulk and then bonded to the hydrogel networks though amide bonds to stich two hydrogels together. To confirm the formation of the bridging structure at interface, the SEM image of the adhered interface was measured, in which two hydrogel surfaces had been tightly bonded together to form a complete network (Figure 1c). Benefiting from this strong adhesive interface, two hydrogels could be stretched together without relative sliding when one layer was lengthened (Figure 1d). In contrast, the interfacial debonding behavior between two hydrogel films occurred for the weak adhesive interface formed by gelatin instead of chitosan as the adhesive, indicating the weak adhesive performance (Figure 1e and Figure S3). The bridging network at the interface enabled the overlapping part of two hydrogels to maintain the identical deformation during



Figure 1 The preparation of adhered hydrogels. a) Schematic illustration of the wet adhesion of aluminum hydroxide nanocomposite hydrogels (Al-NC gels). b) The SEM images of the surface of the as-prepared Al-NC gel. c) The SEM images of the adhered interface. d) The adhered Al-NC gel fabricated by chitosan can withstand large deformation without debonding. e) The adhered Al-NC gel prepared by gelatin is vulnerable to debonding under the large deformation. f) The T-peeling process of the adhered Al-NC gel. g) The force-extension curve of the T-peeling test of the adhered Al-NC gel. stretching. We next quantitatively explored the interfacial adhesion energy by the T-peeling adhesion test. The schematic diagram of peeling samples was illustrated in Figure S2 and the peeling process of the adhered AD10Al2.5 hydrogels (the molar ratio of AA to total monomers was 10 mol% and the weight ratio of Al(OH)3 to H2O was 2.5 %) was showed in Figure 1f. During the peeling process, the crack emerged at adhered interface and then gradually propagated along the unpeeled interface,


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accompanied by stretched fragments across the peeling region. From the force-extension curve of T-peeling test, the interfacial adhesion energy (2216 J m-2) of adhered AD10Al2.5 hydrogel could be obtained (Figure 1g). Besides, the influence of bridging polymer types on the interfacial adhesion energy was investigated (Figure S3). It could be seen that chitosan resulted in a higher interfacial adhesion energy than other bridging polymers (less than 100 J m-2), which might be attributed to higher content of primary amines on the chitosan. By contrast, if only employing the coupling reagents or the bridging polymer (chitosan) alone, the adhesion energy could reach 50 J m-2 and 557 J m-2, respectively. These results indicate that the abundant strong interfacial bridging interactions (covalent bonds) can lead to a high dissociation energy of the adhered interface during the peeling process. On the other hand, the procedure of achieving adhesion may also play a great impact on the adhesion energy.36 We first explored the interfacial adhesion energy of the adhered hydrogels suffering different compressive force during the preparation process. Figure S4 showed that the adhesion energy gradually increased from 1200 to 2216 J m-2 with improving the external compressive force from 0 to 100 N cm-2. However, after the compressive force exceeded 100 N cm-2, increase of the compressive force would diminish the adhesion energy. It could be owing to extrusion of some bridging polymers from the hydrogel-hydrogel interface under a higher compression. We next investigated the variation of the interfacial adhesion energy over the compression time. The adhesion energy reached approximately 1000 J m-2 after compression for 30 minutes and then almost attained the balance value of around 2100 J m-2 after continuation for another 90 minutes (Figure S5). The increase of adhesion energy could result from gradual formation of covalent cross-linked networks across the hydrogel-hydrogel interface over time. In addition, the adhesion energy was also strongly affected by the concentration of the bridging polymer. When the concentration of chitosan solution increased from 0 to 2 wt %, the adhesion energy grew from 140 to 2185 J m-2, whereas the excessive chitosan would undermine the adhesion effect (Figure S6). This was because that the appropriate improvement of the chitosan concentration could enlarge the bridging density across two hydrogels, which led to a higher adhesion energy.24 However, the excessive chitosan concentration could cause the entanglements of chitosan chains on the hydrogel surfaces, limiting the diffusion of bridging polymers and undermining the adhesion



Figure 2 The mechanical properties and adhesion energy of the adhered Al-NC gels with different Al(OH)3 nanoparticle content. a) The stress-strain curves, b) compressive curves of the Al-NC gels having different crosslinking agent content. c) The effect of the Al(OH)3 nanoparticle content on the interfacial adhesion energy of the adhered Al-NC gels. d) The relationship between the interfacial adhesion energy and toughness of the Al-NC gels. property. Furthermore, the medium viscosity chitosan was beneficial to achieving a higher adhesion energy than other kinds of chitosan (Figure S7). During diffusion of bridging polymers, the low viscosity chitosan chains easily penetrated into the hydrogel substrates, while the excessive chitosan diffusion would reduce the bridging structures. In contrast, the high viscosity chitosan chains could form entanglements on the hydrogel surface to undermine their diffusion. 37 The medium viscosity chitosan could balance the relationship between diffusion and bridging structures. Based on these results, the best adhesion performance can be yielded under the optimum conditions (compression force of 100 N cm-2, compression time of at least 8 hours, bridging polymer concentration of 2 wt % and medium viscosity chitosan). In addition to the interfacial bridging interactions, the interfacial adhesion energy of adhered hydrogels is also dependent on the energy dissipation capacity of the hydrogels.25 To examine the


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influence of the mechanical properties of the hydrogels on the interfacial adhesion energy, a series of hydrogels with different toughness were prepared via adjusting the nanoparticle content in the polymer matrix. With the increase of the Al(OH)3 content from 0.5 to 3.0 wt %, the tensile strength and compressive strength gradually increased from 414 to 1091 kPa and 5 to 38 MPa, respectively (Figure 2a). Notably, the tensile strength of the AD10Al2.5 hydrogel was far higher than that of alginate-PAAm adhesive systems,28 implying that our Al-NC gel could endure the greater external force. Besides, the toughness of Al-NC gels, calculated from the area under the stress-strain curve, displayed the same trend to the tensile strength when varying the nanoparticles content. Improving the Al(OH)3 content from 0.5 to 3.0 wt % caused a significant increase of the toughness from 2151 to 6479 kJ m-3 (Figure S8). This could be owing to the fact that increasing the crosslinked density by raising the nanoparticle content enhances the energy dissipation capacity of hydrogels. Additionally, the effect of the nanoparticle content on the interfacial adhesion energy of adhered hydrogels was also explored. It could be found that when the addition of Al(OH)3 rose from 0.5 to 2.5 wt %, the adhesion energy gradually increased from 519 to 2185 J m-2, while further improvement of the Al(OH)3 content would undermine the adhesion performance (Figure 2c). Furthermore, we found that when the Al(OH)3 content exceeded 3 wt %, the hydrogels (AD10Al3.5 and AD10Al4) almost could not be bonded together by this proposed method. In other word, when the Al(OH)3 content was low, the improvement of nanoparticle addition within the hydrogels could lead to a notable increase in the interfacial adhesion energy, since the interfacial adhesion energy relied heavily on bridging interactions between hydrogels and the hydrogel toughness.25,

27, 36

However, the excessive Al(OH)3 content could limit the diffusion of bridging polymers into hydrogels, which resulted in decreasing bridging structures and undermining the adhesion performance. The further research demonstrated that when the Al(OH)3 content did not exceed 2.5 wt %, increase of the toughness of adhered hydrogels resulted in the improvement of the interfacial adhesion energy of adhered hydrogels (Figure 2d), which was in agreement with previously reported studies.24, 25. Based on the above results, it can be concluded that the optimal adhesion energy will be achieved at 2.5 wt % content of Al(OH)3. Besides the above-mentioned T-peeling test, the interfacial adhesion property of the adhered hydrogels can be also measured by other modes of deformation. For instance, an adhered AD10Al2.5 hydrogel could

lift

up

200

g

weight

without

breaking

the


lap

part

due

to

the

high


Figure 3 The shearing test and self-recovery of the adhered hydrogels. a) The adhered AD10Al2.5 hydrogel with a junction contact area of 2×2.5 cm2 can lift up a 200 g weight. b) The lap-shear test process of the adhered AD10Al2.5 hydrogel. c) The influence of the Al(OH)3 content on the shear adhesion strength of the adhered Al-NC gels. d) The relationship between the shearing adhesion energy and the toughness of the Al-NC gels. The shearing adhesion strength shows a linear dependence on the toughness of hydrogels (R2=0.98). e) The self-recovery property of the AD10Al2.5 hydrogel. mechanical strength of this hydrogel and strong interfacial adhesion (Figure 3a). To evaluate the adhesion performance of the lap part, the lap-shear test of the adhered hydrogel was performed (Figure 3b). During the lap-shear test, the adhered hydrogel could be lengthened and then fractured without damaging the lap part, indicating that the lap part was strongly bonded together. Subsequently, we further explored the impact of the nanoparticle content on the shearing adhesion strength of the adhered hydrogels. The shearing adhesion strength of samples increased from 176 to 385 N/m with improving the addition of Al(OH)3 nanoparticles from 0.5 to 3.0 wt % (Figure 3c). The maximum shearing adhesion strength was much superior to those of adhered hydrogels that were prepared by using nanoparticles (~20 N/m) or ionic coordination (172 N/m) as the bridging


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Figure 4 The preparation of hydrogel balloons. a) The inflated hydrogel balloon displays good sealability and recoverability. b) The cylindrical hydrogel balloon can be inflated into more than 100-fold its original volume. c) Three of the same inflated hydrogel balloons can support 1 kg weight without rupture. d) The burst pressure of hydrogel balloons with different thickness. (scale bars: 2 cm) agent.13, 16 The shearing adhesion strength exhibited a linear dependence on the toughness of the hydrogel substrates (Figure 3d). This result could be ascribed to the appearance of fracture in hydrogel substrates rather than the lap part, so the shearing adhesion strength highly relied on the toughness of hydrogels. Based on the outstanding adhesion and good mechanical properties, the Al-NC gels could be assembled into various devices. For example, our transparent AD10Al2.5 hydrogels could be processed into a round hydrogel balloon using the method mentioned here (Figure S9), which was able to turn into a big round sphere after inflation (Figure 4a). Interestingly, this inflated balloon almost remained unchanged in size after being placed in sealed bag for 1 h and then recovered to the original shape without leaving deformation after deflation and relaxation for 1 h. Besides, even after 100 times inflation-deflation cycles the inflated hydrogel balloon could still recovery to its initial state after resting 1 h. These could be owing to good sealability and fast self-recovery of the adhered hydrogels. The loading-unloading test revealed that the energy dissipation capacity (hysteresis loop area) and elastic modulus of the stretched hydrogel could recover to nearly 90% of their original value and their primary state respectively after waiting for 1 h (Figure 3e), indicating that the recovery rate of our hydrogels was much faster than that of the alginate-PAAm hydrogels .28The cylindrical adhered AD10Al2.5 hydrogel balloon was also able to expand up to



more than 100-fold its original volume without rupture (Figure 4b), showing a high stretchability. Besides, our inflated hydrogel balloons could endure high external load (Figure 4c). For example, three same inflated hydrogel balloons could together support 1 kg weight without leakage and rupture. To further investigate the pressure-resistant property of the hydrogel balloons, the burst pressure of the hydrogel balloons with different thickness was measured (Figure S10). During the test process, the hydrogel balloon gradually grew in volume and then ruptured in the center (Figure S11). The burst pressure of hydrogel balloon increased from 2.4 to 8.2 kPa with improving the thickness from 1 to 4 mm (Figure 4d). In short, based on the strong wet adhesion and excellent mechanical property, our Al-NC gels could be refabricated into stretchable balloons that showed superior sealability and pressure resistance.

Figure 5 The fabrication of hydrogel optical lenses. a) The hydrogel balloon (inner diameter of 30 mm and outer diameter of 50 mm) can turn into a convex lens that is able to enlarge or invert the images. b) The AD10Al2.5 hydrogel shows high transparency (over 95% in visible region). c) The relationship between the focal length of hydrogel lens (inner diameter of 30 mm) and liquid content. d) The effect of the distance between the hydrogel convex lens (inner diameter of 30 mm and 6 mL mineral oil) and object on the image status. e) The influence of liquid content within the hydrogel lens on the image status. The distance between the hydrogel lens and object is fixed at 40 mm and the inner diameter of lens is 30 mm


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More importantly, our hydrogel balloons could be also designed into optical lenses due to their combination of the exceptional sealing property, high stretchability and transparency (over 95% in visible light region) (Figure 5b). For instance, the hydrogel balloon (inner diameter of 30 mm and outer diameter of 50 mm) could become a convex lens after being filled with transparent liquids (mineral oil). This hydrogel convex lens could enlarge the size of the virtual object with a logo ‘BIT’ and even inverted the image of distant object (Figure 5a). The hydrogel convex lens could also magnify, blur and even invert the virtual images of the logo with ‘BIT’ when gradually increasing the distance between the hydrogel convex lens and the object (Figure 5d). Besides, the focal length of this hydrogel convex lens could be easily adjusted by controlling the addition of the transparent liquid. For instance, the focal length of the hydrogel convex lens fast decreased and the image was enlarged, followed by being blurred and inverted within 5 seconds when increasing the addition of the mineral oil (Figure 5e). This result presented that the tunable-focus process of our hydrogel lens was more facile than those of electrically tunable elastomer lenses with high voltage. 38, 39

Besides, compared to other convex lenses achieved by regulating the swelling degree of

hydrogels, 33, 34,40, 41 our hydraulically driven hydrogel convex lens could quickly adjust its focal length under ambient conditions. Additionally, the dependence of the focal length on the liquid addition was also examined. For the hydrogel lens, when raising the content of mineral oil from 2 to 8 mL, the focal length decreased from 38.3 to 20.0 mm (Figure 5c). That means our hydrogel convex lens can be rapidly set to a certain focal length by accurately controlling addition of mineral oil in the hydrogel balloon. Thus, this hydraulically driven hydrogel convex lens with a fast and facilely tunable focus will open up a new approach for achieving fast tunable-focus optical devices.

CONCLUSION In summary, highly transparent tough nanocomposite hydrogels with high mechanical strength and quick self-recovery had been prepared. These nanocomposite hydrogels could be strongly bonded together by anchoring bridging polymers to the hydrogel networks though covalent bonds. The interfacial adhesion energy seriously relied on the bridging interactions located at interface and the mechanical properties of the hydrogels. The optimized interfacial adhesion and shearing adhesion strength were up to 2216 J m-2 and 385 N/m, respectively. Owing to the strong wet adhesion and exceptional mechanical properties, our nanocomposite hydrogels could be assembled into high stretchable balloons with quick self-recovery, superior sealability and pressure resistance. Most



importantly, the combination of outstanding sealing property and high transparency also enabled the hydrogel balloons to turn into hydraulically driven fast tunable-focus convex lenses. These adhered hydrogels with the strong adhesion property, high transparency, good mechanical properties and rapid self-recovery will have a great potential in high stretchable sealed devices and fast tunablefocus lenses. EXPERIMENTAL SECTION Materials. The aluminum hydroxide sol (particle size of 2-3 nm) was obtained from Suzhou Nanodispersions Co., Ltd. N,N-dimethylacrylamide (DMAA) monomer was purchased from TCI (Shanghai) Development Co., Ltd, China and further purified by filtering through activated alumina. Acrylic acid (AA), 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), NHydroxysuccinimide (NHS), 4-morpholineethanesulfonic acid (MES), chitosan (low viscosity (400 mPa⋅s)) and 2hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (photoinitiator) were purchased from Aladdin, China and used as received. Other chemical reagents used in this study were also without further purification. Preparation of tough nanocomposite hydrogels. The aluminum hydroxide nanocomposite hydrogels (Al-NC gels) were synthesized following our previous procedure35. In brief, the aluminum hydroxide sol solution containing 0.3 mol L-1 acrylic acid (AA), 2.7 mol L-1 N,Ndimethylacrylamide (DMAA) and photoinitiator were stirred with bubbling N2 for 20 min. Subsequently, the mixture was transferred into the glass mold (100mm×100mm×2mm) or glass tubes (internal diameter of 3mm) and next exposed to the UV light (λ=365 nm) for 8h. Fabrication of the adhered hydrogels. Chitosan (medium viscosity: 200–400 mPa⋅s) serving as the bridging polymer was dissolved into 2.0 wt % MES buffer to prepare the bridging polymer solution and the concentration of the chitosan was fixed at 2 wt % unless otherwise stated. Then, the homogeneous polymer solution containing EDC and NHS (2:1 molar ratio) was rapidly poured on the hydrogel surface, followed by covering the coating solution with other hydrogel. This hydrogel bilayer was pressed under the pressure of 100 N/cm2 for 8 h to prepare the adhesion interface unless otherwise noted. The hydrogel balloons were fabricated by bonding the edge of AD10Al2.5 hydrogels with special shapes together using the proposed method. The hydrogel lenses could be obtained by injecting the mineral oil into the hydrogel balloons though the plastic pipes


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that had been embedded in balloons using cyanoacrylate glue. Sample characterization. The mechanical properties of Al-NC gels were measured on the Shimadzu ASG-J electronic universal testing machine with the tensile speed of 100 mm/min and compressive speed of 5 mm/min. For the loading-unloading tests, the crosshead speed was always fixed at 100 mm/min and the energy dissipation capacity was calculated from the closed-loop area of loading-unloading curves. The T-peeling adhesion tests were performed to measure adhesion energy following the previous reports.27, 36 The back of adhered hydrogels was bonded to a fiberreinforced tape and the peel speed was fixed at 50 mm/min. The peeling adhesion energy was twice the ratio of the peeling force at plateau to the width of the sample. The lap shear tests of the adhered hydrogels with the joint area of 20 mm×20 mm were performed at a speed of 100 mm/min and the shearing adhesion strength (force per width) was calculated by dividing the max load by the width of the sample.16 The thickness of the samples for all the adhesion tests was fixed to 2 mm. The burst pressure of hydrogel balloons was measured by inflating air into the balloons (inner diameter of 30 mm and outer diameter of 50 mm) at an air flow rate of 50 mL/min and probing the rupture internal pressure with pressure gauge. The focal length of the hydrogel convex lens was measured referring to the previously reported method.39 We shined the hydrogel convex lens using white light to form a spot on the screen and then adjusted the screen to obtain a minimal spot. At this moment, the distance between the hydrogel lens and the movable screen with the minimal spot was defined as the focal length.

ASSOCIATED CONTENT Supplementary Information The Supporting Information is available free of charge on the ACS Publications website. The FTIR spectra of hydrogels, schematic diagram of peeling samples, the interfacial adhesion energy of adhered hydrogels prepared with different bridging polymers, compressive forces, compressive time, bridging polymer concentration and polymer molecular weight, the toughness of nanocomposite hydrogels, the preparation of hydrogel balloon, the test method of burst pressure, the burst process of hydrogel balloon. AUTHOR INFORMATION Corresponding Author



*Email: [email protected] ORCID Feibo Li: 0000-0003-2499-5199 Huanjun Li: 0000-0003-1907-2111 Notes: The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financed by Natural Science Foundation of China (21736001 and 21174017) and the Beijing Municipal Natural Science Foundation of China (2102040). REFERENCES

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Strong Wet Adhesion of Tough Transparent Nanocomposite

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