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Letter

Rough Adhesive Hydrogels (RAd gels) for Underwater Adhesion Laura C. Bradley, Nathan D Bade, Lisa Mariani, Kevin T. Turner, Daeyeon Lee, and Kathleen J. Stebe ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08916 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Rough Adhesive Hydrogels (RAd gels) for Underwater Adhesion Laura C. Bradley,1 Nathan D. Bade, 1 Lisa M. Mariani,2 Kevin T. Turner, 2 Daeyeon Lee1* and Kathleen J. Stebe1* 1

Department of Chemical and Biomolecular Engineering, 2Department of Mechanical

Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States *Corresponding Authors: [email protected]; [email protected]

Keywords: hydrogel, adhesion, surface roughness, PHEMA, biocompatible, drainage

Abstract: In this work, underwater adhesion is achieved between biocompatible hydrogels and a suite of substrates. Surface roughness, which is typically detrimental for adhesion in air, is shown to be beneficial for underwater adhesion. Contact between the hydrogels with macroscopically flat substrates, and the resulting non-specific chemical interaction, is facilitated by surface roughness, which enables drainage of the lubricating fluid layer. Hydrogel composition plays an important role in tuning the gel elasticity and interaction with the substrate. Hydrogels that are adhesive on two sides are synthesized for potential use as versatile adhesives in various applications.

Text: Advances in underwater adhesion have inspired recent studies focused on developing materials capable of underwater adhesion for biomedical and environmental applications.1 Most notably, the finding that sea mussels use catechol-moieties2,3 to strongly adhere to surfaces in the ocean has led to the development of a variety of catechol-functionalized materials, including hydrogels, 4 , 5 polymer cements/glues, 6 , 7 and coatings. 8 , 9 The distinct advantage of catechol chemistry is the strong adhesion to surfaces with a range of compositions and mechanical properties. However, recent studies show that oxidation of catechol moieties can produce cytotoxic concentrations of reactive oxygen species, such as hydrogen peroxide, potentially limiting their biocompatibility.10,11

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For biomedical applications, underwater adhesives need to be biocompatible and complementary to soft, wet biological tissues or biomimetic hydrogels. Current alternatives to catechol-based adhesives for biomaterials typically require harsh conditions, such as chemical reaction, heat, light exposure, or non-physiological pH. 12 - 14 It is thus desirable to develop adhesion schemes that do not require harsh conditions or invoke cytotoxic responses. Recently, nanoparticle solutions have been used to adhere hydrogels and biological tissues. 15 , 16 Nanoparticles facilitate adhesion by adsorption of the hydrogel or tissue matrix to the particle surface so that particles bridge the two substrates. Nanoparticles, however, have not been shown to enable adhesion between substrates with pronounced differences in mechanical properties (i.e., between hard and soft surfaces), which is often necessary to implant medical devices in vivo (e.g., the electrodes of neurostimulator devices). Biocompatible adhesives that are compatible to substrates of different chemistries and mechanical properties are critical to universal use in vivo.17 In this Letter, we demonstrate the synthesis of a rough adhesive hydrogel (RAd gel) that exhibits underwater adhesion to both hard and soft substrates. The hydrogel matrix is comprised of

poly(2-hydroxyethyl

methacrylate)-co-poly(ethylene

glycol)

diacrylate

(PHEMA-co-

PEGDA), polymerized by a thermally initiated free-radical mechanism. RAd gels are synthesized in an open-face set-up by polymerizing the reaction solution containing water, monomer (HEMA and PEGDA), and initiator (ammonium persulfate) in a glass Petri dish with a lid. The reaction solution does not fill the entire volume of the Petri dish, leaving an air gap above the liquid. Polymerization in this open-face set-up produces a rough surface on the top side that is in contact with the air gap, and a relatively smooth surface on the bottom side that is in contact with the glass Petri dish (Figure 1a,b). During polymerization, the air gap enables evaporation (primarily of water and a small amount of HEMA) which results in the top (vapor) side of the hydrogel having a higher concentration of polymer (45 wt.% polymer) compared to the bottom (glass) side (30 wt.% polymer) (Figure S1). Confocal microscopy of RAd gels immersed in a dyed water bath illustrates the difference in surface roughness of the two sides (Figure 1c). To quantify the root mean square (RMS) roughness, the top and bottom surfaces of a RAd gel are molded using a silicone elastomer (Smooth-On Mold Star) (Figure S2). Profilometry on these molds measures RMS roughnesses (1.98 mm scan length) of 73±3 µm and 4±1 µm for the rough (top) and smooth (bottom) sides, respectively (Figure 1d). The roughness 2 ACS Paragon Plus Environment

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on the top (vapor) side may be attributed to unconstrained polymer phase separation at the free air-liquid interface, as well as, potentially, an evaporation-induced Bénard-Marangoni convection. 18 , 19 PHEMA is originally chosen as the base of the polymer matrix due to its biocompatibility. 20 Both the rough and smooth sides of RAd gels support attachment and spreading of anchorage-dependent fibroblasts (Figure S3). The rough side of RAd gels can be adhered underwater to a variety of substrates with different composition and mechanical properties. Underwater adhesion is achieved by pressing the hydrogel to the substrate for twenty seconds using a force on the order of 10 N, and then allowing the interface to set for five minutes. The rough hydrogel surface adheres to either the rough or smooth surfaces of other PHEMA-co-PEGDA hydrogels, as well as to polystyrene (PS) Petri dishes, polydimethylsiloxane (PDMS), carbon steel razor blades, and glass coated with fluorinated silane (Supporting Information Videos 1-6). The adhesion to PS is also achieved in PBS buffer or at elevated temperature (80°C) (Supporting Information Videos 7-8). We estimate a lower bound for the shear adhesion strength of 1.8 kPa from single lap shear tests on hydrogels adhered to PDMS; RAd gels do not exhibit adhesive failure and instead break at the edge of the adhered area or within the bulk (Figure S4). The ability of RAd gels to lift a weighted PS Petri dish (adhered area = 1cm2; total weight = 24g) suggests a higher minimum for the shear adhesion strength of 2.4 kPa (Figure 1e, Supporting Information Video 9). The estimated adhesion strength on the order 1 kPa is the same magnitude as previously reported nanoparticle adhesives, and smaller than that of catechol-based systems.15,21 Interestingly, adhesion is only observed for the rough surface; the smooth surface is not adhesive to itself or other substrates. The strong adhesion exhibited by the rough surface contradicts a general rule accepted for adhesion in air where smooth surfaces promote adhesion.22,23 To study the mechanism that allows the rough hydrogel surface to achieve underwater adhesion, we first investigate how contact is made between the hydrogel and substrate. Confocal microscopy images of the rough surface pressed to a PS Petri dish containing dyed water show that large patches of polymer adhere, trapping water between these features (Figure 2a). The rough topography facilitates liquid drainage through interconnected valleys to enable the peaks to make contact with the Petri dish.24 Fluid drainage in our system is analogous to drainage during the approach of a structured surface to a flat substrate in a liquid environment.25 During approach, there is a transition from fluid flowing radially to draining through surface networks. 3 ACS Paragon Plus Environment


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Scaling analysis by Gupta and Frechette reports that the onset of fluid drainage cannot be correlated to a single length scale describing the surface structure.25 The toe pad of torrent tree frogs offers an example from nature of how structured surfaces comprised of densely packed polygonal epithelial cells (10-15 µm wide, spaced ~1 µm apart) achieve adhesion in flooded conditions due to fluid drainage.26,27 In contrast to the rough surface, when the smooth side of RAd gels is pressed to contact a Petri dish, the hydrogel delaminates and floats in solution. Pressing the smooth surface of the hydrogel onto the PS substrate underwater traps a liquid layer which prevents two surfaces from making intimate contact for adhesion.24 The difference in contact is also observed by adhering the two hydrogel surfaces to a PS Petri dish and then dragging the Petri dish through the water bath (Figure 2b, Supporting Information Video 10). The hydrogel adhered by the smooth side delaminates due to a lubricating liquid layer between the hydrogel and PS. In contrast, the hydrogel adhered using the rough side remains secured to the Petri dish, even after increasing agitation. We attribute the adhesion to two key features. The surface roughness of the hydrogels promotes contact with substrates by facilitating drainage of liquid, while the chemistry of adhesion occurs through interactions between the polymer backbone and substrate. Due to the wettability and compositions of the adhered substrates, hydrogen bonding and electrostatics can be eliminated as potential interactions facilitating adhesion. Hydrophobic interactions are probably relevant, and van der Waals attractions may also play a role. We find that the rough side can adhere to substrates with receding contact angles ≥50°, whereas there is no adhesion on fully water-wetting glass surfaces (i.e., water receding contact angle of ~ 0°) (Table S1). Indeed, hydrophobic interactions between PHEMA polymer chains are known to influence the structure and properties of PHEMA-based hydrogels, 28 , 29 and, therefore, are also expected to occur between the polymer matrix of RAd gels and hydrophobic surfaces. Both surface roughness and hydrophobic surface chemistry are required for adhesion; the failure of the smooth side to adhere shows that underwater adhesion of the hydrogels is not possible without the rough surface topography facilitating liquid drainage, and thus contact with the substrate surface. We examine the adhesive property of hydrogels as a function of polymer concentration and surface roughness. Hydrogels are synthesized in a closed-cell comprised of a glass slide as the top plate and a rough mold (Figure S2) as the bottom plate to produce hydrogels with uniform composition containing a smooth and rough surface (Table 1). Small variations in the 4 ACS Paragon Plus Environment

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hydrogel composition from 28 to 32 wt.% polymer reveal a significant difference in adhesion behavior. In all cases, the smooth sides of the hydrogels are not adhesive. The rough sides of hydrogels with 23 and 28 wt.% polymer exhibit underwater adhesion, whereas the rough sides of hydrogels with 32 or 45 wt.% polymer fail to adhere. The transition in adhesion with increasing polymer composition may be attributed to changes in the elastic energy of the bulk hydrogel relative to the adhesive energy at the interface. With increasing polymer concentration, and therefore higher elastic stiffness, there is more elastic strain energy stored in the hydrogel when the rough surface of the hydrogel is pressed into conformal contact and thus the adhesion energy must be higher to hold the surfaces in contact once the applied force is removed. We are able to probe the effect of bulk polymer concentration at constant surface roughness and composition by comparing the hydrogel with 45 wt. % polymer synthesized in the closed-cell and the original hydrogel (i.e., bulk concentration of polymer = 30 wt%) synthesized in the open-face set-up. The former has uniform composition throughout the gel, whereas the latter gives a gel that has a bulk/average concentration of 30 wt% but has a locally high polymer concentration of 45 wt.% at the rough surface (Figure 1 and Figure S1). Despite having the same polymer composition at the rough surface, only the soft hydrogel with the lower bulk polymer concentration (30 wt.%) is adhesive. The contributions of the surface and bulk properties to the adhesion of RAd gels are not independent. This interplay between the surface and bulk has been exploited in other systems by incorporating reversible cross-links in hydrogels to improve adhesion.1,30 The optimization of the RAd gel adhesive behavior and mechanical properties is a focus of our ongoing work. Our observations also raise questions regarding the lengths scales of roughness that best facilitate fluid drainage via surface topography. The surface features on RAd gels (~100 µm) are an order of magnitude larger that features used in biomimetic surfaces (~10 µm); larger channels for drainage may facilitate more rapid contact. Adhesion is also not limited to aqueous systems. Hydrated RAd gels can be adhered to PS in glycerol and hexadecane (Supporting Information Videos 11 and 12). Successful adhesion in glycerol demonstrates RAd gels can support the fluid drainage in viscous environments. This may be attributable to two effects; the large surface features are expected to aide drainage, and a miscible glycerol/water front may reduce the local viscosity of the fluid, as well as change the properties of the hydrogel. Interestingly, successful adhesion in hexadecane indicates miscibility 5 ACS Paragon Plus Environment


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between water and the surrounding fluid is not required. When pressure is applied, water expelled from the hydrogel creates a water film in this hexadecane environment; adhesion relies on drainage of both the hexadecane and this expelled water, which can be influenced by capillarity. Our future work will build on these observations to elucidate the importance of surface roughness in inducing underwater adhesion. Lastly, we are able to induce roughness on both sides to fabricate double-sided RAd gels using an open-face set-up with a rough mold as the bottom surface (Figure 3a). In agreement with our previous results, double-sided adhesive hydrogels can perform underwater adhesion to tether PS, PDMS, or non-adhesive hydrogel substrates (Figure 3b,c). The adhesion is robust enough to withstand bending and folding of the adhesive hydrogel tether (Supporting Information Videos 13-15). In conclusion, we demonstrate the synthesis of RAd gels that adhere underwater to a variety of substrates with different chemistries and moduli. The non-specific chemical interaction of the PHEMA-co-PEGDA polymer matrix eliminates the need for substrates to have a specialty coating or surface chemistry. Rough surface topography is critical to facilitating contact of the hydrogel and substrate, which is counterintuitive to adhesives used in air. The synergistic effects of rough topography with non-specific chemical interactions, in conjunction with biocompatibility of the polymer matrix, opens opportunities for RAd gels to be used as versatile adhesives in biomedical applications.

Acknowledgements: We acknowledge funding from NSF DMR-1120901 (Penn MRSEC), and NSF 1435745. LCB is supported by the Penn Provost’s Fellowship for Academic Diversity and the Africk Family PostDoctoral Fellowship. We also acknowledge Professor Cherie R. Kagan for use of ATR-FTIR, Professor Ivan Dmochowski for use of the confocal microscope, Dr. Richard Grote for assistance with profilometry, and Professor Richard K. Assoian for assistance with cell culture.

Supporting Information: Details on synthesis procedures, characterization methods, single-lap shear tests, and descriptions of the videos are provided in the Supporting Information.


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Figure 1. a) Schematic of the hydrogel synthesis in an open-face set-up comprised of a glass Petri dish and lid. During polymerization, there is evaporation and condensation of water and HEMA monomer. b) Images of the top (vapor) and bottom (glass) sides of RAd gels. c) Crosssection confocal microscopy stacks of the rough and smooth sides of hydrogels immersed in a water bath containing fluorescein sodium salt (green). d) Profilometry scans of the hydrogel topography collected on Smooth-On molds. e) RAd gel adhered underwater by the rough side lifting a weighted PS Petri dish.



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Figure 2. a) Confocal image of the adhered interface between a rough hydrogel surface and a PS Petri dish. Adhesion was performed in a water bath dyed with fluorescein sodium salt (green). b) Series of images depicting underwater adhesion of the hydrogel surfaces to a PS Petri dish under shear. As the Petri dish is moved through the water bath, the hydrogel adhered by the smooth side slips off, whereas the hydrogel adhered by the rough side remains attached.


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Table 1. Adhesive behavior of hydrogels as a function of composition and surface topography. Smooth

Rough

Surface

Surface

23

Not Adhesive

Adhesive

28*

Not Adhesive

Adhesive

32

Not Adhesive

Not Adhesive

45

Not Adhesive

Not Adhesive

Polymer wt %

* denotes the original recipe for the hydrogels previously discussed in detail.

Figure 3. Schematics of a) the open-face set-up to synthesize double-sided adhesive hydrogels, and b) the tethered configuration for testing double-sided adhesion. c) Images of double-sided adhesive hydrogels as tethers for adhering PS (left), PDMS (middle), and non-adhesive hydrogels (right). Scale bars are 25 mm.



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References: (1) Peak, C. W.; Wilker, J. J.; Schmidt, G. A Review on Tough and Sticky Hydrogels. Colloid Polym Sci 2013, 291, 2031–2047. (2) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448, 338-341. (3) Sedó, J.; Saiz-Poseu, J.; Busqué, F.; Ruiz-Molina, D. Catechol-Based Biomimetic Functional Materials. Adv. Mater. 2013, 25, 653–701. (4) Skelton, S.; Bostwick, M.; O'Connor, K.; Konst, S.; Casey, S.; Lee, B. P. Biomimetic Adhesive Containing Nanocomposite Hydrogel with Enhanced Materials Properties. Soft Matter, 2013, 9, 3825–3833. (5) Xu, J.; Strandman, S.; Zhu, J. X. X.; Barralet, J.; Cerruti, M. Genipin-Crosslinked CatecholChitosan Mucoadhesive Hydrogels for Buccal Drug Delivery. Biomaterials 2015, 37, 395-404. (6) Zhao, Q.; Lee, D. W.; Ahn, B. K.; Seo, S.; Kaufman, Y.; Israelachvili, J. N.; Waite, J. H. Underwater Contact Adhesion and Microarchitecture in Polyelectrolyte Complexes Actuated by Solvent Exchange. Nat. Mater. 2016, 15, 407-412. (7) Kim, H. J.; Choi, B.-H.; Jun, S. H.; Cha, S. J. Sandcastle Worm-Inspired Blood-Resistant Bone Graft Binder Using a Sticky Mussel Protein for Augmented In Vivo Bone Regeneration. Adv. Healthcare Mater. 2016, 5, 3191–3202. (8) Yang, S. H.; Kang, S. M.; Lee, K.-B.; Chung, T. D.; Lee, H.; Choi I. S. Mussel-Inspired Encapsulation and Functionalization of Individual Yeast Cells. J. Am. Chem. Soc. 2011, 133, 2795–2797. (9) Scognamiglio, F.; Travan, A.; Turco, G.; Borgogna, M.; Marsich, E.; Pasqua, M.; Paoletti, S.; Donati, I. Adhesive Coatings Based on Melanin-Like Nanoparticles for Surgical Membranes. Colloids Surf., B 2017, 155, 553-559. (10) Meng, H.; Li, Y.; Faust, M.; Konst, S.; Lee, B. P. Hydrogen Peroxide Generation and Biocompatibility of Hydrogel-Bound Mussel Adhesive Moiety. Acta Biomater. 2015, 17, 160– 169. (11) Meng, H.; Liu, Y.; Lee, B. P. Model Polymer System For Investigating the Generation of Hydrogen Peroxide and its Biological Responses During the Crosslinking of Mussel Adhesive Moiety. Acta Biomater. 2017, 48, 144–156. (12) Roy, C. K.; Guo, H. L.; Sun, T. L.; Ihsan, A. B.; Kurokawa, T.; Takahata, M.; Nonoyama, T.; Nakajima, T.; Gong, J. P. Self-Adjustable Adhesion of Polyampholyte Hydrogels. Adv. Mater. 2015, 27, 7344–7348.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Cha, C.; Antoniadou, E.; Lee, M.; Jeong, J. H.; Ahmed, W. W.; Saif, T. A.; Boppart, S. A.; Kong, H. Tailoring Hydrogel Adhesion to Polydimethylsiloxane Substrates Using Polysaccharide Glue. Angew. Chem. Int. Ed. 2013, 52, 6949 –6952. (14) Zhang, T.; Yuk, H.; Lin, S.; Parada, G. A.; Zhao, A. Tough and tunable adhesion of hydrogels: experiments and models. Acta Mech. Sin. 2017, 33, 543–554. DOI 10.1007/s10409017-0661-z. (15) Rose, S.; Prevoteau, A.; Elzière, P.; Hourdet, D.; Marcellan, A.; Leibler, L. Nanoparticle Solutions as Adhesives for Gels and Biological Tissues. Nature 2014, 505, 382-385. (16) Gao, Y.; Han, Y.; Cui, M.; Tey, H. L.; Wanga, L.; Xu, C. ZnO Nanoparticles as an Antimicrobial Tissue Adhesive for Skin Wound Closure. J Mater. Chem B. DOI: 10.1039/C7TB00664K. (17) Discher, D. E.; Janmey, P.; Wang, Y.-L. Tissue Cells Feel and Respond to the Stiffness of Their Substrate. Science 2005, 310, 1139-1143. (18) Erbil, H. Y. Evaporation of Pure Liquid Sessile and Spherical Suspended Drops: A Review. Adv. Colloid Interface Sci. 2012, 170, 67−86. (19) Kang, S.-M.; Hwang, S.; Jin, S.-H.; Choi, C.-H.; Kim, J.; Park, B. J.; Lee, D.; Lee, C.-S. A Rapid One-Step Fabrication of Patternable Superhydrophobic Surfaces Driven by Marangoni Instability. Langmuir 2014, 30, 2828−2834. (20) Zhang,Y.; Chu, D.; Zheng, M.; Kissel, T.; A B. N. J. PerssonE. Tosattigarwal, S. Biocompatible and Degradable Poly(2-hydroxyethyl methacrylate) Based Polymers for Biomedical Applications. Polym. Chem. 2012, 3, 2752–2759. (21) North, M. A.; Del Grosso, C. A.; Wilker, J. J. High Strength Underwater Bonding with Polymer Mimics of Mussel Adhesive Proteins. ACS Appl. Mater. Interfaces 2017, 9, 7866−7872. (22) Fuller, K. N. G., and D. F. R. S. Tabor. The Effect of Surface Roughness on the Adhesion of Elastic Solids. Proc. R. Soc. Lond. A 1975, 345, 327-342. (23) Persson, B. N. J.; Tosatti, E. The Effect of Surface Roughness on the Adhesion of Elastic Solids. J. Chem. Phys. 2001, 115, 5597-5610. (24) Yashima, S.; Takase, N.; Kurokawa, T.; Gong, J. P. Friction of Hydrogels with Controlled Surface Roughness on Solid Flat Substrates. Soft Matter 2014, 10, 3192–3199. (25) Gupta, R.; Fréchette, J. Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels. Langmuir 2012, 28, 14703−14712. (26) Drotlef, D. M.; Appel, E.; Peisker, H.; Dening, K.; del Campo, A.; Gorb, S. N.; Barnes, W. J. P. Morphological Studies of the Toe Pads of the Rock Frog, Staurois parvus (Family: Ranidae)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and their Relevance to the Development of New Biomimetically Inspired Reversible Adhesives. Interface Focus 2015, 5, 20140036. (27) Iturri, J.; Xue, L.; Kappl, M.; García-Fernández, L.; Barnes, W. J. P.; Butt, H.-J.; del Campo, A. Torrent Frog-Inspired Adhesives: Attachment to Flooded Surfaces. Adv. Funct. Mater. 2015, 25, 1499–1505. (28) Refojo, M. F. Hydrophobic Interaction in Poly(2-hydroxyethyl Methacrylate) Homogeneous Hydrogel. J. Polym. Sci. A Polym. Chem. 1967, 5, 3103-3113. (29) Kim, S. H.; Opdahl, A.; Marmo, C.; Somorjai, G. A. AFM and SFG Studies of pHEMABased Hydrogel Contact Lens Surfaces in Saline Solution: Adhesion, Friction, and the Presence of Non-Crosslinked Polymer Chains at the Surface. Biomaterials 2002, 23, 1657-1666. (30) Yuk, H.; Zhang, T.; Lin, S.; Parada, G. A.; Zhao, X. Tough Bonding of Hydrogels to Diverse Non-Porous Surfaces. Nature Materials 2015, 15, 190-196.


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