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"chunk": "# Tough Particle-Based Double Network Hydrogels for Functional Solid Surface Coatings \n\nRiku Takahashi, Kouichi Shimano, Haruka Okazaki, Takayuki Kurokawa, Tasuku Nakajima, Takayuki Nonoyama, Daniel R. King, and Jian Ping Gong\\* \n\nCoating solid surfaces with tough hydrogels is necessary for the practical application of hydrogels in various fields. Here a simple yet versatile method for coating tough double network (DN) hydrogels onto a wide range of solid surfaces, including various materials and geometries is reported. Particlebased double network (P-DN) gels that combine ease-of-use and significantly strong mechanical properties are utilized. The P-DN gel coating process involves two steps. First, the solid surface (plastic, rubber, ceramic, and/ or metal) is treated to form a thin, physically bound primer layer containing radical initiators. The pre-gel solution is then applied to the treated surface, followed by photo-induced polymerization. The P-DN gel coatings show high toughness, with one notable formulation reaching over $\\mathsf{1000}\\mathsf{I m}^{-2}$ in a $90^{\\circ}$ peeling test. The coatings also show high stability against long-term water-storage, elevated temperatures, and solvent exposure. Moreover, it is demonstrated that the P-DN gel-coated surfaces exhibit low friction properties with high wear resistance, by pin-on-flat tests. The simple coating process can be used even on surfaces with complex geometries, including 3D shapes. This work will enable the use of DN gels in applications such as biocompatible lubricants, scratch resistance coatings, and anti-fouling paints.",
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"category": " Results and discussion"
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"id": 2,
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"chunk": "# 1. Introduction \n\nHydrogels consist mostly of water, supported by a 3D crosslinked polymer network. They exhibit many unique properties such as low friction,[1–3] biocompatibility,[4–6] permeability,[7,8] antifouling,[9–11] optical clarity,[12] and so on. However, despite these impressive attributes, synthetically derived hydrogels tend to be very fragile, limiting their applicability. \n\nOver the last 15 years, many strong and/or tough hydrogels have been developed with a variety of chemical structures, including slide-ring gels,[13] nanocomposite gels,[14] double network (DN) gels,[15,16] and hybrid gels.[17–19] These studies have demonstrated that, in general, incorporating energy dissipation mechanisms into crosslinked hydrophilic polymer networks can result in tough hydrogels. For example, chemically cross-linked DN gels are a unique form of interpenetrating network gel with highly contrasting network structures. The first network is rigid and brittle, acting as a sacrificial network, while the second network is soft and stretchable. During deformation, the covalent bonds of the first network rupture extensively dissipate energy, prior to global fracture. In the case of hybrid gels that also consist of two interpenetrating networks, such as alginate-polyacrylamide (PAAm) gels, energy dissipation occurs due to the dissociation of physical bonds of the alginate network.[18] Chemical DN gels are elastic, while hybrid gels based on physical sacrificial bonds are viscoelastic, which results in their mechanical properties having strong temperature and deformation rate dependence. The development of extraordinarily strong and tough hydrogels enables us to explore various practical applications of hydrogels. To achieve these goals, hydrogels must be capable of robustly bonding to a wide variety of solid surfaces, including various materials and complicated shapes. It is necessary to utilize a proper coating method depending on the type of hydrogel. \n\nSeveral attempts have been made to form robust bonds between tough hydrogels and other surfaces. For example, our group has developed a method to robustly bond tough bulk DN hydrogels to porous solids by connecting the bulk surface DN hydrogel to the double networks formed in the solid pores with a soft and stretchable second network that has long chains. High bonding strength ${\\approx}1000\\ \\mathrm{~J~}\\ \\mathrm{m}^{-2}$ , comparable to the fracture strength of bulk DN gels, has been reached.[20] Later, Zhao’s group succeeded in strongly bond PAAm hybrid hydrogels to diverse, nonporous solid surfaces by chemically anchoring the long-chain PAAm network to the surface via silanation.[21] Strong interfacial toughness values of over $1000\\ \\mathrm{J\\m^{-2}}$ , approaching the toughness of the gel itself, have been reached.[18] In addition to this, they successfully achieved elastomer/hydrogel hybrids with strong interfacial toughness (over $1000\\mathrm{~J~m}^{-2})$ ), using elastomer surface modification with benzophenone to activate elastomer surfaces for hydrogel grafting.[22] More recently, Mooney’s group achieved bonding of PAAm-alginate hydrogels to wet biological tissues.[23] In this case, they used a bridging polymer that can bond to the tissues through electrostatic interactions, covalent bonds, and physical interpenetration, which also leads to strong interfacial toughness of over $1000\\mathrm{~J~m}^{-2}$ . These studies have clearly shown that the strength of a bonded gel is not only related to the chemical/physical anchorage at the interface, but also strongly related to the properties of the bulk hydrogels. To create a robust coating, both a strong interface and tough hydrogels are required; the strong interface enables energy transfer from the surface to the bulk hydrogel, which resists detachment through internal energy dissipation mechanisms.[20–24] \n\nThe aim of this study is to establish a simple method to coat DN gels onto diverse nonporous solid surfaces. Previous studies on bulk DN gels have shown that these materials have low surface sliding friction,[2] low wearing,[6] and excellent antifouling to barnacles.[10] The excellent surface properties of DN gels make them functional coating materials for antifouling paints, low friction medical devices, wearing-resistance surface layers, and so on. However, classic DN gels are not easily applicable as a coating material, because their unique structure/properties and two-step network synthesis method results in the following issues: (1) the swollen first network gel is extremely brittle, and thus difficult to handle as a coating, (2) the substantial swelling of the first network after polymerization induces large swelling mismatch at the interface, leading to delamination of the coating, and (3) the multistep solidification fabrication process makes them difficult to coat on surfaces with complicated geometry. \n\nOur strategy to overcome these difficulties is to utilize a particle gel-based double network (P-DN) hydrogel technique.[25,26] This technique relies on the first brittle network hydrogel being dispersed as microparticles in the second monomer solution which is subsequently polymerized. After the polymerization of the second network, a composite structure is formed, where the bulk second network interpenetrates with the first network particles, resulting in covalently trapped double network particles inside a stretchable matrix. The obtained P-DN gels, also called a microgel-reinforced hydrogel, show comparable toughness with normal DN gels that have a bicontinuous DN structure.[26] Since the solidification of P-DN gels is through a one-step polymerization of the second network, the fabrication process of P-DN gels is suitable for large areas and arbitrary shape coatings, in which the swollen first network particle “paste” can be applied freely prior to polymerization of the second network, minimizing swelling mismatch.[27] The “paste” precursor of the P-DN gel enables us to easily obtain tough, free-formable, and large-area hydrogel coatings. \n\nTo achieve bonding, interfacial bridging between the second network of the P-DN hydrogel and the solid surface is required. Our strategy to form the interface bridging is inducing, on the solid surface, a primer layer containing radical initiators by pretreating the surface with a solution containing poly(vinyl acetate) (PVAc) and initiator (Figure 1a).[28] Upon drying, PVAc forms a strong physical coating layer on diverse surfaces (plastic, rubber, ceramic, and metal), with embedded initiator. A similar approach was utilized for coating hydrogels to elastomer surfaces by Zhao and co-workers.[22] This primer method has been shown to be a reliable method to form strong interfaces. We then apply the pre-gel solution that contains the brittle first network gel particles and second network monomer to the surface. To prevent drying and to control the coating thickness, a glass plate with spacer covers the surface of pre-gel solution (Figure 1b). Polymerization of the second monomer takes place from the initiator embedded in the primer layer to form the second network that is strongly bonded to the surface via covalent bonding to the primer layer (Figure 1c and Figure S1, Supporting Information). By using this approach, we can easily obtain robust hydrogel coatings that exhibit functional properties such as high wettability, low friction, and wear resistance (Figure 1d). The approach is further applied to coating diverse materials (plastics, rubbers, ceramics, and metals) as well as surfaces with complex 3D shapes. \n\n \nFigure 1. Schematic illustration of the coating process of particle-based double network hydrogels (P-DN gels) onto a solid surface. a) An acetone solution containing benzophenone initiator and poly(vinyl acetate) (PVAc) was coated on the solid substrate. After evaporation of the acetone solvent, a PVAc primer layer containing initiat the ubstrate. b) The sor solution of P-DN gel, containing monomer of second network, acrylamide (AAm), and particle gels of the firs rk, poly(2-a mide-2-methylpropane sulfonic acid sodium salt) (PNaAMPS), was applied to the solid substrate covered with the th n to maintain a flat surface and prevent drying of the solution during polymerization. Durin etwork was polymerized from the initiator in the primer layer. This network penetrates the first network of the gel par h ydr yer strongly bound to the substrate. d) The fabricated tough hydrogel coating exhibits improved surface propertie l w friction, high wettability, and wear resistance.",
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"category": " Introduction"
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"id": 3,
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"chunk": "# 2. Result and Discussion",
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"category": " Results and discussion"
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"id": 4,
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"chunk": "# 2.1. Mechanical Properties of the P-DN Gel \n\nThe mechanical strength of the P-DN gels was strongly related to the crosslinking concentration of the first network particles for a fixed particle concentration, as presented by the stress– strain curves in Figure 2b. We found that the modulus and fracture stress increased, while the extensibility remained almost constant with increasing first network cross-linker concentration until $4\\mathrm{mol}\\%$ , where the fracture stress of the P-DN gel reached a maximum value, $2.4~\\mathrm{MPa}$ . This result indicates that the P-DN gels exhibited the characteristic double network effect when the cross-linker concentration of the first network was below $4\\mathrm{mol\\%}$ .[15,16] In this region, we envision that the second network gel transfers sufficient load to the first network particles to cause rupture of the first network, which dissipates significant amount of energy.[15,16,25,26,29] Once these particles have been fractured, the second network will continue to stretch until failure, resulting in global fracture of the sample. When the crosslinking concentration in the first network particles was larger than $4\\mathrm{mol}\\%$ , the modulus, fracture stress, and fracture strain all decreased with increasing cross-linker concentration. In this region, we conjecture that the strength of the first network particles became too high, and the second network is not able to transfer enough force to cause fracture of the first network particles prior to second network fracture. This causes catastrophic fracture to occur entirely in the second network, resulting in decreased work of extension. In this case, the first network particle gels do not serve as sacrificial bonds, and the resulting mechanical response no longer exhibits a double network effect. \n\n \nFigure 2. Tough P-DN hydrogel films prepared based on the double network concept. a) Photographs of the fabrication process of particle-based double network gels (P-DN gel). (i) PNaAMPS gel microparticles in the dried powder state; (ii) PNaAMPS particles swollen in AAm aqueous solution to form a “paste”; (iii) Tough P-DN gel film. PNaAMPS particles and PAAm network act as a rigid/brittle first network and soft/ductile second network, respectively. b) Tensile stress–strain curves of P-DN gels with varying first network crosslinking concentration (testing velocity of $\\mathsf{l o o}\\mathsf{m m}\\mathsf{m i n}^{-1}$ ). The particle concentration in the P-DN gel is fixed as $0.075\\mathrm{~g~}\\mathsf{m}\\bar{\\mathsf{L}}^{-1}$ . c) Work of extension of the P-DN gels with varying first network crosslinking concentration. The dark blue, light blue, and gray lines represent different particle concentrations in the P-DN gel. The error bars are standard deviation from the results of 3–5 samples, and were smaller than the symbol unless otherwise present. \n\nSubsequently, we studied the effect of particle concentration in P-DN gels. As shown in Figure 2c, and in agreement with the preceding paragraph, the work of extension reached a maximum around $4\\mathrm{\\mol}\\%$ cross-linker for the different particle concentrations. For the fixed cross-linking density $(4\\mathrm{mol}\\%)$ , the highest work of extension was achieved when the particle concentration was $0.015\\ \\mathrm{g\\mL^{-1}}$ (see Figure S2, Supporting Information). This result indicates that there is an optimum particle concentration in the P-DN gels to effectively exhibit high strength and toughness. This agrees with the result of the conventional double network, which states that the ratio of the first to the second network should be a moderate value to maximize toughness.[30,31] For the rest of the experiments, $4\\mathrm{mol}\\%$ and $0.015\\ \\mathrm{g\\mL^{-1}}$ were chosen for the cross-linker concentration of the particle gels and the particle concentration for the P-DN gels, respectively.",
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"category": " Results and discussion"
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"id": 5,
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"chunk": "# 2.2. Robust Coating of P-DN Gels \n\nHomogenous coating layers were formed on the solid substrate. Figure 3a shows typical photos of a polyethylene (PE) substrate before (Figure 3a-i) and after (Figure 3a-ii) coating of the P-DN gel. The coating strength of the tough P-DN gels was investigated by using a $90^{\\circ}$ peeling test, as shown in Figure 3a-iii. Typical force–displacement curves of the peeling test for samples with varied initiator concentration in the primer layer are shown in Figure 3b. The peeling behaviors can be classified into three cases, depending on the benzophenone concentration in solution for the primer layer formation: (1) completely peeled off without gel fracture, (2) peeled off with gel fracture, and (3) gel fractures without peeling. This result indicates that the initiator concentration in the primer layer not only changes the intrinsic bridging strength at the interface, but also changes the properties of the bulk hydrogel. That is, the failure mode changes are governed by the competition between the stress to fracture the gel $(\\sigma_{\\mathrm{c,gel}})$ and the stress to peel off the interface $(\\upsigma_{\\mathrm{interface}})$ . When the initiator concentration is low $(0.3\\mathrm{-}0.6\\mathrm{\\quad}\\mathrm{wt}\\%)$ , interfacial rupture (case (1)) occurs due to a low density of bridging polymers at the interface $(\\sigma_{\\mathrm{c,gel}}\\gg\\sigma_{\\mathrm{interface}})$ . On the other hand, when the initiator concentration is intermediate $(0.6{-}2.0\\ \\mathrm{wt}\\%)$ , we observed a thin residual hydrogel layer on substrate for all gel samples after the peeling tests, indicating case (2) failure. This result indicates that after the occurrence of a few debonding events at the interface, the interfacial crack shifts into the P-DN gel, causing fracture of the bulk P-DN gel $(\\sigma_{\\mathrm{c,gel}}\\sim\\sigma_{\\mathrm{interface}})$ . In this case, the peeling strength is determined by the tearing energy of the P-DN gel, which is confirmed by comparison with the reported tearing energy of the bulk P-DN gels.[25] Furthermore, we investigated the influence of the coating thickness $(0.5{-}2.0\\ \\mathrm{mm})$ ) on peeling strength for intermediate initiator concentration $(0.6~\\mathrm{wt\\%})$ (see Figure S3, Supporting Information). From this test, we can see a trend of increasing peeling strength with increasing coating thickness in the observed thickness range. It is considered that a thicker coating avoids stress concentrations at the interface, resulting in high adhesion energy. When the initiator concentration is higher $(>3.0\\ \\mathrm{wt}\\%)$ , the density of bridging polymers is significantly high, resulting in case (3) failure, which is the rupture of gel without any interfacial debonding $(\\sigma_{\\mathrm{c,gel}}\\ll\\sigma_{\\mathrm{interface}})$ . In this case, the mechanical properties of the gel coating should decrease due to the high concentration of initiator that leads to the formation of relatively short polymer chains. To maximize the toughness of the coating, we need sufficiently high interfacial strength, and also tough gels which can dissipate bulk energy. This result indicates that if we can increase $\\sigma_{\\mathrm{interface}}$ without decreasing $\\sigma_{\\mathrm{c,gel}}$ , we can further enhance the robust hydrogel coating. \n\n \nFigure 3. Robustness of the P-DN gel coatings on solid surfaces. a) Photographs of an uncoated (i) and P-DN gel coated (ii) polyethylene substrate. To easily visualize the hydrogel coating, the P-DN gel was swollen in water containing a dye, Alcian blue $(0.05~\\mathrm{wt\\%})$ . (iii) A photograph of the $90^{\\circ}$ peeling test of the P-DN coated substrate. Silicone rubber was introduced to prevent elongation of the hydrogel along the peeling direction. Before the peeling test, the P-DN coated samples were immersed in pure water to reach the equilibrium state. b) Normalized force–displacement curves of the peeling tests of P-DN gel films with varying initiator concentration in the primer layer (testing velocity of $30\\mathsf{m m}\\mathsf{m i n}^{-1}.$ ). The values shown in the legend of the plot are the concentrations of benzophenone in the pretreatment solution. The cross-linker concentration of the particles and particle concentration in the P-DN gel were fixed as $4m o l\\%$ and $0.075\\ \\mathrm{g\\mL^{-1}}$ , respectively. Measured force was normalized by the width of the samples. The peeling behaviors can be divided into three cases: (1) completely peeled off without gel fracture, (2) peeled off with gel fracture, and (3) gel fractures without peeling. These cases are governed by the competition between the strength to fracture the gel $(\\sigma_{\\tt c,g e l})$ and the strength to peel off the interface $(\\sigma_{\\mathrm{interface}})$ . \n\nTo verify this assumption, we attempted to increase the $\\sigma_{\\mathrm{interface}}$ by increasing the surface roughness of the substrate, which increases the effective surface area. A hot press method was utilized to obtain the PE substrates with increased surface roughness (Figure 4a). Three types (#30, #150, #220) of commercially available patterned glass substrates (ISHIMOTO. Co., Ltd.) were heated to $100~^{\\circ}\\mathrm{C}$ and the PE substrates were pressed onto the glass substrate with $800\\mathrm{\\Pa}$ of pressure (Figure 4a-i). After cooling the sample to $25~^{\\circ}\\mathrm{C}$ , we obtained substrates that have various roughness, characterized by the area ratio, $S_{\\mathrm{a}}$ , the ratio between the surface area measured by a 3D-laser scanning microscope (KEYENCE) and the pristine (flat) surface area (Figure 4b). Then, the P-DN gel was coated on the rough PE substrates as shown in Figure 4a-iii. Typical force–displacement curves of the peeling test for samples with 0.3 and $0.6~\\mathrm{wt\\%}$ benzophenone initiator concentration in the primer layer are shown in Figure 4c,d, respectively. When the initiator concentration is $0.3\\mathrm{wt}\\%$ , the peeling behavior changes from case (1) to case (2) with increasing surface area of the substrate. Furthermore, in the case of the $0.6~\\mathrm{wt\\%}$ initiator, increasing the surface area changed the peeling behavior from case (2) to case (3), showing high peeling strength of over $1000\\ \\mathrm{J\\m^{-2}}$ . This result indicates that the increase in surface area from the rough surfaces provided high interfacial bonding strength, resulting in robust coatings when combined with the high toughness of the P-DN gel. We note that the bonding strength at the interface between the substrate and the primer layer (PVAc) is stronger than that between the primer layer and the gel or the strength of the bulk P-DN gel. Because the presence of benzophenone in the primer layer, which can cause branching via transfer reaction in polymeric system, may result in some covalent bonding between the primer layer and PE substrates.[32] \n\n \nFigure 4. Surface roughness effect on the peeling strength. a) Schematic illustration for preparation of hydrogel coatings on rough polyethylene (PE) surface. The rough PE substrates were fabricated by a hot press method. b) Surface morphology of the rough PE substrates. Four different roughness values, $S_{a},$ which was determined by measured surface area divided by initial (flat) surface area, were prepared. c,d) Normalized force–displacement curves of the peeling tests of P-DN gel film with 0.3 and $0.6~\\mathrm{wt\\%}$ benzophenone initiator concentrations in the primer layer (testing velocity of $30\\mathsf{m m}\\mathsf{m i n}^{-1}$ ). The cross-linker concentration of particles and particle concentration in the P-DN gel were fixed as $4m o l\\%$ and $0.0\\dot{7}5\\ \\mathrm{g}\\ \\dot{\\mathsf{m L}^{-1}}$ , respectively. Measured force was normalized by the width of the samples. Silicone rubber was used to prevent elongation of the hydrogel along the peeling direction. Before the peeling test, the P-DN coated samples were immersed in pure water to reach the equilibrium state. \n\nBy utilizing the sample prepared with $0.3\\mathrm{wt}\\%$ benzophenone in the primer layer and an optimized pre-gel solution (4 m AAm, $0.01\\mathrm{mol}\\%$ $\\boldsymbol{N,N^{\\prime}}$ -methylenebis(acrylamide) (MBAA), $0.015~\\mathrm{\\mg~\\mL^{-1}}$ dry PNaAMPS gel particles synthesized at $4\\mathrm{mol\\%}$ MBAA), we further investigated the stability of the coatings by exposing the coated samples in various conditions (see Figure S4, Supporting Information). The sample did not show any apparent change even after storage in pure water $(25~^{\\circ}\\mathrm{C})$ for $282\\mathrm{~d~}$ , indicating long-term stability (Figure S4a, Supporting Information). After being immersed in hot water $(80~^{\\circ}\\mathrm{C})$ for $24\\mathrm{h}$ , the P-DN gel coatings still exhibit strong bonding, indicating excellent thermostability (Figure S4b, Supporting Information). The sample immersed in acetone, a poor solvent, for 4 days did not show delamination, indicating good resistance to swelling/ deswelling induced interfacial mismatching (Figure S4c, Supporting Information).",
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"category": " Results and discussion"
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"chunk": "# 2.3. Functional Properties of P-DN Gel Coatings \n\nThe most practical use of tough hydrogel coatings is for modifying the surface properties of solid substrates. By using the coating method introduced here, we can modify solid surfaces to have valuable functions based on hydrogel properties such as high wettability, antifouling, low friction, small molecule retention capacity, and high biocompatibility.[1–12,33,34] Moreover, taking advantages of the high toughness of P-DN gels, we can increase the wear resistance to solid substrates. Herein, to demonstrate the functional properties of P-DN gel coatings such as high wettability, low friction, and wear resistance, we prepared PE substrates coated with tough P-DN gels at the optimized coating condition $(0.3~\\mathrm{wt\\%}$ benzophenone in the primer layer, pre-gel solution of $4\\mathrm{~M~}$ AAm, $0.01\\mathrm{mol}\\%$ MBAA, $0.015~\\mathrm{mg~mL^{-1}}$ dry PNaAMPS gel particles synthesized at $4\\mathrm{mol}\\%$ MBA A). The gel-coated samples were immersed in water for one week after synthesis to reach the equilibrium state (thickness: $300\\upmu\\mathrm{m}$ in the swollen state). \n\nFirst, we measured the contact angles of water on the P-DN gel coated PE, PAAm (traditional single network hydrogel) coated PE, and pristine PE substrate, respectively (Figure 5a). We confirmed that the hydrogel-coated substrates exhibit high wettability due to the hydrophilic surface properties of hydrogels. Second, to evaluate the sliding friction of the samples, we performed a reciprocating pin-on-flat test as shown in Figure 5b.[35] The tests were conducted in pure water $(25~^{\\circ}\\mathrm{C})$ using an instrument with a motor-driven stage that oscillates a flat specimen beneath a fixed alumina ball (diameter: $10~\\mathrm{mm}$ ). The hydrogel-coated samples (substrate size: length $\\times$ width $\\times$ thickness $\\mathbf{\\tau}=100\\ \\mathrm{mm}\\times50\\ \\mathrm{mm}\\times2\\ \\mathrm{mm})$ were fabricated using the method as mentioned above and the bottom surface of the samples were fixed onto the testing area with double-sided tape. The vertical load, average sliding speed, reciprocating cycles, and sliding distance were 1 N, $500\\ \\mathrm{mm\\min^{-1}}$ , 300 cycles, and $35~\\mathrm{mm}$ , respectively. From the measured friction force $(F)$ (see Figure S5, Supporting Information) and vertical load $(\\mathbb{W})$ , we estimated the coefficient of friction $(\\mu)$ from $\\mu=\\operatorname{F}/\\operatorname{W}.$ The static friction, $\\mu_{0}$ , and dynamic friction, $\\mu_{\\mathrm{d}},$ were estimated from $F_{0}$ (max force) and $F_{\\mathrm{d}}$ (average force), respectively, in the time profiles of the friction during sliding. At the initial state, we found that the hydrogel-coated samples showed lower friction than the bare PE substrate, especially the dynamic friction, as shown in Figure 5c-i. This result indicates that utilizing hydrogel coatings was an effective approach to reduce friction under water. Additionally, after 300 cycles of the sliding test, the coefficient of static friction of the bare PE substrate dramatically increased due to surface wearing. However, the hydrogel-coated samples maintained low friction. Indeed, the P-DN gel coating did not show any wear tracks, indicating strong wear resistance compared to the single network PAAm hydrogel (Figure 5c-ii, iii). These results enable us to explore various practical applications of tough hydrogel coatings. \n\n \nFigure 5. Surface properties of the hydrogel coating. a) Contact angles of water on a polyethylene substrate (i), pure PAAm coated (ii), and P-DN gel coated polyethylene substrate (iii). Hydrogel coatings (thickness: $300\\ \\upmu\\mathrm{m};$ exhibit high wettability. b) Schematic illustration of the setup for a friction measurement (pin-on-flat test). c) Coefficient of static and dynamic friction on the PE substrate or hydrogel coated surfaces. (i) After 300 cycles of friction testing, hydrogel coatings maintained low friction, similar to the initial state. (ii) Photographs of the coating surface of PAAm and (iii) P-DN gel after 300 cycles of friction testing. Due to the high toughness of the P-DN gel, the gel coating shows little wear damage. The friction load, velocity, and track length are 1 N, $500\\ m\\ m\\mathrm{~min^{-1}}$ , and $35~\\mathsf{m m}$ , respectively. The inset scale represents $500\\upmu\\mathrm{m}$ . Experimental details are shown in the Supporting Information.",
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"category": " Results and discussion"
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"id": 7,
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"chunk": "# 2.4. Versatility of P-DN Hydrogel Coatings \n\nThe above result suggests that by using a thin primer layer, the P-DN hydrogel coating method can be applied to diverse solid surfaces. Furthermore, taking advantage of the one-step fabrication of P-DN gels, this method has the potential to coat surfaces with complicated 3D shapes. First, we coated various flat substrates by using the method mentioned in Figure 1. We used zirconium oxide, copper, and polybutadiene as representative ceramic, metal, and rubber specimens, respectively. The optimized coating condition $(0.3\\ \\mathrm{wt\\%}\\$ benzophenone in the primer layer, pre-gel solution of $4\\mathrm{~M~}$ AAm, $0.01\\mathrm{mol}\\%$ MBAA, $0.015\\mathrm{mg}\\mathrm{mL}^{-1}$ dry PNaAMPS gel particles synthesized at $4\\mathrm{mol}\\%$ MBA A) was used for the coating. From the $90^{\\circ}$ peeling test, adhesion strength of P-DN gel coating on polybutadiene rubber showed similar peeling strength to the PE substrate (see Figure S6, Supporting Information). Although the primer layer does not possess chemical bonds with the inorganic surface, it is able to adhere strongly due to physical bonds such as Van der Waals forces. The coatings exhibited excellent adhesiveness even after swelling the hydrogels, as shown in Figure 6a. Therefore, using a thin primer layer by simple solution casting provides a general method to pretreat the coating surfaces where chemical treatments may not be possible. These results demonstrate that by coating tough hydrogels onto solid substrates, we can easily obtain functional surfaces which exhibit low friction and high wear resistance. \n\nWe next attempted to coat P-DN gels on surfaces with complicated 3D structures (Figure 6b-i, ii). We used a PE model frog for this experiment. By using a brush (or spray-gun), the primer layer precursor solution $(0.3\\ \\mathrm{wt\\%}$ initiator and $1\\mathrm{\\mt{\\%}}$ PVAc in acetone), and then the pre-gel solution ( $\\mathrm{~4~}\\mathrm{~M~}$ AAm, $0.01\\mathrm{\\mol}\\%$ MBAA, $0.015\\mathrm{\\mg\\mL^{-1}}$ dry PNaAMPS gel particles synthesized at $4\\mathrm{mol}\\%$ MBA A) were applied to the model. After that, UV radical polymerization was carried out under an argon atmosphere with a UV lamp for $^{8\\mathrm{~h~}}$ to form the P-DN hydrogel coating layer. To visualize the coating layer easily, the hydrogel-coated model was dyed with Alcian blue $(0.05\\ \\mathrm{wt\\%})$ . As shown in Figure 6b-iii, we successfully obtained tough P-DN gel coatings on complex solid surfaces with robust adhesion. Because of the nonuniformity of the thickness of the coating and large distribution of particle size of gel (from 10 to $200\\upmu\\mathrm{m}$ ), surface roughness is induced after immersing the painted model in water due to inhomogeneous swelling of the gel. To minimize the swelling inhomogeneity, we can use smaller or homogenously sized particles that can be obtained by sieving or through other means. However, even if the coated surface has some inherent roughness, hydrogel coatings on complicated 3D shapes have great potential for practical use, such as coating the exterior surfaces of ships for antifouling and coating medical equipment for minimally invasive procedures.",
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"category": " Results and discussion"
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"id": 8,
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"chunk": "# 3. Conclusions \n\nIn summary, tough P-DN hydrogel coatings for various solid substrates have been successfully developed. We systematically investigated the mechanical properties of P-DN gels and found the suitable conditions for making tough P-DN gels. By coating the substrates (plastic, ceramic, metal, and rubber) with a primer layer containing proper concentrations of initiator, we can synthesize tough P-DN gels on surfaces with robust bonding. The peeling strength of the hydrogel coating is strongly related to the competition between the strength to break the gel and the strength to peel at the interface. In the toughest peeling case, little debonding occurred at the interface, and the interfacial crack moved into the bulk of the P-DN gel, causing fracture of the bulk P-DN gel. Therefore, using tough P-DN gels as a coating is a novel method to fabricate robust hydrogel coatings. Another useful aspect of using tough hydrogels is that the coatings are sufficiently robust to exhibit wear resistance with low friction. Moreover, taking advantage of the free-formability of P-DN gels, we successfully attained tough P-DN gel coatings on complicated 3D shapes. These demonstrations suggest that hydrogels can be used for a variety of real-world applications where tough coatings are required.",
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"category": " Conclusions"
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"id": 9,
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"chunk": "# 4. Experimental Section \n\nMaterials: 2-Acrylamide-2-methylpropane sulfonic acid sodium salt (NaAMPS) was purchased from Toagosei Co., Ltd. and used as received for the rigid/brittle first network. Acrylamide (AAm) (Jundei Chemical Co., Ltd.) was recrystallized from chloroform and used for the soft/ductile second network. MBAA (Tokyo Kasei Co., Ltd.), as a cross-linker for both NaAMPS and AAm gels, was recrystallized from ethanol. 2-Oxoglutaric acid (α-keto) (Wako Pure Chemical Industries, Ltd.), as an UV initiator for the gelation reaction, was used as received. Benzophenone (Wako Pure Chemical Industries, Ltd.), as an UV initiator for coating of gels on substrates, was used as received. PVAc (Wako Pure Chemical Industries, Ltd.), as an initiator support layer, was used as received. \n\n \nFigure 6. Universality of the hydrogel coating on various solid surfaces. a) Tough P-DN gel coating on (i) ceramic $(Z\\r\\Gamma O_{2})$ , (ii) metal (copper), and (iii) rubber (polybutadiene). The coatings $0.3\\mathrm{\\wt\\%}$ benzophenone in the primer layer, pre-gel solution of 4 m AAm, $0.01\\mathrm{\\mol\\%}$ MBAA, $0.075~\\mathrm{mg~mL^{-1}}$ dry PNaAMPS gel particles synthesized at $4m o l\\%$ MBAA) exhibited excellent adhesiveness, and resist peeling due to friction and bending. b) Freeform P-DN coating on a complicated 3D shape. (i) Schematic illustration of the coating method. The photographs represent (ii) uncoated and (iii) coated samples, respectively. Due to the nonuniformity of the thickness of the coatings, surface roughness was induced by the swelling of the P-DN gel. To easily visualize the hydrogel coating, the P-DN gel was swollen in water containing a dye $(0.05\\mathrm{~wt\\%~}$ Alcian blue). \n\nPreparation of Particle-Based DN Gels: The P-DN gel films were synthesized using a method similar to the one previously described in the literature.[25] PNaAMPS particles and a PAAm network were used as the rigid/brittle first network and the soft/stretchable second network, respectively. To determine the optimal conditions for making strong and tough P-DN gels, P-DN gel films were systematically synthesized by varying the first network crosslinking concentration and particle concentration. Sheet-like P-DN gels were synthesized through a twostep sequential free-radical polymerization.[25] In the first step of the first network particle preparation, MBAA $(0.5{-}6~\\mathsf{m o l}\\%)$ and $\\alpha$ -keto $(0.1\\ m\\circ1\\%)$ were added to 1 m NaAMPS solution (the molar percentages of MBAA and $\\alpha$ -keto were in relative to the NaAMPS monomer). The solution was poured into reaction cells consisting of a pair of glass plates with a $\\textsf{l m m}$ silicone spacer. Photoinduced free radical polymerization was carried out under argon atmosphere with a UV lamp for $\\mathsf{10~h}$ (UV light intensity was $3.9\\ m\\backslash\\forall c m^{-2}$ ). After that, the as-prepared PNaAMPS gels were roughly ground with a spoon into particles and dried using a vacuum oven for $24\\mathrm{~h~}$ . Subsequently, the dried particles were finely ground with a multibead shocker (Yasui Kikai Co., Ltd.), resulting in particles ranging in size from 10 to $200~{\\upmu\\mathrm{m}}$ (Figure 2a-i). Then, the first network particles were added into the AAm aqueous solution $(4~\\mathsf{M})$ containing MBAA $(0.01\\ m\\circ|\\%)$ and $\\alpha$ -keto $(0.01\\ m\\circ|\\%)$ , where the concentration of PNaAMPS dried particles to AAm solution was varied in the range of $0.005{-}0.030\\ m g\\ m L^{-1}$ , to obtain paste-like precursors of the P-DN gels (Figure 2a-ii). After that, the particle gel solution was poured into the reaction cell consisting of a pair of glass plates with a $\\textsf{l m m}$ silicone spacer. The AAm monomers were photopolymerized with the UV lamp for $8\\ h$ to obtain the P-DN gel sheets (Figure 2a-iii). The P-DN gels in pure water exhibit isotropic swelling, showing an equilibrium thickness of $1.5~\\mathsf{m m}$ . \n\nCoating P-DN Hydrogels on Solid Substrates: A $2\\mathsf{m m}$ thick PE plate was immersed in the pretreatment solution $(0.7-6~\\mathrm{wt\\%}$ benzophenone, $1\\mathrm{wt\\%}$ PVAc in acetone) for 5 min, then dried in a vacuum oven under reduced pressure for 5 min to form the primer layer. This cycle was repeated twice and performed in a vial shielded from light at room temperature. Then, the pre-gel solution containing $4\\mathrm{~M~}$ AAm, $0.01\\ m\\mathrm{o}1\\%$ MBAA (in relative to AAm), $0.075\\mathrm{\\mg\\mL^{-1}}$ PNaAMPS gel particles synthesized at $4m o l\\%$ \n\nMBA A, was poured onto the pretreated substrates. After that, a flat glass plate was placed on the pre-gel solution with a $0.5\\ \\mathsf{m m}$ thick spacer to prevent evaporation of the solution and control the coating thickness. UV radical polymerization was carried out under an argon atmosphere with a UV lamp for $\\mathsf{10~h}$ , during which the PAAm network was formed and the P-DN gel was bonded onto the solid surface via the primer layer. It should be noted that the pre-gel solution did not contain any radical initiator, and the polymerization of the PAAm network was initiated by the benzophenone in the primer layer. Due to the restriction in the lateral direction, the coated P-DN gel swelled only in the thickness direction in pure water, resulting in a thickness of $1.0\\mathsf{m m}$ . \n\nVarious types of solid surface substrates, such as ceramic (zirconium oxide, $Z r O_{2}\\mathrm{,}$ ), metal (copper, $\\mathsf{C u}$ ), and rubber (polybutadiene), were also coated with P-DN gels using a $0.3\\mathrm{wt\\%}$ benzophenone concentration. Other conditions were the same as that for the PE coating. Before use, all substrates were washed with pure water. \n\nTensile Experiments: To characterize the mechanical properties of the P-DN gels, uniaxial tensile tests were performed at equilibrium water swelling conditions using a tensile-compressive tester (Tensilon RTC1310A, Orientec Co.). The P-DN gel sheet, with a thickness of $1.5\\ \\mathsf{m m}$ , was cut into dog bone shape (gauge length: $12\\mathsf{m m}$ , width: $2~\\mathsf{m m}$ ) and the sample was stretched along the length direction at an extension velocity of $\\mathsf{l o}0\\mathsf{m m}\\mathsf{m i n}^{-1}$ . The tensile stress was defined as the ratio between the force generated during the elongation and the initial crosssectional area. Tensile strain was defined as the ratio between the length change during elongation and the initial length of the sample. The work of extension was calculated from the area below the stress–strain curve. \n\n$90^{\\circ}$ Peeling Tests: The peeling strength between the P-DN gel and PE substrate was measured using a standard $90^{\\circ}$ peeling test (ISO 8510–1) with a mechanical testing machine (Tensilon RTC-1310A, Orientec Co.). As part of the sample preparation process, half of the PE substrate was masked with adhesive tape to prevent chemical bonding to the substrate, which creates the arm for the peeling test. Then, the P-DN gels were coated onto the substrate, forming chemical bonds with the unmasked region of the substrate. A sample with a gel layer coating thickness of $\\mathsf{1.0~m m}$ was cut to specific dimensions (length: $\\mathsf{100~m m}$ , width: $5~\\mathsf{m m}$ ) by using a cutting machine (Dumb Bell Co., Ltd.). To prevent stretching of the P-DN gel, a relatively stiff silicone rubber sheet was bonded onto the back surface of the hydrogel by the same method as the coating process for P-DN gels onto the solid substrates. The PE substrate was fixed to the jig of the tester using an instant glue (Toagosei Co., Ltd.). The samples were tested according to a standard $90^{\\circ}$ peeling test with a constant peeling velocity of $30\\ m m\\ m i n^{-1}$ . When the measured force reached a plateau, the value was treated as the peeling force. The peeling strength was determined by dividing the peeling force with the width of the sample.",
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"category": " Materials and methods"
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"id": 10,
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"chunk": "# Supporting Information \n\nSupporting Information is available from the Wiley Online Library or from the author.",
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"category": " References"
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"id": 11,
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"chunk": "# Acknowledgements \n\nR.T. and K.S. contributed equally to this work. This research was financially supported by a Grant-in-Aid for Scientific Research (S) (No. 17H06144) and Grant-in-Aid for JSPS Fellows (No. 15J01078) from Japan Society for the Promotion of Science (JSPS). R.T. was supported by MEXT through Program for Leading Graduate Schools (Hokkaido University “Ambitious Leader’s Program”).",
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"category": " Acknowledgements"
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},
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"id": 12,
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"chunk": "# Conflict of Interest \n\nThe authors declare no conflict of interest.",
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"category": " References"
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"id": 13,
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"category": " References"
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