Hydrogel coatings exhibit versatile applications in biomedicine, flexible electronics, and environmental science. However, current coating methods encounter challenges in simultaneously achieving strong interfacial bonding, robust hydrogel coatings, and the ability to coat substrates with controlled thickness. This paper introduces a novel approach to grow a double-network (DN) tough hydrogel coating on various substrates. The process involves initial substrate modification using a silane coupling agent, followed by the deposition of an initiator layer on its surface. Subsequently, the substrate is immersed in a DN hydrogel precursor, where the coating grows under ultraviolet (UV) illumination. Precise control over the coating thickness is achieved by adjusting the UV illumination duration and the initiator quantity. The experimental measurement of adhesion reveals strong bonding between the DN hydrogel coating and diverse substrates, reaching up to 1012.9 J/m(2) between the DN hydrogel coating and a glass substrate. The lubricity performance of the DN hydrogel coating is experimentally characterized, which is dependent on the coating thickness, applied pressure, and sliding velocity. The incorporation of 3D printing technology into the current coating method enables the creation of intricate hydrogel coating patterns on a flat substrate. Moreover, the hydrogel coating's versatility is demonstrated through its effective applications in oil-water separation and antifogging glasses, underscoring its wide-ranging potential. The robust DN hydrogel coating method presented here holds promise for advancing hydrogel applications across diverse fields.

Puncture of soft solids involves the process of piercing through a soft solid material using a sharp object, which is of great interest in biomedical and industrial applications. In this work, we investigate the puncture of soft solids by a commercial medical needle using polyacrylamide (PAAm) hydrogel as the model material. We measure the relationship between the depth of puncture and force exerted on the needle as it penetrates and retracts from the PAAm hydrogel at different speeds, and interpret the results of the experiments. We find that the puncture force fluctuates with the increasing puncture depth during needle penetration, while the retraction force decreases almost linearly with the retraction depth. Both the puncture and retraction forces, as well as the amplitude and period of the puncture force fluctuation, are positively related to the loading speed. We estimate the size of the crack, which is difficult to directly recognize from experiments, induced by puncture using the critical puncture depth at which the needle first pierces the hydrogel. We find that the size increases with the puncture speed. Moreover, we obtain a work balance relationship between the energy done by the puncture force minus friction (Fp-Fr) and the sum of the energy required for crack propagation and the elastic energy stored in the hydrogel bulk. These findings from our research contribute to a comprehensive understanding of deep indentation and puncture in soft materials. These results carry important implications for the design of biomedical and industrial devices, where the puncturing of soft solids is a critical procedure.

Applications of stretchable polymeric soft materials, such as elastomers and hydrogels, in fields of biomedicine and soft devices frequently require strong bonding between these materials or these materials and other substrates. Methods of achieving the strong interfacial bonding have been well developed, but the fatigue behaviors of these bonding interfaces under cyclic large deformation have been rarely explored in both experiments and theories. In this work, an experimental methodology is presented to study the interfacial fatigue fracture of firmly bonded elastomer bilayers enabled by topological entanglements under cyclic large deformation. The relationship between the interfacial fatigue crack propagation speed v and the energy release rate G is obtained. Three regions are identified in this relationship: crack initiation, stable crack propagation, and catastrophic crack propagation regions. The threshold of energy release rate G0 for the crack initiation under cyclic large deformation is about 35 times smaller than the critical energy release rate Gc for the catastrophic crack propagation. Both in the crack initiation and catastrophic crack propagation regions, the logarithmic v increases with increasing the logarithmic G nonlinearly. While in the stable crack propagation region, the experimental points of logarithmic v at various logarithmic G yield a linear relationship with a positive slope. It is demonstrated that by designing the structure of the bonding interface or the bonding edge, the interfacial fatigue fracture of elastomer bilayers can be alleviated. The experimental methodology presented in this work can be utilized to study the interfacial fatigue fracture between stretchable materials of other types. The findings in this work help to reveal the mechanism of interfacial fatigue fracture between stretchable materials and the development in anti-interfacial fatigue bonding methods.

Underwater adhesion involves bonding substrates in aqueous environments or wet surfaces, with applications in wound dressing, underwater repairs, and underwater soft robotics. In this study, we investigate the underwater adhesion properties of a polyacrylic acid hydrogel coated substrate. The underwater adhesion is facilitated through hydrogen bonds formed at the interface. Our experimental results, obtained through probe-pull tests, demonstrate that the underwater adhesion is rapid and remains unaffected by contact pressure and pH levels ranging from 2.5 to 7.0. However, it shows a slight increase with a larger adhesion area. Additionally, we simulate the debonding process and observe that the high-stress region originates from the outermost bonding region and propagates towards the center, spanning the thickness of the target substrate. Furthermore, we showcase the potential of using the underwater adhesive hydrogel coating to achieve in-situ underwater bonding between a flexible electronic demonstration device and a hydrogel contact lens. This work highlights the advantages of employing hydrogel coatings in underwater adhesion applications and serves as inspiration for the advancement of underwater adhesive hydrogel coatings capable of interacting with a wide range of substrates through diverse chemical and physical interactions at the interface.

In this work, a coarse-grained model is adopted to explore the fracture toughness of a dual cross-linked hydrogel which consists of a physically cross-linked network and a chemically cross-linked network. By calculating the fracture energy, the optimized fracture toughness of the hydrogel appears at the intermediate content of the chemical network. To understand it, the structure change of both the chemical network and the physical network is first characterized during the tensile process. For the chemical network, the fraction and rate of broken bonds gradually improve with increasing content of the chemical network while the strain range where the bond breakage occurs is reduced. For the physical network, the number of clusters and the interaction energy first increase and then decrease with increasing strain. This reflects the breakage and reformation of the physical network, which dissipates more energy and improves the fracture energy. Furthermore, by stress decomposition, the stress is mainly borne by the physical network at small strain and the chemical network at large strain, which proves their synergistic effect in enhancing the hydrogel. Then, the number of voids is calculated as a function of strain. It is found that the voids initiate in the weak region at small strain while in the position of the bond breakage at large strain. Moreover, the number of voids decreases with increasing content of the chemical network at small strain. Finally, the effect of the strength of the chemical network or the physical network on the fracture toughness is discussed. The optimized fracture toughness of hydrogel appears at the intermediate strength.