Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer–based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high–water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.

Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically, chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.

The Mullins-type damage behaviors in double network hydrogels have attracted a broad research interest in recent years. However, most of current works focus on characterizing and modeling the uniaxial deformation behaviors of these materials. In this work, we combine experimental and theoretical approaches to investigate the anisotropic damage behaviors of double network hydrogels revealed in multi-axial deformation conditions. We demonstrate that an isotropic damage model based on the eight-chain model and the network alteration theory fails to capture the stress response in multi-axial loading tests. An anisotropic damage theory based on the microsphere model has also been developed, while both the affine and non-affine approaches are adopted to obtain the micro-macro mapping. The results show that the affine microsphere model cannot describe the experimental results in pure shear and unequal biaxial tests. Remarkably, the non-affine microsphere model with three parameters captures all the important features of the experimental observations. This is because the non-affine model accurately predicts the directional damage of the primary cross-linked network. The non-affine microsphere model is also able to describe the damage cross-effect in double network hydrogels. The developed theoretical framework can promote the fundamental understanding of the anisotropic damage behaviors in various types of tough gels.

The soft actuators similar to the periodic motion of biological organisms are accomplished based on assembling multiple unidirectional memory effect shape memory alloys (SMAs) components or introducing biasing elements, which give rise to the complex structure and difficult control of soft actuators. Herein, inspired by the power‐amplified biological systems, beneficial with the advantages of 3D integrated molding technology, an integrated SMA‐polydimethylsiloxane (PDMS) composite structure (SPCS) is proposed, which can achieve periodic heterogeneous actuation only by controlling the SMA phase transformation and the PDMS strain potential energy distribution states. To test the feasibility of the mechanism, a theoretical model of SPCS deformation is conducted. The results of numerical feasibility analysis show that the factors affecting SPCS deformation mainly involve the excitation current strength of SMA, PDMS structure thickness and its distribution state. The experimental results show that the current intensity mainly affects the deformation rate of SPCS, and the thickness of PDMS is not only the key to realize the periodic deformation of SPCS but also the orderly arrangement of PDMS structure thickness is helpful for SPCS to achieve periodic heterogeneous deformation. These demonstrate that the proposed mechanism can inspire the design of soft actuators, smart wearable equipment, and medical devices.

The basic pore unit model is extended to predict the strained pore size characteristics of woven slit-film geotextiles subjected to unequal biaxial tensile strains. The strained per cent open area (POA) and analytical pore size are expressed as functions of the weft strain and the warp strain to weft strain ratio. The influence of the biaxial tensile strain on pore size characteristics is evaluated in three woven slit-film polypropylene geotextile samples using image analysis under the warp strain to weft strain ratios of 1, 2, 3 and 4. It is shown that the experimental POA and O95 increased significantly with increasing strain at different warp strain to weft strain ratios, and the PSD curves moved toward the direction of large open sizes. The analytical models of POA and pore size can accurately predict the increasing trend of POA and O95. Moreover, unequal biaxial tensile strains can significantly change the shape of the pores, which may influence the results of the pore size obtained by indirect methods. A larger warp strain to weft strain ratio can lead to a larger change in the pore shape when the length to width ratios of initial pores are close to 1.