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Porous hydrogels with high porosity can achieve high matter diffusion and transport efficiency. Achieving both ultrahigh porosity and excellent mechanical properties in porous hydrogels remains a long‐standing challenge, considerably hindering their use in high load‐bearing elastic scaffolds such as artificial articular cartilages and meniscus replacements. Herein, a novel crystalline‐constrained multi‐solvents template preparation method is proposed for synthesizing highly porous hierarchical polyvinyl alcohol hydrogels (P‐exogel) with excellent mechanical properties. The enhanced mechanical characteristics of the P‐exogel are attributed to the crystallinity‐induced network anti‐swelling effect of the pore walls during dynamic template removal. The P‐exogel exhibits a hierarchical and interconnected pore structure with high porosity (81.69%), resulting in ultrafast surface wetting with water (in less than 10 ms). Moreover, the P‐exogel exhibits good tensile breaking strength (2.47 ± 0.53 MPa) with elongation of more than 400%, high toughness (5.61 ± 1.65 MJ m−3), and excellent elastic recovery performance. The pre‐crack testing results further confirm the ultrahigh fracture resistance (18.73 ± 2.52 KJ m−2) of the P‐exogel, which almost surpasses those of all reported and commercial porous hydrogels. The unique network microstructure of the P‐exogel ensures its highly efficient and reversible liquid absorption and release ability during dynamic loading–unloading processes, confirming its great application potential in high load‐bearing elastic scaffolds. [ABSTRACT FROM AUTHOR]

The last few decades have witnessed the great progress in surface modification through the use of functional polymer coatings. Surface-grafted polymers with thickness ranging from several nanometers to micrometers have been proven to significantly improve the surface properties of materials, thus enabling diverse, customizable, and controllable performances. Consequently, surface-grafting has become a key tool in scientific research on surface/interface and in surface engineering applications. The interface adhesion and friction between materials and their environments can be precisely controlled by grafting specially designed polymer coatings on material surfaces. As a result, the use of surface-grafted polymers to control the adhesion and friction of materials has attracted extensive attention across various disciplines, from polymer chemistry, physics, and materials science to biology and medical science. This review starts with a discussion of functional surfaces in nature that exhibit unique adhesion and friction phenomena. It then introduces the fundamental principles of tribology and the adhesion and friction behaviors of polymer surfaces. It covers different methods for producing polymer coatings and the corresponding strategies for controlling adhesion and friction. Finally, the challenges and barriers that prevent broader application of surface-grafted polymers are discussed and an outlook of future opportunities is presented. [Display omitted] [ABSTRACT FROM AUTHOR]

Directional transportation of objects has important applications from energy transfer and intelligent robots to biomedical devices. Although breakthroughs in liquid migration on 2D surfaces or 3D tubular devices have been achieved, realizing smooth/on‐demand transportation of constrained solids within a 3D cavity environment under harsh pressurized environment still remains a daunting challenge, where strong interface friction force becomes the main obstacle restricting the movement of solids. Inspired by typical feeding mechanism in natural esophagus system which synergistically couples a lubricating mucosa surface with the peristaltic contraction deformation of the cavity, herein, this challenge is addressed by constructing an esophagus‐inspired layered tubular actuator with a slippery inner surface and responsive hydrogel matrix to realize spherical solid propulsion by photo(thermo)‐induced cavity deformation. The as‐constructed tubular actuator containing Fe3O4 nanoparticles exhibits local volumetric shrinkage upon NIR‐irradiation, which can generate large hydrodynamic pressure and considerable mechanical extrusion force (Fdriving force ≈ 0.18 N) to overcome low interface friction force (ffriction force ≈ 0.03 N), enabling on‐demand transportation of constrained (pressure: 0.103 MPa) spherical solids over a long distance in an arbitrary direction. This actuator is anticipated to be used as bionic medicine transportation devices or artificial in vitro esophagus simulation systems, for example, to help formula eating‐related physiotherapy plans for patients and astronauts. [ABSTRACT FROM AUTHOR]

The development of intelligent underwater soft actuators by using stimuli‐responsive polymer materials is of immense scientific interest because of its wide application potential. Inspired by the hunting mechanism of sundews in nature, having rosettes of leaves that are sticky and capable of actuating, a novel structural hydrogel actuator made by high mechanical strength hydrogel poly(N‐isopropyl acrylamide‐acrylic acid)/Fe (PNIPAAm/PAAc‐Fe) with asymmetric structures in its two sides is fabricated, which is defined as structural hydrogel film. The as‐prepared hydrogel actuator can demonstrate reversible bending in response to temperature change (below/above the lower critical solution temperature of PNIPAAm) or encountering fast solvent exchange between ethanol and water, along with the switching of surface friction/adhesion force. During responsive bending, the actuator can generate large actuation force of ≈21 mN. Meanwhile, the combination of fast‐responsive bending, switchable surface friction, and strong actuation force endows the actuator excellent capacity to reversibly catch and release objects underwater. Especially, the existing fibrillar hydrogel pillars on actuator makes it possible to be used as a manipulator for realizing precise transportation of small objects underwater. The current actuator shows fantastic actuation capacity similar to natural sundews, and provides inspiration for developing novel soft robotics underwater. Inspired by the predation mechanism of sundews, a novel structured hydrogel actuator with fast‐responsive bending behavior, switchable surface friction, and strong actuation force, which endows the actuator excellent capacity to reversibly catch and release objects with different sizes, weights, and surface chemistries underwater, is developed. [ABSTRACT FROM AUTHOR]

Anisotropic friction of widespread biological surfaces with micro‐ and nanostructures oriented to supporting layer is proved to be crucial for the purpose of locomotion or transporting items in nature, which is therefore attracting extensive attention. Herein, variation of supporting layer rigidity induced dynamic control of anisotropic friction of biomimetic surface with structures oriented to supporting substrate is demonstrated. The biomimetic surface with oriented hook‐like spines microstructures embedded in polymeric substrate is fabricated by three‐dimensional (3D) printing followed by transfer reproduction. Incorporation of Fe3O4 nanoparticles with excellent near‐infrared (NIR)/infrared (IR) photothermogenesis in the substrate results in its variable rigidity with light on/off, which achieves the tunable anisotropic friction performance of the biomimetic surface. A proof‐of‐concept device with the function of Velcro type strip is demonstrated to unsnap the fastener by remote NIR/IR light, revealing its promising for designing intelligent control devices. It is believed that this novel anisotropic friction controllable system with photothermogenesis‐induced supporting layer rigidity variation will provide wide and potential applications in various fields including sensors, robots, and other bionic devices. Biomimetic surface with microstructures oriented to substrate is fabricated with three‐dimensional (3D) printing followed by transfer reproduction, where photothermal Fe3O4 nanoparticles are incorporated into the substrate. With the supporting layer rigidity variation induced by photothermogenesis of Fe3O4, the resultant surface realizes light triggered tunable frictional anisotropy, which will be promising in sensors, robots, and many other bionic devices. [ABSTRACT FROM AUTHOR]