The design of materials that can mimic the complex yet fast actuation phenomena in nature is important but challenging. Herein, we present a new paradigm for designing responsive hydrogel sheets that can exhibit ultrafast inverse snapping deformation. Dual-gradient structures of hydrogel sheets enable the accumulation of elastic energy in hydrogels by converting prestored energy and rapid reverse snapping (<1 s) to release the energy. By controlling the magnitude and location of energy prestored within the hydrogels, the snapping of hydrogel sheets can be programmed to achieve different structures and actuation behaviors. We have developed theoretical model to elucidate the crucial role of dual gradients and predict the snapping motion of various hydrogel materials. This new design principle provides guidance for fabricating actuation materials with applications in tissue engineering, soft robotics, and active medical implants.

The design of smart hydrogel actuators fully constructed from natural polymers for assessing the biomedical applications is important but challenging. Herein, an extremely simple, green, and ultrafast strategy is presented for preparing robust gradient all-polysaccharide polyelectrolyte complex hydrogel actuators. Driven by diffusing of low molecular weight chitosan into high molecular weight sodium alginate solution, a nanoporous, ultrastrong, and gradient chitosan/sodium alginate complex hydrogel film with adjustable thickness can be directly generated on the interface of two solutions within minutes. The as-prepared film can provide superfast temperature, ionic strength, and pH-triggered programmable deformations, and perform a distinct sequential double folding behavior due to the competitive effect between complexed and noncomplexed segments of polyelectrolyte. Besides, patterning Ca 2+ to locally crosslink sodium alginate in the film enables various more complex shape transformations. This green and simple diffusion-driven strategy provides significant guidance for fabricating bio-friendly actuators with applications in drug delivery, tissue engineering, soft robotics, and active implants. drug delivery, [6] tissue engineering, [7] biomimetic actuators, [8] medical devices, [9] etc. [10] The 3D deformation of self-shaping hydrogel actuator stems from their preprogrammed heterogeneous structures, [11] which cause the in-plane and/or out-ofplane mismatches in volume change upon the trigger of external stimuli. [12] Despite the considerable progress that has been made recently in terms of the sophistication of the shapes and the diversity of the function for self-shaping hydrogels, [13] their applications in biomedicine and engineering remain challenging. The major limitations include the unsatisfactory biocompatibility and biodegradation of synthetic polymers involved in existing hydrogel actuators, as well as their poor mechanical properties and long characteristic response time. [14] Currently, there is a growing demand for developing a broadly applicable method toward designing the ultrastrong hydrogel actuators composed entirely of natural polymers to assess the practical applications.Natural polyelectrolytes are the ideal candidate materials for the hydrogel actuators. Besides inherently prominent advantages of renewability, biocompatibility, and biodegradability for nature polymers, [15] they possess the ability of changing their own conformations in response to external stimuli (e.g., pH and IS). [16] To integrate polyelectrolytes into macroscopic materials, layer-by-layer (LBL) assembly is the most commonly used fabrication method, which is carried out by sequential adsorption of oppositely charge polyelectrolytes onto various substrates. [17] However, the time-consuming LBL technique can only generate ultrathin films on a nanometer scale that cannot be used as freestanding material unless by chemical posttreatment, and thus is inapplicable for the fabricating of biocompatible hydrogel actuators. [...

Human skin is the largest organ, and it can transform multiple external stimuli into the biopotential signals by virtue of ions as information carriers. Ionic skins (i-skins) that can mimic human skin have been extensively explored; however, the limited sensing capacities as well as the need of an extra power supply significantly restrict their broad applications. Herein, we develop self-powered humanlike i-skins based on gradient polyelectrolyte membranes (GPMs) that can directly and accurately perceive multiple stimuli. Prepared by a hydrogel-assisted reaction-diffusion method, the GPMs exhibit gradient-distributed charged groups across polymer networks, enabling one to generate a thickness-dependent and thermoresponsive self-induced potential in a hydrated situation and in a humidity-sensitive self-induced potential in a dehydrated/dried situation, respectively. Consequently, the GPM-based i-skins can precisely detect pressure, temperature, and humidity in a self-powered manner. The coupling of mechano-electric and thermo-electric effects inherent in GPMs provides a general strategy for developing innovative self-powered ion-based perception systems.

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Controllable growth of CNTs/CNFs on carbon fiber surface without degradation of tensile strength of carbon fibers.

The design of materials that can mimic encoded shape evolution in nature is important but challenging. Here we present a simple yet versatile strategy for programming the sequential deformation of hydrogel sheets to acquire desired actuation motions and geometric shapes. The method relies on the dual-gradient structure-enabled snapping deformation of hydrogels through the accumulation and burst release of elastic energy, as well as the patterning of the prestored energy in gels. Pretreating distinct regions of the hydrogel sheets with different durations of the same stimulus (or with different stimuli) allows for locally prestoring chemical energy that can be converted to temporospatially patterned elastic energy and abruptly released to drive the successive snapping of different regions of hydrogels in predefined onset sequences. The sequence of energy release (i.e., the sequence of snapping deformation) of the local regions for hydrogels can be reprogrammed by different local prestimulation methods, which allows one gel to deform into various defined geometric configurations. The general mathematic criteria are developed to predict the energy release and snapping of the hydrogels. This work can provide guidance for the design of new-generation actuators and soft robotics.

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