Research Progress of Solvent-based Smart Actuator Materials

Recently, smart actuator materials have drawn widespread research attention due to their important applications in soft robots, artificial muscles, sensors, or micro hand device preparation. In nature, there are many examples of actuator materials. For example, sea cucumbers can alter the stiffness of their dermis within seconds to obtain survival advantages and the venus flytrap can close their leaves in a second for efficient prey capture. Pinecones and flowers respond to their environment by opening and closing with the relative humidity changes. Inspired by these natural creatures, synthetic polymer microactuators such as polymer hydrogels and polymer composites are widely developed due to their important applications based on their response to external stimuli, such as light, heat, electronic, magnetic, solvent and humidity. In this work, we review the research progress of solvent-based smart actuator materials. There are mainly two kinds of solvent-actuator based on the fabrication method and actuator mechanism: one is a two-layer structure membrane formed by active layers-support layers with different expansion coefficients. The active layer is volumetrically expanded under the action of a solvent, and the support layer is a passive holder. The other is made of rigid material skeleton with a flexible material to make a single-layer composite membrane filler. The ionic gradient or the pore structure gradient of the material itself gives rise to the directional driving behavior with a varying solvent binding gradient. Otherwise, the membrane's bending drive behavior has been achieved by inducing a single material to form an infiltration gradient by a solvent infiltration process. Solvent-based smart actuator materials are prepared by introducing moisture or solvent-responsive molecules in a polymeric material to form a bilayers or monolayer structure. The material is distorted by volume deformation due to humidity or solvent field action. At present, a great deal of research work has been devoted to converting the mechanical deformation of solvent-based smart actuator materials into electric energy and developing related intelligent application in energy transformation, liquid switching, biomimicry, transportation of liquids and smart sensing. The paper presents a pioneering outlook for the further development of the solvent actuator materials.