Stimulus-active polymers can vary their shapes (configuration or dimension) or produce mechanical power in response to external stimuli such as heat, electricity, light, magnetic field, and water. In the past decade, many breakthroughs have been made in developing stimulus-active polymers with novel stimulus-active mechanisms. With proper designing, complicated movements such as swimming, inchworm walking, rotation, and bending can be achieved. Stimulus-active polymers can be applied in a wide range of areas from hi-tech areas to daily life. A few papers have been written on specific type of stimulus-active polymers, but have not covered all stimulus-active polymers. Hence, this article aims to present a brief overview of the different mechanisms and fabrication strategies of typical stimulus-active polymers. The various applications of different stimulus-active polymers are also briefly summarized in the article.

We report a new phenomenon in which the reversible formation and disruption of a cellulose nano-whisker (CNW) percolation network in an elastomeric thermoplastic polyurethane (TPU) matrix leads to an unprecedentedly rapidly switchable shape-memory effect (SME) that may be activated by water. The materials have been fully characterized to investigate the SME phenomenon using a number of different experimental techniques including cyclic tensile deformation, dynamic mechanical analysis, FTIR and polarized Raman spectroscopy. A model is developed in which it is shown that exposure to water allows breakup of the CNW percolation network so that the flexible elastomer matrix can be deformed to the desired shape. The CNW percolation network reforms after drying to provide a fixing force for the temporary shape. The entropy elasticity of the TPU matrix then enables rapid shape recovery when the CNW percolation network is disrupted again during wetting. This completely athermal water-sensitive SME mechanism is totally different from traditional ones, in which the water or other solvents are used as plasticizers to lower the glass transition temperature of shape memory polymers, so as to allow triggering of the shape recovery at room temperature or lower. The reported work provides a novel and effective strategy to achieve rapidly switchable shape recovery in a material by a simple wetting process and fixing through an easily applicable programmed drying process.

Electronic skins can monitor minute physiological signal variations in the human skins and represent the body’s state, showing an emerging trend for