Fig. 1. Conceptual scheme of the strong contractive materials based on mechanical energy storing method.

(A) Frog leaping off a rock through prestoring and subsequent releasing of elastic potential energy. (Photo credit: Purchased from vjshi.com under royalty-free license.) (B) Elastic potential energy was initially stored (red area) in elastic structures by contraction of muscle via consuming chemical energy in the body and then instantaneously releasing based on releasing (blue area) of elastic potential energy. (C) Schematics of the design strategy of shrinking materials via storing energy during stretching and then releasing the energy: storing elastic potential energy in the material by mechanically stretching it and locking it at the stretched state through forming new chemical bonds under external stimulus-1 (i). Upon external stimulus-2, the newly formed bonds break, and the prestored energy is released, resulting in rapid and powerful contraction of the material, which generates large force (ii). The green images are confocal microscopic images of the hydrogel material.