Shape memory polymer actuators have attracted great attention in the past few
decades for developing soft robots, artificial muscles, and biomedical implants,
among others. Herein, under a tutorial perspective, a series of shape memory
actuators that display unidirectional movement upon exposure to specific
external stimuli is explored. The mechanistic details associated with the
movement of each system are also highlighted. In general, an overview of basic
design principles, the chemical structure of molecular switches, the strategies
used to achieve such controlled and unidirectional movement, as well as key
experimental aspects, is provided. The most critical challenges of the existing
devices that limit their practical applicability are also discussed. This is of utmost
importance because such limitations must be overcome to fabricate micromotors
that can generate complex movements, other than simple stretching/contraction
or bending, to perform complex tasks.
