Trends in polymeric shape memory hydrogels and hydrogel actuators

Recently, smart hydrogels with either shape memory behavior or reversible actuation have received particular attention and have been further developed into sensors, actuators, or artificial muscles. These three-dimensional polymer networks with the capability of shape deformations show a volume phase transition after being triggered by external physicochemical stimuli. Here, we review the recent advancements and the different types of shape memory hydrogels (SMHs). In addition, stimuli-responsive hydrogel actuators have been investigated with a special focus on their stimulation, their motion-deformation strategies and on the fabrication technologies adopted in hydrogel-based actuators, and finally their applications are described and discussed using specific examples.