A pH-Responsive Liquid Crystal Hydrogel Actuator with Calcium-Induced Reprogrammable Shape Fixing

Soft polymer actuators have myriad applications and have therefore gained considerable attention in recent years. However, it remains challenging to prepare soft actuators with predefined shapes. Here, a bilayer polymer actuator with a (re)programmable shape is prepared from a microporous anisotropic polypropylene scaffold and a thin, pH-responsive liquid crystalline network (LCN) layer. The hydrogen bonds between dimerized benzoic acid derivatives in the LCN can be disrupted by an alkaline treatment, resulting in a pH-responsive LCN hydrogel layer. The pH-responsive actuation is governed by both the anisotropic mechanical properties of the scaffold and the cross-link density of the LCN hydrogel. Ca2+ ions can be used to chemically cross-link the actuator resulting in an initial programmed shape. The shape fixing can be reversed by removing the Ca2+ ions with an ethylenediaminetetraacetic acid (EDTA) solution. The shape fixing can be performed locally, resulting in pH-responsive actuators with three-dimensional initial configurations of choice.