Polyelectrolyte soft actuators: 3D printed chitosan and cast gelatin

With increasing utilization of robots in daily tasks, especially in biomedical and environmental monitoring applications, there would be demands for soft, biodegradable, or even edible actuators that provide more versatility than conventional rigid materials (e.g., metals and plastics). Polyelectrolyte hydrogels produce mechanical motion in response to electrical stimulus, making them good candidates for implementation of soft actuators. However, their conventional fabrication process has so far hindered their applicability in a broad range of controlled folding behaviors. A novel application of 3D printing in biodegradable and biocompatible soft robots is presented in this study. It is observed that the contactless electroactive polyelectrolyte structures demonstrate reversible bending through polarity changes of electrodes. Edible gelatin and chitosan hydrogels are chosen as potential candidates of polyelectrolyte actuators. Actuation of 3D printed chitosan and its performance are compared with those of conventional cast film gelatin. The printing parameters are optimized for fabrication of the desired geometrical model and the printing effects on actuation performance are analyzed. It is demonstrated that the rectilinear hollows made by 3D printing improve the functionality of actuation in chitosan compared with a cast film gelatin actuator. 3D printed polyelectrolyte actuators will open a new chapter of soft biodegradable robots for preserving sustainability while offering custom geometrical, functional, and control properties.