Thermally triggered soft actuators based on a bilayer hydrogel synthesized by gamma ray irradiation

Abstract

Hydrogel soft actuators, which have a bilayer structure consisting of a temperature-responsive active layer and a passive layer, were prepared by a gamma ray radiation-induced polymerization and cross-linking process. We synthesized single layer hydrogels of poly(N-isopropyl acrylamide) (PNIPAAm), poly(N-isopropyl acrylamide-co-acrylic acid) (PNIPAAm-PAAc), or poly(acrylamide) (PAAm) and bilayer hydrogels of PNIPAAm/PAAm and PNIPAAm-PAAc/PAAm with a range of cross-link densities. It was confirmed that PNIPAAm-based hydrogels could be synthesized without any additives through effective polymerization and cross-linking reactions under a gamma ray irradiation dose >20 kGy. Most hydrogels had a gel fraction >80%, which gradually increased with gamma ray irradiation dose and cross-linker content. All PNIPAAm-based hydrogels showed a high equilibrium water content (EWC) of >90%. As the cross-link density within the hydrogel matrix increased, the EWC gradually decreased, while the mechanical strength tended to increase linearly. It was found that the temperature-responsive bending actuation of the PNIPAAm-based hydrogel with a bilayer structure could be controlled by monomer composition, cross-link density, and the geometrical parameters of hydrogel. In addition, all A549 cells exposed to the extracts of PNIPAAm/PAAm and PNIPAAm-PAAc/PAAm hydrogels exhibited >77% cell viability. This gamma ray radiation-induced hydrogel with bilayer structure provides a new strategy for the construction of stimuli-responsive and shape-transformable hydrogel actuators for various applications including biomimetic robotics, wearable devices, medical devices, and environmental sensors.