Low-intensity near-infrared light-tunable gold nanorod-incorporated hydrogel actuator system for remotely controlled human–robot interface applications

Abstract

Light-responsive soft actuators have recently drawn attention as the need for lightweight remotely controlled actuator systems with high environmental adaptability and safe human–robot interaction interface characteristics has increased. In this study, we focused on a near-infrared (NIR) light-tunable hydrogel soft actuator system capable of fast and precisely controllable mechanical actuation even under the irradiation of NIR light with low intensity. We designed a bilayer type hydrogel soft actuator consisting of a poly(N-isopropylacrylamide) (PNIPAAm) active layer containing gold nanorods (AuNRs) as a photothermal agent and poly(acrylamide) (PAAm) passive layer. AuNRs with an aspect ratio of 4.12 showed the largest longitudinal plasmon resonance absorption peak in the NIR 880 nm range, which was used as a photothermal agent. The AuNR-incorporated PNIPAAm/PAAm hydrogel with a bilayer structure (PNIPAAm-AuNR/PAAm) exhibited more than 70% gel fraction and temperature-sensitive swelling behaviors. In particular, rod-shaped PNIPAAm-AuNR/PAAm hydrogels showed rapid bending deformation by a photothermally induced phase transition of PNIPAAm upon 880 nm laser irradiation, but a PNIPAAm/PAAm bilayer hydrogel without AuNRs did not show a light-induced bending movement. The NIR-responsive bending actuation of the hydrogel can be precisely controlled by cross-linking density, content of photothermal agent, NIR laser intensity, and thickness ratio of the active/passive layer. It was observed that the full bending deformations of the hydrogel into a ring shape through an arc occurred within a minute under NIR irradiation with a lower intensity of less than 1.0 W/cm2. Hydrogels showed a cell viability of more than 95% in the biocompatibility test, indicating no significant cytotoxicity. Therefore, these PNIPAAm-AuNR/PAAm hydrogels are promising soft actuator materials that can be controlled by low-intensity NIR irradiation for remotely controlled human–robot interaction interface applications.