Thermally driven phase transition for reversible diving/surfacing hydrogel devices

Underwater robots and vehicles are highly interested research area because their potential applications in sensing and search. A challenge for the underwater devices is realization of three-dimensional locomotion in water. Herein we report a diving/surfacing device made from thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel. The PNIPAM hydrogel device shows fast and reversible diving/surfacing cycles in response to heating and cooling. The interaction between PNIPAM and water molecules makes density change of devices, resulting in diving/surfacing locomotion. Above the lower critical solution temperature (LCST), the hydrogel surfaces through deswelling process. Conversely, below the LCST, the hydrogel dives through swelling process. We also demonstrated the reversible diving/surfacing cycles of the PNIPAM hydrogel device through electrical stimulation. These results suggest the potential of controllable PNIPAM hydrogel devices for underwater micro robots.