Light-Responsive, Steerable Hydrogel Micro-Crawlers.

Abstract

Mimicking the crawling abilities of living organisms in robotics is a challenging feat, requiring complex feedback circuits to manage the many moving parts whilst retaining balance and precision. This is especially true on the microscale, where the downsizing of rigid parts and circuitry presents many new problems. In nature, it can be seen that many soft-bodied organisms have evolved strategies to spatiotemporally control friction between their bodies and environment in order to achieve locomotion. In this research, microscopic (100 μm) hydrogels are demonstrated to crawl by reciprocal actuation coupled to spatiotemporal modulation of friction between the gel and the surface. Thermo-responsive polyethylene glycol diacrylate (PEGDA) -crosslinked poly-nisopropyl acrylamide (PNIPAm) gels are loaded with gold nanoparticles such that a localised shrinking response occurs when irradiated with laser light. Off-center modulation of the shrinking response, combined with a hysteresis in the friction of the gel between shrinking and expanding cycles, results in a crawling motion. Crawlers produced this way are demonstrated to be steerable and have the ability to push small cargo along a surface. The new principle of crawling enables designing soft micro-robots that can crawl by single degree of freedom actuation and have the potential to be further modified with sensors or other devices.