Mimicking the locomotive abilities of living organisms on the microscale, where the downsizing of rigid parts and circuitry presents inherent problems, is a complex feat. In nature, many soft-bodied organisms (inchworm, leech) have evolved simple, yet efficient locomotion strategies in which reciprocal actuation cycles synchronize with spatiotemporal modulation of friction between their bodies and environment. We developed microscopic (∼100 μm) hydrogel crawlers that move in aqueous environment through spatiotemporal modulation of the friction between their bodies and the substrate. Thermo-responsive poly-n-isopropyl acrylamide hydrogels loaded with gold nanoparticles shrink locally and reversibly when heated photothermally with laser light. The out-of-equilibrium collapse and reswelling of the hydrogel is responsible for asymmetric changes in the friction between the actuating section of the crawler and the substrate. This friction hysteresis, together with off-centered irradiation, results in directional motion of the crawler. We developed a model that predicts the order of magnitude of the crawler motion (within 50%) and agrees with the observed experimental trends. Crawler trajectories can be controlled enabling applications of the crawler as micromanipulator that can push small cargo along a surface.

Faculty of Chemical Engineering UCT Prague Prague Czech Republic

Guangdong Provincial Key Laboratory of Optical Information Materials and Technology and Institute of Electronic Paper Displays South China Academy of Advanced Optoelectronics South China Normal University Guangzhou China

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Prague Czech Republic

Netherlands Organization for Applied Scientific Research Materials Solutions Eindhoven The Netherlands

Process and Energy Laboratory 3ME Faculty TU Delft Delft The Netherlands

Van't Hoff Laboratory for Physical and Colloid Chemistry Debye Institute for Nanomaterials Science Utrecht University Utrecht The Netherlands

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