Development of environment-responsive self-propelling robots with shape-preserving exoskeletons for directed...

Autonomous soft robots capable of converting chemical energy into mechanical motion possess significant potential for applications such as targeted transport and environmental monitoring. In particular, systems that autonomously modulate their behavior in response to environmental stimuli are essential for practical implementation. Self-propelling droplets driven by the Marangoni effect are representative of chemical propulsion; however, their trajectory is inherently stochastic due to the random formation of interfacial tension gradients, making directional control difficult. Thus, establishing stable rectilinear motion remains a critical challenge. In this study, we have developed a pH-responsive gel robot powered by self-propelling droplets and propose a design strategy based on structural constraints to enhance directional controllability. Specifically, 1-pentanol-containing poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAm-co-AAc) gels are synthesized and evaluated in a pH 12 solution. At high pH, deprotonation of the acrylic acid carboxyl groups induces significant swelling through electrostatic repulsion within the polymer network. This pH-dependent expansion triggers the release and diffusion of internal 1-pentanol into the surrounding aqueous phase, creating interfacial tension gradients that induce Marangoni convection. While unconstrained gels exhibit random motion, the introduction of a 3D-printed resin exoskeleton geometrically restricts the reaction interface and localizes the convection formation. Particle Image Velocimetry (PIV) confirms that the convection field is aligned along the robot's central axis; thereby, stable straight-line propulsion was achieved. These results demonstrate that integrating pH-responsive swelling with mechanical constraints provides a robust design principle for programming the motion of chemically driven soft robots.