Light-steerable locomotion using zero-elastic-energy modes

The creation of synthetic materials mimicking the dynamic behavior of living organisms is an active area of research. Driving these materials out of equilibrium is crucial for achieving life-like properties such as autonomy and adaptability. This principle is central to soft actuators and soft robotics, where self-sustained, periodic motions can result from the interplay between structural deformation and energy dissipation. Energy input (light, heat, chemical reactions) enables out-of-equilibrium functions like locomotion, object manipulation, and signal transduction. Zero-elastic-energy modes (ZEEMs), arising from mechanically frustrated systems with built-in strains, are particularly interesting for generating continuous motions under continuous external stimulation. While self-sustained ZEEM-based motions have been demonstrated, controlling motion parameters like velocity and direction remains a challenge. Dynamic steerability is vital for advanced soft robotics, allowing for adaptation and interaction with dynamic environments, especially for autonomous swimming in the Stokes regime. This study aims to address these challenges.

Previous research has explored various methods to drive soft materials out of equilibrium, including the use of ZEEMs. ZEEMs have been observed in diverse systems, from quantum mechanics to DNA topology. In materials science, ZEEMs are found in mechanically frustrated structures that exhibit continuous motion under continuous external stimuli. Self-sustained ZEEM-based locomotion has been demonstrated in various materials, offering potential for miniaturized autonomous soft robotics. However, a major limitation is the poor controllability of movement parameters in systems driven far from equilibrium. Achieving unidirectional locomotion typically requires asymmetry in geometry or deformability, which are difficult to adjust post-fabrication. The development of dynamically steerable self-sustained locomotion is crucial for advanced soft robotic functionalities.

The researchers fabricated a light-responsive liquid crystal elastomer (LCE) torus. (Photo)thermally responsive LCE fibers were created by injecting a monomer mixture into silicone tubes, followed by oligomerization via an aza-Michael addition reaction. A non-polymerizable liquid crystal was added to reduce viscosity. Photopolymerization under mechanical strain ensured monodomain alignment. The LCE fibers were post-cured on a hot plate to evaporate the non-polymerizable liquid crystal. The thermal expansion coefficient (α) of the LCE could be controlled by applying either stretching or twisting during polymerization, resulting in either inverting (α < 0) or everting (α > 0) rotational motion. Disperse Red 1 (DR1), a light-absorbing molecule, was incorporated to make the fibers light-sensitive. The fibers were then looped and glued to form a torus. The direction of rotation was determined by the sign of α. The researchers then investigated the torus's behavior in various environments (terrestrial and fluidic) under different illumination conditions. Detailed characterization of thermal expansion, light sensitivity, and mechanical properties were performed using various techniques including infrared imaging, dynamic mechanical analysis, and video analysis. Particle image velocimetry was used to analyze fluid flow.

The LCE torus demonstrated self-sustained rotation under constant light or heat exposure due to the formation of ZEEMs. The rotation direction was controlled by the sign of the thermal expansion coefficient (α), which was determined by the fabrication process. Oblique illumination created friction asymmetry, enabling light-steerable terrestrial locomotion. The direction of movement depended on the sign of α. The torus also exhibited swimming capabilities in viscous fluids. In glycerol, the interaction of bubbles formed due to photothermal heating with the surface influenced the translocation behavior. In non-polymerized PDMS (creating a Stokes regime with Re ≈ 0.0001), the torus demonstrated light-steerable three-dimensional swimming. The swimming direction was controlled by the illumination direction, with the viscous drag forces on the inner and outer layers of the torus dictating the direction of movement. The relationship between translational displacement and rotational angle was consistent with theoretical predictions for Stokes regime swimmers. The torus also showed the ability to dynamically reorient itself in response to an inhomogeneous light field, allowing for complex three-dimensional swimming trajectories.

The results demonstrate a novel approach to creating light-steerable soft robots based on ZEEMs. The ability to control locomotion direction using light offers significant advantages for applications such as targeted drug delivery or micro-manipulation. The achievement of light-driven swimming in the Stokes regime addresses a longstanding challenge in artificial microswimming, surpassing the limitations of previous designs. The use of a toroidal structure minimizes energy loss during rotation, leading to enhanced swimming efficiency. The findings contribute significantly to the field of soft robotics and active matter, opening new avenues for designing sophisticated, dynamically controllable soft locomotors.

This study successfully demonstrated a light-fuelled elastomeric torus capable of self-sustained rotational and translational motion. The direction of motion was precisely controlled by manipulating light. Three-dimensional steerable swimming in the Stokes regime was achieved, surpassing the limitations of previous artificial microswimmers. This work contributes to the development of advanced soft robots with potential applications in various fields.

The study primarily focused on a toroidal geometry. Further research is needed to explore the applicability of this approach to other geometries and materials. The fabrication process may require further optimization for large-scale production. The effect of the bubbles formed in glycerol on the locomotion needs further investigation.