Self-Resetting Soft Ring Enables Autonomous and Continuous Leaping under Uniform Light (2510.09529v1)

Abstract: Jumping is an efficient locomotion strategy to traverse cluttered, uneven, or unstable environments in nature, yet replicating continuous, autonomous leaping in soft robots remains challenging due to limited energy storage and reliance on human intervention or latches. Here, we report a millimeter-scale, self-resetting soft ring that achieves repeated vertical and stable horizontal leaps under uniform infrared illumination without external control. The ring-shaped liquid crystal elastomer body twists to store elastic energy, which is suddenly released when a rigid tail strikes the ground, propelling the robot. During the airborne phase, the twisted body autonomously untwists, resetting for the next cycle. By tuning geometric asymmetry and the center of mass, the robot transitions between crawling, directional leaping, and vertical jumping. Optimized configurations yield vertical jumps exceeding 80 body heights and directional horizontal leaps over 3 body lengths. Beyond controlled motion on flat ground, the robot demonstrates resilient and robust locomotion across slopes, parallel hurdles, and diverse cluttered natural terrains including grass, wet sand, and mulch. This work establishes a new paradigm of twisting-enabled, photothermally powered soft robots capable of autonomous, continuous leaping, with potential applications in environmental navigation, swarm robotics, and unstructured terrain navigation.

• The paper presents a self-resetting soft ring that autonomously leaps using photothermal-induced torsional twisting and elastic energy release.
• The design integrates a hybrid liquid crystal elastomer structure with a rigid tail to store energy and control trajectories through geometric asymmetry and mass distribution tuning.
• The robot exhibits robust performance with leaps up to 80 body heights and directional jumps over 3.1 body lengths, demonstrating adaptability in unstructured environments.

Autonomous and Continuous Leaping in Soft Robots via Self-Resetting Soft Rings

Introduction and Motivation

The paper presents a millimeter-scale, self-resetting soft ring robot capable of autonomous, continuous leaping under uniform infrared (IR) illumination. The work addresses a persistent challenge in soft robotics: achieving repeated, untethered, and autonomous jumping without the need for latches, time-varying external stimuli, or manual resetting. Traditional soft jumpers, often inspired by biological systems, typically require either active control, external modulation, or are limited to single-use launches due to inefficient energy storage and lack of autonomous resetting mechanisms. The proposed design leverages a liquid crystal elastomer (LCE) ring with a rigid tail, enabling cyclic elastic energy storage and release through twisting, and automatic resetting via geometric inversion symmetry.

Design Principles and Mechanism

The core innovation is a hybrid LCE ring structure, fabricated by joining the ends of a soft LCE rod with a rigid aluminum tube to form a triangular tail. The tail introduces geometric asymmetry (binding angle $\beta$), which is critical for energy storage and release dynamics. Under uniform IR illumination, the LCE segment undergoes anisotropic photothermal contraction, inducing torsional twisting and storing elastic energy. The rigid tail resists this twist, accumulating strain until a critical threshold is reached, at which point the tail snaps and strikes the ground, releasing the stored energy and launching the robot.

During the airborne phase, the twisted LCE segment autonomously untwists due to the ring's inversion symmetry, resetting the geometry for the next cycle. This closed-loop process enables continuous, autonomous leaping under constant illumination, with no need for external control or latching mechanisms.

Locomotion Phase Diagram and Performance Regimes

The interplay between the binding angle $\beta$ and light intensity $w$ governs the robot's locomotion mode, as mapped in a comprehensive phase diagram. Three primary regimes are identified:

• Vertical Jumping: Achieved with sharp binding angles ($30^\circ \leq \beta \leq 80^\circ$) and moderate $w$, enabling jumps exceeding 80 body heights (BHs) due to a self-locking effect that maximizes energy storage.
• Directional Leaping: Intermediate angles ($80^\circ < \beta < 110^\circ$) and optimal $w$ produce forward leaps up to 3.6 body lengths (BLs), with launch angles near $50^\circ$.
• Crawling: Near-circular rings ($110^\circ < \beta \leq 130^\circ$) and low $w$ result in insufficient energy storage, yielding slow crawling with minimal lift.

Light intensity modulates actuation power and thermal softening, with an optimal window ($w \approx 0.49$ W/cm$^2$) maximizing elastic energy storage and leap performance. Excessive intensity leads to overheating and loss of mobility.

Center of Mass Tuning for Trajectory Control

A critical insight is that stable, directional leaping requires alignment of the center of mass (CoM) with the thrust vector at liftoff. Misalignment induces torque, causing backflips and unstable trajectories. The authors introduce a tunable head weight (copper-foil tube) to shift the CoM forward, parameterized by the weight ratio (wt%). Increasing wt% suppresses flipping, converts vertical jumps into forward leaps, and stabilizes trajectories, even on inclined surfaces.

Optimal performance is achieved at $\beta = 80^\circ$ and wt% = 25%, yielding launch angles of $40^\circ$–$50^\circ$ and velocities exceeding 0.4 m/s, with leap distances up to 3.1 BLs. This configuration outperforms previous electrohydrostatically driven soft jumpers in normalized leap distance.

Robustness in Unstructured and Cluttered Environments

The robot demonstrates robust, self-adaptive locomotion across a range of challenging environments:

• Obstacle Navigation: The soft ring can clear parallel hurdles, leverage tail-obstacle interactions for collision-assisted leaps, and recover from head-on collisions due to geometric symmetry.
• Cluttered Terrains: In simulated "pillar forests" (randomly scattered tacks), the robot self-adjusts posture and escapes trapping through successive jumps.
• Natural Surfaces: On grass, wet sand, and mulch, the robot maintains continuous leaping, with terrain compliance and irregularity modulating trajectory stability and heading control. Wet sand provides the most stable trajectories, while grass enables the highest but least directed leaps.

Theoretical and Practical Implications

The work establishes a generalizable strategy for autonomous elastic energy recycling in soft active matter. Twisting is shown to be a compact and efficient mode of energy storage and release, superior to bending or stretching for cyclic actuation at small scales. The design principles—geometric asymmetry, mass distribution tuning, and inversion symmetry—are broadly applicable to other soft active materials and actuation modalities, including photothermal, magnetic, and electroactive systems.

Potential applications include distributed environmental sensing, swarm robotics, and navigation in unstructured or hazardous terrains where conventional robots are ineffective. The design is amenable to scaling and integration into multi-agent systems.

Conclusion

This paper demonstrates a self-resetting, photothermally powered soft ring robot capable of autonomous, continuous leaping under uniform illumination. The integration of torsional energy storage, geometric inversion symmetry, and CoM tuning enables robust, repeatable, and adaptive locomotion across diverse terrains and obstacles. The findings provide a blueprint for the development of autonomous soft machines with enhanced mobility and resilience, with significant implications for future research in soft robotics, active matter, and distributed robotic systems.