Session: 13-15-02: Mechanics of Soft Materials II

Paper Number: 173949

Autonomous Locomotion of Non-Equilibrium Stimuli-Responsive Soft Materials 

One recent impetus of developing stimuli-responsive soft materials (SRSMs) is to use them for soft robotics. However, achieving robotic locomotion of SRSM typically requires sophisticated control of external stimuli in coordination with their movement. In this talk, I will demonstrate complex and autonomous locomotion of SRSMs under simple stimuli control by steering their non-equilibrium thermodynamic processes. SRSMs change their shapes, structures or functions in response to external stimuli through non-equilibrium processes, including diffusion, reaction, viscoelastic relaxation, etc. These processes not only determine the response speeds of SRSMs, but also govern how they spatiotemporally evolve their shapes, structures and functions. In this talk, using shape memory polymers (SMPs) and liquid crystal elastomers (LCEs) as model SRSMs, we demonstrate autonomous jumping and crawling via modulating their non-equilibrium processes.

Jumping is a crucial ability that enhances the mobility of natural creatures, enabling them to navigate intricate terrains effectively. In nature, achieving jumping often requires complex and specialized shapes that can store and release energy efficiently. This study investigates a novel mechanism for achieving jumping behavior in soft robots using SMPs without requiring high energy input or extra structures through experiments and finite element analyses. Specifically, the proposed design employs a spiral structure, which can form self-contact through non-equilibrium heat transfer and viscoelastic processes when the sample is subjected to local moderate heating, resulting in storing and rapidly releasing elastic energy, and thereby efficient jumping. By incorporating liquid metal particles into SMPs, the thermal response speed is significantly enhanced, which, in turn, enables faster actuation. Moreover, a double spiral design inspired by human biomechanics is implemented to further control the jumping direction and to increase the jumping height. SMP spirals are also adapted for projectile launching, mimicking the seed dispersal mechanisms of plants, wherein a small load is thrown upon sudden energy release. These findings provide valuable design guidelines for future jumping soft robots, emphasizing the importance of material choice, efficient energy storage, and control mechanisms to achieve optimal jumping performance.

Crawling is another essential mode of locomotion for many natural organisms, which inspires scientists to develop soft crawling robots capable of navigating challenging environments based on SRSMs. However, existing crawling systems are often limited by their dependence on specially tailored friction interactions, dynamic non-uniform stimuli control, complex fabrication involving heterogeneous materials, or extremely slow crawling speeds, which hinder practical applications. In this study, we present an autonomous, unidirectional, untethered, and sustainable soft crawler made of photothermal LCEs by patterning inhomogeneous photothermal responses. Consequently, we are able to tune the spatiotemporal bending of the LCE strip on the ground under light illumination, which results in distinct frictional forces at the two ends, leading to unidirectional movement. This approach enables autonomous crawling without complex fabrication and controls. Furthermore, experimental results and finite element analyses are employed to determine the optimal geometric and material compositions for maximizing crawling displacement, thereby providing valuable insights into developing more efficient autonomous soft robots. Additionally, multiple cycles of sustainable crawling motion were achieved by applying cyclic illumination, demonstrating the capability for continuous locomotion. This work provides guidelines on using simple material systems and controlling non-equilibrium processes to achieve crawling motion, contributing to the advancement of sustainable and efficient soft robots.

Presenting Author: Lihua Jin University of California, Los Angeles

Presenting Author Biography: Lihua Jin is an associate professor in the Department of Mechanical and Aerospace Engineering at the University of California, Los Angeles (UCLA). Before joining UCLA in 2016, she was a postdoctoral scholar at Stanford University. In 2014, she obtained her PhD degree in Engineering Sciences from Harvard University. Prior to that, she earned her Bachelor’s and Master’s degrees from Fudan University. Lihua conducts research on mechanics of soft materials, stimuli-responsive materials, instability and fracture, soft robotics, and biomechanics. She was the winner of the Haythornthwaite Research Initiative Grant, Extreme Mechanics Letters Young Investigator Award, Hellman Fellowship, NSF CAREER Award, ACS PMSE Early Investigator Award, Sia Nemat-Nasser Early Career Award, and SES Huajian Gao Young Investigator Medal.

Authors:

Boliang Wu University of California, Los Angeles
Lihua Jin University of California, Los Angeles