Liquid Crystal Elastomer Used in Soft Robots Can Fracture and Change Direction Unexpectedly

UCLA mechanical engineers have discovered that liquid crystal elastomer, a material commonly used in soft robotics, can fracture in an unexpected way. As a crack propagates, it can spontaneously change direction, deviating from its initial path due to internal molecular realignment. The team also developed a model that predicts the complex paths these fractures can take.

Findings from the study, published in the Proceedings of the National Academy of Sciences, could help scientists design more durable liquid crystal elastomers, or LCEs, leading to stronger, more resilient soft robots and other pliable devices.

LCEs are composed of a cross-linked polymer network embedded with rod-like liquid crystal molecules that naturally align in a preferred direction. This alignment gives the material direction-dependent mechanical properties, while the material’s viscoelastic nature allows it to behave like a liquid or an elastic solid depending on how quickly it is stretched.

Mechanics of Soft Materials Laboratory/UCLA
Video of a liquid crystal elastomer sample under stress, showing deformation and crack propagation

“Interestingly, the liquid crystals in LCEs can reorient their microstructure alignment when they’re pulled or stressed,” said study principal investigator Lihua Jin, an associate professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering. “This realignment results in fracture behavior that is not seen in traditional soft materials.”

To study the phenomenon, the researchers conducted systematic experiments and developed a predictive model that captures the interplay between material deformation and internal molecular reorientation. The model successfully predicted fracture behavior in various sample geometries and loading conditions.

“Understanding how LCEs fail can help improve how they are deformed and how they actuate,” said Jin, who directs the Mechanics of Soft Materials Laboratory at UCLA. “Ultimately, it could enhance the design of robust, high-performance soft materials for a variety of emerging applications.”

Beyond LCEs, the research could have broader implications for understanding materials whose internal microstructure changes in response to stress. For example, blood clots exhibit crack propagation perpendicular to the alignment of the platelets and proteins that form them. The new model could help in the design of advanced biomaterials for wound healing and other medical applications.

Funded by the National Science Foundation, the study’s first author Yu Zhou and co-author Chen Wei are both UCLA doctoral graduates previously advised by Jin.