Researchers at the University of Michigan, led by Assistant Professor Suraj Shankar, have proposed a new theoretical framework that could help synthetic soft materials move in ways that more closely resemble living tissue. The work focuses on how internal feedback between chemistry and mechanics can generate fast, irregular motions such as twitching or shivering, behaviors commonly observed in muscles but rarely achieved in engineered materials.
Sarkar, S., Ash, B., Wu, Y., Boechler, N., Shankar, S., & Mao, X. (2025). Mechanochemical Feedback Drives Complex Inertial Dynamics in Active Solids. Physical Review Letters, 135(25), 258301. https://doi.org/10.1103/19rh-3whq
Living tissues are capable of producing rapid, forceful motion despite being soft and flexible. Muscles contract and relax repeatedly, hearts beat continuously, and cells respond dynamically to their environments. By contrast, most synthetic soft materials are passive. When deformed, they gradually return to their original shape as energy is dissipated through internal friction. This damping limits how quickly and powerfully such materials can move, making it difficult to design soft machines that rival biological performance.
Assistant Professor Suraj Shankar from University of Michigan stated,
“Imagine you have a gel that’s shivering or twitching. That’s physically what this sort of chaotic behavior would look like for an actual material.”
The Michigan team’s model addresses this limitation by treating energy loss not as a problem to be eliminated, but as a component that can be counterbalanced through active processes. Their framework describes materials that contain chemical reactions capable of injecting energy into the system. Crucially, these reactions are sensitive to mechanical forces, meaning that stress or strain within the material can alter reaction rates. This creates a feedback loop in which mechanical deformation influences chemistry, and chemical activity in turn affects mechanical motion.
In conventional soft materials, inertia is often neglected because damping dominates the dynamics. The new model shows that when chemical feedback is sufficiently strong, inertia becomes relevant again. Under these conditions, the material does not simply relax back to equilibrium. Instead, it can enter regimes of complex motion that never exactly repeat, a hallmark of chaotic dynamics. In physical terms, this could correspond to irregular oscillations or localized twitching throughout the material.
The researchers emphasize that “chaos” in this context does not imply randomness, but rather deterministic behavior that is highly sensitive to initial conditions. In biological systems, such sensitivity is common and often useful, allowing tissues to respond quickly and adaptively to changing demands. The modeling work suggests that similar responsiveness could be engineered into synthetic materials by carefully coupling force-sensitive chemistry with mechanical design.
Although the study is theoretical, it builds on experimental results reported elsewhere. Previous work in soft matter and materials science has demonstrated individual elements of the proposed feedback loop. Some polymers change color or chemical state under stress, while others undergo shape changes driven by internal reactions. What has been missing is a unified framework that shows how these elements could work together to sustain fast, lifelike motion rather than isolated responses.
The authors argue that combining these components is chemically and physically plausible with existing techniques. Advances in mechanochemistry, active gels, and responsive polymer networks provide potential pathways toward realizing such materials experimentally. If successful, the approach could inform the design of soft actuators, adaptive structures, and bio-inspired machines that operate efficiently without rigid components.
Beyond robotics and mechanical systems, the framework may also be relevant for understanding active matter more broadly, including collective behavior in cells and tissues. By showing how mechanical resistance, chemical energy input, and inertia interact, the work provides a common language for describing both biological and synthetic active solids.
For engineers and material scientists, the study highlights a shift in perspective. Instead of designing soft materials to suppress dissipation and irregularity, future systems may intentionally harness feedback and instability to achieve useful motion. While practical implementations remain a challenge, the model offers a roadmap for moving closer to materials that do not just deform, but actively behave.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
