Sound reprograms material stiffness remotely

American and French scientists have discovered a way to remotely alter the mechanical properties of materials using only sound. The technology, published in the journal Nature Communications, could revolutionize the development of protective equipment, medical implants, and artificial muscles capable of adjusting their stiffness on demand.

The research and who’s behind it

The study is a collaboration between the University of California, San Diego (UC San Diego), the University of Michigan, and the French National Centre for Scientific Research (CNRS), affiliated with Le Mans University. Professor Nicholas Boechler, from UC San Diego’s Department of Mechanical and Aerospace Engineering, is one of the corresponding authors, alongside Xiaoming Mao of the University of Michigan and Georgios Theocharis of CNRS.

What “kinks” are and why they matter

At the heart of the discovery is a concept called a mechanical kink—a kind of internal boundary within a material. On each side of this boundary, the building blocks of the material are the same, but they are oriented differently in space. This subtle difference is enough to create regions with completely distinct mechanical properties: on one side, the material may be soft; on the other, rigid.

These kinks appear, for example, where metals undergo permanent bending or where strands of DNA separate. Controlling their position is essentially controlling the material’s behavior. The problem is that, in most materials, these kinks are trapped by energy barriers and are difficult to move in a predictable way.

The innovation: a material without energy barriers

The team overcame this obstacle by designing a model material in which moving the kink requires no energy—a rare and unusual property. This was possible because the material’s behavior is dictated by its structure rather than its chemical composition.

In this material, wherever the kink is located, that region is soft, while the rest becomes progressively stiffer. If the kink is at one end, that tip is soft and the stiffness increases exponentially toward the opposite end. By moving the kink to the middle, the material becomes soft at the center and rigid at both ends. Boechler described the mechanism as an “acoustic tractor beam” capable of altering the material’s stiffness profile on demand.

The full-scale experiment

To validate the theory, the team built a full-scale experimental model: a chain of rotating disks stacked and connected by springs. Each disk represents an atom, and the springs simulate the bonds between atoms. A single disk positioned differently from the others represents the kink.

When short pulses of acoustic waves were sent through the structure, the kink was pulled toward the sound source, advancing a few disks at a time. With each new pulse, it moved a bit further. When longer, continuous vibrations were applied, the kink traveled across the entire length of the chain, completely reversing which end was soft and which was rigid. The experiments also confirmed that only certain sound frequencies trigger the motion—others have no effect at all.

What the simulations revealed

Computer simulations helped explain the physical mechanism behind the phenomenon. When a sound wave reaches the kink, part of it is reflected while another part passes through the region. Even so, this interaction transfers linear momentum to the kink, allowing it to keep moving. Currently, researchers can only pull the kink—not push it—but they emphasize that this level of control already surpasses anything previously achieved in the field.

Future applications and next steps

The researchers acknowledge that, for now, this remains a theoretical model. “If something like this can be turned into a real material, you can imagine structures that adapt in real time—materials you reprogram using sound,” Boechler said. Potential applications include materials with tunable stiffness, structures capable of changing shape, and more robust signal transmission systems.

The next steps involve exploring three-dimensional versions of the system and investigating whether similar effects could exist at much smaller scales—down to the atomic level. The study’s first author is Kai Qian, also from UC San Diego.

The research marks a fundamental advance in materials science: demonstrating that sound can be used not just to measure or communicate, but to rewrite the internal behavior of a material. Although still far from clinical or industrial applications, the discovery lays the conceptual groundwork for a new generation of smart, adaptive materials—from protective armor to robotic muscles and medical implants that respond to their environment without requiring internal power sources.