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Scientists Use Sound to Remotely Control Material Stiffness

 Imagine being able to make a material soft in one area and stiff in another — all by sending a pulse of sound. It sounds like science fiction, but a team of researchers has just demonstrated a way to do exactly that. Their groundbreaking study, published in Nature Communications, reveals how acoustic waves can be used to remotely control the internal behavior of materials, potentially paving the way for adaptive protective gear, robotic muscles, and even medical implants that adjust their stiffness on demand.

The research was co-led by the University of California San Diego, the University of Michigan, and the French National Center for Scientific Research (CNRS) at the Laboratory of Acoustics of Le Mans University. By studying how sound interacts with a material’s internal structure, the team discovered a method to move “mechanical kinks” — tiny boundaries within materials that determine whether regions are soft or stiff — in a precise and predictable way.

What Are Mechanical Kinks?

To understand this discovery, it helps to know what mechanical kinks are. Kinks are boundaries inside materials where the internal structure changes slightly. On either side of a kink, the atoms or building blocks of the material may be the same, but they are arranged differently in three dimensions. This subtle shift can drastically change how the material behaves.

Kinks are found in many materials. For example, they appear where metals permanently bend or where strands of DNA separate. They act as “decision points” for how a material deforms. If scientists can move a kink, they can effectively reshape the material’s properties, making some areas soft and others stiff. However, controlling kinks has historically been very challenging because they are often trapped by energy barriers, making their motion unpredictable.

Sound as a Tool to Move Kinks

Previous studies had hinted that sound waves could move kinks, but the motion was typically chaotic and hard to control. This new study changes that. The researchers developed a material model in which moving a kink costs no energy — a rare and unusual property. In this model, the material’s behavior is determined by its structure rather than its composition.

In practical terms, this means that wherever a kink is located, that region of the material is soft, and the rest becomes progressively stiffer. Moving the kink to different positions changes the stiffness profile:

  • Move it to one end, and that end becomes soft while stiffness increases toward the opposite end.

  • Move it to the middle, and the center is soft while both ends are stiff.

“It’s like creating an acoustic tractor beam that moves a kink and changes the way a material feels — while creating gradients of stiffness — on demand,” explained Nicholas Boechler, a professor at the UC San Diego Jacobs School of Engineering and co-corresponding author of the study.

Because the model material has no energy barriers, sound waves can move the kink predictably, step by step. A small pulse moves the kink slightly; another pulse moves it further. Essentially, it’s remote control for a material’s internal state.

Demonstrating the Concept

To demonstrate their concept, the researchers built a life-sized experimental model: a chain of stacked, rotating disks connected by springs. Each disk represented an individual atom, and the springs mimicked atomic bonds. One disk, arranged differently, represented the kink.

When short pulses of sound were applied, the kink moved toward the sound source a few disks at a time. Longer pulses could move the kink continuously along the chain, flipping which side was soft and which was stiff.

“The level of control we achieved surpasses anything previously done,” Boechler noted. Interestingly, only certain sound frequencies caused the kink to move; other frequencies had no effect. Computer simulations showed that when a sound wave reached the kink, part of the wave reflected and part passed through. Yet, enough momentum transferred to move the kink forward.

Why This Matters

This study opens the door to a variety of future applications:

  1. Tunable Materials: Materials whose stiffness can be adjusted on the fly could improve protective gear, making helmets, armor, or padding adapt to different levels of impact.

  2. Robotics: Robotic muscles that change stiffness could move more naturally and adapt to different tasks, improving performance and safety.

  3. Medical Implants: Implants that can adjust their stiffness could better integrate with surrounding tissue or adapt to changing conditions in the body.

  4. Shape-Shifting Structures: Buildings or devices that adapt their mechanical properties in response to environmental conditions could be possible.

While the current demonstration is a “toy model,” it shows what becomes possible when materials are designed with new structural properties. Boechler emphasized, “Fundamental research like this often leads to technological breakthroughs. We’ve shown what is possible if you can control a material’s internal structure using sound.”

Next Steps

The team plans to explore three-dimensional versions of the model and investigate whether similar effects can occur at atomic scales. Achieving kink control at the atomic level could revolutionize materials science, offering unprecedented ways to design materials from the ground up.

The study was a collaborative effort, with Xiaoming Mao from the University of Michigan and Georgios Theocharis from CNRS as co-corresponding authors. Additional contributors include Kai Qian (first author), Nicolas Herard, Nan Cheng, Francesco Serafin, and Kai Sun.

A Fundamental Breakthrough

This research represents a fundamental shift in how scientists think about controlling materials. Instead of relying solely on composition, engineers can now consider how structure alone can dictate behavior. The ability to remotely control mechanical kinks with sound could lead to innovations across multiple fields, from aerospace engineering to wearable technology.

In the words of Boechler, “We’re at the beginning of a new way to interact with materials. If we can take this from a model to a real material, the possibilities are enormous. You could reprogram a material on demand, simply using sound.”

In the future, “smart” materials that adapt to their environment may not just exist in science fiction—they may be controlled by the sound pulses around us.

Reference: Kai Qian et al., Observation of mechanical kink control and generation via acoustic waves, Nature Communications (2026). DOI: 10.1038/s41467-026-68688-7

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