A team led by UC San Diego has shown that sound waves can act like a remote control for a material’s stiffness by moving tiny internal features called kinks. The work hints at future protective gear, soft robots and implants that can adapt on demand.
Imagine body armor that softens as you move but stiffens in a split second on impact, or medical implants that adjust to your body in real time. A new study suggests that one day, sound waves could be the remote control that makes such shape-shifting materials possible.
A team of researchers co-led by the University of California San Diego, the University of Michigan and the French National Center for Scientific Research (CNRS) at Laboratory of Acoustics of Le Mans University has demonstrated a way to use sound to change how a material behaves. By carefully tuning acoustic waves, they were able to move tiny internal features called mechanical kinks, which in turn switch regions of a material from soft to stiff and back again.
The work, published in Nature Communications, shows that sound can do this in a controlled, predictable way — a key step toward real-world applications such as adaptive protective gear, robotic “muscles” and smart implants.
At the heart of the research is the kink, a kind of internal boundary inside a material. On either side of a kink, the material is made of the same basic building blocks, but those blocks are oriented differently in three dimensions. That subtle difference can dramatically change how the material bends, stretches or resists force.
Mechanical kinks show up in many places in nature and technology, from the spots where metals permanently bend to the points where DNA strands separate. Materials scientists have long known that if you can move a kink, you can reshape how a material behaves — deciding which parts are soft, which are stiff and where it will deform.
The problem is that in most materials, kinks are stuck. They face energy barriers that pin them in place. Past attempts to move them with sound waves often led to chaotic, hard-to-predict motion.
In the new study, the team took a different approach. They designed and modeled a special kind of one-dimensional material in which moving the kink costs essentially no energy. Instead of relying on the material’s chemical composition, they engineered its structure so that the kink could slide freely.
In this model material, the kink acts like a moving “soft zone.” Wherever the kink sits, that region is soft, while the rest of the material becomes progressively stiffer. Slide the kink to one end, and that end softens while stiffness ramps up toward the opposite side. Move it to the middle, and the center becomes soft with stiff regions toward both ends.
Co-corresponding author Nicholas Boechler, a professor in the Department of Mechanical and Aerospace Engineering at the UC San Diego Jacobs School of Engineering, described the concept in vivid terms.
“The idea here is that we’ve essentially made an acoustic tractor beam that moves a kink and changes the way a material feels — while creating gradients of stiffness — on demand,” Boechler said in a news release.
Because there are no energy barriers in this design, the researchers could use sound waves not just to nudge the kink, but to do so step by step in a highly controlled fashion.
“We showed that if you send acoustic waves in from one side, they actually pull the kink toward where the sound came from,” Boechler added. “You can send a small pulse, and the kink moves a little. Send another pulse, and it moves a little more. It’s basically remote control for the material’s internal state.”
To move beyond theory, the team built a life-sized experimental model: a chain of stacked, rotating disks connected by springs. Each disk stood in for an atom, and the springs mimicked the bonds between them. One disk was arranged differently from the rest, representing the kink.
When the researchers sent short pulses of acoustic waves into the chain, the kink shifted a few disks at a time toward the sound source. Each additional burst of vibration pushed it farther along. When they applied longer vibrations, the kink traveled continuously across the entire length of the chain, flipping which side of the chain was soft and which was stiff.
The team also found that only certain sound frequencies could move the kink; others passed through without effect. Computer simulations showed that when a sound wave hits the kink, part of the wave reflects and part passes through, but the interaction still transfers enough momentum to keep the kink moving.
“Right now, this is a toy model,” added Boechler. “If something like this could be made into a real material, you could imagine structures that adapt on the fly — materials you can reprogram using sound.”
That vision includes materials with tunable stiffness, shape-changing structures and new ways to transmit signals robustly through a material by steering kinks instead of relying on traditional electronics.
The next steps for the researchers include exploring three-dimensional versions of their system and asking whether similar kink control could happen at much smaller scales, potentially down to the level of actual atoms.
“This is fundamental research,” Boechler added. “But fundamental discoveries are often what end up advancing technology in the long run. Our work shows what becomes possible when you design materials with genuinely new properties.”
If those possibilities pan out, future engineers might not just design what a material is made of, but also how it can be reprogrammed — with sound as the dial that turns stiffness up or down on demand.