New chain mail-inspired material morphs between liquid and solid states

PASADENA, Calif. — The line between solid and liquid just got a lot blurrier. Scientists at the California Institute of Technology have engineered materials that can transition between flowing like water and becoming rigid as plastic, potentially revolutionizing everything from protective gear to medical devices. These materials, which can also respond to electrical charges and absorb impacts, represent a fundamental shift in how we think about matter.

Most materials we encounter in daily life are either consistently solid (like a wooden table) or liquid (like water). But these new materials, called polycatenated architected materials, or PAMs for short, blur that line. They’re made up of rings or cage-like structures that interlock like pieces of a 3D puzzle, allowing them to move and slide against each other until they lock into place when enough force is applied.

The research, published in Science, draws inspiration from an unexpected source: medieval chain mail armor. While chain mail is made of simple metal rings linked together to form a protective mesh, these new materials take the ancient concept to new heights.

“With PAMs, the individual particles are linked as they are in crystalline structures, and yet, because these particles are free to move relative to one another, they flow, they slide on top of each other, and they change their relative positions, more like grains of sand,” says study author Chiara Daraio, a professor at Caltech, in a statement.

The research began with Wenjie Zhou, a former chemist turned postdoctoral researcher in Daraio’s lab. The team created their initial designs using computer modeling, translating crystal lattice patterns into structures where the fixed particles were replaced by interlocked rings or multi-sided cages. These designs were then 3D printed using various materials, including acrylic polymers, nylon, and metals, resulting in prototype cubes and spheres about two inches across. To test their creations, the researchers subjected them to various types of stress.

“We started with compression, compressing the objects a bit harder each time,” says Zhou. “Then we tried a simple shear, a lateral force, like what you would apply if you were trying to tear the material apart. Finally, we did rheology tests, seeing how the materials responded to twisting, first slowly and then more quickly and strongly.”

When subjected to certain forces, the materials showed almost no resistance, similar to how water flows. Under other conditions, they would suddenly become rigid and strong. This dual personality makes them unique among engineered materials and opens up possibilities for numerous applications.

“PAMs can be very different from one another,” explains Daraio. “You can print them in squishy materials or hard ones. You can change the shape of each particle, and you can change the lattice that you use to connect these particles. Each of these parameters affects the behavior of the resulting material.”

What makes these materials particularly revolutionary is their scalability. While most materials behave differently at different sizes—think of how a large sheet of paper falls compared to a tiny piece of confetti—PAMs maintain their core properties whether they’re the size of a golf ball or smaller than a human hair. This consistency across scales is rare in materials science and makes PAMs especially valuable for engineering applications.

The microscopic versions of these materials point toward perhaps the most exciting future applications. At this scale, individual building blocks are about the size of a human blood cell, allowing for potentially revolutionary medical applications. These tiny structures could be used to create devices that navigate through the body’s narrowest passages, changing shape as needed to deliver medications or perform minimally invasive procedures.

Another promising application lies in protective gear. Current technologies often force a trade-off between comfort and safety. Motorcycle jackets, for example, typically use rigid armor inserts that can be uncomfortable during normal wear. PAMs could create gear that remains flexible and comfortable during regular use but instantly stiffens upon impact, providing better protection when needed.

The research could also apply to robotics. Traditional robots often rely on rigid components connected by joints, limiting their flexibility and adaptability. Materials that can transition between fluid-like and solid-like states could lead to robots that can flow through tight spaces like a liquid but solidify to perform tasks requiring strength and precision.

“We can envision incorporating advanced artificial intelligence techniques to accelerate the exploration of this vast design space. We are only scratching the surface of what is possible,” says study author Liuchi Li, now an assistant professor at Princeton University.

The development of PAMs represents what Daraio calls “a fascinating frontier that promises to redefine what materials are and how they behave.” Their work bridges a gap in materials science between granular materials and elastic deformable materials, creating something entirely new that combines properties of both.

As researchers continue to explore the possibilities of these materials, the applications seem limited only by imagination. From protective gear that adapts to impacts to medical devices that navigate through blood vessels, PAMs represent not just a new material but a new way of thinking about what materials can do.