Researchers at the California Institute of Technology have devised a new class of materials that can shift seamlessly between behaving like a fluid and a solid. These materials, known as polycatenated architected materials (PAMs), are made up of interlocking rings or cage-like structures that can rearrange themselves in response to external forces. Imagine a suit of chainmail armor that flows like water when touched lightly but locks into place when struck with force. This is not far off from what PAMs can do.

The term polycatenated architected materials refers to the way they are built: not from solid blocks or rigid lattices, but from interlocked rings or cage-like particles that form three-dimensional networks. When gently deformed, they can stretch and twist, changing their structure like non-Newtonian fluids. But under greater stress, they stiffen and absorb energy like traditional foams or lattices. This unique behavior could lead to new applications in soft robotics, impact-resistant materials, and shape-morphing structures.

A Material That Defies Convention

Traditional materials are often rigid, with fixed properties determined by their chemical composition. PAMs are different because they derive their behavior not from what they’re made of, but from how their internal structures are arranged. The researchers, led by Wenjie Zhou and Chiara Daraio, designed these materials by translating the intricate geometries of crystalline networks into 3D structures made of interlocking particles.

“PAMs are really a new type of matter,” said Daraio, a professor of mechanical engineering and applied physics at Caltech. “They don’t fit neatly into the categories we’ve used for centuries.”

The inspiration for PAMs comes from an ancient invention: chain mail. For centuries, warriors wore armor made of interlocking metal rings, creating a flexible yet durable mesh. PAMs take this idea to a new level. Instead of simple rings, they are made up of multiple intricate, interlocking shapes. So, they consist of rings, cages, and other geometric forms arranged in three-dimensional lattices. These structures are designed on computers and brought to life using 3D printers.

Zhou, a postdoctoral researcher in Daraio’s lab, has spent two years studying these materials. “I was a chemist, and I wanted to make these structures at a molecular scale, but that proved too challenging,” Zhou says. “In order to get answers to the questions I had about how these structures behave, I decided to join Chiara’s group and study PAMs at a larger scale.”

From Concept to Reality

The team 3D printed prototypes of PAMs in various materials, including acrylic polymers, nylon, and metals. They made PAMs at both macro and microscales. Most of the prototypes were small cubes or spheres, about the size of a golf ball. Then, they put these materials through a series of tests — compressing, twisting, and shearing them to see how they responded.

The results were astonishing. Under some conditions, PAMs behaved like fluids. “Imagine applying a shear stress to water,” Zhou explains. “There would be zero resistance. Because PAMs have all these coordinated degrees of freedom, with the rings and cages they are composed of sliding against one another as the links of a chain would, many have very little shear resistance.” But when compressed, the same materials became rigid, behaving like solids.

This makes PAMs exquisitely unique. Most materials fall into one of two categories: solids or granular matter. Solids, like metals or crystals, have fixed structures. Granular materials, like sand or rice, are made up of individual particles that can move freely. PAMs, however, straddle the line between these two worlds.

“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,” Daraio explains. “This transition between fluid and solid-like behavior is what makes them so fascinating.”

The researchers found that by tweaking the geometry of the particles and the way they’re connected, they could control the critical strain at which jamming occurs. This means that PAMs can be designed to absorb energy more efficiently or to morph into specific shapes under certain conditions.

The Road Ahead

The potential applications for PAMs are vast. Their ability to absorb energy efficiently makes them ideal for protective gear, such as helmets or body armor. They could also be used in packaging, where cushioning is critical, or in biomedical devices and soft robotics, where flexibility and responsiveness are key.

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

As scientists continue to explore their properties, these materials could soon find their way into our everyday lives, reshaping the world in ways we can only begin to imagine.

The study appeared in the journal Science.