Recent research from the Caltech lab led by Chiara Daraio, a professor of Mechanical Engineering and Applied Physics, has resulted in a captivating new form of matter. This material, known as PAM (short for polycatenated architected materials), displays unique properties, behaving like a fluid under certain stresses and like a solid under others. Its potential applications span from protective gear like helmets to biomedical devices and robotics.
While PAMs don’t naturally occur, their fundamental concept is reminiscent of the ancient technique of creating chain mail, where small metal rings are interconnected to form a flexible armor. In contrast, PAMs can be thought of as an advanced version of chain mail. They utilize interlocking shapes and consist of diverse forms that are interconnected to create highly complex three-dimensional structures. Researchers, including Daraio and her team, have produced these innovative materials using 3D printing technology and discovered behaviors that are not observed in traditional materials.
Wenjie Zhou, a postdoctoral researcher in mechanical and civil engineering, has been focused on these materials for the past two years in Daraio’s lab. Originally trained as a chemist, Zhou aimed to create micro-scale structures but found it too difficult. This led him to join Daraio’s group to explore PAMs on a larger scale and investigate their unique behaviors.
The PAMs developed by Daraio’s team were first simulated on a computer. They aimed to mimic the lattice patterns found in crystals, but with fixed particles replaced by intertwining rings or multi-sided cages.
Using diverse materials like acrylic polymers, nylon, and metals, the team printed these lattice structures in 3D. Once they were tangible—usually in cube or sphere shapes measuring around 5 centimeters (2 inches)—the PAMs were subjected to different types of physical stress. “We began with compression,” Zhou notes, “applying increasing pressure on the materials. Next, we applied a simple shear force, similar to when you try to tear something. Finally, we conducted rheology tests to observe how they reacted to twisting, starting with slow movements and gradually increasing the speed and intensity.”
Interestingly, in some cases, PAMs acted like liquids. “If you think about applying shear stress to water,” Zhou explains, “there’s no resistance whatsoever. Since PAMs feature numerous coordinated movements, the rings and cages slide past one another similar to links in a chain, resulting in very little shear resistance.” However, when these structures are compressed, they can become completely stiff, mimicking the properties of solid objects.
This adaptability sets PAMs apart. “PAMs represent an entirely new category of material,” says Daraio. “Typically, we distinguish between solids and granular materials. Solid materials are often illustrated by crystalline lattices like those seen in traditional atomic or chemical structures. Conversely, granular materials consist of separate particles that move and slide independently, such as rice or flour.”
PAMs blur the lines between these classifications. “PAMs link individual particles as in crystalline structures. Yet, since these particles can move relative to one another, they can flow, glide, and change positions much like grains of sand,” Daraio clarifies. “PAMs can vary significantly from one another; you can produce them with soft or hard materials, alter the shapes of particles, and modify the lattice structures that join them. Each of these variations impacts the material’s overall behavior. Nevertheless, they consistently exhibit a transition between fluid and solid characteristics—this transition occurs under different conditions but is always present.”
“For the last two to three decades, architected materials have emerged as an important subfield in materials science and engineering,” adds Daraio. “Yet PAMs stand out as they blend attributes of granular and elastic materials. We have theoretical frameworks for each of these categories, but none that addresses materials that fall between the two. This is an exciting new frontier that is poised to redefine our understanding of materials and their behaviors.”
Currently, the applications of PAMs are mainly conjectural but nonetheless compelling. “These materials possess distinct energy-absorption abilities. Since each component can shift, rotate, and rearrange relative to one another, they can dissipate energy very effectively,” Daraio mentions, indicating that, for uses like helmets and other protective gear, PAMs may outperform conventional foam materials. They also show promise for packaging and situations requiring cushioning or stabilization.
Research involving microscale PAMs has revealed that they can expand or contract in response to electrical charges as well as physical forces, hinting at potential applications in biomedical devices or soft robotics.
Liuchi Li, co-author and now an assistant professor at Princeton University, is excited about the future possibilities of PAMs: “We can imagine leveraging advanced artificial intelligence to accelerate the exploration of the vast design potential. We’re only beginning to uncover what these materials can do.”
This research is documented in the journal Science under the title “3D polycatenated architected materials.” The co-authors include Zhou, Daraio, Sujeeka Nadarajah, Hujie Yan (MS ’24), Aashutosh K. Prachet, and Payal Patel from Caltech; Li from Princeton; and Anna Guell Izard and Xiaoxing Xia (PhD ’19) of Lawrence Livermore National Laboratory (LLNL). Computational resources were supplied by Caltech’s High-Performance Computing Center, and the research was supported by the Army Research Office, the Gary Clinard Innovation Fund, LLNL, and the U.S. Department of Energy.
