Novel materials from the 3D printer with unique properties

A research team at the California Institute of Technology (Caltech) led by Chiara Daraio has developed a new class of materials that are neither classically granular nor crystalline. These so-called PAMs (polycatenated architected materials) react either like liquids or like solids, depending on the load. The innovative materials, which are produced using 3D printing, could be used in areas such as protective clothing, robotics or medical technology.

“We started with compression,” Wenjie Zhou, postdoctoral scholar research associate in mechanical and civil engineering explains, “compressing the objects a bit harder each time. 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.”

PAMs are based on the idea of interlocking structures, similar to chain mail, but in highly complex three-dimensional patterns. The materials are produced by 3D printing, using polymeric, metallic or other materials. By linking rings or cages, lattice structures are created that have different mechanical properties depending on the load. Tests have shown that PAMs can react like liquids under shear stress, as the elements slide freely against each other. Under pressure, on the other hand, they harden and behave like solids.

“Imagine applying a shear stress to water,” Zhou says. “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.”

These dynamic properties make PAMs a unique class of materials that cannot be categorized as either elastic or granular materials. The researchers were able to show that the material properties can be specifically adapted through the choice of shape, connection and material of the individual components. This opens up a wide range of potential applications, from energy-absorbing helmets to actuators in medical technology.

At this point, potential uses for PAMs are largely speculative but nevertheless intriguing, Daraio says: “These materials have unique energy-absorption properties. Because each element can slide and rotate and reorganize relative to each other, they can dissipate energy very efficiently,” making them better candidates for use in helmets or other forms of protective gear than the currently used foams. This property makes them similarly attractive for use in packaging or in any environment where cushioning or stabilization is required.

Co-author Liuchi Li (PhD ’20), now assistant professor of civil and environmental engineering at Princeton University, is enthusiastic about the future of PAMs: “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.”

The research results were published in Science. Daraio sees great potential in the further development of these materials, particularly through the use of artificial intelligence to further explore the design options of PAMs. The work was supported by Caltech’s High-Performance Computing Center and funding from the US Department of Energy and the Army Research Office. The results mark a promising step in materials research and could have a significant impact on future technologies.