In the evolving fields of materials science and 3D printing, engineers at Lawrence Livermore National Laboratory (LLNL) are exploring novel ways to create materials and structures that adapt and respond to their environments.
A recent study featured on the cover of Science, conducted in collaboration with the California Institute of Technology (CalTech) and Princeton University, has introduced a revolutionary class of materials known as 3D polycatenated architected materials (PAMs). These intricate structures can behave with both solid and liquid-like properties and have the potential to impact industries ranging from engineering to medicine.
"Polycatenated" describes how these new architected materials are built-multiple interconnected loops or cages form a flexible and resilient framework, akin to chain mail armor-enabling dynamic responses to external forces. When subjected to specific stressors, these networks show a transformative capability, expanding, contracting or morphing into entirely new shapes.
"Architected materials can be designed and 3D printed to have specific internal structures that define their properties but are generally rigidly connected repeating units-the interaction between neighboring units is limited," explained LLNL staff scientist and co-corresponding author Xiaoxing Xia. "PAMs, however, have interlocked building blocks that cannot be separated but can move with much greater freedom compared to rigid lattices. This gives them the ability to behave like both a liquid and a solid under different conditions."
The science behind PAMs
The team, including LLNL engineers Xia and Anna Guell Izard, conducted experiments examining PAMs' mechanical properties under various conditions. One key finding was gravitational relaxation-where PAMs change shape in response to gravitational forces, suggesting applications in stimuli-responsive materials, energy-absorbing systems and morphing architectures, particularly in low or zero-gravity environments.
"In space, a little bit of electrostatic interaction can expand a large, interlocked network, potentially shielding a space station or satellite or deploying flexible solar panels or telescopes," Xia said. "When submerged in liquid, PAMs-especially micro-PAMs-remain highly mobile with minimal influence of gravity, making them useful for micro-robotic or bio-implantable applications."
Additionally, PAMs respond differently depending on their orientation along specific crystallographic axes, the researchers reported. When tested on a flat surface and oriented along multiple axes, they displayed distinct relaxed outline shapes, highlighting the critical role of internal structure in mechanical response.
The researchers also demonstrated PAMs' length-scale-independence, fabricating them at both macro and microscale levels while maintaining consistent mechanical responses. This scalability suggests the PAMs' behavior can apply to structures ranging from microscopic medical devices to large-scale architectural components, they reported.
Future vision and the role in energy absorption
Researchers said PAMs could impact engineering by enabling lightweight, durable structures that can withstand extreme conditions. Aerospace engineers, for instance, could design aircraft components that balance strength and efficiency. Additionally, the PAMs ability to absorb energy, redistribute stress and deform predictably could be ideal for protective gear such as helmets and body armor.
"Normally lattice structures are used for lightweight applications; when one unit is fractured, cracks can propagate easily, causing catastrophic failure," Xia said. "Since PAMs are not rigidly connected, elastic or shock waves struggle to transmit from one unit to another, making them excellent for energy absorption under impact. Also, their shear-thinning behavior-solid-like under zero or low frequency vibration but liquid-like under high frequency-could also mitigate vibrations mitigation during rocket launches."
In the medical field, PAMs could enhance prosthetics and implants, adapting shape and stiffness to a user's movements for a more natural experience. They could enable precise drug delivery systems by altering their shape to release medication where needed, the researchers said.
Electrostatic responsiveness and smart materials
Another notable aspect of the study is the role of electrostatic forces in PAM behavior. The team coated microscale PAM samples with a thin copper layer to enhance electrical conductivity. Xia recalled observing intriguing movement when he placed a micro-PAM in water and later noticed under a scanning electron microscope (SEM) that electron beams charged up the rings, causing them to repel each other due to electrostatic repulsion.
"The rings were shaking a little bit and some rings started to levitate slightly," Xia said. "Because the charge in SEM was minimal, I bought a small Van de Graaff generator, which provided much more charge. Each ring or cage immediately repelled the others, causing the entire structure to expand and stand up due to the electrostatic forces."
This rapid, reversible transformation suggests potential applications in smart systems that react to electrical signals. PAMs could be used in robotics that change shape or stiffness in response to electrical inputs. In wearable tech, they could form the clothing or devices that adjust in real time for enhanced comfort and functionality.
Challenges and future work
Despite their potential, PAMs face challenges in large-scale production. Variations in fabrication techniques can affect material properties, and micro-PAMs fabrication has proven particularly difficult due to limitations in 3D printing.
"For most 3D printing methods, you cannot print unsupported structures," Xia said. "In the end, we embedded PAMs in thin supporting lattices and used oxygen plasma to remove them-requiring a lot of patience."
To address this, LLNL researchers are developing new printing techniques to streamline fabrication. Songyun Gu, a postdoc in LLNL's Materials Engineering Division, has successfully made larger interlocked networks using a parallel 3D printing setup. Recent mechanical tests done by Guell Izard show these structures exhibit greater toughness and resilience than traditional octet lattices because cracks struggle to propagate, Xia said.
The team continues to investigate PAMs' unique properties and long-term behavior under varying environmental conditions, such as temperature, humidity and chemical exposure, which will help ensure their real-world durability and performance.
The LLNL portion of the work was funded by the Laboratory Directed Research and Development program under Xia's project titled "Large-Scale, Two-Photon, Three-Dimensional Printing Enabled by Metaoptics." Additional funding for the collaboration came from the Gary Clinard Innovation Fund and the Army Research Office.
Other co-authors included first author Wenjie Zhou, Sujeeka Nadarajah, Hujie Yan, Aashutosh Prachet, Payal Pate, Chiara Daraio of Caltech and Liuchi Li of Princeton.
