LLNL researchers develop 3D-architected materials with liquid-solid properties
February 17, 2025
Researchers from the US-based institutes Lawrence Livermore National Laboratory (LLNL), the California Institute of Technology, and Princeton University have introduced a new class of materials known as 3D polycatenated architected materials (PAMs) in a study featured on the cover of Science. These intricate structures – both metals and polymers – exhibit solid and liquid-like properties.
‘Polycatenated’ describes how these new architected materials are built: multiple interconnected loops or cages form a flexible and resilient framework, similar to chain mail armour. This structure enables dynamic responses to external forces; when subjected to specific stressors, these networks demonstrate 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 neighbouring 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 team conducted experiments to examine PAMs’ mechanical properties under various conditions. A key finding revealed gravitational relaxation: where PAMs change shape in response to gravitational forces. This behaviour may suggest potential applications in stimuli-responsive materials, energy-absorbing systems, and morphing architectures, especially 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.”
The researchers found that PAMs exhibit different responses based on their orientation along specific crystallographic axes. Tests on flat surfaces revealed that when oriented along multiple axes, the PAMs displayed distinct relaxed 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 micro-scale levels while maintaining consistent mechanical responses. This scalability suggests the PAMs’ behaviour can apply to structures ranging from microscopic medical devices to large-scale architectural components, they reported.
Researchers say PAMs could impact engineering by enabling lightweight, durable structures that can withstand extreme conditions. For example, aerospace engineers could design aircraft components that perfectly balance strength and efficiency. The PAMs’ ability to absorb energy, redistribute stress, and deform predictably makes them ideal for protective equipment like helmets and body armour.
“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 behaviour – 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 also enable precise drug delivery systems by altering their shape to release medication where needed, the researchers said.
Another notable aspect of the study is the role of electrostatic forces in PAM behaviour. The team coated micro-scale 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.”
The researchers have stated that 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.
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 Additive Manufacturing.
“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 manufacturing techniques to streamline fabrication. Songyun Gu, a postdoc in LLNL’s Materials Engineering Division, has successfully made larger interlocked networks using a parallel Additive Manufacturing setup. Xia shared that recent mechanical tests done by Guell Izard show that these structures exhibit greater toughness and resilience than traditional octet lattices because cracks struggle to propagate.
The team continues to investigate PAMs’ unique properties and long-term behaviour under varying environmental conditions, such as temperature, humidity and chemical exposure.
