Research team discovers how to turn 3D-printed polymer into stronger, ductile hybrid carbon microlattice material

The four main types of samples studied in this work, namely fabricated, low-carbon, partially carbonized and over-carbonized micronets. Credit: James Utama Surjadi and others, Affair (2022). DOI: 10.1016/j.matt.2022.08.010

Developing a lightweight material that is both strong and highly ductile has been considered a long-cherished goal in the field of structural materials, but these properties are often mutually exclusive. However, researchers at the City University of Hong Kong (CityU) have recently discovered a direct, low-cost method of turning commonly used 3D-printable polymers into ultra-strong, lightweight, biocompatible hybrid carbon microlattices, which can have any shape or size. and are 100 times stronger than the original polymers. The research team believes that this innovative approach can be used to create sophisticated 3D parts with customized mechanical properties for a wide range of applications, including coronary stents and bioimplants.

Metamaterials are materials designed to have properties not found in natural materials. 3D architectural metamaterials such as microlattices combine the benefits of lightweight structural design principles with the intrinsic properties of their constituent materials. Fabricating these microgrids often requires advanced manufacturing technologies such as additive manufacturing (commonly known as 3D printing), but the range of materials available for 3D printing remains quite limited.

“3D printing is becoming a ubiquitous technology for producing geometrically complex components with unique and tunable properties. Strong and resilient architectural components typically require metals or alloys to be 3D printed, but are not easily accessible due to high cost and the low resolution of commercial metal and raw material 3D printers Polymers are more accessible, but typically lack mechanical strength or toughness We found a way to turn these weaker, more brittle 3D printed photopolymers into ultra-strong 3D architectures comparable to metals and alloys just by heating them under the right conditions, which is amazing,” said Professor Lu Yang from the Department of Mechanical Engineering (MNE) and the Department of Materials Science and Engineering (MSE) at CityU, who led the research. .

A new method to increase strength without compromising ductility

Until now, the most effective approach to increasing the strength of these 3D-printable polymer networks is pyrolysis, a heat treatment that transforms whole polymers into ultra-strong carbon. However, this process deprives the original polymer network of almost all its deformability and produces an extremely brittle material, like glass. Other methods of increasing the strength of polymers also often compromise their ductility.

The team led by Professor Lu found a “magic-like” condition in the pyrolysis of 3D-printed photopolymer microlattices, which resulted in a 100-fold increase in strength and doubled the ductility of the original material. His findings were published in the scientific journal Affair under the title “Hybrid carbon microgrids with lightweight and ultra-resistant 3D architecture”.

They found that by carefully controlling the heating rate, temperature, duration, and gas environment, it is possible to simultaneously improve the stiffness, strength, and ductility of a 3D-printed polymer microlattice dramatically in a single step.

Demonstration of 3D printed partially charred core coronary stents. Credit: James Utama Surjadi and others, Affair (2022). DOI: 10.1016/j.matt.2022.08.010

Through various characterization techniques, the team found that simultaneous improvement in strength and ductility is only possible when the polymer chains are “partially carbonized” by slow heating, where incomplete conversion of the polymer chains to pyrolytic carbon occurs, producing a hybrid material in which both loosely cross-linked polymer chains and carbon fragments coexist synergistically. The carbon fragments serve as reinforcing agents that strengthen the material, while the polymer chains restrain the fracture of the composite.

The ratio of polymer fragments to carbon is also crucial for optimum strength and ductility. If there are too many carbon fragments, the material becomes brittle, and if there are too few, the material loses strength. During experiments, the team successfully created an optimally carbonized polymer network that was more than 100 times stronger and more than twice as ductile as the original polymer network.

Benefits beyond improved mechanical properties

The research team also found that these “hybrid carbon” micronetworks showed improved biocompatibility compared to the original polymer. Through experiments monitoring cell behavior and cytotoxicity, they showed that cells grown on the hybrid carbon microarrays were more viable than cells seeded on the polymer microarrays. The improved biocompatibility of hybrid carbon networks implies that the benefits of partial carbonization may go beyond the improvement in mechanical performance and potentially improve other functionalities as well.

“Our work provides a low-cost, simple, and scalable route to fabricating lightweight, strong, and ductile mechanical metamaterials with virtually any geometry,” said Professor Lu. He envisions that the newly invented approach can be applied to other types of functional polymers, and that the geometric flexibility of these engineered carbon-hybrid metamaterials will allow their mechanical properties to be tailored to a wide range of applications, such as biomedical implants, mechanically robust scaffolds for micro-robots, energy harvesting and storage devices.

Professor Lu is the corresponding author and Dr. James Utama Surjadi, a postdoc in his group, is the first author of the paper. Collaborators include Professor Wang Zuankai, Senior Lecturer in the Department of MNE, and Dr. Raymond Lam Hiu-wai, Associate Director and Associate Professor in CityU’s Department of Biomedical Engineering.