The 3D printing industry has been trying to make lightweight materials more durable and ductile for a long time. Regarding this, a move came from the University of Hong Kong, also known as CityU.

Researchers have discovered a low-cost, direct method to turn commonly used 3D printable polymers into lightweight, ultra-tough, biocompatible hybrid carbon microlattices. More importantly, these microlattices are 100 times stronger than regular polymers.

The results were published very recently in Matter on September 1.

According to the study team, this novel method can be utilized to develop complex 3D parts with customized mechanical properties for a variety of applications, such as coronary stents and bio-implants.

"3D printing is becoming a ubiquitous technology for producing geometrically complex components with unique and tuneable properties. Strong and tough architected components usually require metals or alloys to be 3D printed, but they are not easily accessible owing to the high cost and low resolution of commercial metal 3D printers and raw materials," explained Professor Lu Yang in the Department of Mechanical Engineering. He also led the research.

"Polymers are more accessible but typically lack mechanical strength or toughness. We found a way to convert these weaker and brittle 3D-printed photopolymers into ultra-tough 3D architectures comparable to metals and alloys just by heating them under the right conditions, which is surprising."

Microlattices are an example of a 3D architected metamaterial that combines the inherent qualities of its component materials with the advantages of lightweight structural design concepts. In many cases, advanced fabrication is needed to create these microlattices, claims CityU.

Surjadi, James et al. / CityU 

As said, the most effective approach for increasing the strength of these 3D printable polymer lattices is pyrolysis.

Professor Lu discovered a "magic-like" circumstance in the pyrolysis of the 3D-printed photopolymer microlattices, which caused a 100-fold increase in strength and a two-fold increase in ductility of the original material.

To get the best strength and ductility, the proportion of polymer to carbon fragments is also essential. The material loses strength if there are too few carbon pieces, and it becomes brittle if there are too many. The researchers successfully developed an ideally carbonized polymer lattice throughout the study.

The research team also found that these "hybrid carbon" microlattices showed improved biocompatibility compared to the original polymer.

"Our work provides a low-cost, simple, and scalable route for making lightweight, strong and ductile mechanical metamaterials with virtually any geometry," said Professor Lu.

The research was supported by CityU, the Hong Kong Institute for Advanced Study, the Shenzhen Science and Technology Innovation Committee, and the National Natural Science Foundation of China.

A lightweight material with simultaneous high strength and ductility can be dubbed the "Holy Grail" of structural materials, but these properties are generally mutually exclusive. Thus far, pyrolytic carbon micro/nanolattices are a premium solution for ultra-high strength at low densities, but intrinsic brittleness and low toughness limits their structural applications. Here, we break the perception of pyrolyzed materials by demonstrating a low-cost, facile pyrolysis process, i.e., partial carbonization, to drastically enhance both the strength and ductility of a three-dimensional (3D)-printed brittle photopolymer microlattice simultaneously, resulting in ultra-high specific energy absorption of up to 60 J g−1 (>100 times higher than the original) without fracture at strains above 50%. Furthermore, the partially carbonized microlattice shows improved biocompatibility over its pure polymer counterpart, potentially unlocking its biomedical and multifunctional applications. This method would allow a new class of hybrid carbon mechanical metamaterials with lightweight, high toughness, and virtually any geometry.