Maria Burke
A simple way to make light yet strong biocompatible carbon composites of any shape and architecture has been reported by a team at the City University of Hong Kong. The method turns commonly used 3D printable polymers into hybrid carbon microlattices. It could be used to create parts with tailored mechanical properties for applications including coronary stents and bio-implants.
While 3D printing has provided researchers with a way of making geometrically complex components with tuneable properties, they typically lack mechanical strength. A thermal treatment approach produces ultra-strong carbon. However, pyrolysis leaves the original polymer lattice with little deformability and produces an extremely brittle material.
‘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,’ explains team leader Lu Yang. ‘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.’
By carefully controlling the rate and duration of heating rate, temperature, and gas environment, the team discovered it is possible to simultaneously enhance the stiffness, strength and ductility of a 3D-printed polymer microlattice in a single step (Matter, 2022; doi: 10.1016/j.matt.2022.08.010). By experimenting with these conditions, the team created an optimally carbonised polymer lattice that was over 100 times stronger and over two times more ductile than the original polymer lattice. They also reported that cells cultured on the hybrid carbon microlattices were more viable than cells seeded on the polymer microlattices, meaning the microlattices were more biocompatible.
The team found that incomplete conversion of polymer chains to pyrolytic carbon produces a hybrid material containing loosely crosslinked polymer chains and carbon fragments. The carbon fragments reinforce the material, while the polymer chains restrict the fracture of the composite. The ratio of polymer to carbon fragments is crucial to obtaining optimal strength and ductility, they found. If there are too many carbon fragments, the material becomes brittle; too few and the material lacks strength.
Cheng Huiming of the Institute of Metals Research in China notes that partial carbonisation has often been seen to drastically enhance both the strength and ductility of some specific kinds of polymer materials, especially for 3D-printed polymer microlattices. Nevertheless, the ‘low-cost, facile’ pyrolysis process is innovative. ‘Strategies in other reported work to fabricate mechanically robust composites often require two or more constituents to be mixed together. However, the partial carbonisation process applied in this work essentially forms a hybrid carbon/polymer composite using only one starting material, which reduces processing time, cost, and defects within the sample during printing.’
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