Researchers at the City University of Hong Kong (CityU) have figured out a way to make 3D-printed polymer lattice parts 100 times stronger than before.
Compared to traditional heat treatments that strengthen plastic printed objects at the expense of deformability, CityU's approach simply carbonizes them partially to make them stronger and twice as ductile. Using their process, the team says, it is possible to achieve complex 3D printing with mechanical properties tailored to specific applications, such as coronary stents or bioimplants.
"It's amazing that we have found a way to convert fragile and fragile 3D printed photopolymers into ultra-tough 3D structures that rival metals and alloys simply by heating them under the right conditions," CityU professor Lu Yang said. "Our work provides a low-cost, simple, and scalable route to fabricate lightweight, strong, and malleable mechanical metamaterials with virtually any geometry."
Chasing the "Holy Grail" of Materials
According to CityU scientists, the development of a polymer that is lightweight, yet ultra-high-strength and ductile at the same time is considered the "holy grail" of materials research and development, but these properties are often "mutually exclusive".
This is because pyrolysis, a process commonly used to convert plastic parts into reinforced carbon by heating in an inert atmosphere, strips almost all deformability of the original polymer. While the team acknowledges that other plastic strengthening methods exist, they say these also result in "inherent brittleness and low toughness" that "limit the structural applications [of the final part]".
In particular, these shortcomings limit the fabrication of parts from "metamaterials," which are designed to have properties not found in natural raw materials. Certain iterations of these can be used to create micro lattices that combine lightweight structural designs with the qualities of the materials they are made of, but the researchers say their 3D printing capabilities are still limited.
"Strong and tough architectural components often require metals or alloys for 3D printing, but they are not readily available due to the high cost and low resolution of commercial metal 3D printers and raw materials," Yang added. "Polymers are more readily available, but often lack mechanical strength or toughness."
Develop polymers that are 100 times tougher
In the process of studying 3D printed polymer lattices, the CityU team said they had come up with a way to heat them to a "magic-like" state of partial carbonization. By carefully controlling the heating rate, temperature, duration, and gas environment of the pyrolysis process, the scientists found that the stiffness, strength, and ductility of the micro lattices could be increased in a single step.
The researchers made this discovery through a series of characterization techniques that revealed that slow heating causes the material's polymer chains to undergo an incomplete transformation during the pyrolytic transformation. This creates a hybrid material in which structurally reinforced carbon fragments and loosely cross-linked polymer chains that prevent the composite from cracking coexist synergistically.
Through further R&D, the researchers went on to discover that the ratio of polymer to carbon fragments is also critical for producing parts optimized for strength and ductility. Putting their theory to the test, the team created several test prints in which they were able to iteratively develop a carbonized lattice that was 100 times stronger and twice as ductile as before.
As an added bonus, the researchers' "hybrid carbon" micro lattices also exhibited better biocompatibility than their base polymers and were even shown to support cellular bioactivity better. With this in mind, the team believes their process could be used to expand the functionality of a variety of other polymers and unlock new 3D printing materials for medical, robotic, and energy devices.
Examples of coronary stents 3D printed from carbonized materials