Vat photopolymerization is a 3D printing technique in which a light-sensitive resin is poured into a vat, and then selectively hardened into a desired shape using a laser or UV light. But this process is mostly used only with light-sensitive polymers, which limits its range of useful applications.
While some 3D printing methods have been developed to convert these printed polymers into tougher metals and ceramics, Daryl Yee, head of the Laboratory for the Chemistry of Materials and Manufacturing in EPFL's School of Engineering, explains that materials produced with these techniques suffer from serious structural setbacks. "These materials tend to be porous, which significantly reduces their strength, and the parts suffer from excessive shrinkage, which causes warping," he says.
Now, Yee and his team have published a paper in Advanced Materials that describes a unique solution to this problem. Rather than using light to harden a resin that is pre-infused with metal precursors, as previous methods have done, the EPFL team first creates a 3D scaffold out of a simple water-based gel called a hydrogel. Then, they infuse this 'blank' hydrogel with metal salts, before chemically converting them into metal-containing nanoparticles that permeate the structure. This process can then be repeated to yield composites with very high metal concentrations.
After 5-10 'growth cycles', a final heating step burns away the remaining hydrogel, leaving behind the finished product: a metal or ceramic object in the shape of the original blank polymer that is unprecedently dense and strong. Since the hydrogels are only infused with the metal salts after fabrication, the technique allows a single hydrogel to be transformed into multiple different composites, ceramics, or metals.
"Our work not only enables the fabrication of high-quality metals and ceramics with an accessible, low-cost 3D printing process; it also highlights a new paradigm in additive manufacturing where material selection occurs after 3D printing, rather than before," Yee summarizes.
Targeting advanced 3D architectures
For their study, the team fabricated intricate mathematical lattice shapes called gyroids out of iron, silver, and copper, demonstrating their technique's ability to produce strong yet complex structures. To test the strength of their materials, they used a device called a universal testing machine to apply increasing pressure to the gyroids.
"Our materials could withstand 20 times more pressure compared to those produced with previous methods, while exhibiting only 20% shrinkage versus 60-90%," says PhD student and first author Yiming Ji.
The scientists say their technique is especially interesting for the fabrication of advanced 3D architectures that must be simultaneously strong, lightweight, and complex, like sensors, biomedical devices, or devices for energy conversion and storage. For example, metal catalysts are essential for enabling reactions that convert chemical energy into electricity. Other applications could include high-surface area metals with advanced cooling properties for energy technologies.
Looking ahead, the team is working on improving their process to facilitate uptake by industry, notably by further increasing the density of their materials. Another goal is speed: the repeated infusion steps, while essential for producing stronger materials, make the method more time-consuming compared to other 3D printing techniques for converting polymers to metals. "We are already working on bringing the total processing time down by using a robot to automate these steps," Yee says.
