3D printing method “grows” complex, ultra-strong materials inside a water-based gel

Vat photopolymerization is a 3D printing technique in which a light-sensitive resin is poured into a vat and then selectively cured into the desired shape using a laser or UV light. But this process is mainly only used with photosensitive polymers, which limits its range of useful applications.

While some 3D printing methods have been developed to turn these printed polymers into stronger metals and ceramics, Daryl Yee, head of the Materials Chemistry and Manufacturing Laboratory at EPFL’s Faculty of Engineering, explains that materials produced with these techniques suffer from serious structural setbacks.

“These materials tend to be porous, which greatly reduces their strength, and the parts suffer from excessive shrinkage, which causes warping,” he explains.

Now, Yee and his team have published a paper in Advanced materials which describes a unique solution to this problem.

Rather than using light to cure a resin pre-infused with metal precursors, as previous methods have done, the EPFL team first creates a 3D scaffold from a simple water-based gel called a hydrogel. Then, they infuse this “virgin” hydrogel with metal salts, before chemically converting them into metal-containing nanoparticles that permeate the structure. This process can then be repeated to produce composites with very high metal concentrations.

After five to ten “growth cycles,” a final heating step burns off the remaining hydrogel, leaving behind the finished product: a metal or ceramic object shaped like the original virgin polymer, with unprecedented density and strength. Since the hydrogels are only infused with the metal salts after manufacturing, the technique allows a single hydrogel to be transformed into several different composites, ceramics or metals.

“Our work not only enables the manufacturing of high-quality metals and ceramics with an accessible and inexpensive 3D printing process; it also highlights a new additive manufacturing paradigm where material selection happens after 3D printing, rather than before,” summarizes Yee.

Target advanced 3D architectures

For their study, the team made complex mathematical lattice shapes called gyroids from iron, silver and copper, demonstrating the ability of their technique to produce strong but 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 pressure 20 times greater than that produced with previous methods, while having a shrinkage of only 20% compared to 60 to 90%,” explains the doctor. student and first author Yiming Ji.

The scientists say their technique is particularly interesting for manufacturing advanced 3D architectures that must be strong, lightweight and complex, such as sensors, biomedical devices or energy conversion and storage devices. For example, metal catalysts are essential for enabling reactions that convert chemical energy into electricity. Other applications could include large surface area metals with advanced cooling properties for energy technologies.

Looking ahead, the team is working to improve its process to facilitate its adoption by industry, notably by further increasing the density of its materials. Another goal is speed: Repeated infusion steps, while essential for producing stronger materials, make the method more time-consuming than other 3D printing techniques for converting polymers into metals.

“We are already working to reduce the total processing time by using a robot to automate these steps,” says Yee.