Additive Manufacturing 3D Printing Process Enables Materials to Grow

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Researchers at EPFL have developed a 3D printing process that allows metals and ceramics to grow within a water-based gel. The result is dense yet complex structures suitable for the next generation of energy, biomedical, and sensor technologies.

Vat photopolymerization is a 3D printing technique in which a photosensitive resin is poured into a vat and then selectively cured into a desired shape using a laser or UV light. However, this process is mostly used with photosensitive polymers, which limits its practical range of applications.

Although some 3D printing methods have been developed to convert these printed polymers into more robust metals and ceramics, Daryl Yee, head of the Laboratory for the Chemistry of Materials and Manufacturing at EPFL's School of Engineering, explains that the materials produced with these techniques suffer from serious structural issues. "These materials tend to be porous, which significantly reduces their strength, and the parts experience excessive shrinkage, leading to deformations," he says.

Composites With Very High Metal Concentrations

Yee and his team have now published a solution to this problem in the journal Advanced Materials. Instead of using light to cure a resin containing metal precursors, as in previous methods, the EPFL team begins by creating a 3D scaffold from a simple water-based gel, known as a hydrogel. This 'empty' hydrogel is then infused with metal salts, which are chemically transformed into metal-containing nanoparticles that permeate the structure. This process can be repeated to achieve composites with very high metal concentrations.

After 5-10 'growth cycles,' the remaining hydrogel is burned off in a final heating step, leaving behind the finished product: a metal or ceramic object in the shape of the original polymer framework that is dense and strong.

Since the hydrogels are only infused with metal salts after production, the technique allows a single hydrogel to be converted into different composites, ceramics, or metals. "Our work not only enables the production of high-quality metals and ceramics using an accessible and cost-effective 3D printing process, but also highlights a new paradigm in additive manufacturing, where material selection occurs after 3D printing rather than before," Yee summarizes.

Our materials were able to withstand 20 times the pressure compared to those produced with previous methods, while showing only 20 percent shrinkage as opposed to 60 to 90 percent.

Yiming Ji

For their study, the team fabricated complex mathematical lattice structures, known as gyroids, from 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 were able to withstand 20 times the pressure compared to those produced using previous methods, while showing only 20 percent shrinkage as opposed to 60 to 90 percent," says doctoral student and lead author Yiming Ji.

For the Production of Advanced 3D Architectures

The scientists emphasize that their technique is particularly interesting for the production of advanced 3D architectures that need to be strong, lightweight, and complex at the same time, such as sensors, biomedical devices, or devices for energy conversion and storage. For example, metal catalysts are crucial for enabling reactions that convert chemical energy into electricity. Other applications could include metals with large surface areas and advanced cooling properties for energy technologies.

Looking ahead, the team is working on improving their process to facilitate industrial adoption, particularly by further increasing the density of their materials. Another objective is speed: while the repeated infusion steps are crucial for producing stronger materials, they make the method more time-consuming compared to other 3D printing techniques that convert polymers into metals. "We are already working on reducing the overall processing time by using a robot to automate these steps," says Yee.

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