Engineers at Caltech have developed a method for 3D printing pure and multicomponent metals, at a resolution that is, in some cases, an order of magnitude smaller than previously possible. The process, which uses water-based chemistry and 3D printing, was described in a paper titled “Additive Manufacturing of Micro-Architected Metals Via Hydrogel Infusion,” which was published in the journal Nature. The research was funded by the US Department of Energy, the Resnick Sustainability Institute, the Masason Foundation, and Caltech’s AI4Science initiative.
The new process can be used for a variety of metals – even multiple types in the same manufactured part – with only minor adjustments. It has the potential to pave the way for fabricating tiny components for microelectronic mechanical systems (MEMS) – precise components for vehicles and space applications, heat exchangers, or biomedical devices.
The problem, when 3D printing with metals, particularly those with high thermal conductivity such as copper, is that the metals transmit heat so well that even with a finely focused laser, the heat spreads out and melts powder outside the desired area – lowering the possible resolution of the print.
A team led by then-graduate students Max Saccone, now a postdoctoral researcher at Stanford University; Rebecca Gallivan, now a postdoctoral researcher at ETH Zurich; Daryl Yee, incoming assistant Professor at EPFL, Switzerland; and Kai Narita, working in the lab of Caltech’s Julia R. Greer, cultivated a different approach to the problem: Instead of printing metals directly, they 3D print a hydrogel and use it as a scaffold for metal-containing liquid precursors. Kai Narita has launched a start-up company named 3D Architech that is licensing the new technology from Caltech.
“We had to develop a new way of doing it, and we couldn’t rely on heat to build our structures,” said Max Saccone.
Hydrogels are materials made from flexible polymer chains that do not dissolve in water and are used for products such as soft contact lenses. Light from a low-powered ultraviolet lamp is capable of triggering a chemical reaction in liquid polymers, causing them to harden by inducing cross-linking of the polymer chains. If you repeat the process over and over in a specific pattern, you can form desired microscopic shapes.
The Caltech researchers then infuse the 3D printed hydrogel scaffolds with metal salts dissolved in water, causing the metal ions to infiltrate the hydrogel – not just coat its surface. Then, in the ‘reaction’ portion of the process, the researchers burn away the hydrogel portion of the structure in a furnace that reaches 700 to 1100 degrees Celsius, depending on the material. Because the melting point of all metals is higher than the combustion temperature of the hydrogel, the metal remains intact.
The heat not only removes the hydrogel, but also causes the overall structure to shrink as the hydrogel burns off, resulting in an even tinier metal structure. With this process, in addition to pure metals, the team can 3D print metal alloys and multicomponent metallic systems, with feature sizes around 40 microns – or less than half the width of a human hair.
“One of the exciting things is that it works with a variety of metals with just slight tuning of the ‘reaction’ phase of the process and creates new opportunities for microscale materials engineering,” said Rebecca Gallivan. While developing the process, the team produced 3D printed structures made from copper, nickel, silver, and various metal alloys.
“The hydrogel infusion additive manufacturing process, or HIAM, as we coined it, establishes a pathway to create metallic materials in an entirely new, much more environmentally friendly way at unprecedented precision levels,” said Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics, and Medical Engineering; Fletcher Jones Foundation Director of the Kavli Nanoscience Institute; and a pioneer in the field of nanoscale- and microscale-architected materials.