A lattice of 3D printed nickel. The entire structure is printed in 150-nm layers, and the final structure is 6 µm high. (Credit: Greer Lab)

For the first time, it is possible to create complex nanoscale metal structures using 3D printing. The process, once scaled up, could be used in a wide variety of applications, including building tiny medical implants. It also opens the door to the creation of a new class of materials with unusual properties that are based on their internal structure.

The group 3D prints structures out of a variety of materials, from ceramics to organic compounds. Metals, however, have been difficult to print, especially when trying to create structures with dimensions smaller than around 50 µm, or about half the width of a human hair. They used organic ligands-molecules that bond to metal-to create a resin containing mostly polymer, but which carries along with it metal that can be printed, like a scaffold.

The researchers bonded together nickel and organic molecules to create a liquid that looks a lot like cough syrup. They designed a structure using computer software, and then built it by zapping the liquid with a two-photon laser. The laser creates stronger chemical bonds between the organic molecules, hardening them into building blocks for the structure. Since those molecules are also bonded to the nickel atoms, the nickel becomes incorporated into the structure. In this way, the team was able to print a 3D structure that was initially a blend of metal ions and nonmetal, organic molecules.

The structure was put into an oven that slowly heated it up to 1,000 C (~1,800 F) in a vacuum chamber. That temperature is well below the melting point of nickel (1,455 C, or about 2,650 F) but is hot enough to vaporize the organic materials in the structure, leaving only the metal. The heating process, known as pyrolysis, also fused the metal particles together.

In addition, because the process vaporized a significant amount of the structure's material, its dimensions shrank by 80 percent, but it maintained its shape and proportions.