When Mark A. Skylar-Scott says he and his Harvard University colleagues can make metal filaments in midair, he stresses that they aren’t defying gravity. The team’s silver strands start attached to a substrate, but the researchers can draw them out through three-dimensional space to create intricate patterns. Led by Skylar-Scott and Jennifer A. Lewis, the researchers are forging the 3-D silver wires on demand using a nanoparticle ink and laser light (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1525131113). The approach could help create custom transmitters and electrical connections for biosensors and other miniaturized devices, the researchers say. The ink consists of silver nanoparticles mixed with polyacrylic acid. The polymer chains prevent the silver particles from clumping while also serving as a binder to give the ink the consistency of toothpaste, Skylar-Scott says. A computer-controlled nozzle moves in 3-D to squeeze the ink into the focal point of an infrared laser, which moves in concert with the nozzle. The laser’s heat vaporizes the polymer and sinters the nanoparticles into a solid metal wire. This laser-assisted method enables the team to directly write wire patterns onto low-cost substrates that can’t support conventional high-temperature annealing.