After printing, we bathe the structure in a solution of rat cardiac cells and support cells called fibroblasts, and the cells attach to the collagen-infused area. We store the bio-bot in an incubator that is 98°F (about the body temperature of a living rat) and that has proper CO2 levels for the cells to survive. A few days later, the growing cells form a sheet and start beating about once a second—the rate that a rat's heart beats. Each contraction bends the longer leg down, propelling the robot forward. The whole thing moves about 15 millimeters per minute (and only in one direction). In the future, we'd like to try bird heart cells, which beat five times as fast as rat ones and thus might make the bio-bot faster. We'd also like to make the robots more intelligent, so they could assist in surgery or be used to find toxins or parasites and release chemicals to neutralize them.