They started the construction of the robo-ray's body with a layer of transparent elastic polymer--the aforementioned "breast implant." Then they genetically encoded rat heart cells to respond to flashes of blue light, and aligned the cells along the ray's "fins" in a serpentine pattern. These muscles would allow the fins to flex downward, but to ensure they would return to the starting position, the team reverse-engineered a stingray's physiology to create a skeleton made of gold. Because, Parker says, "Cells like gold, just like people do." A final layer of polymer, and the biohybrid stingray was complete. At just 16 millimeters long, and weighing just ten grams, the tiny robot looked a little like a transparent coin with a tail.