Part-animal, part-machine powered by genetically-engineered rat cells and may lead to a better understanding of the artificial heart. As live muscle robots work only in nutrient-filled solutions warmed to a rat’s body temperature, the challenge ahead is keeping them going in a natural environment. However, roboticists and engineers are excited because the stingray both shows how biological cells can be used and how heart muscle can flush and flow liquid around it. Marine biologists are more excited by the fact it can help them understand the muscles in rays.