Inside the chests of 41 million people around the world, failing hearts gradually become less effective at the vital task of pumping blood. Some heart failure patients get on the list for a transplant, while others receive metallic pumps called ventricular assist devices (VADs) to help their faltering organs. But VADs increase the risk of blood clots, which can form as the liquid flows over surfaces made of metal and plastic. In search of a better pumping assistant, an international team of researchers invented a silicone sleeve that slips over the heart's exterior, thus keeping the robot from contact with flowing blood as it rhythmically squeezes the organ. The sleeve's design is inspired by the arrangement of real heart muscles, with an inner layer of material that contracts using concentric rings and an outer layer that contracts in a helical fashion.