Although artificial hearts are stymied by complications, left ventricular assist devices (LVADs) are extending lives. Doctors began implanting them two decades ago to keep heart failure patients alive while they waited for weeks or months for an available transplant organ. Today improved designs are being installed as final fixes. Indeed, the distinction between an LVAD used as a bridge to transplant and as a permanent aid "is disappearing," says Kiyotaka Fukamachi, head of the Cleveland Clinic's Cardiovascular Dynamics Laboratory. "Some patients who received an LVAD as a bridge have been living with it for two or three years."

A healthy left ventricle pumps freshly oxygenated blood through the aorta to the body. LVADs help the ventricle or take over its operations if the chamber is weak or has stopped functioning. First-generation designs, which still prevail, are pulsatile: an implanted pump pushes blood in pulses like a natural heart. Second-generation LVADs are smaller, relying on a rotor that continuously streams blood. Engineers are evaluating experimental, third-generation devices that use magnetically levitated rotors, reducing moving parts.