Scientists are studying how to tap the energy naturally created by people's bodies—such as heat, sound and movement—to power medical devices without the need to change batteries.

The development, still years from becoming a reality, could spare some of the millions of people with implanted devices like pacemakers from undergoing surgery to replace rundown batteries. Other products, including hearing aids, insulin pumps and pain-management devices, could be made to function without changing batteries, or at least sharply extend...