Implantable electronic devices potentially offer a rapid and accurate way for doctors to monitor patients with particular medical conditions. Yet powering such devices remains a fundamental challenge: batteries are bulky and eventually need recharging or replacing. Jia Hao Cheong at the A*STAR Institute for Microelectronics, Singapore, and his co-workers are developing an alternative approach that eliminates the need for a battery. Their miniature devices are based on wireless power-transfer technology.