Wireless power for medical implants

With advancing technology, most components of electronic medical implants can be made smaller and smaller. But power sources have lagged behind and are still centimeters in size. That limitation complicates many practical devices: In deep-brain stimulation, for example, electrodes in the brain are connected by wire to a power source implanted in the chest. It’s possible to wirelessly transfer energy to a device from a source outside the body. But because of the way electromagnetic fields interact with biological tissue, most straightforward wireless powering schemes are prohibitively inefficient for powering a millimeter-sized implant more than 1 cm beneath the skin surface. Now Ada Poon , John Ho, and colleagues at Stanford University have devised a new method of wireless power transfer that works with tissue’s dielectric properties rather than against them. In a theoretical model of the multilayer structure shown in the figure, the researchers found that a pattern of crescent-shaped currents, modulated at 1.6 GHz, produces propagating waves that converge at the site of a microimplant 5 cm from the surface. Surprisingly, the scheme is robust to the heterogeneities present in real tissue: In experiments on a pig cadaver, a 500-mW source (about as powerful as a cell phone) transferred 200 µW to an implant in either the heart or the brain. (A typical cardiac pacemaker uses just 8 µW.) Teaming up with colleagues in Stanford’s medical school, the researchers wirelessly powered miniature pacemakers implanted in live rabbits, and they’re preparing their first study on human patients. (J. S. Ho et al., Proc. Natl. Acad. Sci. USA 111, 7974, 2014 .)