Wireless Power for Minuscule Medical Implants

Medical implants like pacemakers, deep brain stimulators, and cochlear implants could someday be joined by still more bioelectronic gadgets—devices that regulate insulin levels, control appetite, lower blood sugar, or treat brain injuries (see “Nerve-Stimulating Implant Could Lower Blood Pressure”).

But before we’re all riddled with electronics, researchers have to figure out how to power it all. Pacemaker batteries are too clunky for tiny devices saddled up to nerves, and existing wireless methods, such as those used for cochlear implants, won’t work with devices buried deep in the body.

That’s where electrical engineer Ada Poon and her team at Stanford University say they might be able to help. The group has developed a new method of sending magnetic fields well below skin level to power devices that would otherwise need batteries.

Wireless systems like the one used in cochlear implants sit permanently on the skin and derive power from electromagnetic induction, in which a current running through a coil of wire generates a magnetic field that then induces a current in a nearby device. The problem is that a field generated this way decays exponentially with distance from the generating coil, so it only works with devices close to the skin’s surface.

Poon and her team found a way to use electromagnetic induction through biological tissue without that exponential decay. They call the technique midfield wireless powering (as opposed to near-field, which refers to the exponentially decaying radiation, and far-field, which refers to the kind of radiation emitted from a cell tower).

The key, Poon says, is that instead of using a coil of wire, they use a flat plate adorned with a specially designed four-line pattern of conductive material. When they send current through the plate, that pattern produces a magnetic field capable of propagating through biological material without decaying over a short distance. The plate would most likely sit on the skin, providing constant power to an implant.

Morris Kesler, vice president of research and development at WiTricity, a Massachusetts-based company that develops wireless powering systems, says Poon’s technique would be particularly useful for powering tiny devices.

To test their new powering scheme, the Stanford group implanted a pacemaker about the size of a grain of rice in a rabbit and then powered the device using a plate about six centimeters on a side. The setup worked with about 0.1 percent efficiency—meaning that nearly all the energy sent from the conductive material to the pacemaker was wasted. Nonetheless, Poon says that is sufficient for this kind of low-power medical device. It also met safety regulations limiting the amount of radiation delivered to a given amount of tissue in humans.

In the future, Poon says, the group plans to develop flexible versions of the plate that will be more comfortable against skin. One of her graduate students is also designing plates that will penetrate materials other than biological tissue.

Kip Ludwig, the program director for neural engineering at the National Institute of Neurological Disorders and Stroke at the National Institutes of Health, says Poon’s method is promising but years from any clinical application. Still, there is so much promise in bioelectronics, he says, and the powering issue needs to be addressed.