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Wireless system transfers power inside the body

27 May 2014  | Tom Abate

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The article describes the work of Poon's interdisciplinary research team that included John Ho and Alexander Yeh, electrical engineering graduate students in Poon's lab; Yuji Tanabe, a visiting scholar; and Ramin Beygui, associate professor of cardiothoracic surgery at Stanford University Medical Center.

The crux of the discovery involves a new way to control electromagnetic waves inside the body.

Electromagnetic waves pervade the universe. We use them every day when we broadcast signals from giant radio towers, cook in microwave ovens, or use an electric toothbrush that recharges wirelessly in a special cradle next to the bathroom sink.

 batteryless electrostimulator

A batteryless electrostimulator next to medicinal pills for size comparison. The new powering method allows the device to be wirelessly powered deep inside the body.

Before Poon's discovery, there was a clear divide between the two main types of electromagnetic waves in everyday use, called far-field and near-field waves.

Far-field waves, like those broadcast from radio towers, can travel over long distances. But when they encounter biological tissue, they either reflect off the body harmlessly or get absorbed by the skin as heat. Either way, far-field electromagnetic waves have been ignored as a potential wireless power source for medical devices.

Near-field waves can be safely used in wireless power systems. Some current medical devices like hearing implants use near-field technology. But their limitation is implied by the name: They can transfer power only over short distances, limiting their usefulness deep inside the body.

What Poon did was to blend the safety of near-field waves with the reach of far-field waves. She accomplished this by taking advantage of a simple fact—waves travel differently when they come into contact with different materials such as air, water, or biological tissue.

For instance, when you put your ear on a railroad track, you can hear the vibration of the wheels long before the train itself because sound waves travel faster and further through metal than they do through air.

With this principle in mind, Poon designed a power source that generated a special type of near-field wave. When this special wave moved from air to skin, it changed its characteristics in a way that enabled it to propagate—just like the sound waves through the train track.

She called this new method mid-field wireless transfer.

In the experiment, Poon used her mid-field transfer system to send power directly to tiny medical implants. But it is possible to build tiny batteries into microimplants, and then recharge these batteries wirelessly using the mid-field system. This is not possible with today's technologies.

Co-author Ho noted, "With this method, we can safely transmit power to tiny implants in organs like the heart or brain, well beyond the range of current near-field systems."


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