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A millimeter-scale, wirelessly powered cardiac device
Date:8/31/2012

ugh the bloodstream. To get there she needed to overturn some long-held assumptions about delivery of wireless power through the human body.

Her device works by a combination inductive and radiative transmission of power. Both are types of electromagnetic transfer in which a transmitter sends radio waves to a coil of wire inside the body. The radio waves produce an electrical current in the coil sufficient to operate a small device.

There is an indirect relationship between the frequency of the transmitted radio waves and the size of the receive antenna. That is, to deliver a desired level of power, lower frequency waves require bigger coils. Higher frequency waves can work with smaller coils.

"For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one big hurdle," explained Kim.

Ignoring consensus

Existing mathematical models have held that high frequency radio waves do not penetrate far enough into human tissue, necessitating the use of low-frequency transmitters and large antennastoo large to be practical for implantable devices.

Ignoring the consensus, Poon proved the models wrong. Human tissue dissipates electric fields quickly, it is true, but radio waves can travel in a different wayas alternating waves of electric and magnetic fields. With the correct equations in hand, she discovered that high-frequency signals travel much deeper than anyone suspected.

"In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor," said Kim. "If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant."

According to their revised models, the researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second.

"In this high-frequency range, we can
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Contact: Andrew Myers
admyers@stanford.edu
650-736-2245
Stanford School of Engineering
Source:Eurekalert

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