Cardiac Implants Could Potentially Be Powered Wirelessly

The researchers then demonstrated wireless power transfer to a millimeter-sized device implanted five centimeters inside the chest, on the surface of the heart, a depth once thought out of reach for wireless power transmission.

The device works by a combination of inductive and radiative electromagnetic transfer using radio waves that are received by a coiled antenna inside the body. The radio waves produce an electrical current in the coil that is sufficient to operate a small device. The amount of power available is indirectly related to the frequency of the transmitted radio waves, and the size of the receiving antenna. To deliver a certain level of power, lower frequency waves require bigger coils; but higher frequency waves can work with smaller coils.

Existing mathematical models, however, hold that high frequency radio waves do not penetrate far enough into human tissue, necessitating the use of low-frequency transmitters and large antennas that are too large to be practical for implantable devices. But according to the researchers, the models are wrong - although human tissues dissipate electric fields quickly, radio waves can travel as alternating waves of electric and magnetic fields. Using revised models, the researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second, allowing them to increase power transfer by about 10 times over earlier devices.

The discovery meant that the team could shrink the receive antenna by a factor of 10 as well, to a scale that makes wireless implantable devices feasible. At the optimal frequency, a millimeter-radius coil is capable of harvesting more than 50 microwatts of power, well in excess of the needs of a pacemaker. The researchers also designed an innovative transmit antenna structure that delivers power efficiently, regardless of orientation of the antennas, precisely focusing the radio waves at the point inside the body where the device rests, thus increasing the electric field where it is most needed, but canceling it elsewhere. The study was published online on August 13, 2012, in Applied Physics Letters.

“To achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor. If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant,” concluded senior author Ada Poon, PhD, and colleagues of the department of electrical engineering. “With proper system design, power sufficient to operate typical cardiac implants can be received by millimeter-sized coils.”

The researchers added that the research is a major step toward a future in which all implants are driven wirelessly. Beyond the heart, they believe that devices that could benefit from wireless power transfer might include pillcams that travel the digestive tract, permanent pacemakers, and precision brain stimulators. In fact, the devices could potentially be used for virtually any medical applications for which device size and power matter.

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Stanford University