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A Millimeter-Scale, Wireless Cardiac Device

Experimental heart implant could fit on the head of a pin (Aug. 31)

The Stanford University School of Engineering in Stanford, Calif., announced on August 31 that a team of scientists has demonstrated the feasibility of a super-small, implantable cardiac device that obtains its power not from batteries but from radio waves transmitted from outside the body. The implanted device is contained in a cube just eight-tenths of a millimeter in radius. It could fit on the head of a pin.

The team’s findings were published in the journal Applied Physics Letters.

In their paper, the researchers described wireless power transfer to a millimeter-sized device implanted 5 centimeters inside the chest on the surface of the heart — a depth once thought out of reach for wireless power transmission.

The engineers say the research is a major step toward a day when all implants will be driven wirelessly. Beyond the heart, they believe such devices might include swallowable endoscopes (so-called "pillcams" that travel the digestive tract), permanent pacemakers, and precision brain stimulators.

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 antennas — too large to be practical for implantable devices. The researchers found, however, that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second. This discovery meant that the team could shrink the “receive” antenna to a scale that makes wireless implantable devices feasible. At that optimal frequency, a millimeter-radius coil is capable of harvesting more than 50 microwatts of power — well in excess of the needs of a recently demonstrated eight-microwatt pacemaker.

With the dimensional challenges solved, the team found themselves limited by other engineering constraints. First, electronic medical devices must meet stringent health standards, particularly with regard to tissue heating. Second, the team found that “receive” and “transmit” antennas had to be optimally oriented to achieve maximum efficiency. Differences in alignment of just a few degrees could produce troubling drops in power.

The team responded by designing a “transmit” antenna structure that delivers consistent power efficiency regardless of the orientation of the two antennas.

In addition, the new design focuses the radio waves precisely at the point inside the body where the implanted device rests on the surface of the heart, increasing the electric field where it is needed most, but canceling it elsewhere. This helps reduce overall tissue heating to levels well within current standards.

For more information, visit the Stanford University Web site.

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