The discovery meant that the team could shrink the receive antenna by a factor of ten as well, to a scale that makes wireless implantable devices feasible. At that 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 recently demonstrated eight-microwatt pacemaker.
With the dimensional challenges solved, the team found themselves bound in by other engineering constraints. First, electronic medical devices must meet stringent health standards established by the IEEE, 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.
"This can't happen medical devices," said Poon. "As the human heart and body are in constant motion, solving this issue was critical to the success of our research."
The team responded by designing an innovative slotted transmit antenna structure. It resembles a swastika, but delivers consistent power efficiency regardless of orientation of the two antennas.
The new design serves additionally to focus 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 the IEEE standards. Poon has applied for a patent for the antenna structure.
|Contact: Andrew Myers|
Stanford School of Engineering