Earlier this year, Stanford electrical engineer Ada Poon, PhD, made headlines when she publicly showcased a tiny wireless chip, driven by magnetic currents, that is small enough to travel through the bloodstream. Now comes news that Poon and colleagues have demonstrated the feasibility of a millimeter-sized, implantable cardiac device that runs on radio waves transmitted from a small power device on the surface of the body.
A Stanford Report story published today discusses the significant engineering challenges that researchers had overcame in designing the device:
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.
Ignoring the consensus, Poon proved the models wrong. Human tissues dissipate electric fields quickly, it is true, but radio waves can travel in a different way – as 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.
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, much higher than previously thought.
The discovery meant that the team could shrink the receiving 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 recently demonstrated 8-microwatt pacemaker.
Researchers say the work is a major step in advancing the development of a new class of medical devices, ranging from swallowable endoscopes to precision brain stimulators, that can be implanted into the body and powered wirelessly.