Engineers at Stanford University (Palo Alto) have shown that a miniature implantable cardiac device can forgo the use of batteries and derive its power from radio waves transmitted from outside the body. A cube-shaped device 8/10 mm in diameter, the device was implanted on the surface of the heart—5 cm within the chest. Until now, this depth was considered beyond the reach of wireless power transmission.
Developed by assistant professor of electrical engineering Ada Poon, the device combines inductive and radiative power transmission, forms of electromagnetic transfer in which a transmitter sends radio waves to a wire coil inside the body. The radio waves produce an electrical current in the coil capable of powering small devices. At the optimal frequency, a millimeter-size radius coil can harvest more than 50 mW of power, more than that required by a 8-mW pacemaker.
A challenge the engineers faced was to disprove the conventional wisdom—based on existing mathematical models—that because high-frequency radio waves do not penetrate far into human tissue, low-frequency transmitters and large antennas would be necessary to power implantable medical devices. Such components, however, would be too large for such devices.
The Stanford team learned that while tissues dissipate electric fields quickly, radio waves can travel as alternating waves of electric and magnetic fields. Based on that knowledge, they developed new mathematical equations showing that high-frequency signals travel much deeper than previously thought. As a result of this discovery, the team was able to shrink the receiver antenna by a factor of 10.
For more information on this technology, see this Stanford Engineering article by Andrew Myers.