07 Sep 2012, BioSpectrum Bureau , BioSpectrum
Singapore: In a breakthrough research in wireless implants, a team of engineers at Stanford University, US, demonstrated the feasibility of a super-small, implantable cardiac device that gets its power from radio waves. The radio waves are transmitted from a small power device on the surface of the body. The implanted device is contained in a 0.8 millimeter cube area on the side.
The findings, published in the journal Applied Physics Letters, hold a lot of promise in bringing revolutionary changes to devices such as swallowable endoscopes that travel to the digestive system, permanent pacemakers and precision brain stimulators.
Ms Ada Poon, assistant professor, electrical engineering, led the project and was assisted by Sanghoek Kim and John Ho, both doctoral candidates in her lab. It was funded by C2S2 Focus Center, one of six research centers funded under the Focus Center Research Program, a Semiconductor Research Corporation entity.
They demonstrated wireless power transfer to a millimeter-sized device implanted five centimeters inside the chest on the surface of the heart. The device works by a combination of inductive and radiative transmission of power. Both are types of electromagnetic transfer in which a transmitter sends radio waves to a coil of wire inside the body. The radio waves produce an electrical current in the coil sufficient to operate a small device. To deliver a desired level of power, lower frequency waves require bigger coils, while higher frequency waves can work with smaller coils.
Existing models held that high-frequency radio waves do not penetrate far enough into human tissue, necessitating the use of low-frequency transmitters and large antennas. However, research by Ms Poon and her team proved that high-frequency signals can travel much deeper than anyone suspected.
The discovery meant that the team could shrink the receiving antenna 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 eight microwatt pacemaker.
The team also designed an innovative slotted transmitting antenna structure that allows delivery of consistent power efficiency. Ms Poon has applied for a patent of the design. The design also focuses the radio waves at the point where the implanted device rests, increasing the electric field where it is needed most, but canceling it elsewhere. This helps to reduce overall tissue heating to levels well within the standards approved by the Institute of Electrical and Electronics Engineers.