A team of engineers at Stanford has demonstrated the feasibility of a super-small, implantable cardiac device that gets 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 pin.
the researchers demonstrated wireless power transfer to a millimeter-sized device implanted five 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 are 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. The devices could potentially be used for virtually any medical applications for which device size and power matter.
The latest device works by a combination 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.
There is an indirect relationship between the frequency of the transmitted radio waves and the size of the receiving antenna. That is, to deliver a desired level of power, lower frequency waves require bigger coils. Higher frequency waves can work with smaller coils.
xisting 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. 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.
The discovery meant that the team could shrink the receive antenna by a factor of 10 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.
The team responded by designing an innovative transmit antenna structure that delivers 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 device rests on the surface of the heart, increasing the electric field where it is most needed, but canceling it elsewhere. This helps reduce tissue heating to levels well within the IEEE standards.
This story is reprinted from material from Standford University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.