Implantable devices can benefit greatly from an efficient way to transfer electric power to their location within a human body. Current methods are fairly limited, requiring large coils that keep devices bulky and more difficult to implant.
Now a group of researchers from Stanford’s Department of Electrical Engineering have shown, at least in theory, that one can build a wireless electricity transfer system that is both small and efficient, while safe for use within the human body. They showed that using radio waves at higher frequencies and staying within the “mid-field” regime (somewhere between radiating and evanescent waves), should allow for creation of a power transfer system with small coils that don’t need to matched up in size to each other.
Some details from APS Physics:
The large coil is needed because, instead of relying on “radiating” waves (conventional radio waves), which don’t penetrate well into biological tissue, the systems use so-called evanescent fields. These are short range and don’t extend more than several times the source’s dimensions. So the source must be large enough that its evanescent field will extend to the implanted receiver—but then the receiver must be of a similar size to the source, because otherwise this kind of power transfer is inefficient.
Ada Poon and colleagues at Stanford University in California looked for a way to reduce the size of the receiver, ideally so that implants need be no bigger than a millimeter or so. Such an improvement could also allow more capabilities to be packed into a smaller volume. This battery-free approach could work in situations where “a device turns on when activated by an external source but is otherwise dormant,” says Poon. “These might include stimulators for treating neurological disorders, sensors that are powered during read-out, or locomotive drug-delivery systems that are inside the body for a short time.”
Poon’s team has shown that, despite the issue of tissue absorption, radiating modes can after all be used to boost power transfer—which makes it less necessary to match the size of the source and receiver. Their theoretical analysis s shows that, although these radiating modes can’t travel very far without being absorbed, there is an optimal middle ground where both evanescent and radiating modes can deliver power even to a tiny receiver. This “mid-field” regime can be reached by using higher frequencies, in the gigahertz range, rather than the kilohertz to megahertz range of existing technologies.
More at APS Physics: Focus: Wireless Power for Tiny Medical Devices
Study in Physical Review Letters: Midfield Wireless Powering of Subwavelength Autonomous Devices