SAN JOSE, Calif. — Ada Poon still recalls the day she read the report from the safety compliance lab. The Stanford assistant professor had gotten validation for her six years of research on millimeter-scale medical implants that were now shown to have commercial potential.
"I was quite excited," says Poon of her work exploiting living animal tissue as a medium to deliver more than 200 microwatts of power over a distance of 5 centimeters or more.
"We knew it was safe in simulation," Poon tells us. "We kept the output power to 500 mW, which is the same as a cellphone, and we did measurements to check the temperature rise. We did all this validation, but in the end to be really sure and articulate our point that our technique was safe, I said let's do third-party testing."
Today Poon and colleagues submitted a paper to the National Academy of Sciences on their work in so-called midfield wireless power for a medical implant. It describes a way to deliver power through nearly 5 cm of tissue to a 2 mm microstimulator implanted on a rabbit's heart.
Current inductive coupling methods rely on near-field coupling between an implant and an external device with nothing more than a thin layer of skin in between. Poon's team took a different approach, embracing propagation of the 1.6 GHz signal through biological tissue rather than trying to avoid it.
The Stanford researchers call their approach "mid-field" wireless. Essentially, they converted "electromagnetic waves from the 'evanescent' or 'near-field' type, which are safe but decay rapidly away from the source, to the 'propagating' or 'far-field' type, which carry energy away with much farther reach," explains John Ho, a Stanford graduate student and co-author of the paper
The more than 200 microwatts the technique delivers far exceeds the 8 mW consumed by today's pacemakers. Poon and others foresee the advent of a class of microstimulators the size of a grain of rice that may someday be more effective for some ailments than today's implants or drugs.
"Our powering method could be applicable to a broader class of devices that have yet to be developed, such as implantable diagnostic sensors or localized drug delivery tools," says Ho. "Part of the reason they are not commercially used today may be because of the bulkiness of existing implants.
"One intriguing application is in an emerging class of medicines called 'electroceuticals.' Tiny devices that directly modulate neural activity in the body may provide more effective treatments for some disorders than drugs. Much more research is required to understand the neural basis for diseases and develop electronic treatments, but all such approaches will require ways to safely transfer power."
The Stanford microstimulator implant measures 2mm across, about the size of a grain of rice.
Next page: From theory to surgery