Stanford Engineering assistant professor Ada Poon demonstrated a tiny, wirelessly powered, self-propelled medical device capable of controlled motion through blood. The device drives electrical current directly through the fluid, which in the presence of an external magnetic field creates a directional force that pushes the device forward. This type of device is capable of moving at just over half-a-centimeter per second.
That was the news picked up by bloggers from Poon’s presentation at the International Solid-State Circuits Conference (ISSCC). However, what caught my attention is her work on inductive transcutaneous energy transmission. From Stanford’s press release:
“For fifty years, scientists have been working on wireless electromagnetic powering of implantable devices, but they ran up against mathematics. According to the models, high-frequency radio waves dissipate quickly in human tissue, fading exponentially the deeper they go.
Low-frequency signals, on the other hand, penetrate well, but require antennae a few centimeters in diameter to generate enough power for the device, far too large to fit through all but the biggest arteries. In essence, because the math said it could not be done, the engineers never tried.
Then a curious thing happened. Poon started to look more closely at the models. She realized that scientists were approaching the problem incorrectly. In their models, they assumed that human muscle, fat and bone were generally good conductors of electricity, and therefore governed by a specific subset of the mathematical principles known as Maxwell’s equations — the “quasi-static approximation” to be exact.
Poon took a different tack, choosing instead to model tissue as a dielectric — a type of insulator. As it turns out human tissue is a poor conductor of electricity. But, radio waves can still move through them. In a dielectric, the signal is conveyed as waves of shifting polarization of atoms within cells. Even better, Poon also discovered that human tissue is a “low-loss” dielectric — that is to say little of the signal gets lost along the way.
She recalculated and made a surprising find: Using new equations she learned high-frequency radio waves travel much farther in human tissue than originally thought.
“When we extended things to higher frequencies using a simple model of tissue we realized that the optimal frequency for wireless powering is actually around one gigahertz,” said Poon, “about 100 times higher than previously thought.”
More significantly, however, her revelation meant that antennae inside the body could be 100 times smaller and yet deliver the same power.
Poon was not so much in search of a new technology; she was in search of a new math. The antenna on the device Poon demonstrated at the conference yesterday is just two millimeters square; small enough to travel through the bloodstream.”