Wirelessly powered medical implant propels itself through the bloodstream
By Darren Quick
February 23, 2012
With the wait still on for a miniaturization ray to allow some Fantastic Voyage-style medical procedures by doctors in submarines, tiny electronic implants capable of traveling in the bloodstream show much more promise. While the miniaturization of electronic and mechanical components now makes such devices feasible, the lack of a comparable reduction in battery size has held things back. Now engineers at Stanford University have demonstrated a tiny, self-propelled medical device that would be wirelessly powered from outside the body, enabling devices small enough to move through the bloodstream.
While the benefits of medical implants have already been realized with devices such as artificial pacemakers and cochlear implants, which are stationary within the body, energy storage continues to limit such devices. With half of the volume of implants often consumed by the battery, the locations in which they can be placed are limited. Additionally, batteries also need to be periodically replaced, which generally requires a surgical procedure.
Developing implants capable of traveling through the bloodstream not only requires an energy source to power the device's medical functions, but also its propulsion system - something that today's batteries are unable to deliver in a form factor that is small enough to fit inside arteries.
The obvious approach would be to remove the battery from the device altogether and look to wireless electromagnetic power delivery. This is just what many scientists have been working on for fifty years. While such wireless power transmission technology has recently entered the mainstream through wireless chargers for consumer devices such as mobile phones, it wasn't believed the technology could be made small enough to be compatible with tiny implantable devices.
The problem is that, according to mathematical models, high frequency waves that would require antennas small enough to be used in such devices were believed to dissipate quickly in human tissue, fading exponentially the deeper they go. At the same time, antennas to harness enough power from low-frequency signals, which are able to penetrate the human body well, would need to be a few centimeters in diameter, making them OK for larger devices, but too large to fit in all but the biggest arteries.
However, when electrical engineer Ada Poon looked at the models more closely she realized they were calculated assuming that human muscle, fat and bone were generally good conductors of electricity. Realizing that human tissue is actually a poor conductor of electricity but that radio waves could still move through it, Poon decided to redo the models with human tissue as a type of insulator called a dielectric. Her new calculations revealed that high-frequency waves travel much farther in human tissue than previously 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."
This meant that antennae inside the body could be 100 times smaller while delivering the same amount of power. This finding enabled Poon to create an antenna of coiled wire small enough to be placed inside the body and receive power from a radio transmitter outside the body. The transmitter and the antenna are magnetically coupled so that any change in current flow in the transmitter induces a voltage in the coiled wire.
Poon has created two types of wirelessly powered devices that are able to propel themselves through the bloodstream. One creates a directional force by driving an electrical current directly through the blood to push itself forward at a velocity of just over half a centimeter (0.2 inches) a second. The second type switches current back-and-forth in a wire loop to produce a swishing motion to propel the device forward.
Poon's research could finally enable the development of medical implants capable of traveling through the bloodstream to deliver drugs to a specific area, perform analyses, and maybe even zap blood clots or remove plaque from arteries.
"There is considerable room for improvement and much work remains before these devices are ready for medical applications," said Poon. "But for the first time in decades the possibility seems closer than ever."
Poon recently demonstrated one of her tiny, wirelessly powered, self-propelled devices at the International Solid-State Circuits Conference (ISSCC). The animation below produced by Carlos Suarez at StrongBox3d shows how such a device might move through the bloodstream.
Source: Stanford University
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