February 23, 2012
Tiny Device Can Swim Through Your Bloodstream
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Engineers at the Stanford University School of Engineering have for the first time demonstrated a wirelessly powered medical device so small that it can be implanted in the human body and propel itself through the bloodstream, a feat scientists have been trying to accomplish for more than fifty years.
“Such devices could revolutionize medical technology,” said Poon. “Applications include everything from diagnostics to minimally invasive surgeries.”
She said these medical devices could travel through the body delivering drugs to where they need to go, performing analyses, and even zapping blood clots or removing plaque from sclerotic arteries.
These tiny wireless devices could one day replace most of today´s implements that are run on large, heavy batteries that must be changed periodically. And most of the current devices in use have batteries that take up half the volume of the device.
“While we have gotten very good at shrinking electronic and mechanical components of implants, energy storage has lagged in the move to miniaturize,” said co-author Teresa Meng, a professor of electrical engineering and of computer science at Stanford. “This hinders us in where we can place implants within the body, but also creates the risk of corrosion or broken wires, not to mention replacing aging batteries.”
The wireless devices are much different. A radio transmitter sending signals to the device would remain outside the body. The device picks up the signals using a tiny coiled wire antenna. The two are magnetically coupled such that any change in current flow in the transmitter induces a voltage in the antenna. The power can both run the device and propel it.
Although its sounds like an easy task to accomplish, Poon said it was anything but. She first had to upend some long-held assumptions about the delivery of wireless power inside the human body. According to scientific models, high-frequency radio waves dissipate quickly in human tissue, fading exponentially the deeper they travel.
But on the other hand, low-frequency signals penetrate easier. However, these require larger antennas -- a few centimeters in diameter -- to generate enough power for the device, far too large to fit through most arteries in the body. So, based on the models telling engineers that it could not be done, they never tried.
But then, engineers, namely Poon, looked at the models more closely and realized that scientists were approaching the problem incorrectly. 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, taking a different approach, chose to model tissue as a dielectric -- a type of insulator. As it turned out, human tissue is a poor conductor of electricity. But, radio waves can still penetrate through them. In a dielectric, the signal is conveyed as waves of shifting polarization of atoms within cells. She also discovered that human tissue is a “low-loss” dielectric -- meaning little of the signal gets lost along the way.
Using the new models, 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, about 100 times higher than previously thought,” said Poon.
And more significantly, the antenna inside the body could be 100 times smaller and induce the same amount of power. The antenna Poon demonstrated was just two millimeters square; small enough to travel through the bloodstream.
Poon developed two types of self-propelled devices. One drives electrical current directly through the fluid to create a directional force that pushes the device forward, moving at about a half-centimeter per second. The other type switches current back and forth in a wire loop to produce swishing motion similar to the motion of a kayaker paddling upstream.
“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´s research was supported and funded by C2S2 Focus Center, Olympus Corporation, and Taiwan Semiconductor Manufacturing Company.
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