Prototype of Stanford's wireless, self-propelled chip measures 3 mm wide x 4 mm long. (Photo by Steve Fyffe)

Researchers at Stanford University (Palo Alto, CA) have developed a wireless, self-propelled miniature medical implant that can perform controlled movements in blood. Led by Ada Poon, an assistant professor of electrical engineering, the team is developing a class of medical devices that can be implanted or injected into the human body and powered using electromagnetic radio waves instead of batteries and cables. Unlike stationary medical devices, the Stanford researchers' invention is capable of traveling through the bloodstream to deliver drugs and perform a host of other applications.

"While we have gotten very good at shrinking electronic and mechanical components of implants, energy storage has lagged in the move to miniaturize," remarks Teresa Meng, a professor of electrical engineering and computer science. "This hinders us in where we can place implants within the body and also creates the risk of corrosion or broken wires, not to mention replacing aging batteries."

Poon's device consists of a radio transmitter located outside of the body that sends signals to a receiver located inside the body that picks up the signal using a coiled-wire antenna. Because the transmitter and antennas are magnetically coupled, changes in current flow in the transmitter induce a voltage in the other wire. Transferred wirelessly, this voltage can be used to power an implantable device and cause it to move through the bloodstream.

Until now, scientists have experimented with high-frequency radio waves to electromagnetically power implantable devices wirelessly. However, high-frequency radio waves dissipate quickly in human tissue, fading exponentially the deeper they go. While low-frequency signals penetrate well, they have required antennas measuring a few centimeters in diameter—far too large to fit through any but the largest arteries.

In an effort to surmount these obstacles, Poon began to challenge the assumption that human muscle, fat, and bone are generally good conductors of electricity and therefore governed by the quasi-static approximation of Maxwell's equations. Instead of basing her work on this assumption, Poon decided to model tissue as a dielectric—a type of insulator. From that vantage point, she determined that human tissue is in fact a poor conductor of electricity but that radio waves can still move through them. In addition, Poon discovered that human tissue is a 'low-loss' dielectric. In other words, little of the signal gets lost along the way.

Concluding that radio waves travel much farther in human tissue than originally thought, Poon and her team realized that the optimal frequency for wireless power is approximately 1 gH—about 100 times higher than previously thought. This conclusion enabled the scientists to use antennas inside the body that are 100 times smaller than previous models yet deliver the power needed by the medical device.

Thus far, the Stanford researchers have developed two types of self-propelled devices. Capable of moving about half a centimeter per second, one device drives electrical current directly through the fluid to create a directional force that pushes the device forward. The other type switches current back-and-forth through a wire loop to produce a swishing motion that resembles the movements of a kayaker.