But hasn’t stopped researchers from attempting to develop devices that could do much the same thing. Some have opted for pills that propel themselves with mechanical parts, like jarringly sharp insect-inspired legs or moving clamps that perform an inchworm crawl. Others are exploring more passive schemes that use magnetic fields to guide ferromagnetic objects through the bloodstream.
A team led by Ada Poon of Stanford University has devised a different approach. After convincing themselves that ultra-small wireless antennas can receive a fair amount of power even after transmissions pass through human tissue, Poon and colleagues built a 3 mm x 4 mm prototype chip that exploits an external magnetic field to actively propel and steer itself.
The chip harnesses the Lorentz force, the force that arises when an electric charge moves in a magnetic field. One scheme (illustrated above) uses electrodes at the rear of the chip to run a current through a fluid. The other uses a loop of wire attached to the chip. Alternating the direction of current in the loop will allow the chip to wiggle itself forward by virtue of asymmetric drag.
Experimenting in water, Poon’s team found they could propel the chip at speeds of 0.53 centimeters per second with a magnetic field that’s about 1% as strong as the field in an MRI. Stanford graduate student Anatoly Yakovlev presented the chip designs on Tuesday at the IEEE International Solid-State Circuits Conference in San Francisco, Calif.
Poon says this approach to locomotion requires less energy and will be easier to miniaturize than mechanical locomotion. And unlike approaches that use passive magnetic materials, Poon says her team's current propulsion schemes shouldn’t need strong or complex magnetic fields to work. At its present size, she says the chip is suitable for the stomach or digestive track and perhaps the larger vessels of the body’s venous system.
Attendees of Yakovlev’s talk brought up a few safety concerns, including the possibility that the chip’s electrodes could create unwanted chemical reactions. But Poon says careful selection of electrode materials will cut down on that risk and that the biggest foreseeable danger is that the device might get lost as it’s guided through the body. This is unlikely to be much of an issue in the digestive system, but “for motion through the blood stream, the danger is much higher because the device must be removed after use,” Poon says. She says adding feedback control to the chip to assist with navigation might help prevent an operator from losing the device.
Poon says that it should be fairly straightforward to shrink down the device and lists drug delivery and diagnostic imaging and sensing as potential applications. But we’re still a far way from Richard Feynman’s “swallow the surgeon” vision of the future or even in vivo tests of the device. In the short term, Poon says the locomotion schemes the team has devised could help improve existing medical equipment, by, for example, helping guide the tips of catheters used in cardiovascular surgery.
Image courtesy of Ada Poon
Rachel Courtland, an unabashed astronomy aficionado, is a former senior associate editor at Spectrum. She now works in the editorial department at Nature. At Spectrum, she wrote about a variety of engineering efforts, including the quest for energy-producing fusion at the National Ignition Facility and the hunt for dark matter using an ultraquiet radio receiver. In 2014, she received a Neal Award for her feature on shrinking transistors and how the semiconductor industry talks about the challenge.