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Lifesaver. Ho and colleagues' design for a wirelessly powered pacemaker outfitted with a tiny receiver antenna, pictured alongside grains of rice.

Lifesaver. Ho and colleagues' design for a wirelessly powered pacemaker outfitted with a tiny receiver antenna, pictured alongside grains of rice.

Courtesy of Austin Yee

Tiny, Wireless Pacemaker Implanted in Rabbit

The days of clunky, battery-operated pacemakers may soon be over. Researchers have built a wirelessly powered pacemaker the size of a grain of rice and successfully implanted it in a rabbit. If the results hold up, a new generation of smaller and safer medical implants could be on the market in the next 5 to 10 years.

Today’s pacemakers, cochlear implants, and other internal medical devices require batteries that are either built into the implant or connected by long wires, making them bulky and requiring surgery any time the batteries need to be replaced or the wires need repair. Recently, researchers have worked to make pacemakers smaller; one recent design is so tiny that doctors can use a catheter to guide it through a vein that starts in the thigh to implant it directly inside a patient’s heart. But no matter how small a pacemaker is, its batteries still need to be replaced eventually.

An alternative, first suggested in the 1960s, is to power a pacemaker by transmitting radio waves to it from outside the body using Tesla coils, the doughnut-shaped metal coils Nikola Tesla originally proposed as an alternative to standard electrical power lines. In this scenario, the batteries required to run the transmitter would remain outside the body, eliminating the need for surgery to replace them. In theory, a pacemaker powered with a Tesla coil could run indefinitely with no tinkering. And by eliminating internal batteries, a pacemaker could be much smaller—just a simple electronic circuit and a tiny receiver coil. In practice, however, current designs are so inefficient that to power a pacemaker, the transmitting coil mounted on a patient’s chest has to send about 100 watts through the skin—more than enough to burn.

“Safety is probably the single most important problem with wireless power,” says John Ho, lead author of the new study and an electrical engineering graduate student at Stanford University in California. The key to a safer wireless pacemaker, he says, is designing a better transmitter. After some initial research, he and his colleagues realized that a Tesla coil isn’t actually the best choice: Because it shoots energy in many different directions, doctors would need to crank up its power to dangerous levels in order to be sure a fraction of that energy would be absorbed by the pacemaker. To make wireless power a reality, researchers need a design that focuses energy directly on an implant, he says.

To begin to address the problems, Ho, Qualcomm engineer Sanghoek Kim, and Stanford electrical engineer Ada Poon last year decided to completely rethink transmitter design. They first devised a series of equations that the electrical currents in an optimal transmitter would have to satisfy and showed that the ideal currents followed semicircular paths and switched back and forth 2 billion times a second, about the same frequency at which cellphones broadcast. Based on that observation, they played around until they found a transmitter design that came close to producing the optimal electrical currents: a 6-centimeter square plate with four trident-shaped cutouts arranged in a circle, operating at 1.6 gigahertz, and, like earlier designs, placed on the skin above an implant.

Now, Ho and his team have built a model of their device and used it to transmit power to a tiny receiver coil mounted on a 2-millimeter-long pacemaker embedded in simulated human hearts and brains. Their tests showed they could run the implant with about 100 times less power than Tesla coil–based designs, they report online today in the Proceedings of the National Academy of Sciences. In a second test, the team implanted a pacemaker in a rabbit and used it to successfully control its heart rate without burning the animal’s skin.

“It’s a very interesting idea,” says Vivek Reddy, director of Arrhythmia Services at Mount Sinai Hospital in New York City and a researcher who’s performed clinical trials on capsule-sized, battery-powered pacemakers that fit inside the heart. Still, the idea of wirelessly transmitting power raises issues such as whether patients could be trusted to replace the transmitter’s batteries or to properly position it over a medical implant without a doctor’s help. When it comes to pacemakers, implanting both the device and its transmitter may be safer for patients because doctors could then monitor and control its operation, Reddy says. Still, he says, the technology should be valuable for cochlear implants or other implanted devices where a dead battery isn’t a matter of life and death.