Bulky batteries have long been a thorn in the side of designers trying to miniaturize implantable medical devices. But a new method for powering implants wirelessly could help bring pacemakers and other implantable products down to size.
A team led by assistant professor Ada Poon at the Stanford University school of engineering has demonstrated that it is possible to wirelessly power a cardiac device implanted 5 cm into the chest—deeper than ever thought possible.
Shown are computer models of power delivery to the heart from a 200MHz low-frequency transmitter (left) and a 1.7GHz high-frequency transmitter (right). Red indicates greatest power; blue is least. Image courtesy JOHN HO, STANFORD SCHOOL OF ENGINEERING
The team’s device uses a combination of inductive and radiative power transmission to send radio waves to a coil inside the body. The bigger the coil, the lower frequency the radio waves need to be in order to harvest the same amount of power. But it was previously thought that high-frequency radio waves could not penetrate far enough into the body. Tissues heating increases with frequency, so others who have developed wirelessly powered implantable devices have erred on the side of caution, opting for lower frequency power and hence requiring a bigger device, says John Ho, a PhD candidate in the department of electrical engineering at Stanford and member of the research team. But using mathematical modeling, the Stanford researchers proved that much higher frequencies can be used than initially thought.
The Stanford team proved that it is possible to wirelessly charge an implanted cardiac device using a frequency of 2 gHz. This would allow them to scale their device down from centimeters to just .8 mm in size—smaller than the head of a pin. The battery typically accounts for about half the size of an implantable medical device, so eliminating it allows for the creation of a much smaller device.
“Current devices that are able to power through the chest wall are too big—usually a couple of centimeters in size, and that’s not good for the patient,” Ho says. “If the device is too big, then, for one, you aren’t able to localize measurements.”
The example device the researchers described in a paper published in the journal Applied Physics Letters was a pacemaker, but Ho says the technology could also be used for electrical sensing to treat cardiac arrhythmia or for neural stimulators implanted in the brain.
"Another particularly fascinating application that our research group is working on are tiny devices that can swim through the bloodstream to monitor physiological parameters, deliver drugs, or even perform surgery," Ho says. "It is crucial for such devices to be tiny—this is possible only with wireless powering."
So far, the team’s work has involved proving that such a wirelessly powered implantable cardiac device could work using a computer model. Their next step is to build a prototype and test it in an animal model, which they’re hoping to do by the end of the year.
—Jamie Hartford is the associate editor of MD+DI. Follow her on Twitter @readMED.