Wirelessly powered implantable device can propel itself through bloodstream

A team led by electrical engineer Ada Poon is developing a new class of medical devices that can be implanted or injected into the human body and powered wirelessly using electromagnetic radio waves. "This means no batteries to wear out and no cables to provide power," says Poon. "Such devices could revolutionise medical technology. Applications include everything from diagnostics to minimally invasive surgeries." While the idea of implantable medical devices is not new, most of today's implements are challenged by power, namely the size of their batteries, which are large, heavy and must be replaced periodically. "While we have gotten very good at shrinking electronic and mechanical components of implants, energy storage has lagged in the move to miniaturise," explains Teresa Meng, a professor of electrical engineering and 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." Poon's devices are different in that they consist of a radio transmitter outside the body sending signals to an independent device inside the body that picks up the signal with an antenna of coiled wire. The transmitter and the antenna are magnetically coupled such that any change in current flow in the transmitter produces a voltage in the coiled wire - or, more accurately, it induces a voltage. This means that the power is transferred wirelessly. The electricity runs electronics on the device and propels it through the bloodstream, if so desired. Using new equations, Poon and her team found that 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 realised that the optimal frequency for wireless powering is actually around 100GHz," she says, "about 100 times higher than previously thought." More significantly, however, the revelation meant that antennae inside the body could be 100 times smaller and yet deliver the same power. Poon has 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. This type of device is capable of moving at just over 1/2cm per second. The second type switches current back and forth in a wire loop to produce swishing motion similar to the motion a kayaker makes to paddle upstream. "There is considerable room for improvement and much work remains before these devices are ready for medical applications," Poon concludes. "But for the first time in decades the possibility seems closer than ever."