Tiny wireless device powers itself through bloodstream

Engineers at Stanford University have demonstrated how a tiny, externally controlled, wirelessly-powered medical device, is able to propel itself through blood, in a manner reminiscent of the 1966 film Fantastic Voyage, where a microscopic submarine and scientific crew are injected into the bloodstream of a man. Assistant professor and electrical engineer Ada Poon heads the Poon Research Group at Stanford University School of Engineering. She and her team pursue new ways to use wireless communication and integrated circuit technologies in medicine. Earlier this year, at the International Solid-State Circuits Conference (ISSCC) in San Francisco, before an audience of her peers, Poon presented a study that suggests the day when we are invited to "swallow the surgeon" as part of a diagnostic test may be closer than we imagined. Poon said in a Stanford communication in March: "There is considerable room for improvement and much work remains before these devices are ready for medical applications. But for the first time in decades the possibility seems closer than ever." Small implantable medical devices have been around for a while, but most of them are limited by power constraints: their batteries are large and heavy and have to be replaced now and again. They take up nearly half the size of the device. Poon's lab is developing a new type of device that can be implanted or injected into the body and powered wirelessly via electromagnetic radio waves transmitted remotely from outside the body. Requiring no battery or cables means it can be small and unencumbered. Co-author of the study, Teresa Meng, is professor of electrical engineering and also of computer science at Stanford. She said while implant technology has become proficient at shrinking electronic and mechanical parts, energy storage has lagged behind. "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," she explained. Poon said such devices could "revolutionize medical technology", and offer applications from diagnostics to minimally invasive surgery. Stationary versions include devices like drug pumps, cochlear implants, pacemakers, heart probes, and pressure sensors. However, devices like those Poon's lab is developing are designed to travel through the bloodstream. Such applications offer many uses, including drug delivery, analysis of target sites, and maybe even breaking up blood clots or zapping away plaque in sclerotic arteries. For its energy source, the device that Poon's lab is working on relies on a radio transmitter sited outside the body to send it signals while it travels inside the body. The signals arrive at the device's tiny coiled wire antenna which is magnetically coupled to the body of the device so that any change in current flow in the external transmitter induces a voltage in the coiled wire, thereby wirelessly producing the energy needed for propulsion and working the device. This simple description belies the challenges that were overcome to create such a device. One such challenge involved overturning some established assumptions about delivering power wirelessly into the human body. Much of the mathematics behind models that test the feasibility of getting electromagnetic waves to create energy in an implanted device assumes that human tissue is a good conductor of electricity and would therefore dissipate the high-frequency radio waves before they could reach such a device. But when Poon plugged in a different assumption, that human tissue is a dielectric, a type of insulator, the equations worked. And in fact, as it turns out, human tissue is a poor conductor of electricity, but because it is of the dielectric type, it still allows radio waves to travel through it. Poon's lab also discovered that human tissue is a "low-loss" dielectric, a great benefit for their application because this means little of the electromagnetic signal gets lost on the way to the implanted device. And when they plugged their new assumptions into the equations they made a surprising discovery: the high frequency radio waves travel much further in human tissue than the original models were suggesting. It wasn't so much a case of new technology, it was more about putting new mathematics into the technology. They also discovered that the optimal frequency for powering the device wirelessly was around 100 times higher than previously thought, that is about 1 gigahertz. A real benefit of this discovery is that the antenna could be about 100 times smaller than previously thought, and still be capable of generating the same amount of power for the device. The antenna that Poon and colleagues showed at the conference is just two millimetres square, which allows it to travel in the human bloodstream. The Poon team has created two prototype versions: one moves at half a centimetre per second and relies on driving electrical current directly through the medium to propel the device; and the other moves like someone paddling a kayak, it swishes side to side as a current generated back and forth in a wire loop propels it forward. Funds from the C2S2 Focus Center, Olympus Corporation, and Taiwan Semiconductor Manufacturing Company helped pay for the research.