New Biosensor Chip Picks Up Heart Signals Remotely
By Willie D. Jones
NOTE: This is an abstract of the entire article, which appears in IEEE Spectrum Online.
Click here to read the entire article.
A look inside the housing of a prototype of the Electric Potential Integrated Circuit (EPIC) electrocardiogram tracking device
shows that it is actually an array of 16 sensors that together
ensure accuracy and reliability.
Medical technologists are in hot pursuit of methods for unobtrusively monitoring the body, and video-game-system makers are on the same trail. This kind of sensing is poised to become even less obtrusive with the introduction of a new technology that detects the voltage change in the body’s muscles and nerves without electrical contact. In October, Plessey Semiconductors of Roborough, England, began shipping samples of its Electric Potential Integrated Circuit (EPIC), which measures minute changes in electric fields. In videos demonstrating the technology, two sensors placed on a person’s chest delivered electrocardiogram (ECG) readings. No big deal, you say? The sensors were placed on top of the subject’s sweater, and in future iterations, the sensors could be integrated into clothes or hospital gurneys so that vital signs could be monitored continuously – without cords, awkward leads, hair-pulling sticky tape, or even the need to remove the patient’s clothes.
Though the technology is being introduced to pick up heart signals, EPIC’s developers say it is also good at sensing the electrical activity of skeletal muscles, including those that control the eyes. Derek Rye, Plessey’s marketing director, says that researchers are working to include this ability in computer interfaces for assisting the disabled; it would give quadriplegics the ability to control a cursor on a computer screen or operate a motorized wheelchair with a series of eye movements. It could also benefit amputees, who often have residual electrical activity in the muscle at the amputation site. Researchers say that EPIC could serve as a noninvasive interface between the nerves and muscles and a prosthesis, allowing it to respond just as a natural limb would.
How does it work? Plessey describes EPIC as essentially a very sensitive, contactless digital voltmeter capable of measuring millivolt changes in electric fields. The sensor’s input impedance is tens of gigaohms, so the act of taking a measurement does not throw off the quantity being measured. Also in the secret sauce is filtering technology that ensures the output does not include signals other than the one you’re attempting to pick up. So when you’re trying to do an ECG trace, the readout doesn’t show the results of the muscular movement involved in respiration.
The technology for being able to sense what the body is doing without physical or electrical contact was developed over the past decade by researchers at the University of Sussex, in England. Robert Prance, head of the team that invented EPIC, says it was initially created as a noninvasive noncontact sensor to aid in their own fundamental physics research. The team had been bedeviled by a vexing electric field measurement problem. Because the researchers feared that sensitive equipment would be damaged by electric discharges, they had to use either large laboratory electrometers that needed frequent recalibration or detectors capable of measuring only in increments of hundreds of volts.
The breakthrough, says Prance, was more the result of independent thinking rather than a technical advance. “Getting away from the obsession with measuring DC data was a big step, intellectually as well as technologically,” he says. “Often it’s AC info that people want – very often in the form of how it changes with time. ECG, for example, doesn’t require DC information. Once we decided that we were interested in AC, we were able to use different circuit topologies that freed us up from problems such as DC offsets, temperature and time drift, and the inability to move the sensors or the person being measured without recalibrating the system.” Just as important, says Prance, was feeding in a stable input bias current that didn’t compromise input impedance.