By Duncan Graham-Rowe

A super-sensor that can monitor a person's heartbeat from a meter away has been created by a team at the University of Sussex in Brighton, who only set out to improve the accuracy of electrocardiograms.

Via stick-on contacts, EKGs measure minute voltages on the surface of the skin that correspond to electrical activity in the heart. EKGs effectively act like very sensitive voltmeters, measuring the waveform of your heartbeat and displaying the trace on a screen or printout.

But the problem with EKGs is that the skin contacts distort the electrical measurements because they drain the current slightly. So you get a smaller and distorted signal. It's best not to make contact with the skin.

Work was done on an alternative that measures a parameter called "displacement current". Unlike a standard conduction current of moving electrons, displacement current is a measure of the changing electric field in the air, generated by the shifting voltages on the skin surface.

Under Wraps

To accurately measure this subtle current in the air without shorting it you need a sensor with an even higher resistance (or more properly, impedance) than that of the air gap between the body and the sensor. Otherwise, the sensor will drain the electrical signal just like an EKG contact sensor does. This is no mean feat because the impedance of air is extremely high.

The sensor itself is a small copper disc about a centimeter across. The team used a number of electronic feedback techniques to make its impedance a hundred times higher than that of air. Although they're keeping the details under wraps while their patents are pending. They claim their sensor produces the most sensitive EKGs ever.

Burns Victims

While measuring electrical activity from a distance is certainly useful, perhaps for monitoring burns victims who cannot be touched, the resolution of the sensor is far higher the closer it is to the body. So researchers have been experimenting with a finger sensor that makes contact with the skin but is electrically insulated from it.

They have managed to detect heart signals that would normally be impossible to pick up without surgery, such as the His-Purkinje discharge - a weak current that travels from the atrium to the ventricle. This signal would normally need electrodes placed within an artery to detect it.

Using arrays of the sensors, the team hopes to create novel images of the body's electrical activity.

Measurement Science and Technology January 2002 13:163