YOU鈥橰E sitting in the surgery bending your doctor鈥檚 ear about your latest
ailment. What you don鈥檛 know is that the doctor is in fact already measuring
your vital signs without even touching you. How come? She鈥檚 using an
ultra-accurate sensor that can capture clinical data at a distance.
True, this scenario is still a dream, but early results are encouraging. The
super sensor can already monitor a person鈥檚 heartbeat from a metre
away鈥攕omething of a bonus for the team at the University of Sussex in
Brighton, who only set out to improve the accuracy of electrocardiograms.
Via stick-on contacts, ECGs measure minute voltages on the surface of the
skin which correspond to electrical activity in the heart. ECGs effectively act
like very sensitive voltmeters, measuring the waveform of your heartbeat and
displaying the trace on a screen or printout.
Advertisement
But the problem with ECGs, says Terry Clark, an electrical engineer at the
University of Sussex, is that the skin contacts distort the electrical
measurements because they drain the current slightly. 鈥淪o you get a smaller and
distorted signal,鈥� he says. 鈥淚t鈥檚 best not to make contact with the skin.鈥�
Working with engineers Robert Prance and Christopher Harland, Clark set about
developing an alternative that measures a parameter called 鈥渄isplacement
current鈥�. 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.
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 ECG contact sensor does, says Prance.
This is no mean feat because the impedance of air is extremely high.
The sensor itself is a small copper disc about a centimetre across. The team
used a number of electronic feedback techniques to make its impedance a hundred
times higher than that of air. Although they鈥檙e keeping the details under wraps
while their patents are pending, says Clark, their basic idea is revealed in the
current issue of the journal Measurement Science and Technology (vol
13, p 163).
They claim their sensor produces the most sensitive ECGs ever. Ary
Goldberger, a cardiologist at Harvard Medical School in Massachusetts, says this
claim needs to be independently verified, but he agrees that if the Sussex
team鈥檚 idea works, the sensor could be the basis of a major advance in remote
medical sensing. And Goldberger鈥檚 colleague Isaac Henry predicts that DIY
monitors could well be launched one day.
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 Clark鈥檚 team 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鈥攁 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鈥檚 electrical activity.