Friday, November 18, 2011

Plessey Semiconductor Electric Potential Integrated Circuit

The electrocardiogram, or ECG, is one of the most common and useful tools for diagnosing heart arrhythmias. Russ Hobbie and I discuss the ECG in Chapter 7 (The Exterior Potential and the Electrocardiogram) of the 4th edition of Intermediate Physics for Medicine and Biology. The November issue of the magazine IEEE Spectrum contains an article by Willie D. Jones about new instrumentation for measuring the ECG. Jones writes
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.
Apparently the Plessey device is an ultra high input impedance voltmeter. The electrode is capacitively coupled to the body, so no electrical contact is necessary. You can learn more about it by watching this video. I don’t want to sound like an advertisement for Plessey Semiconductors, but I think this device is neat. (I have no relationship with Plessey, and I have no knowledge of the quality of their product, other than what I saw in the IEEE Spectrum article and the video that Plessey produced.)

According to the Plessey press release, “most places on earth have a vertical electric field of about 100 Volts per metre. The human body is mostly water and this interacts with the electric field. EPIC technology is so sensitive that it can detect these changes at a distance and even through a solid wall.”

I don’t have any inside information about this device, but let me guess how it can detect a person at a distance. The body would perturb a surrounding electric field because it is mostly saltwater, and therefore a conductor. In Section 9.10 of Intermediate Physics for Medicine and Biology, Russ and I explain how a conductor interacts with applied electric fields. For the case of a dc field, the conducting tissue completely shields the interior of the body from the field. To understand how a body could affect an electric field, try solving the following new homework problem
Section 9.10

Problem 34 ½ Consider how a spherical conductor, of radius a, perturbs an otherwise uniform electric field, Eo. The conductor is at a uniform potential, which we take as zero. As in Problem 34, assume that the electric potential V outside the conductor is V = A cosθ/r2Eo r cosθ.
(a) Use the boundary condition that the potential is continuous at r=a to determine the constant A.
(b) In the direction θ=0, determine the upward component of the electric field, - dV/dr.
(c) The perturbation of the electric field by the conductor is the difference between the fields with and without the conductor present. Calculate this difference. How does it depend on r?
(d) Suppose you measure the voltage in two locations separated by 10 cm, and that your detector can reliably detect voltage differences of 1 mV. How far from the center of a 1 m radius conductor can you be (assuming θ=0) and still detect the perturbation caused by the conductor?
You may be wondering why there is a 100 V/m electric field at the earth’s surface. The Feynman Lectures (Volume 2, Chapter 9) has a nice discussion about electricity in the atmosphere. The reason that this electric field exists is complicated, and has to do with 1) charging of the earth by lightning, and 2) charge separation in falling raindrops.

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