BARE electrodes for measuring tissue oxygen tension polarographically have several disadvantages. The electrode current depends not only on the oxygen tension at the cathode surface but also on the diffusion constant of oxygen and the blood flow pattern in the tissue in that region. In addition, the electrode current slowly falls when the bare cathode is exposed to tissue or blood. Clark1 overcame this latter difficulty by using electrodes covered with a membrane for measurements in blood. The remaining difficulties in tissue can be overcome by using a membrane-covered electrode together with a pulsed voltage2. In this way the region around the cathode surface which is depleted of oxygen, due to the oxygen reduction process, is limited to the membrane and the thin saline layer behind it and does not extend into the tissue, where there is an unknown diffusion constant and blood flow pattern. The membrane-covered electrode, in contrast with the bare electrode, may be calibrated in vitro before use and the calibration will be valid for conditions in vivo. A number of membrane materials have been tried, for example collodion, rubber, polyethylene, polytetrafluoroethylene and ‘Cellophane’. In the past we have found the most suitable membranes to be latex rubber sheaths. A disadvantage is that the electrodes must be mounted in the sheaths shortly before use, which can be a very lengthy operation. Furthermore, the rubber can move relative to the cathode and cause a change in calibration. Bartels et al.3,4 reported that electrodes for use in blood samples can be covered with polystyrene and used with a continuously applied voltage. We have found that if our electrodes are coated with polystyrene and are used with a pulsed voltage, the more exacting requirements for measurements in tissue are met, and the disadvantages associated with rubber membranes are avoided. Such polystyrene-covered electrodes are independent of stirring effects outside the membrane, have a time response of a few seconds, are not poisoned by tissue and are easy to prepare. Further advantages are that there is no need for a membrane holder in the electrode assembly and the electrode tip can be made small and streamlined. Electrodes with an overall diameter of only 0.3 mm have been made and used satisfactorily in tissue.
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About this article
Biomedical Engineering (1968)