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Abstract

A closed tonometer for subcutaneous insertion is disclosed having a small closed equilibrating bath. Three noble metal electrodes, preferably platinum (Pt), communicate through electrical conductors to the bath at spacings exceeding 0.2 mm. Each noble metal electrode is typically exposed to the bath by embedding the noble metal in a dielectric and cutting both dielectric and electrode so that the exposed electrode is flush with and a part of the closed bath wall. The bath is contained within an oxygen permeable membrane -- preferably Silastic.RTM. a registered trademark of Dow Corning Corporation of Midland, Michigan. The bath contains a sodium chloride (NaCl) solution, preferably 0.9%, and small amounts of a hydroquinone such as a tenth of a gram of pure ascorbic acid per 30 ml of NaCl solution. One of the noble metal electrodes becomes a quinhydrone reference electrode preserving a precise voltage differential with respect to the electrolyte bath. This voltage of the quinhydrone reference electrode is passed through a voltage follower amplifier to a voltage adder where it is added to a reference bias voltage. Output from the adder passes to a second electrode serving as an anode. The anode supplies current to the enclosed bath sufficient to maintain the bath at constant voltage with respect to the cathode. The current at the cathode is thus solely a function of oxygen tension equilibrated from the measured tissue. There results a tonometer of miniature dimension having true analytical output.

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First Claim