In-vivo continuous measurement of interstitial pH for intensive care application

An optical sensor for the continuous detection of pH in the interstitial fluid was developed. The pH sensing layer is immobilised on the internal wall of a glass capillary which is in series with the microdialysis catheter. Phenol red is the pH indicator and is covalently bound directly to the glass surface by means of the Mannich reaction. An optoelectronic unit, which makes use of a light emitting diode at 590 nm as source and a photodiode as detector, is used for the interrogation of the glass capillary. Optical fibres (core diameter: 200 ?m) are used to couple the sensing capillary with the unit. Effect of the ionic strength was studied and the performance of the sensor in contact with dialysed blood was carefully investigated. Particular attention was given also to the pH recovery rate, which is given by the ratio between the hydrogen ion concentration in the perfusate and the hydrogen ion concentration in the analysed medium. The pH sensor works in the range 6-8 pH units and it is characterised by an accuracy of 0.07 pH units and a response time of the order of the minute. Long term stability was checked and the sensor is perfectly working for a period of three days, when exposed to dialysed blood. In vivo tests on animals and on volunteers are described.