Calibration of the viscometric glucose sensor before its use in physiological liquids--compensation for the colloid-osmotic effect.

The function of the recently described viscometric affinity sensor (VAS), which measures glucose due to its strong effect on the viscosity of a sensitive liquid containing Concanavalin A (ConA) and dextran, was analysed for osmotic and colloid-osmotic effects on the glucose reading. The suction of low- and high-molecular weight osmotica on the membrane of the microdialysis fibre was measured using a membrane osmometer built from the microdialysis probe of the VAS. The reduction of the sensor read-out in blood plasma can be completely explained by a change in small osmotic volume fluxes through the dialysis membrane, which affect the ConA concentration and the viscosity after the flow of the sensitive liquid through the dialysis probe. The measuring error could be prevented by the presence of the polyethylene glycol 6000 at an isotonic concentration in the glucose standard solutions used for sensor calibration.

Recording of subcutaneous glucose dynamics by a viscometric affinity sensor.

AIMS/HYPOTHESIS: To provide a nonenzymatic sensor for glucose monitoring in subcutaneous tissue. METHODS: A continuously working affinity sensor based on the glucose-dependent viscosity of a sensitive liquid containing dextran and concanavalin A has been designed by arranging a microdialysis probe, two flow-resisting capillaries and two pressure transducers in a linear flow system. It allows synchronous processing of the viscosity of the sensitive liquid at the standard glucose concentration and the glucose concentration to be measured. In preliminary human trials the sensor was implanted into the subcutaneous tissue of the forearm and its read-out was compared with capillary blood concentrations. RESULTS: In vitro, the viscometric sensor shows a linear and long-term stable dependence on the glucose concentration without detectable drift. At the applied flow rate of the sensitive liquid (about 5 microliters/h) the technical delay is 5 to 10 min. The read-out of the implanted sensor followed the dynamics of the capillary blood glucose concentrations with a time-shift of 10 to 15 min but showed a systematic error when based on precalibration with polymer-free glucose solutions. After appropriate in vivo calibration, the read-out was in good or acceptable coincidence with capillary blood concentrations according to the error grid method and did not show any detectable reduction of sensitivity during the periods of measurement (up to 44 h). CONCLUSION/INTERPRETION: The viscometric-affinity sensor is an efficient tool for current research on glucose monitoring in the subcutaneous tissue and can potentially be further developed for routine clinical use.