ENZYME ELECTRODES FOR IN VIVO APPLICATION; KINETIC PROPERTIES; STERILIZATION, AND GEOMETRY

Abstract

Using glucose oxidase/hydrogen peroxide electrodes, covered with Cuprophane and sealed with cellulose acetate or polyurethane, there are three major difficulties in practical application of subcutaneous implantable glucose electrodes. These are the lack of knowledge concerning kinetic properties between different sensor preparations and their influence on the response characteristics in vivo, the suitable sterilization procedure, and the sensor geometry. Response times of the sensors in vitro were between 1 and 5 min (testgeie 3 of according to nonlinear regression analysis, NLRA), in dependence on qualities and thickness of the covering layer. The time constants T resulting from in vivo measurings subjected to NLRA at increases and decreases of glucose were 28+8 and 15+2 min, in blood, 26.5+5 and 18+2 min in plasma, and 53+10 and 2644 min in subcutaneous tissue. As a practicable methode to sterilize enzyme sensors, gamma irradation in the presence of hydrogen peroxide has been used at doses of 0.6 kGy + 0.1 % H202 for safe killing Pseudomonas aeruginosa. Increasing H202- concentration (at about 1 %), however, reduces the sensitivity by influencing the enzyme activity. Overcoming problems at sensors implantation site caused by the size of the sensor (d = 2 mm, 1 = 20 mm) most probably, a miniaturized dualelectrode was constructed, where the working electrode (Pt-anode, d < 0.5 mm) has been prepared as an enzyme electrode, only. This electrode arrangement has shown excellent electrochemical characteristics as well as in the pOz - and in the H202-polarogran.