Comparative assessment of chemical and gamma-irradiation procedures for implantable glucose enzyme electrodes.

Chemical and gamma-irradiation sterilisation were examined in this study for implantable needle electrodes. Exposure to isopropyl alcohol (IPA) led to response elevation, but time-dependent exposures up to 30 min variously to chlorhexidine, H2O2, HCl, HCl/IPA, and alcoholic iodine/potassium iodide, all caused substantial time-dependent response degradation. Sterility was not assessed for such electrodes. High dose (30 KGy) gamma-irradiation also compromised sensor response, with exposed electrodes exhibiting approximately 30% reduction in response, comparable in magnitude to 10 min exposure to chemical sterilisation. However, a lower dose (25 KGy) gamma-irradiation was well tolerated and subsequent microbiological testing (following contamination with Streptococcus epidermidis and Staphylococus aureus) proved device sterility with no culture growth following 7 days incubation at 37 degrees C in brain heart infusion medium.

Stabilized needle electrode system for in vivo glucose monitoring based on open flow microperfusion.

Preliminary in vitro studies and in vivo performance of amperometric glucose needle enzyme electrodes incorporating an open microflow technique, in which the sensor surface in subjected to a flow of fluid, are reported. Initially using a slow flow (60 microliters h-1) of isotonic phosphate buffer over the enzyme electrode tip, an interface was created which reduced cellular/protein fouling for electrode measurements in whole blood. Here a minor reduction in electrode response (apparent only at high glucose concentration) occurred which was not cumulative and therefore not associated with fouling. The protection afforded by the moving aqueous film was independent of fluid composition; the use of isotonic/hypertonic buffer, addition of anticoagulant (1% m/v heparin) or enhanced fluid viscosity (addition of 1% v/v glycerol) did not affect the system. Implantation of the electrode and its microflow cannula into subcutaneous tissue in rats was associated with a decreased buffer flow (30 microliters h(-1)) and had the effect of (i) reducing electrode stabilization (30 min), (ii) accelerating 'pick up' of tissue glucose changes after intravenous glucose (1-2 min lag) or insulin (3-7 min lag) and (iii) achieving a correlation between tissue and blood glucose values under dynamic conditions (r2 = 0.98, y = 0.99x + 0.23). Reassessment of the electrode response in vitro, following a 4 h monitoring period, provided a sensor response within 3% of the original electrode sensitivity, indicating little or no surface fouling and avoiding the requirement for repeated in vivo calibrations at least over the initial implantation period.

Open flow microperfusion: approach to in vivo glucose monitoring.

A slow flow of liquid over the working tip of a classical glucose needle electrode allows a significant reduction in surface fouling with minimal sample dilution. The fluid flow technique enables relatively drift-free in vivo operation for up to four hours (electrode responses in vitro following explanation of the device after a 4 h monitoring period are within +/- 5% of original values), exhibits a close correlation with blood glucose levels and is unique in requiring no in vivo calibration.