Subcutaneous glucose monitoring by means of electrochemical sensors: fiction or reality?

Amperometric glucose oxidase/hydrogen peroxide sensors were inserted subcutaneously into the neck of normal and diabetic dogs (n = 10), to elucidate the conditions for stable long-term functioning. Their output current was observed in parallel with measurements of plasma glucose concentrations and their function was checked by means of induced alterations in glycaemia. After between 14 and 96 h the experiments were terminated due to losses in the apparent sensitivity of implanted sensors and/or increasing oscillations following stable measurements. This was accompanied by an inflammatory reaction which was analysed on the basis of the clinical picture and histology. In most cases there was a bacterial ingrowth from the normal skin flora of dogs. The inflammatory exsudate contained only 23 +/- 17% of the simultaneous steady state plasma glucose concentration, which was significantly different from the glucose level in the fluid obtained from non-irritate subcutaneous tissue (95 +/- 12%, separate set of experiments). The in vitro calibration of sensors exhibited essentially comparable sensitivities before and after the in vivo application. No differences in reported findings related to the biomaterials used (polyurethane versus cellulose acetate), the presence of diabetes, the history of individual electrodes and the effective duration of a given experiment were discernible. We conclude that the functional bioinstability of subcutaneous glucose sensors is largely due to the inflammatory tissue reaction which alters the effective glucose concentration within the measuring compartment of the electrodes; these drawbacks may be overcome by further miniaturization including implantable telemetric devices allowing the closure of the skin.