The development and application of glucose electrodes based on "wired" glucose oxidase
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The sensitivity of rapidly responding “wired” glucose sensing microelectrodes with a tailored outer membrane was examined in vivo. It is shown that these microsensors maintained in vivo sensitivities similar to those in buffer before the implantation and after the explantation when implanted intravenously or intraperitoneally for several hours in the rat. Maintenance of the sensitivity in vivo provides the basis for pre-calibrating the microsensors in vitro, lifting the requirement of in vivo calibration. The in vitro calibration results can be used by the patients as a reference in judging the reliability of sensor readings. The causes of much more rapid decaying current in the “wired” glucose oxidase-based microelectrodes in serum compared to their sensitivity in buffer were studied. Urate and transition metal ions were identified as two main causes for the loss in current. Urate is electrooxidized to dimeric or trimeric products, which precipitate in the electrocatalytic film, reducing the mobility of the redox sites of the polymer. Transition metal ions coordinatively crosslink pyridine or imidazole functions of the redox polymers and also inhibit glucose oxidase. Implantable “wired” glucose microsensors with novel polyionic membrane for mass transport-controlling were developed and tested in the jugular veins and in the intrascapular subcutaneous region of non-diabetic SpragueDawley rats. The micromembranes were assembled by sequentially chemisorbing polyanions and polycations on miniature enzyme electrodes. The sequential chemisorption process allowed the simultaneous tailoring of their sensitivity, dynamic range, drift and selectivity. The membranes also retained transition metal ions that bound to and damaged the redox polymer “wiring” the enzyme. All of the in vivo data points measured by the microsensors were clinically accurate after correcting the lag time of the glycemia in the subcutaneous fluid behind that of blood withdrawn from the vein. The “wired” glucose enzyme microelectrode was also used as the microanode in developing a compartmentless miniature biofuel cell intended to be an implanted mini power source for medical microdevice. The glucose-oxidizing anode was combined with a high current output “wired” laccase microcathode, which catalyzes the electro-reduction of O2 to water. The areas of the anode and the cathode of the cell are much smaller than any previously reported fuel cell. The power density of the cell exceeds by a factor of five that of the highest power density of earlier reported glucose-air biofuel cells. The effects of different buffers, oxygen concentration, pH and chloride on cell current were investigated.