Medtronic Diabetes has developed an electrochemical sensor for long-term implantation in the superior vena cava called the Vascular Glucose Monitoring System (http://www. medtronicdiabetes.com). The distal tip of the flexible vascular catheter contains an oxygen
electrode that is covered with the enzyme glucose oxidase. A second oxygen electrode is used to measure the partial pressure of oxygen in central venous blood. The concentration of glucose is calculated by comparing the oxygen concentration adjacent to the enzyme (change in local oxygen concentration due to oxidation of glucose) to the oxygen concentration in blood. Porous membranes protect the enzyme from fouling and ensure an adequate supply of oxygen.
The catheter is attached to an electronics module that is implanted within the subcutaneous tissue below the clavicle, similar to a pacemaker. The implanted module contains the power source, microprocessor and transceiver for receiving calibration data from an external reference glucose meter, and transmitting glucose data to an external display (Fig. 6). Excellent accuracy and sensor stability have been demonstrated in animal and human trials. Unfortunately, the harsh chemical environment of the bloodstream has caused premature sensor failure from membrane fouling and enzyme degradation, limiting long-term clinical application (38,39,40).
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