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Continuous monitoring with implantable sensors using an open microflow approach

Research has focussed on needle type enzyme electrodes utilizing immobilized oxidase enzymes with detection of hydrogen peroxide. In recent years, the basic structure has been optimized to achieve sensor responses that are independent of sample variables such as viscosity and stirring. Enhanced stability in blood has been achieved by controlling the properties both of external and internal barrier membranes of the device, using, respectively, polyurethane and a sulphonated polyether ether sulphone; such inner membrane protection appears to be due to rejection of electroinactive passivating species. Some form of inner membrane appears vital for stability quite beyond any selectivity criteria.

Irrespective of the membrane materials used, some loss of sensitivity in blood or tissue is inevitable and in tissue response does not correspond to blood values. Accordingly, an Open Microflow system has been devised, whereby an ultrafine flow of solution over the sensor tip maintains separation of the sample and sensor while allowing a substrate (eg glucose) to diffuse to the detector surface. During in vivo use, the procedure allows reliable continuous determination of body glucose levels following a short 30 min to 60 min stabilization period with no apparent requirement for in vivo calibration or evident drift (explantation response within 3% of original). The latter feature is of particular relevance to the ease of use of a device, giving clinical reliability, during least short term monitoring. Open microflow in tissue implanted devices does not require the use of a pump, the fluid requirement is minimal and the external cannula for channelling the fluid flow is of the diameter of standard needles used for venepuncture. In tissue, the novel fluid interface helps to minimize the tissue inflammatory response and to maintain tissue hydration for good local solute mass transfer characteristics. Of possible relevance is the use of a stainless steel pseudo reference electrode to avoid the local toxic effects due to silver/silver chloride.

Reported by

University of Manchester
Hope Hospital Eccles Old Road
M6 8HD Salford
United Kingdom
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