sensors, one implanted intravascularly in the jugular vein and one implanted subcutaneously in the rat’s back, with the measured glucose concentration in withdrawn blood samples, following an intravenous insulin injection. The prediction of a model translating the subcutaneously measured glucose concentration to the blood glucose concentration is also shown.

Separationless immunoassays in whole blood present a number of challenges. Hydrogen peroxide, the substrate of the commonly used immunolabeling peroxidases, is eliminated by catalase and catalase-like blood constituents, and proteins may quickly foul the sensors. Additionally, mass transport both to and within the sensors defines the time required for separationless immunoassays. The problems are alleviated in a redox hydrogel-based separationless immunoassay. The redox polymer used is a copolymer of acrylamide and N-vinylimidazole complexed with osmium-4,4′-dimethyl-2,2′-bipyridine and treated with hydrazine to provide hydrazide functions for cross-linking (PAA-PVIOs-Hz, Figure 3). The redox polymer is then mixed with avidin, choline oxidase, and an amine-reactive cross-linker, 400 MW poly (ethylene glycol) diglycidyl ether, and deposited on a 3 mm diameter glassy carbon electrode. As the mixture dries, the reactive groups on the cross-linker react with terminal amines on the polymer and proteins to form a rugged adhering film on the electrode. The avidin of the film provides a means for attaching a biotin-labeled antibody. Although avidin has amine-containing lysine residues in its biotin-binding regions, these are not accessible to most cross-linking agents [16]. Thus, the cross-linking reaction has little effect on the biotinbinding activity of avidin. Choline oxidase is an 80 kDa enzyme with covalently bound FAD [17]. The redox-active centers of choline oxidase, unlike those of most other redox enzymes, are buried deeply in the interior of the protein shell and thus are not affected by cross-linking with the hydrogel [18].