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The surplus of enzyme activity is a main prerequisite for the proper long-term function of enzymatic biosensors based on a diffusion-controlled process at any time. Long-term functional stability in vitro could be reached with sensor preparations using human serum albumin (HSA, Rhodalbumin) and glutaraldehyde (GDA, 25%,) as a mixture with glucose oxidase (GOD, EC 188.8.131.52., Aspergillus niger, 300 IU/mg) covered by polyurethane (PUR, Tecoflex EG 80 A) as a membrane with well-defined diffusion qualities. A very rapidly decreasing sensitivity has been observed after sensor implantation. As a reason for this, a reversible enzyme inhibition has been hypothesized, underlined by a slow restoration of the sensitivity up to the original one over a period of 5 days after sensor explantation. The same immobilization procedure on the surface of electrochemical sensors has been used very successful in the case of lactate oxidase (LOD, Pedicoccus species, 35 IU/mg). Dependent on the covering membrane lactate measurements in the range of 0.05 up to 50 mM lactate, concentration in milk and products of that can be realized. Further research has been pointed at the development of such immobilization methods which guarantee sufficient enzyme stability at in vivo conditions, too.