Processes and devices for continuously monitoring levels of analyte

In a process and devices or continuously measuring the concentration of an analyte in tissue. The tissue is provided with a hollow fiber having a pore size between the size of the analyte and the size of macromolecules, hollow fiber is perfused with a solution compatible with the tissue, and that solution is supplied from the hollow fiber through a reactor for the analyte. The reactor has a second hollow fiber having a pore size between the size of the analyte and the size of macromolecules, and the second hollow fiber is located in a container which contains macromolecular reactants for detecting the analyte and a signal producing system coupled therewith. The perfusion fluid is fed from the reactor to an eliminator for the analyte, whereby at least 95% of the remaining analyte is removed from the perfusion fluid and the perfusion fluid is reintroduced into said first hollow fiber. The process and devices operate as a closed circuit and therefore this method can be continued over a longer period of time than known methods, without the need to replace reservoirs.