In this paper a device, based on urease-loaded microspheres, is presented. The first task of this work was the optimization of a procedure for the alginate microspheres realization, having a radius as close as possible to the optimal one necessary to achieve the maximum enzyme exploitation. This optimal radius was calculated theoretically through a mathematical model which describes the concentration of substrate (urea) inside the microspheres on the assumption of a diffusion-reaction mechanism. The enzyme-loaded microspheres were successfully tested in a prototypal device aimed at the depletion of urea from a circulating fluid simulating blood flow: the results showed that urea concentration in the circulating fluid drops down to less than 25% of the initial value after 5 h.