Abstract: Apparatus comprising a micro-rheometer (1) with a microchannel (2) and a sensor array arranged along the microchannel to measure rheological properties of a fluid. The sensor array comprises a plurality of pairs of electrodes (8, 8?), each pair being placed face to face to function as an electronic switch when the fluid flows through them. It further comprises a data acquisition system (10) with an electronic circuit in which each pair of electrodes is connected to an amplifier electronic circuit (11) to ensure an ultra-low electrical current flow through the short-circuit created by the fluid and the pair of electrodes, to avoid damaging the fluid. The invention may be used as a small portable device for medical diagnosis in diseases associated to changes in blood viscosity, operating in a wide range of shear rates.