A microfluidic device comprising one, two, three or more enclosed microchannel structures, each of which comprises a) an inlet arrangement with an inlet opening for liquid, b) a reaction microcavity (RM.sub.1) downstream to the inlet opening, and c) an upstream section that is located between the inlet opening and the reaction microcavity (RM.sub.1) and is defined between two planar substrates made of plastic material which substrates are common for said microchannel structures. The characteristic feature is that the inner wall in said upstream section exposes a hydrophilic surface area with an underlying base layer comprising metal oxide. In one aspect the microfluidic device as such is covered.