A microfluidic device comprising one, two or more microchannel structures (101a-h), each of which comprises a reaction microcavity (104a-h) intended for retaining a solid phase material in the form of a wet porous bed. Each of said one, two or more microchannel structures comprises the solid phase material in a dry state together with a bed-preserving agent comprising one or more compounds having bed-preserving activity.