A low temperature method of bonding two polymer sheets (2, 3) without adhesive, at least one of said polymer sheets comprising a microstructure (1) or a network of microstructures, comprises the steps of treating at least a portion of one surface of one of said polymer sheets by using a cold plasma or a laser beam so as to physically activate said portion at low temperature, placing the two polymer sheets in contact, with the activated portion of said one sheet in contact with the other sheet, and subjecting said sheets to pressure and to a temperature below the melting and/or glass transition temperature of either of said polymer sheets, thereby bonding said sheets and forming a sealed micro-structure and/or network of micro-structures. The method is used to fabricate a micro-analytical device for use in biological and/or chemical applications.