a low temperature method of bonding two polymer sheets without adhesive , at least one of said polymer sheets comprising a microstructure 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 .