A method of impregnating a polymeric substrate with an antimicrobial substance or precursor thereto, in which said substance is impregnated into said substrate as a solution, an emulsion or a suspension in a supercritical fluid. Additionally, there is provided a method of impregnating a substantially transparent polymeric substrate with an antimicrobial substance or precursor thereto, wherein the polymeric substrate is capable of being swelled by a swelling agent which contains dissolved, suspended or emulsified therein said antimicrobial substance or precursor thereto, so as to permit impregnation of the polymeric substrate with the antimicrobial substance or precursor thereto. There is also provided a device obtained by such methods.