Microporous membranes are smooth, sponge-like films containing a narrow distribution of pores with a size between 0.1 and 0.5 microns. When supported on a laminate, this thin layer can provide a true sterile barrier with 99.999% bacterial retentivity. A new polymerization process using ultraviolet light or an electron beam has achieved high production speeds and chemical versatility in a sterilized product. The process itself uses nontoxic, nonexplosive chemicals and is closed with virtually no waste products. -- AATA