Disclosed herein is a shelf-stable, two-part formula for making an antimicrobial biphasic polymer. Some variations provide a two-part formula for fabricating a biphasic polymer, wherein the two-part formula consists essentially of (A) a first liquid volume, wherein the first liquid volume comprises: a structural phase containing a solid structural polymer; a transport phase containing a solid transport polymer; a chain extender; a curing catalyst; a first solvent; and (B) a second liquid volume that is volumetrically isolated from the first liquid volume, wherein the second liquid volume comprises: a crosslinker that is capable of crosslinking the solid structural polymer with the solid transport polymer; and a second solvent. An antimicrobial agent (e.g., quaternary ammoniums salts) may be contained in the first liquid volume or in the second liquid volume. Methods of making and using the antimicrobial biphasic polymer are described.