Saturation of water with CO2 at high pressure results in formation of an acidic medium (carbonic acid) with pH value of about 3 (at pressures up to 30 MPa and room temperature). Such pressures may be routinely generated with typical sc CO2-extraction equipment commonly used nowadays in practice of biomedical laboratories in research centres and hospitals. Carbonic acid is capable to dissolve chitosan materials with different DAs (16–33%, at least) up to concentrations of a few mg per ml. GA-stabilised bovine pericardium post-treated in carbonic acid (with or without dissolved chitosan) demonstrates a set of improved properties, which are highly relevant to the usage of this collagen tissue as a material for biological prosthetic heart valves.