Structural and mechanical characteristics of collagen tissue coated with chitosan in a liquid CO2/water system at different pressures.

Chitosan coatings of biological heart-valve prostheses enhance their biocompatibility, resistance to pathogenic microflora and lifetime. Collagen tissues can be coated with chitosan in aqueous solution acidified, to make chitosan soluble, with H2CO3 formed from a coexisting liquid CO2 phase under pressure. The advantage of H2CO3 is that it can be easily removed after the coating procedure. This study assessed the effects of 6-50 MPa CO2 pressure during the coating procedure on the structure and mechanical properties of the resulting biocomposite matrices. The dependence of chitosan adsorption on CO2 pressure was bell-shaped, reaching a maximum adsorption of 0.8 mass % at 40 MPa. Tissue surface became highly porous upon pressure treatment. At 50 MPa, the pores merged to form furrows with lengths of several hundred micrometers, accompanied by collagen fibril reorganisation. Chitosan coating did not affect tissue tensile strength in the axial direction, but increased it by 75% in the radial direction in the tissue coated at 50 MPa pressure. Strain at break, a measure of elasticity, increased in both directions by up to 100% upon coating with chitosan. CO2 pressure of 30-50 MPa seems thus optimal in terms of chitosan incorporation and tissue mechanical properties.