Supercritical CO2:

To minimize the problem of host immune rejection, residual lipids, free proteins, cells, and nucleic acids need to be removed as completely as possible. Supercritical fluids (SCF) will be applied to remove these materials. SCF technology has been around for over a century. They are fluids over their critical temperature and pressure. They exhibit good solvent power. The wide application of SCF is related to their “tunable” properties. This means that their properties can be changed easily by altering pressure and temperature. They exhibit both gas-like transfer properties and liquid-like solvent properties. Like gases, SCF have little surface tension and has diffusion coefficient (transfer resistance) 10 to 100 times that of liquids. Due to their high diffusivity and low viscosity, they can penetrate extremely tiny crevices in a porous material to dissolve organic compounds.

Carbon dioxide (CO2) is a very attractive supercritical fluid due to the fact that it is cheap, abundant, nonflammable, nontoxic, and recyclable. The critical temperature of CO2 is 31.265°C, and the critical pressure is 72.9atm. Proteins will not be denatured at this critical temperature. Therefore, supercritical CO2 (ScCO2) technology can be used to completely remove cells, lipids, and free proteins from the original source materials while retain the structural integrity of the collagen scaffold. This process has little effects on the mechanical strength and natural porosity of the material.