(395f) Bi-Modal Porous Poly(ε-caprolactone) Scaffolds Fabricated Via Two-Step Depressurization Supercritical CO2 Foaming

Poly(ε-caprolactone) (PCL) is widely used as scaffold in bone tissue engineering due to its lasting degradation rate that matches the rate of natural bone growth. To mimic the extracellular matrix environment of natural bone tissue, emphasis has been laid on the pore morphology and structure of PCL scaffolds, and it demonstrates that a macroporosity of a few hundred microns is beneficial for cell adhesion, proliferation and differentiation; while a microporosity of tens of microns is necessary for nutrients and waste transportation. Therefore, formation of a bi-modal porosity is of particular interest in the fabrication of novel PCL scaffolds. Supercritical carbon dioxide foaming (sc-CO₂ foaming) is an alternative as solvent-free green process for scaffold fabrication. In this paper, a novel two-step depressurization sc-CO₂ foaming was designed, in which two different depressurization rates were utilized in one single foaming process to induce pores formation at different scales and attain bimodal porous scaffolds. Effects of important processing parameters, such as foaming temperature, saturation pressure, intermediate pressure, saturation time, holding time and depressurization rate etc., on pore morphology and structure of PCL scaffolds were investigated in details. Results indicated that intermediate pressure had dominant impact on the final pore morphology. Decreasing the intermediate pressure from 12 MPa to 5 MPa, the macropore size would increase from 130 μm to 404 μm. Interestingly, when intermediate pressure reached 9 MPa, only monomodal porosity was observed. Studies implied that nucleation size and macropore density was mainly determined in the first depressurization step by intermediate pressure and pore growth was more affected by the second depressurization. In summary, two-step depressurization sc-CO₂ foaming is verified available to produce bimodal porous PCL scaffolds with suitable pore structure and fine porosity. The plasticizing effect of sc-CO₂ during foaming causes the reduction of PCL polymer glass transition temperature (T_g) and melting point (T_m), which avails processing polymers at a relatively low operating temperature, making it potential in protein growth factor incorporation for cells growth.