Scaffolds as one of the most important elements in tissue engineering are attracting much attention in last decades. Bi-modal pore structure, appropriate pore diameter, high porosity and fine mechanical strength are essential for tissue engineering scaffolds. For example, macro-pores of 200-400 µm is necessary to facilitate bone cell attachment, and micro-pores of less than 10 µm could promote the delivery of bio-factors. The combination of macro- and micro-pores renders a scaffold suitable for cell proliferation and migration. Scaffolds fabricated by an effective method should meet above requirements.
In this paper, supercritical CO2 (scCO2) foaming was introduced to fabricate tissue engineering scaffolds, which has the advantages of avoiding the use of organic solvents and high temperature. Submicron hydroxyapatite (HA) particles were used as nucleation agent, and bi-modal porous PLGA scaffolds were successfully obtained. The effects of foaming temperature and pressure, depressurization rate, and HA amount on the pore structure of scaffolds were studied in detail. In general, foaming temperature and pressure had an effect on the solubility and diffusion of CO2 in PLGA to regulate the formation of pores, and scaffolds with larger interconnected pores could be observed at a lower depressurization rate. With the increase of HA amount from 5% to 20%, heterogeneous nucleation in scCO2 foaming process was enhanced to form smaller pores. The average size of macro-pores ranged from 995±226 µm to 148±39 µm, and micro-pores from 20±5 µm to 104±66 µm. Specifically, all of macro-pores and micro-pores of fabricated scaffolds in this work were interconnected, and the porosity of scaffold was from 75.61% to 96.55±0.11%. Furthermore, the compression modulus of scaffolds could be tailored from 2.67±0.37 MPa to 25.07±8.02 MPa by adjusting operating conditions to be applied in soft or hard tissues.
Supercritical CO2 foaming, in which scCO2 has an ability to extract organic solvent and load thermo-sensitive drugs or proteins, is a green technique to process polymer and prepare bioactive scaffolds in one step. This novel fabrication method is expected to be promising in tissue engineering.