| Scaffolds are one of the key elements in tissue engineering, and preparation methods of scaffolds are attracting much attention in the research field. A novel process based upon supercritical carbon dioxide (SC-CO2) foaming makes it possible to fabricate porous polymer scaffolds without using any organic solvents or high temperature, which presents the potential applications in tissue engineering. The process lays great importance in the solubilization of SC-CO2into the polymer matrix, and then the phase separation induced by a thermodynamic instability under pressure quenching generates nuclei, which grow into pore structures via mass transfer. In this thesis, porous poly (lactic acid-co-glycolic acid)(PLGA) scaffolds with controllable pore size and interconnectivity were fabricated and characterized using SC-CO2foaming and particle leaching technology.Firstly, PLGA macro-cellular and micro-cellular foams were fabricated using SC-CO2foaming method. Some key factors, i.e. polymer chemical composition (the mole percentage of glycolic acid in the polymer was15and50respectively), molecular weight (50kDa and200kDa) and the processing parameters including foaming pressure (from10MPa to25MPa), temperature (from35℃to85℃) and depressurization rate (0.05MPa/s and0.20MPa/s) on the scaffold structure were investigated in detail. The results showed that higher molecular weight tended to more uniform pores, the pore size and interconnectivity increased with the increase of lactic acid content in the polymer. Higher pressure led to higher cell density and smaller pores, and pore coalescence occurred while depressurizing at a slow rate. Glass transition temperature (Tg) of the polymer decreased under SC-CO2, and large polygonal cells were formed when the foaming temperature was set below45℃. Differently, higher foaming temperature resulted in globular cells, with cell size decreased sharply. However, further increasing the temperature led to only slightly larger pore sizes. It is possible to fabricate scaffolds with pore sizes range from5to500micro-meters by changing the parameters of foaming process. Furthermore, porous PLGA scaffolds with multi-scale open pore structure were fabricated using SC-CO2foaming combined with particle leaching technology. Effect of NaCl particle size and amount on the pore size and interconnectivity of scaffolds were investigated. The experimental results demonstrated that the SC-CO2foaming/particle leaching technology allowed for the design and fabrication of bi-modal porous PLGA scaffolds and the architecture of foamed PLGA scaffolds can be advantageously manipulated through the incorporation of NaCl pore-forming agent. In particular, the scaffolds showed100-250μm macro-pores by leaching out the raw NaCl particles from PLGA, coupled with5-10μm micro-pores, which were obtained by SC-CO2foaming process of PLGA. It was found that the total porosity and interconnectivity of porous polymer scaffolds can be greatly modified by adding porogen particles. Generally, increasing quantities of NaCl particles extended the interconnectivity of the scaffold pore structure, and the interconnectivity could achieve as high as80-90%by SC-CO2foaming/particle leaching technology.Finally, the degradation behavior and biocompatibility was carried out on the porous PLGA scaffolds. It was shown that the morphology and weight of the scaffold can be maintained over a long time (approximately12weeks when the composition of lactic acid and glycolic acid is85:15) at the initial stage of the degradation, and the degradation rate of PLGA copolymer can be changed by adjusting the content of lactic acid and glycolic acid. In vitro cell culture was performed using MC3T3-E1subclone and MTT assay was taken to assess the cell toxicity of the scaffolds. The biological characterization indicated that the PLGA scaffolds can be potentially used in tissue engineering without cell toxicity. |
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