Effect Of Nucleating Agent On Mass Transfer And Morphology Of Porous Scaffold Prepared By Supercritical CO₂ Foaming

Scaffold material is one of the three elements of tissue engineering.Tissue engineering has very demanding requirements for scaffold materials.It not only needs to have good biocompatibility,degradability and suitable mechanical properties,but also has to meet certain physical properties,such as suitable pore size and porosity.Compared to the typical techniques,the supercritical CO₂foaming process for preparing bone tissue engineering scaffolds can effectively avoid the disadvantages of the traditional preparation methods such as the need for a large number of organic solvents and harsh process conditions.In this thesis,three different sizes of superparamagnetic iron oxide nanoparticles（SPIONs）and SPIONs with different polymer surface modification were introduced in the poly（lactate-glycolic acid）（PLGA）matrix as nucleating agents,for the preparation of magnetic composite scaffolds with multi-mode pore size structure by supercritical CO₂foaming method.The effects of SPIONs nucleating agent on mass transfer and morphology during supercritical CO₂foaming were discussed in detail.The specific contents of this study were as follows:Firstly,SPIONs with different sizes and polymer modifications were prepared to be used as nucleating agents for supercritical CO₂foaming for subsequent studies.Specifically,three different sizes of citric acid（CA）modified SPIONs were prepared by co-precipitation method.According to the difference of their CO₂-philicity,polyvinyl acetate（PVAc）and polycaprolactone（PCL）were selected to replace citric acid（CA）to prepare PVAc and PCL modified SPIONs,respectively.The modified SPIONs with different PVAc contents were also prepared by changing the addition amount of polymer.The synthesis of target SPION was proved by the characterization including infrared spectroscopy,thermogravimetric analysis,and dynamic light scattering.The results showed that the synthesized SPIONs had uniform size and the relatively low PDI values indicated that SPIONs were evenly dispersed in water without obvious agglomeration.Then,different sizes of CA coated SPIONs were introduced as nucleating agents to strengthen the role of heterogeneous nucleation in the supercritical CO₂foaming process.The multi-modal porous scaffolds which could provide magnetic effects for the scaffolds were successfully prepared.The influence of the addition amount of SPIONs,the foaming temperature,the pressure and the depressurization rate on the structure of the prepared scaffold was investigated and discussed in detail.The results indicated that the scaffolds with different sizes of SPIONs as nucleating agents had good porosity（58.68%～94.97%）and compressive strength（0.54 MPa～11.40 MPa）.The results indicated that the average pore size of the scaffolds decreased with the decrease of the introduce SPIONs size.The high density of dual-mode aperture structure of the scaffolds material was easier to fabricated when the amount of SPIONs content increased.The reason might be that,the dissolving capacity of CO₂in PLGA matrix was improved at high pressure and low temperature,resulting in the increasing mass transfer flux,which eventually led to the formation of micropores.However,when the depressurization rate was slow,the pores were easy to merge and form large pore structure.Finally,SPIONs coated with CA,PCL,PVAc were employed as nucleating agents to prepare PLGA/SPIONs multi-modal porous scaffolds via supercritical CO₂foaming process.The effects of polymer with different degree of CO₂-philicity modified SPIONs on mass transfer and the morphology of the scaffold were investigated.The results revealed that the addition of nucleating agents coated with CO₂-philicity polymer could effectively increase the solubility of CO₂on the polymer-nucleating agent interface,and the mass transfer flux,resulting in obtaining the microporous structure.However,the difference of polymer modification amount had little effect on the scaffold structure.Therefore,foaming with PVAc-SPIONs as nucleating agent could obtain microporous structure with high porosity under mild conditions,while foaming with CA-SPIONs and PCL-SPIONs as nucleating agent was easier to obtain scaffold materials with multi-mode pore structure.Soaking pressure also had signifacant influence on scaffold structure.The surface modification of SPIONs has little influence on foaming processunder high soaking pressure.At that condition scaffolds with high density could be fabricated with three different kinds of nucleating agents by supercritical CO₂foaming process.