Effect Of Technological Conditions On Mass Transfer And Performance Of Magnetic Scaffolds Prepared By Supercritical CO₂ Foaming

Tissue engineering is a technology that simulates natural behavior and is committed to restoring,maintaining and improving tissue function to develop biological substitutes.Scaffold material is one of the three elements of tissue engineering.Bone tissue engineering scaffolds are very important tools in bone healing.In recent years,biological scaffolds instead of autologous bone transplantation have attracted extensive attention.The preparation of tissue engineering scaffolds with high porosity,multi-modal cell structure and good mechanical properties has become the goal pursued by researchers.Meanwhile,the introduction of magnetic materials into scaffolds can further improve the performance of scaffolds.Supercritical carbon dioxide（SC-CO₂）foaming is a new green preparation process.Unfortunately,the obtained scaffolds have low porosity and single cell structure.Therefore,the SC-CO₂foaming process is improved and superparamagnetic iron oxide nanoparticles（SPIONs）are introduced as nucleating agent to prepare magnetic bone tissue engineering scaffolds in this study.The effects of operation conditions of three modes,namely SC-CO₂temperature rise constant pressure foaming mode,two-step depressurization foaming mode,and coordinated cooling and depressurization foaming mode,on the mass transfer in the foaming process and the performance of the scaffold were discussed in detail.The specific research contents of this study are as follows:（1）Poly（lactic-co-glycolic acid）（PLGA）and PLGA/SPIONs magnetic scaffolds were prepared by SC-CO₂temperature rise constant pressure foaming mode.The effects of the additional SPIONs content,saturation temperature,foaming temperature,foaming pressure and depressurization rate on the cell structure,porosity,compression modulus and thermal properties of the composite magnetic scaffolds were systematically studied.The results showed that an obvious multi-mode cell structure was formed when the difference between saturation temperature and foaming temperature was more than 20℃at low pressure and high depressurization rate.The introduction of SPIONs into the scaffold greatly improved the porosity and compression modulus of the scaffold.The porosity of the scaffold was less affected by saturation temperature and foaming temperature,but more affected by foaming pressure and depressurization rate.Lower foaming pressure and depressurization rate were conducive to the formation of scaffolds with high porosity,and scaffolds prepared with higher foaming pressure and depressurization rate had higher compression modulus.Based on these results,the controllable PLGA/SPIONs multi-mode cell structure scaffolds with average diameters ranging from 11.93μm-790.52μm,porosity ranging from 62.98%-93.23%and compression modulus ranging from 0.38 MPa-11.12 MPa could be obtained.（2）SC-CO₂two-step depressurization foaming mode was employed to prepare PLGA/SPIONs magnetic scaffolds with PLGA as foaming polymer and SPIONs as nucleating agent.The effects of initial pressure,intermediate pressure,foaming temperature,1st depressurization rate and 2nd depressurization rate on the cell structure,porosity and compression modulus of the magnetic scaffolds were systematically studied.The results showed that it was easy to form multi-mode cell structure required by bone tissue engineering when the saturation temperature was above 45℃,the difference between the initial pressure and the intermediate pressure was above 4 MPa,and the 1st depressurization rate was high.Multi-mode cell structure was observed at three different 2nd depressurization rates.The porosity of the scaffold was significantly affected by foaming temperature and 2nd depressurization rates.The higher the foaming temperature or the 2nd depressurization rate was,the higher the porosity was.Finally,by changing the foaming parameters to control the mass transfer flux,the multi-mode cell structure scaffolds could be successfully prepared.The pore size,the porosity,and the compression modulus of the prepared scaffolds were in the range of 10.52μm-651.86μm,63.11%-92.56%,and 1.37 MPa-14.22 MPa,respectively.（3）PLGA/SPIONs magnetic scaffold was prepared by SC-CO₂coordinated cooling and depressurization foaming mode using PLGA and SPIONs as raw materials.The effects of foaming temperature,saturation temperature,saturation pressure and depressurization rate on the cell structure,porosity and compression modulus of the magnetic scaffold were systematically studied.The results showed that when the difference between saturation temperature and foaming temperature was more than 15℃,the multi-mode cell structure with high porosity could be formed.With the increase of foaming temperature and foaming pressure,more high-density pore structures were formed in the scaffold,resulting in the decreasing porosity and the enhancing compression modulus.It also indicated that the scaffolds prepared at moderate depressurization rate had a multi-mode cell structure,and their compression modulus and porosity could meet the requirements of bone tissue engineering scaffolds well.According to these results,adjustable PLGA/SPIONs multi-mode cell structure scaffolds with various average diameters（7.75μm-361.26μm）,a wide range of porosity（64.69%-95.85%）and mechanical properties（1.16 MPa-18.81 MPa）could be obtained.