Comparative Study And Optimization Of Long-term Osteogenic Behavior Of Polymer Composite Scaffolds With Different Degradability

Scaffolds composed of polymers and nano-hydroxyapatite(n-HA)have received extensive attentions in the field of bone repair,however,there is a lack of in-depth and long-term comparative study on the effect of scaffold degradability on bone reconstruction.The reconstruction of natural bone is still ongoing between 6 months and 2 years after surgery,whereas current in vivo osteogenesis studies were normally carried out within 6 months.Therefore,it is of great scientific significance to study the effect of scaffold degradability on long-term bone remodeling in vivo,and it will provide a reference for the design of bone repair scaffolds that meet the clinical needs.In this study,three polymers with different degradation properties,namely,nondegradable polyamide 66(PA66),slowly degradable polycaprolactone(PCL)and fast degradable poly(lactic co-glycolic acid)(PLGA),were selected.Then three porous scaffolds with similar porosity and pore structure,n-HA/PA66,n-HA/PCL and nHA/PLGA,were prepared respectively by compounding the three polymers with nanohydroxyaptite(n-HA),an inorganic component of natural bone.The in vitro degradability,cytocompatibility and the in vivo long-term osteogenic differences over a period of 1 to 2 years of the three scaffolds were comparatively investigated to figure out the scientific questions about the relationship between the scaffold degradability and long-term bone remodeling.The in vitro degradation results demonstrated the degradable characteristics of the three composite scaffolds,i.e,non-degradable n-HA/PA66,slowly degradable nHA/PCL and fast degradable n-HA/PLGA,which were consistent with the experimental design.The results of cell experiments presented good cytocompatibility of the three scaffolds.The in vitro degradation and cell experiments suggested that the three scaffolds were suitable for the subsequent long-term in vivo comparative study.The long-term comparative study showed that,although the fast degradation of the nHA/PLGA scaffold was beneficial to early osteogenesis,the fast degradation could cause the loss of scaffold structural integrity and subsequent reduction of bone volume after 3 months.Although the early bone formation of n-HA/PA66 and n-HA/PCL scaffolds was lower than that of n-HA/PLGA scaffold,the final bone volume of nHA/PA66 and n-HA/PCL scaffolds exceeded that of n-HA/PLGA scaffold at/after 12 months.The long-term comparative investigations have drawn the following important conclusions: i)Degradation in early stage of scaffolds can provide space,and Ca/P source for bone tissue growth,which is conducive to early osteogenesis;however,scaffold collapse caused by rapid degradation will cause the loss of scaffold mechanical support,which is not conducive to the subsequent bone reconstruction.ii)The structural integrity maintenance of n-HA/PA66 and n-HA/PCL scaffolds plays a positive role in the final bone reconstruction,a relatively stable scaffold integrity,together with favorable matrix molecular characteristics and hydrophilicity,may be more important for long-term osteogenesis besides the effect of scaffold pore structure,rather than the pursuit of fast scaffold degradation.iii)The long-term in vivo comparative research suggests a scientific principle for future design and application of clinically applicable bone scaffolds,namely,the scaffolds must have a relatively stable osteogenic space and scaffold interface,or at least have a scaffold degradation speed slower than the time of bone reconstruction completion.Based on the conclusions from the long-term comparative study,a polymer matrix with a degradability more suitable for long-term osteogenesis was designed for the subsequent study.The polymer matrix includes two polymers,PCL and PLGA,of which PCL degrades relatively slowly and could maintain the structural integrity of the scaffold,and PLGA would degrade rapidly.In addition,under the presence of Lglu template,Sr-doped HA whiskers(Sr HAW)with uniform size and good dispersion were prepared,which were used for strengthening polymer matrices and promoting bone formation.And nano Sr-doped HA crystals(n-Sr HA)were also prepared by hydrothermal method.Through parameter optimization,Sr HAW,n-Sr HA,PCL and PLGA were combined in a ratio of 1.5:1.5:5:2 to prepare the scaffold(named as SHPP)with a gradient degradation and a compressive strength similar to human cancellous bone.The SHPP scaffold could maintain its structural integrity when gradually degraded and continuously released osteogenic active ingredients of Ca and Sr ions.Afterwards,lipopolysaccharide(LPS),an immunoregulatory active factor,was physically adsorbed on the SHPP scaffold.The in vitro and in vivo studies on the scaffolds showed that: i)Sr ions could indirectly promote angiogenic differentiation of endothelial cells by mediating macrophage phenotypic expression,and directly promote osteogenic differentiation of bone marrow mesenchyml stem cells(MSCs),thus promoting the in vivo bone formation.ii)The introduction of LPS could recruit more cells,including macrophages,to infiltrate into the scaffolds and interact with the scaffolds;iii)The gradient degradation and structural integrity maintaining of the scaffold could endow the SHPP and SHPP/LPS scaffolds with more continuous bone structure formation.The gradient degradability avoided the rapid collapse of the whole scaffold structure,also ensured the continuous release of osteogenic Ca,Sr ions.The SHPP scaffold owns a stronger ability to maintain structural integrity when subjected to external forces or implanted in vivo,which plays an important role in maintaining space for bone tissue growth.iv)The SHPP/LPS scaffold,an "early pro-inflammatory and late anti-inflammatory" model proposed and prepared in this study,could integrate the osteogenic effects of LPS and Sr ions to start and improve the process of tissue repair,which show great potential for application in bone regeneration.