Research On The Preparation Of Hydroxyapatite/Poly(L-lactide) Composites And Its Properties

In this work,lactic acid was used as the raw material to synthesize high-molecular-weight poly?L-lactide??PLLA?through the ring-opening polymerization of lactide.Then,the poly?L-lactide?and the modified nano-HA(PLA-CD-n-HA,OPLA-ESBO-n-HA and modified whisker-like hydroxyapatite?PLA-CD-w-HA?were respectively used to prepare modified HA/PLLA composites.The effects of different modification methods of hydroxyapatite,different morphology of hydroxyapatite on both the mechanical properties and biological properties of HA/PLLA composites were studied.In the process of producing PLLA through ring-opening polymerization L-lactide.The optimization process of synthesis of L-lactide from pyrolysis oligomers PLA intermediates was explored,and the purification process of crude lactide was optimized.Effect of catalysts and additives on the ring opening reaction?ROP?of L-lactide is evaluated.During the modification of HA and the synthesis of HA/PLLA composites,different coupling methods were used to modify the HA with different morphology,and then PLA was grated on the HA surface via the in-situ ROP of L-lactide.With the grafting of PLA,the surface-modified HA material was successfully obtained and different HA/PLLA composited with modified HA and PLLA were also obtained.A variety of characterization methods?XRD,FT-IR,TGA,TEM,SEM,EDS,contact angle,?-potential,AFM,etc.?were used to characterize HA modified by different modification methods and modified HA with different morphology.The mesenchymal stem cells?MSCs?were used to test the biological properties of various materials?HA modified by different modification methods,modified HA with different morphology and modified HA/PLLA composites?,such as the survival rate of MSCs cells on the material,cells adhesion and osteogenic differentiation performance.At the same time,the mechanical properties of the modified HA/PLLA composites were tested.The specific work is as follows:?1?The effects of the comonomer type,the amount of comonomer,the reaction time,the reaction temperature,the initial molecular weight of the prepolymer,and the molecular weight of the residual polymer after the reaction during the pyrolysis of oligomeric lactic acid?OPLLA?and the telechelic OPLLA were investigated.The experimental results show that the increase of the terminated hydroxyl concentration can increase the rate of pyrolysis reaction and lactide yield,also could shorten the reaction time.The results showed that the terminal groups of the OPLLA and telechelic OPLLA intermediate have the greatest influence on the pyrolysis reaction.The reaction conditions for the synthesis of L-lactide from the thermal degradation of OPLLA and telechelic OPLLA intermediates are as follows:OPLLA-?OH?₄ is the optimized intermediate,reaction time is 35min,reaction temperature is 210°C,pyrolysis The yield of crude lactide reached 92.8wt%in the reaction,and the selectivity to L-lactide was as high as 93.5%.Purification of crude lactide.After four times of crystallization under low temperature conditions,L-lactide with high purity can be obtained.And it is applied in the subsequent ring-opening polymerization of L-lactide.In comparison with the catalytic results of several zinc salt catalysts in the ring-opening polymerization of L-lactide,an excellent catalyst can be obtained:zinc acetylacetonate.When?-cyclodextrin was used as a assisstant and zinc acetylacetonate was used as catalyst,the weight-average molecular weight?Mn?of PLLA obtained by the ROP of lactide was 10,742,and the polymer dispersity index?PDI?was 1.2,indicating that the obtained poly?L-lactide?had a relatively large molecular weight and a comparative uniform molecular weight distribution.This result shows that zinc acetylacetonate is a good lactide ring-opening catalyst,while?-cyclodextrin is a good additive.?2?The successful introduction of?-CD/citric acid network on the surface of HA helped the grafting of PLA on its surface.The graft amount reached 11.7wt%.By grafting PLA/?-CD/citric acid network on HA surface,the surface properties of HA were improved,hydrophobicity was enhanced,biocompatibility and bioactivity were also improved.The obtained PLA-CD-HA material was basically non-toxic to cells and can adhere well to MSCs,and favor the extracellular matrix?ECM?mineralization of MSCs.It was found that the ECM mineralization of MSCs was improved.The stained area was more than 60%,which is about 40%higher than that of control group.It indicating that the PLA-CD-HA has positive effects on the osteogenic differentiation of MSCs into osteoblast-related cells.The PLA-CD-HA-72/PLLA composite material are obtained by fill the PLLA matrix with PLA-CD-HA-72.The excellent mechanical properties are obtained because PLA-CD-HA-72 has smaller particles and more uniform distribution than HA and CD-HA in the PLLA matrix.When the content of PLA-CD-HA-72 in the composite was 2.5wt%,the prepared PLA-CD-HA-72/PLLA composite had higher tensile strength?41.7MPa?and better breaking elongation?13.2%?.The introduction of PLA/CD/citric acid network effectively improved the tensile strength and break elongation of modified HA/PLLA composite.PLA-CD-HA-72/PLLA composties possess both good mechanical properties and biological performance,which enable it to become a good orthopedic or dental biomaterial.?3?CaCl₂,Na₃PO₄ and epoxy soybean oleic acid?ESBO?were coprecipitated to introduce reactive groups on the surface of HA.ESBO could grafted on the surface of HA,and OPLA was grafted on it.The introduction of OPLA and ESBOcan improve the surface properties of HA.The hydrophilicity of OPLA-ESBO-HA was reduced,and the particles were suspended in chloroform for a longer time.At the same time,through the bioactivity and biocompatibility performance test,it was found that the OPLA-ESBO-HA has better performance than the ESBO-HA and unmodified HA,including cell viability,cell adhesion and osteogenic differentiation.It was found that the ECM mineralization of MSCs was improved.The stained area was more than 25%,which is about 10%higher than that of control group.This shows that OPLA-ESBO-HA has a positive effect on inducing the osteogenic differentiation of MSCs into bone-related cells.The HA/PLLA,ESBO-HA/PLLA and OPLA-ESBO-HA/PLLA composites were obtained by melt extrusion/injection molding method or solvent blending/hot pressing method.The tensile strength and flexural strength of the composites obtained by melt extrusion/injection were slightly lower than those of PLLA prepared by this method,but they can still be maintained at relatively high levels.However,the tensile strength of OPLA-ESBO-HA/PLLA composites prepared by solvent blending/hot pressing mehod was higher than that of HA/PLLA composites,but the break elongation was smaller.After solvent blending,OPLA-ESBO-HA particles can fully blended with the PLLA matrix in the solvent,and the OPLA and ESBO on the HA surface can form a tie layer with the PLLA matrix,thus,the tensile strength of it increased.However,solvent blending/hot pressing method can only prepared thin polymer films,which is a great limitation.Therefore,after a certain amount of OPLA-ESBO-HA and PLLA are firstly mixed in solvents and dried.The obtained material could be used in extrusion molding method,which is expected to improve mechanical properties of OPLA-ESBO-HA/PLLA.OPLA-ESBO-HA has excellent biocompatibility,and its composite has relatively moderate mechanical properties,making this material also a potential filler for bioabsorbable polymers.?4?w-HA was prepared by optimized the reaction conditions via single factor method.The optimized reaction conditions were as follows:Ca:EDTA:H₂O₂:NH₄OH:1:1:6:16,temperature was 100°C,reaction time was 24h,and concentration of H₂O₂was about 30%.The PLA/CD/citric acid network was successfully modify the w-HA.Compared with the OPLA,PLA and OPLA/CD/citric acid networks,the PLA-CD-w-HA had higher cell viability,cell adhesion and osteogenic differentiation of MSCs.After co-cultured with the PLA-CD-w-HA for 72 hours,the survival rate of MSCs is nearly 100%.MSC cells can adhere well to PLA-CD-w-HA material.At the same time,PLA-CD-w-HA has a strong effect on inducing the ECM mineralization of MSCs,and the stained area exceeds 55%,which is 35%higher than the control group.It was demonstrated that PLA-CD-w-HA has a positive effect on inducing differentiation of MSCs into bone-related cells.However,in this experiment,the induction of Sr on the MSCs bone induction is not obvious.Comparison of different modification methods found that PLA/CD/citric acid network modification method is more effective than other methods in the modification of HA.The introduction of?-CD has extremely important significance.Among the six composite materials obtained by solvent blending and hot pressing,PLA-CD-w-HA has the best dispersibility in the PLLA matrix,and the short inorganic fibers are dispersed uniformly in the PLLAmatrix,effectively increasing the tensil strength of the PLA-CD-w-HA/PLLA composites.The composite material also has good adhesion to MSCs.These properties all make PLA-CD-w-HA/PLLA have a good application prospect in the field of bone tissue engineering.