Study On Shape Memory Behaviors Of Lactic Acid-based Polyester Composites

Poly(lactic acid)(PLA)and its copolymers have good biocompatibility and biodegradability,which make them currently to be the most widely used biomaterials in scaffold-based tissue engineering.The idea that scaffolds have shape memory characteristics,is of great interest and significance for the tissue engineering strategy because it facilitates minimally invasive surgery for the in vivo implantation of scaffolds.Therefore,this dissertation carries out studies on the shape memory characteristics of aliphatic polyesters including poly(L-lactide)(PLLA),poly(L-lactide-co-glycolide)(PLGA)and poly(L-lactide-co-?-caprolactone)(PLCL).These polymers have different glass transition temperatures(Tg),which can be modulated by incorporating different additives to provide the possibility of developing scaffolds with temperature-responsive shape memory behaviors triggered by body temperature for in vivo applications.Targeting bone tissue engineering,in this study,several attempts have been conducted.? Nano-scaled hydroxyapatite(HA),silicon dioxide(SiO2)and halloysite nanotubes(HNTs)are the common inorganic components applied in promoting bone regeneration.Herein,they are surface modified by silane coupling agent or PLA grafting,composited with PLLA and PLCL,and their influences on the shape memory behaviors of resulted composites are investigated.?Composite films composed of PLGA and poly(D-lactide)(PDLA)are prepared by adjusting the contents of L-lactide unit in PLGA,the molecular weights of PDLA,and the blending ratios of PLGA and PDLA.For these preparations,the formation of PLLA/PDLA stereocomplex(SC-PLA)and its modulation on the shape memory behaviors of the composite films are studied.? The of PLCL(75:25)copolymer demonstrates its Tg close to human body temperature,accordingly,PLCL/HA and PLCL/PDLA composite scaffolds are prepared to investigate their temperature-responsive shape memory behavior and shape recovery ability at 37?.The main findings of this dissertation are summarized as follows:PLLA/HA,PLLA/SiO2 and PLLA/HNTs composite films were prepared by solution casting and solvent evaporation.These non-modified inorganic nanoparticles can be well dispersed in the PLLA matrix,which is ascribed to hydrogen interactions between the the ester groups in PLLA and the surface hydroxyl groups on the surfaces of inorganic particles.Compared to the pure PLLA,the Tg and thermal stability of all the composites are improved,as well as,their mechanical properties and shape memory ability become better.Among them,the PLLA/HA composite film shows the highest shape recovery rate(Rr=70.5%),significantly higher than that of PLLA film(60.8%).Subsequently,HA nanoparticles were surface modified with silane coupling agent KH550,PLLA or PDLA,the grafting ratios were characterized by FTIR and TGA,being 1.9%,4.8%and 5.1%,respectively.They were composited with PLCL(75:25)to prepare PLCL/HA,PLCL/HA-KH550,PLCL/HA-PLLA and PLCL/HA-PDLA porous scaffolds by the method of solution casting and freeze drying.Compared to non-modified HA,all the modified HAs display significant enhancement effect on the physicochemical properties of PLCL-based composite scaffolds.The thermal stability and compressive strength of composite scaffolds are proportional to the interface bonding strength of between the HA and the PLCL matrix,showing the trend of PLCL<PLCL/HA<PLCL/HA-KH550<PLCL/HA-PLLA<PLCL/HA-PDLA.The results prove that HA-PDLA can form a strong interface bond through the stereocomplex interaction of PDLA and the PLLA segment in PLCL.Further study was conducted on the relationship between pore sizes and shape memory properties of PLCL/HA-PDLA composite porous scaffolds.The results show that the scaffold exhibits good cyclic compression performance only when the pore size is big enough(approx.>100 ?m).Under 37?,the Rr of PLCL/HA-PDLA scaffold measured by DMA reachs 93.8%,which is higher than that of PLCL scaffold(91.3%).Finally,blends of PLGA(50:50),PLGA(75:25)and PLGA(90:10)with PDLAs(Mw=5000 or 20000)were prepared by altering the incorporated amounts of PDLA.Based on them,the effects of SC-PLA on the shape memory properties of PLGA/PDLA blended films were studied.The study found that PLGA(75:25)and PLGA(90:10)can form SC-PLA with PDLA due to the longer PLLA segment in the polymer molecular chain,while PLGA(50:50)is not detected having such an interaction with PDLA.The formation of SC-PLA enhances the compatibility of PLGA and PDLA.As the content of PDLA rises,the crystallinity of the formed SC-PLA also gradually increases.The mechanical properties,thermodynamic properties and shape memory ability of the blended films are all impro-ved.Among them,the Rr of PLGA(90:10)/PDLA blended film is close to 100%,and PDLA-5K is more beneficial to improve the shape memory ability of the blends than PDLA-20K.On the basis of these findings,the porous scaffold composed of PLCL(75:25)and PDLA shows good shape memory behavior and rapid shape recovery ability at 37?.In summary,the interaction between inorganic nanoparticles and polymers can be used as an effective means to improve the shape memory performance of composite materials.In particular,the HA modified by surface PDLA grafting can enhance the interface binding strength with PLCL through PLLA/PDLA stereocomposite interaction,which is able to significantly improve the deformation recovery ability of the PLCL/HA-PDLA composite at 37?.Based on the formation of SC-PLA,PDLA can also be directly blended with PLGA or PLCL to improve the shape memory ability of corresponding blends and achieve rapid shape recovery at physiological temperature.The results of this dissertation provide new ideas for the design of intelligent bone repair materials.