Study On The In Situ Synthesis Of Nanoapatite/polylactic Acid Composite Under The Polymer Modulation

With the development of industry, transportation, sports and the aging of thepopulation, defection, losses and disfunction of bone tissues has gradually become aserious threat to public health and the clinical therapy to these diseases has attractedmuch research interest. Until now, autograft and allograft bone repair are used inclinics to restore bone defect. Autograft is the gold standard for bone repairing inclinics. But the disadvantages of autograft are obvious: limited resources, creatingsecond trauma and the possibility to induce donor tissue syndrome. Allograft maycause immuno-rejection and the risk to infect diseases from the donor. Therefore,artificial materials are used more and more in Clinics to replace bone defects.By organizing organic materials and inorganic salt in a sophisticated way, naturalbone obtains its structure and physical strength. From a viewpoint of materialsscience, bone is a kind of natural composite composed of inorganic materials andpolymers. Therefore, as a material to restore defective bone, inorganicmaterials/polymer composites are better than pure ceramics, metals and polymersbecause the improvement in physical properties and biological properties.Composite showes its advantage as bone substitute.Because of its excellent biocompatibility, biodegradability, feasibility ofprocessing, polylactic acid (PLA) has been used widely in the field of biomedicalmaterials field as surgical suture, fasten plate in bone fracture, drug controlled release system and scaffold in tissue engineering. However, it has the disadvantageof lack of bioactivity and poor cell compatibility, which results in non-infectiveinflammation due to the high local acidic concentration. Apatite is the main naturalinorganic component of bone with bioactivity, which combined with bone withstrong chemical bonding when implanted in bone tissue. But its disadvantage is thebrittleness, difficulty in processing and the incompatibility with bone in mechanicalproperty. If apatite is compounded with PLA, the composite will possess theirindividual advantages. Therefore, hydroxyapatie/poly(lactide)(HA/PLA) compositehas attracted much research interest.Mineralization of bone realizes the modulation of bone microstructure inmolecular level. By the interaction of organic materials and inorganic materials onthe interlace and the precise modulation of crystal nucleation and growth inmolecular level, organic materials (mainly collagen) bond with salt (mainly boneapatite). The interface between them is strengthened and crystal growth is controlled,which satisfied the biological and biomechanical demand of bone. Theunderstanding of this process provides new ideas on the preparation of newcomposite, which is to prepare inorganic material/organic material composites bythe modulation of growth of inorganic crystal with organic materials in molecularlevel.In this study, in situ synthesis method was used to prepare nano-HA/PLA(n-HA/PLA) composite by the modulation effect of PLA based on the structure ofnatural bone, in which two phases were evenly distributed with chemical bonding inbetween. And the obtained composite possesses good bioactivity and otherbiological properties.Nano-HA/PLA was prepared with in situ synthesis by the interaction of theterminal carboxyl groups (-COOH) group and -OH group and carbonyl group insidePEA with calcium ions to modulate apatite growth. The results indicated that in theprocess of composite formation, the terminal COOH group, OH group and carbonylgroup in PLA might be the nucleation sites. Apatite nucleated and grew on the PLA surface and its morphology and growth direction were modulated by the PLA.Therefore, the further nucleation and growth of apatite was controlled by the 3-Dstructure of PLA. The composite obtained was homogenous with n-HA evenlydistributed in PLA matrix, which realized nano-sized HA composite. The content ofn-HA in the composite is controllable and chemical bonding and force of molecularinteraction was found between the two phases. However, the modulation ability ofPLA is limited because there is only terminal carboxyl group in PLA molecule andthe interaction between carbonyl in PLA and calcium ions is weak. New method isnedded to be developed.The peroxide groups were introduced onto PLA particle surfaces viaphotooxidization with H₂O₂ oxidant. The results showed that the peroxide groups onPLA particle surfaces could be controlled by oxidation time, and the maximumcontent of peroxide groups produced on the PLA surfaces was about 5.6×10^-2mmol/g with energy of 3000×100μJ/cm² after 50min of oxidation. After 50min. thecontent of peroxide groups decreased. Then the PMAA was grafted onto the PLAsurface via UV induced polymerization and PMAA-modified PLA (PMAA-PLA)was obtained. The graft weight increased with the grafting time under the sameoxidation time, irradiation dose and monomer concentration. When grafting timewas the same. the grafting weight increased with the oxidation time.The carboxyl groups (-COOH) on PMAA-PLA surface were easily ionized into-COO in alkaline aqueous solution, which resulted in PMAA-PLA microparticlesurface with high negative charge. PMAA-PLA nanosphere suspension with anaverage particle size of 133.1±2.3nm in Na₂HPO₄ aqueous solution was stable for afew weeks without any deposit. The absolute value of zeta potential of PMAA-PLAnanosphere suspension was affected by pH value of suspension system. When thepH value decreased to 5, PMAA-PLA nanosphere suspension still had high negativezeta potential (-79.8mV) with no deposit. Only when the pH decreased to 3.0. theabsolute value of zeta potential of PMAA-PLA nanosphere suspension was less than30mV and flocculent PMAA-PLA deposit could be observed. Nano-hydroxyapatite/poly(lactide) (n-HA/P-PLA) composites were prepared bythe carboxylic groups, -OH groups and carbonyl groups on the PLA molecules andintroduced carboxylic groups of PMAA on PMAA-PLA controlling the growth ofapatite crystals. Several analyses about FTIR, XRD, TEM, SEM and XPS suggestedthat the PMAA-PLA could manipulate the nucleation and growth of n-HA crystals,control the morphology, size and growth orientation of n-HA crystals as well as theirdistribution over the organic phase. There might be chemical bonds and moleculesinteraction in obtained n-HA/P-PLA composites between the two phases. Theobtained n-HA/PLA composites synthesized by mixing n-HA/P-PLA compositeswith PLA not only strengthened combination between PLA molecules and twophases of HA and PLA but also guaranteed homogenous n-HA crystals distributionin n-HA/PLA composites and bioactivity of obtained composite.Bone apatite could be formed on the surface of n-HA/PLA composites in vivo andin SBF, which indicated that n-HA/PLA composites had good bioactivity. Calciumrelease and calcium precipitation of n-HA/PLA composites in physiologicalenvironment further confirmed bioactivity and bone binding ability of n-HA/PLAcomposites.The in vivo and in vitro experimental results showed that the n-HA/PLAcomposites could be degraded by simple hydrolyzation and the degradation rate invivo was quicker than in vitro because of blood fluidity, enzymatic activity and stressaction. The acidic environment due to the degradation of PLA could be neutralizedor buffered by n-HA, which lowered the intrinsic catalysis inside the composites.Molecular weight decreased quickly in 2 weeks in in vitro experiment, then itdecreased slower, but the loss of weight was lagged a little. The pH value reducedslower, which might be arised by alkalescent n-HA.The results of biological evaluation in vivo suggested that the composites hadexcellent biocompatibility, biodegradability, bioactivity and osteogenicity. At 2weeks, there was a dense fibrous capsule surrounding the composites and the fibrouscapsule didn't become thicker with time. At 4 weeks, the newly formed immature bone grew along the surface of the composite and bone directly contacted with thesamples without intervening fibrous tissue. In undecalcified specimens, osteoblastsand osteoclasts were observed between the composite surface and lamellar bone(woven bone), and new bone formed which took on different structure andorientation from the old bone. The histological results of pure HA control rods wassimilar with that of the composite rods. After 1-2 weeks of implantation, new bonewrapping the implant was clearly visible. The investigation of long timeimplantation about the obtained n-HA/PLA composites needed to be studied further.