Study On The Growth Mechanism Of Hydroxyapatite Nanowires Prepared By Hydrothermal Method And The Performance Of Hydroxyapatite Nanowires/PLA Scaffold

At present,bone defect is still the research focus in the field of bone tissue engineering（BTE）,which resulting in infections,trauma and an aging population,which seriously affects the life and work of patients,and even threatens their lives.Now,the clinical treatment options for bone defects are mainly based on autologous bone and allogeneic bone for transplantation of the defective area.However,there are limitations that restrict its wide application in bone tissue engineering.Therefore,the preparation of biolactive scaffold materials has become a top priority to solve the problem of bone defects.However,scaffolds prepared from a single material can hardly meet the requirements of bone tissue engineering,so suitable methods are utilized to composite different materials to maximize the material properties.In this paper,Hydroxyapatite nanowires were prepared by oil bath method and hydrothermal method,and the differences in morphology and crystallization properties between them were compared.Secondly,in terms of scaffold preparation,hydroxyapatite nanowires with different mass fractions were combined with Polylactic acid（PLA）by vacuum freeze-drying technology,and its morphology,water absorption,porosity,degradability and compressive strength were tested to meet the requirements of bone tissue engineering.Finally,according to the proliferation of MG-63 cells in different scaffold extracts and their growth on the scaffold surface,the biocompatibility of the scaffold was evaluated.The experimental methods and conclusions are as follows:（1）According to the structural characteristics of HAp,calcium source and phosphorus source solution are sequentially added into a mixed system prepared by oleic acid,ethanol and sodium hydroxide solution,and then put into an oil bath pot at180℃,and different reaction times are controlled.SEM results show that HAp prepared by oil bath method is linear in morphology with smaller diameter,shorter length and serious agglomeration.The TG results showed that the thermal stability of the material increased with the increase of reaction time,from 53%at 1 h to 85%at 24h.In addition,XRD and FTIR results showed that the product obtained by the oil bath method was HAp-OCP-DCPD ternary phase with poor crystallinity,so the hydroxyapatite nanowires prepared by the oil bath method were not suitable for material mechanism study.（2）Hydroxyapatite nanowires were synthesized according to the ratio of reactants in the oil bath method.After preparation,they were put into a 180℃oven to react for different times.SEM results showed that the morphology of hydroxyapatite nanowires changed from granular to linear under hydrothermal conditions.When the reaction time is 24 h,the length,flexibility and dispersion of hydroxyapatite nanowires are stable.The results of FTIR and XRD show that the structure of the precipitate before heating is consistent with the crystallographic characteristics of Amorphous calcium phosphate（ACP）.With prolonging the reaction time,ACP in the reaction system is transformed into HAp crystal,and the crystallinity is improved.TG results show that time has a positive effect on the thermal stability of the material,increasing from 33.3%in the initial stage to 96.2%in 24 hours.（3）The formation mechanism of hydroxyapatite nanowires obtained by hydrothermal method is analyzed according to crystallography theory.In the initial stage of the reaction,calcium oleate precursors were formed first in the system,and oleate ions were adsorbed on the（001）crystal plane of HAp with positive charge.With the extension of the reaction time,oleate ions and Ca²⁺,PO₄^3-generate HAp nanocrystals under alkaline conditions.Due to the large amount of HAp nanocrystals formed,nanocrystals become the main growth element and grow in a certain direction and crystal plane It is required to grow into crystallites by superposition.When the crystal growth is advanced,the nanocrystals are consumed,the solution concentration tends to be stable,and the saturation decreases.The superposition growth is completed and a relatively long reaction time is passed under the hydrothermal high temperature and high pressure reaction conditions,so that the branch chain is gradually lengthened The crystal morphology changes to linear to form long-range ordered crystals,so that the hydroxyapatite nanowires are arranged parallel to each other in the longitudinal direction into a partially ordered structure.In addition,the polar hydrophilic end of the oleic acid molecule binds to Ca²⁺,thereby exposing its hydrophobic end and avoiding the aggregation of hydroxyapatite nanowires in solution.（4）Using vacuum freeze-drying technology,hydroxyapatite nanowires prepared by hydrothermal method were mixed with PLA to prepare a porous scaffold material with good biological activity and mechanical properties.When the content of hydroxyapatite nanowires increased from 0%to 35%,the roughness of the scaffold surface deepened and the porosity decreased,but it did not affect the connectivity of the scaffold pores.Because hydroxyapatite nanowires have good water absorption performance,the water absorption capacity of the scaffold is obviously improved,which is beneficial to the input of nutrients and the discharge of metabolic waste.In addition,the compressive strength of the scaffold increased first and then decreased,and reached the maximum when the content was 15%.（5）SEM results of cytotoxicity and co-culture of cells in scaffolds showed that MG-63 cells proliferated in the extract and grew well,which indicated that hydroxyapatite nanowires/PLA scaffolds prepared in this study had good biocompatibility.