Preparation And Surface Modification Of Inorganic Reinforced PCL Composite Bone Scaffolds By Selective Laser Sintering

Bone tissue engineering provides a new way to solve the problem that autogenous bone and allogeneic bone can not meet the increasing demand of bone repair materials due to the increasing number of patients with bone defects.Bone scaffolds are the key factors of bone tissue engineering that not only support surrounding tissues,but also provide growth environment for cells and new bones.Polycaprolactone(PCL)is often used as bone scaffold material because of its good biocompatibility and biodegradability.However,PCL scaffolds also have the shortcomings of inadequate biological activity and low mechanical strength.To overcome these shortcomings,PCL was used as the matrix,egg shell powder(ES)and multi-walled carbon nanotubes(MWCNTs)were added simultaneously to enhance the mechanical and biological properties of PCL,and three-dimensional porous composite bone scaffolds were prepared via selective laser sintering(SLS);Chitosan(CS)/ hydroxyapatite(HA)gel was used to modify the surface of PCL bone scaffold prepared by SLS,so as to further improve its bioactivity and cell proliferation;The morphology,phase composition,thermal properties,hydrophilicity,bioactivity,biodegradability,cytotoxicity and cell proliferation of scaffolds were examined.The main research contents and results are as follows:(1)Firstly,porous bone scaffolds were prepared by selective laser sintering(SLS)with PCL as the matrix and ES powder as a reinforced phase.PCL and PCL/3ES bone scaffolds were prepared with different process parameters to obtain the optimal SLS process parameters.The experimental results showed that SLS processing did not destroy the phase composition and molecular functional group structure of raw materials,the addition of ES improved the crystallinity and thermal stability of PCL,improved the hydrophilicity and the cell proliferation of the scaffold,but the compressive strength of the scaffold increased little and the porosity decreased.Secondly,MWCNTs with different contents were added into PCL matrix,and mixed evenly by machine,then three-dimensional porous bone scaffolds were prepared by SLS.Measurement results showed that the size accuracy and porosity of scaffolds decreased with the increase of MWCNTs content;However,the mechanical properties of bone scaffolds were significantly enhanced by the addition of MWCNTs.The compression,tensile and bending strength of bone scaffolds increased first and thendecreased with the increase of MWCNTs content.When the content of MWCNTs was0.5wt%,the compression,tensile and bending strength of bone scaffolds reached the maximum 10.56,14.36 and 14.84 MPa,respectively,which increased by 35.4%,31.1%and 38.2% respectively compared with the pure PCL samples.The enhancement mechanism of MWCNTs on mechanical properties of bone scaffolds was discussed from a micro-perspective,and that was further demonstrated by molecular dynamics simulation at the level of molecule.At last,in order to improve both of the mechanical and biological properties,ES and MWCNTs were added into PCL matrix,and the composite porous bone scaffolds were prepared by SLS,and then the morphology,porosity,compressive strength,tensile strength,hydrophilicity,biological activity,cytotoxicity and cell proliferation of bone scaffolds were tested.The results showed that: the compressive strength and tensile strength of PCL/3ES/0.5MWCNTs bone scaffolds increased by 30.8% and 22.4%,respectively,compared with pure PCL bone scaffolds;PCL/3ES/0.5MWCNTs bone scaffolds exhibited better hydrophilicity;however,the porosity decreased from 88% to 72%,but still more than 70%,which could provided the necessary space for cell migration and proliferation;In mineralization experiments,apatite crystals were precipitated on PCL/3ES/0.5MWCNTs bone scaffolds,but not on PCL bone scaffolds.In vitro cell experiments showed that the relative cell proliferation rates of PCL/3ES/0.5MWCNTs bone scaffolds were 94.7%,and their cytotoxicity were grade 1,while the relative cell proliferation rates of PCL bone scaffolds were 74.7%,and their cytotoxicity were grade 2.The above test results showed that the PCL/3ES/0.5MWCNTs bone scaffolds prepared by SLS have better mechanical and biological properties,good porosity and potential for clinical application.(2)Hydroxyapatite(HA)is the main component of human and animal bones and it has excellent biological activity and bone induction.Nano-hydroxyapatite(n-HA)powder was added into PCL powder matrix to improve the biological properties of bone scaffolds because of its high specific surface area,which was beneficial to cell adhesion.Porous PCL/n-HA bone scaffolds were prepared by SLS,and then the morphology,mechanical properties,cytotoxicity and proliferation of bone scaffolds were tested and analyzed.The results showed that the addition of n-HA leaded to the deterioration of sintering process properties of powders,and with the increase of n-HA content,the surface of bone scaffold was rougher,the accuracy was lower,and the porosity was smaller;The compressive strength of bone scaffold increased slightlywhen it contained 1wt% n-HA,but decreased more with the increase of n-HA content;However,in vitro cell experiments showed that the cell proliferation rate of bone scaffold extract with n-HA was significantly higher than that of pure PCL,and increased with the increase of n-HA,and their Cytotoxicity was improved from grade2 to grade 1.These results suggest that the addition of n-HA could promote cell growth and proliferation,and improve the cytotoxicity of bone scaffolds.(3)Most of the cells adhere to the bone scaffold surface,so the properties of bone scaffold surface materials have an important impact on cell adhesion and proliferation.The PCL bone scaffolds fabricated by SLS were immersed in the CS/HA suspension of in-situ synthesis.Then by ways of vacuum immersion,low-speed centrifugation and freezing gel,a CS/HA gel was adhered to the surface and pores of the PCL bone scaffold to improve their biological activity and cell proliferation.The morphology,phase composition,mechanical properties and biological properties of the scaffolds were analyzed by means of Scanning Electron Microscope(SEM),X-ray Diffraction(XRD),compressive strength measurement,in vitro degradation and in vitro cell experiments.XRD spectra showed that HA was produced by in situ synthesis;The SEM images of the composite bone scaffold displayed that CS/HA gel had been formed by freezing gel and adhered closely to the surface of PCL bone scaffold;The mechanical properties of PCL-CS/HA bone scaffolds were slightly lower than those of PCL scaffolds,but still met the requirements of cancellous bone;The surface water contact angle of bone scaffold modified by CS/HA decreased from 65 to 42 degrees,which improved the hydrophilicity of bone scaffold and enhanced the cell adhesion on the bone scaffold surface;The addition of CS/HA accelerated the degradation rate of the scaffold and neutralized the acidic substances produced during the degradation of PCL,which improved the biocompatibility of bone scaffold;The results of cell experiment in vitro showed that PCL-CS/HA immersion solution had the most active cells and the largest number of proliferation,indicating that the addition of CS and HA could effectively enhance the cell proliferation activity.