| NiTi alloys have been recently considered as excellent bio materials with potential use in the fields of orthopaedic implant and cardiovascular applications due to their unique properties,such as shape memory effects,superelasticity,good corrosion resistance and biocompatibility.However,the mismatch of elastic modulus between NiTi and bones.This large difference in elastic modulus between implant material and the surrounding human bones leads to an inhomogeneous stress distribution on the bone,and then may cause bone resorption and weakens bone locally which results in aseptic implant loosening.A porous form of NiTi alloy has attracted much attention as potential biomaterials for human hard tissue repair and replacement.However,it is difficult for the current fabrication methods to obtain high porosity,large pore size,high strength and good superelasticity.Besides,porous NiTi alloy has poor bioactivity,which cannot induce the osteoblast grow into the implants actively.Several surface treatments utilizing bioactive ceramic coating on the metal implant have attracted extensive attention.But the main disadvantages of the current coating techniques are their susceptibility to degrading,wearing,nonuniform coating and peeling off from the metal substrate.In this work,to investigate advance technique of obtaining high strength,bioactivity,large pores and good superelasticity,combining NiTi alloy with good corrosion resistance,biocompatibility and superelasticity and hydroxyapatite(HA)ceramic with with good bioactivity and osteogenic activity to be a porous NiTi-HA biocomposite by spark plasma sintering(SPS).To meet needs in clinical,based on the bionic structural design,radial gradient porous NiTi-HA composites were also prepared by SPS.The pore characteristics and pore forming mechanism have been discussed,and the effect of HA ceramic contents and fabrication process on pore structure evolution,mechanical properties,superelastic behaviors,corrosion resistance and bioactivity have been investigated.A process of combing use of SPS technique and sieved NH4HCO3 was developed to successfully fabricated the porous NiTi alloy with 39%-61%porosity,100-415 ?m average pore size,low elastic modulus(7.5?18.1 GPa)and high strength(260-1183 MPa)without binder by optimazed sintering temperature and 5-20wt.%spacehoklier.Porous NiTi alloys with high porosity have interconnected pore structure and consisted of nearly single NiTi phase(B2 and B19).The work showed that the enhaced elements diffusion effect of SPS mechanism would improve the quality between particles and obtain uniform NiTi phase within a few minutes.On the basis,porous NiTi-HA composites with 29%?37%prosity,5.6?8.1 GPa elastic modulus,57?225 MPa compressive strength and over 100?m average pore size were fabricated.The biocompability assay showed that introducing HA ceramic into NiTi matrix enables enhanced bioactivity.As the increase of HA,the homogeneity and the thickness of apatite layer were improved greatly,and the Ni ions release rate decreased.Composites have no adverse reaction to ROS1728 osteoblasts.Cell adhesion and proliferation experiments demonstrate healthy spreading as well as proliferation of the cells on the composites,and some cells were found growth inside the pores.According to the IL-6 levels,the composites do not cause any obvious inflammatory response.However,the increase of HA would lead to deteriorate mechanical properties and superelasticity.The mechanical properties of composites are worse than that of alloy with the same pores.Thus,in order to further improve strength and superelasticity under high porosity,low modulus and high bioactivity,radial gradient composite consisting of a central NiTi micropore core and an outer porous NiTi-HA composite layer were rapidly fabricated by SPS.The gradient composites with 49%porosity together with 380?m average pore size exhibit a low elastic modulus(16.9 GPa),good bioactivity,and yet maintain highly enhanced strength(1300 MPa)and excellent superelastic recovery strain.It is possible to point out that in this work,the control of the volume fraction between the central solid and the outer porous layer is crucial to tune the mechanical properties of the radial gradient porous composites.To sum up,this work fabricated porous NiTi biocomposites with high porosity and good bioactivity by a process of modified SPS and space holder technique,which optimized the SPS of fabricating porous NiTi,and overcome the disadvantages of low porosity,uncontrollable pore structure and necessary binder.Furthermore,by bionic hierarchical design,radial gradient NiTi-HA composites with a combine of low modulus with high strength were fabricated.Altogether,this strategy to design materials with function-structure integration and suggests that properly designed function-structure integrated NiTi composites may be promising as advanced bone implants. |
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