| Titanium and its alloys have long been widely used as biomedical materials on account of their excellent mechanical properties and reasonable biocompatibility.According to the structural distinction,titanium biomaterials can be clarified into solid titanium and porous titanium.The solid titanium possesses a critical issue related to the mismatch of the elastic modulus between Titanium implant and cortical/cancellous bone,which can lead to stress shielding over the surrounding bone.Porous titanium with interconnected structure can reduce the stress-shielding effect due to its low elastic modulus,and can also provide inner spaces for the ingrowth of new bone tissue and vascularization.The conventional metallurgical methods to fabricate porous titanium are mainly powder sintering technique,having the disadvantages of low ductility that may lead to breaking of the porous titanium implants in the body's environment under uncertain overloading or accidental impact.A new entangled titanium material with flexible framework developed recent years has overcome this problem.However,due to the flexibility,its yielding strength and elastic modulus(when the porosity exceeds about 50%)are lower than that of the porous titanium manufactured by conventional methods.Therefore,inhibiting the structural flexibility by fixing the cross wire nodes may improve the stiffness of the entangled titanium material,and any biocompatible material with adequate strength and elasticity can be selected as a bonding agent so as to fix the free nodes in the entangled structure.In this case,in order to improve the mechanical properties of the entangled titanium material,an attempt to fix the free nodes in the entangled structure has been made.A biomaterial,Bisphenol A glycidyl methacrylate(BisGMA),commonly used in dental restorations and some bone cements was selected as a bonding material,because it can provide a strong bonding strength.The entangled titanium material enhanced by BisGMA has been fabricated successfully in this paper,and the structural characterization and mechanical properties of the BisGMA enhanced porous titanium with the entangled wire structure have been discussed systematically.Through the structural analysis of the BisGMA enhanced porous titanium,we found that the free cross wire nodes were fixed and the entangled and through-connected porous structure was still well maintained because BisGMA was mainly coated on the joints of the near cross titanium wires.The titanium wires of the transverse and longitudinal section showed different characteristics.With the same porosity,the average pore size of the longitudinal section was smaller than that of cross section.With the increase of BisGMA and decrease of titanium wire diameter and initial porosity,the porosity amount,porosity diameter and porosity interconnectivity of the sample decreased.Similar to the majority of porous metal materials,The compressive stress-strain curve of BisGMA enhanced porous titanium exhibited a three-stage stress-strain behavior,which is elastic stage,platform stage and densification stage.The difference is that the elastic stage was a curve;and the yielding point was not obvious when the volume fraction of BisGMA was comparatively low,and the stress still increased during the platform stage.The enhanced materials with the BisGMA content of 0-20% exhibited the elastic modulus in the range of 0.2-1.3GPa and the yielding strength in the range of 7.29-53 MPa,which were comparable to that of cancellous bones.The elastic modulus and yielding strength of the enhanced materials increased with the increase of BisGMA content and the decrease of titanium wire diameter and initial pososity.The BisGMA enhanced porous titanium exhibited a special three-stage tensile stress-strain curve: pseudo elastic stage,pseudo platform stage and loosing stage,which is different to that of other porous metals.The tension strain of BisGMA enhanced porous titanium exceeded 80%,much higher than that of other porous metals.With the increase of BisGMA content or decrease of titanium wire diameter,the elastic modulus and yielding strength increased.When the BisGMA content was 0%,the elastic modulus and yielding strength were 0.05 GPa and 1.8MPa,when the BisGMA content increased to 5%,the elastic modulus and yielding strength reached 0.67 GPa and 2.7MPa,respectively,With the same compression strain,the energy absorption capacity of the sample increased with the increase of BisGMA content,and decreased with the increase of titanium wire diameter or initial porosity;while with the same BisGMA content or titanium wire diameter or initial porosity,the energy absorption capacity increased with the increase of compression strain.The samples exhibited noticeable negative poisson ratio effect under tensile test,which could be explained by a new auxetic model with the features of indented structure and fixed cross wire nodes.The simulated body fluid experiment indicated that BisGMA enhanced porous titanium has good corrosion resistance and bioinertia,suggesting potentials for load-bearing bio applications. |
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