Laser Micromachining Of Medical Titanium Surface And The Bioactivity Study

Titanium alloys have been widely used in aerospace,biomedical and chemical industry because of its high specific strength,excellent corrosion resistance and biocompatibility. Laser gas nitrided(LGN) layer was fabricated on the surface of the titanium irradiated by a continuous wave Nd-YAG laser.Pulse Nd-YAG laser was used to micromachine pure titanium and laser gas nitrided titanium.SEM,EDAX,XRD,potentiostat and lock amplifier were used to study the microstructure,composition,surface morphology and electrochemical corrosion properties.The ability of deposition of bone-like apatite and biological activity including hemolysis and cytotoxicity were also investigated before and after laser micromachining.Dense TiN strengthedned metal base composite layer was obtained on the surface of titanium with proper laser irradiation parameters.The outmost of the modified layer is TiN with the thickness of 1～2μm.The modified layer distributed by graduation.Laser gas nitrided layer was dense and free of crack.There was a good metallurgical bonding between the modification layer and the NiTi alloy substrate.Electrochemical test showed that the corrosion potential of titanium was increased from - 0.23 V to - 0.15 V while the corrosion current was reduce from 2.5×10^-6 A/cm² to 7.6×10^-8 A/cm² which indicated that the corrosion reaction was inhabited effectively through laser gas nitriding.The optimized parameters of laser micromachining are the power of 8W,frequency of 10Hz,pulse number of 4 and the pulse width of 1ms.Regularly patterned holes with the diameter of 350～370μm,depth of 450μm and hole pitch of 500μm were fabricated on pure titanium and LGN samples.The diameter of the LGN sample increased at the depth of about 250μm.The bottom of the hole is circular.The shape is favorable for the bone to grow into the holes.Biomineralization tests showed that the LGN layer is excellent in biocompatibility. After immersion in SBF for 7 days,the Ca/P atomic ratio of the deposition on titanium and LGN samples were respectively 1.07 and 1.23 very close to that of bone-apatite,which has the Ca/P atomic ratio of 1.67.The hemolysis of titanium,LGN sample,micromachined Ti and LGN Ti were 1.2%,9 %,3%and 1.7%which are less than 5%,meeting the requirement of biomedical materials.Cytotoxicity test showed that the cytotoxicity grade of the four samples is one after cell cultivation in the 50%infusions for five days,which indicated that all the materials are biocompatible.