Laser Forming And Degradation Properties Of Iron-based Bone Implants

With the aging of China’s population,the acceleration of the pace of life and the rapid development of transportation and sports,the number of patients with bone defects is increasing,and the demand for bone implants continues to grow.Iron（Fe）is considered as a potential bone implant material due to its natural degradability,good biocompatibility and excellent mechanical properties.However,it is difficult to prepare porous structure and slow degradation.Selective laser melting（SLM）is an additive manufacturing technology,showing great advantages in the manufacture of personalized,porous medical products.Therefore,this study proposed to use SLM to prepare Fe bone implants.Firstly,SLM forming equipment was built,and then the laser forming process of iron bone implants was explored.Finally,alloy elements Zn and Ca Cl₂were introduced to improve its degradation and biological properties.The main research contents are as follows:1.A SLM system based on degradable Fe-based bone implants was constructed.The forming process of pure iron was studied by the method of fixed variables.The laser scanning galvanometer focused spot size was about 70μm,the parameter scanning spacing was 0.06 mm,and The thickness of powder coating is 0.05 mm.The laser energy density in the forming process of pure iron was studied.The variable parameters are set as laser power and scanning speed.The laser powers are:50 W,100 W,150 W,200 W,250 W,300 W;Scanning speeds are:100 mm/s,200mm/s,300 mm/s,400 mm/s.There are a total of 24 sets of data.First,the optimal laser process parameters were determined by observing the molding quality of the sintered surface.In addition,The deep mechanism of the influence of process parameters on forming quality was discussed.The optimal laser process parameters are the scanning speed of 200 mm/s and the laser power of 200 W.Under these parameters,the Fe powder could be completely melted and had good liquid-phase flow properties.,a smooth and flat surface is always obtained.2.The degradation rate of Fe bone implant was accelerated by introducing low potential element Zn to reduce the overall potential of Fe matrix.Firstly,Fe/Zn supersaturated solid solution powders were prepared by mechanical alloying（MA）,and then Fe/Zn alloy products were prepared by SLM.After the powder is treated with MA,not only the lamellar spacing of particles decreases,leading to the decrease of atomic diffusion distance,but also the formation of a large number of lattice defects and distortions promote atomic diffusion.Finally,the advantages of SLM melting and cooling retain this supersaturated solid solution phase.Electrochemical corrosion experiments show that the corrosion resistance of Fe matrix decreases and the corrosion rate increases from 0.09 mm/year to 0.112 mm/year.3.The degradation performance of iron bone implant was further improved by using Ca Cl₂self-breakdown surface passivation film.Ca Cl₂itself has strong hydrophilicity,which improves the hydrophilicity of matrix and promotes the invasion of corrosion solution.What’s more,Ca Cl₂can dissolve and release large amounts of chloride ions quickly.Chloride ions can effectively destroy the passivated layer on the surface,leading to lattice expansion and then excessive tensile stress at the interface,which will lead to uneven force on the passivated film in the sag or bulge at the interface,breaking through its stress limit and causing the passivated film to break.Results:The degradation product layer of Fe matrix doped with Ca Cl₂presents porous structure,which indirectly increases the exposed area of the matrix and accelerates the degradation of the matrix,and the corrosion rate further increases from 0.22 mm/year.In addition,cell tests in vivo demonstrated that Fe/CaCl₂ bone implants have good biocompatibility.