Study Of The Effect Of Porosity And Cell Size On The Performance Of 3D Printed Zn-Mg Alloy Scaffolds

ObjectiveZinc has received a lot of attention because of its moderate degradation rate and non-hydrogen-releasing degradation behavior,which is better than that of Mg and Fe.However,the mechanical properties of pure zinc are insufficient and its biocompatibility needs to be further improved.It has been found that alloying can significantly improve the properties of pure zinc,among which zinc-magnesium alloys have become a hot topic of research in the field of bone tissue engineering in recent years due to their good mechanical properties and biocompatibility.With the rapid development of manufacturing technology and materials science,the research on bone defect repair implants has become more and more refined and precise.After implantation,the conventional block structure may cause complications such as loosening or even dislodging of the implant and resorption of bone tissue around the implant due to the stress masking effect of the elastic modulus that is not compatible with the natural bone tissue at the implantation site,which ultimately leads to surgical failure.Studies have shown that the porous structure design of implants can effectively avoid stress masking,and zinc alloy porous scaffolds produced by additive manufacturing can be personalized to match the morphology of bone defects,achieve precise regulation of mechanical properties and have good biocompatibility and degradability,and have great potential in repairing bone defects.The structural characteristics of porous scaffolds,such as porosity,unit size,pore diameter and strut diameter,are important factors affecting their biological effects,degradation rate,mechanical properties and osteogenic activity.However,the quantitative relationship between the porous structural parameters of Zn alloy scaffolds and their degradation rate,mechanical properties,and biocompatibility is not clear so far.In view of this,we prepared Zn-Mg alloy porous scaffolds with different porosity and unit sizes(40%-2 mm,60%-2 mm,80%-2 mm,60%-1.5 mm,60%-2.5 mm)by L-PBF.The mechanical properties,degradation behavior,and osteogenic activity of Zn-Mg scaffolds with different porous structures were systematically evaluated in vitro and in vivo.The relationships between the porous structure parameters and the mechanical properties,degradation rate and osteogenic activity of Zn-Mg alloy scaffolds were thus summarized.It provides a feasible reference and direction for the research and application of Zn-Mg alloy porous implants for repairing bone defects.Methods1.Five different structural porous Zn-2Mg alloy scaffolds with porosity and cell size of 40%-2mm,60%-2mm,80%-2mm,60%-1.5mm,60%-2.5mm,respectively,of Gyroid design configuration were prepared by laser powder bed fusion(L-PBF)technique,and the samples were denoted as G-40-2,G-60-2,G-80-2,G-60-1.5,and G-60-2.5,respectively.2.The morphology and elemental composition of each group of scaffolds were analyzed by Scanning Electron Microscope(SEM)and Energy Dispersive Spectroscopy(EDS).The mechanical properties were evaluated by compression experiments.The finite element models of five different structural scaffolds(40%-2mm,60%-2mm,80%-2mm,60%-1.5mm,60%-2.5mm)were used to calculate the permeability using Ansys Fluent finite element analysis platform.3.Hank’s Balanced Salt Solution(Hank’s)immersion method(including PH measurement,weight loss method and compression experiments)was used to analyze the corrosion behavior,degradation rate and mechanical properties of each group of structural supports.4.The effects of each group of scaffold immersion solution on the proliferation viability and osteogenic differentiation mineralization of mouse preosteoblasts(MC3T3-E1)were examined by CCK-8,live-dead staining,ALP and alizarin red assays.5.Five different structures of cylindrical porous Zn-2Mg alloy scaffolds were implanted into the rat femoral condylar bone defect model,and the rats were euthanized after 30,60 and 90 days of implantation,and the femurs were collected,and the femoral structures containing the implanted scaffolds were scanned by Micro-CT and analyzed using CT Analyser 1.15.2.2 software to calculate the volume of new bone tissue within 0.4 mm around the implants and within the material pores and the volume loss of the implanted scaffold6.The growth-in and scaffold degradation behavior of the new bone tissue was observed by hard tissue sectioning and staining of the rat femur,and whether pathological changes occurred by sectioning of the rat visceral tissue.Results1.Through SEM observation,it can be seen that the pores of each group of samples are interconnected,the surface is uniform and smooth,and no obvious cracks and bulges are found.In the immersion experiment,the degradation process of the five structures is relatively stable,and the degradation rate trend is G-80-2>G-60-2>G-40-2,G-60-1.5>G-60-2>G-60-2.5.The mechanical properties(compressive strength and elastic modulus)are G-40-2>G-60-2>G-80-2,G-60-1.5>G-60-2>G-60-2.5.2.Using the finite element numerical simulation method,the stress distribution trend of five different structures is calculated as G-40-2>G-60-2>G-80-2,G-60-1.5>G-60-2>G-60-2.5,which is consistent with the experimental results of external compression.The permeability trend is G-80-2>G-60-2>G-40-2,G-60-2.5>G-60-2>G-60-1.5.3.In vitro cellular assays showed that G-60-1.5 cells were less active due to the higher zinc ion concentration,while all other groups had good cytocompatibility and osteogenic activity.4.In vivo experiments demonstrated that none of the animals showed significant rejection of the scaffolds in each group,and Micro-CT scans showed that G-40-2 had higher bone volume fraction(BV/TV),trabecular number(Tb.N),trabecular thickness(Tb.Th),and lower trabecular separation(Tb.Sp).2.5 had a higher number of trabeculae(Tb.N)and lower trabecular separation(Tb.Sp)although there was little difference in bone volume fraction(BV/TV).Therefore,its osteogenic trend was G-40-2 > G-60-2 > G-80-2 and G-60-1.5 > G-60-2 >G-60-2.5,while G-80-2 had a higher scaffold volume loss rate with G-60-1.5,which was consistent with the results of in vitro submersion experiments.5.Hard tissue sections showed that there were more new bone growth in G-40-2 and G-60-1.5,which was consistent with the results of Micro-CT.G-80-2and G-60-1.5 showed more degradation products,consistent with the results of in vitro immersion test.Visceral sections showed that there were no pathological changes in the animals implanted in each group of samples.ConclusionsIn this study,Zn-2Mg alloy scaffolds with different porosity and unit size were successfully prepared by 3D printing technology,and the effects of structural parameters(porosity,cell size)on the mechanical properties,degradation behavior,biocompatibility and osteogenic activity of Zn-2Mg alloy scaffolds were systematically investigated.The main conclusions are as follows:1.By changing the structural parameters(porosity,cell size)of the implanted scaffold,the mechanical properties can be effectively regulated to avoid the stress shielding effect and match the mechanical requirements of the bone defect.Under the same condition of cell size,the smaller the porosity and the thicker the struts,the higher the strength and elastic modulus;under the same condition of porosity,the smaller the cell size and the more and finer the struts,the higher the strength and elastic modulus.2.Structural parameters(porosity,cell size)are important factors affecting the degradation behavior of 3D printed porous Zn-Mg alloy.Under the same condition of unit size,the larger the porosity,the higher the permeability and the larger the specific surface area,the faster the degradation rate;under the same condition of porosity,the smaller the unit size,the lower the permeability but the larger the specific surface area,the faster the degradation rate.3.Structural parameters(porosity,cell size)are important factors affecting the osteogenic performance and osseointegration effect of 3D printed porous Zn-Mg alloy.Under the same conditions of cell size,the smaller the porosity and the larger the surface area,the better its osteogenic effect;under the same conditions of porosity,the smaller the cell and the larger the surface area,the better its osteogenic effect.