Exploring the interconnectivity of biomimetic hierarchical porous Mg scaffolds for bone tissue engineering:Effects of pore size distribution on mechanical properties,degradation behavior and cell migration ability

Interconnectivity is the key characteristic of bone tissue engineering scaffold modulating cell migration,blood vessels invasion and transport of nutrient and waste.However,efforts and understanding of the interconnectivity of porous Mg is limited due to the diverse architectures of pore struts and pore size distribution of Mg scaffold systems.In this work,biomimetic hierarchical porous Mg scaffolds with tailored interconnectivity as well as pore size distribution were prepared by template replication of infiltration casting.Mg scaffold with better interconnectivity showed lower mechanical strength.Enlarging interconnected pores would enhance the interconnectivity of the whole scaffold and reduce the change of ion concentration,pH value and osmolality of the degradation microenvironment due to the lower specific surface area.Nevertheless,the degradation rates of five tested Mg scaffolds were no different because of the same geometry of strut unit.Direct cell culture and evaluation of cell density at both sides of four typical Mg scaffolds indicated that cell migration through hierarchical porous Mg scaffolds could be enhanced by not only bigger interconnected pore size but also larger main pore size.In summary,design of interconnectivity in terms of pore size distribution could regulate mechanical strength,microenvironment in cell culture condition and cell migration potential,and beyond that it shows great potential for personalized therapy which could facilitate the regeneration process.

Effects of dynamic flow rates on degradation deposition behavior of Mg scaffold

Degradability of bone tissue engineering scaffold that matching the regeneration rate could allow a complete replacement of host tissue.However,the porous structure of biodegradable Mg scaffolds certainly generated high specific surface area,and the three-dimensional interconnected pores provided fast pervasive invasion entrance for the corrosive medium,rising concern of the structural integrity during the degradation.To clarify the structural evolution of the three-dimensional(3D)porous structure,semi-static immersion tests were carried out to evaluate the degradation performance in our previous study.Nevertheless,dynamic immersion tests mimicking the in vivo circulatory fluid through the interconnected porous structure have yet been investigated.Moreover,the effects of dynamic flow rates on the degradation deposition behavior of 3D porous Mg scaffolds were rarely reported.In this study,Mg scaffolds degraded at three flow rates exhibited different degradation rates and deposition process.A flow rate of 0.5 m L/min introduced maximum drop of porosity by accumulated deposition products.The deposition products provided limited protection against the degradation process at a flow rate of 1.0 m L/min.The three-dimensional interconnected porous structure of Mg scaffold degraded at 2.0 m L/min well retained after 14 days showing the best interconnectivity resistance to the degradation deposition process.The dynamic immersion tests disclosed the reason for the different degradation rates on account of flow rates,which may bring insight into understanding of varied in vivo degradation rates related to implantation sites.

Effect of grain size on the mechanical properties of Mg foams

The grain size of Mg foams was innovatively refined without alteration of pore structure and relative density by subjecting multi-axial forging(MAF)process to Ti-Mg composite,an intermediary product of the fabrication process of Mg foams where the spherical Ti particles were utilized as the replication material.The feasibility of the MAF process and the grain size effect on the mechanical properties of Mg foams were discussed.The results showed that,with the appropriate strain of 0.24 applied in the MAF process,Ti-Mg composites returned to original physical appearance without generating microcracks.And complete recrystallization was achieved after heat treatment,with the grain size of the MAFprocessed Mg foams two to three orders of magnitude smaller than that of as-cast foam.The mechanical properties of Mg foams were enhanced extensively after grain refinement with the yield strength and the plastic collapse strength increased by 147%and 50.7%,respectively.A revised model integrated by the Hall-Petch law and Gibson-Ashby model was proposed,which gave a good estimation of the yield strength and the plastic collapse strength of Mg foams from the compressive behavior of the corresponding parent material,though a knockdown factor of 0.45 was introduced for the yield strength.