Direct ink writing of porous Fe scaffolds for bone implants: Pore size evolution and effect on degradation and mechanical properties

Existing research suggested 300∼600 μm as the optimal pore size range for metallic bone implants. The human bones, however, have numerous pores smaller than 300 μm for bone ingrowth, cell growth and migration, fluid flow, and so forth. The bone implant with such small-sized pores has been rarely manufactured and its property is unclear. In the paper, biodegradable Fe scaffolds of different pore sizes (50 μm, 100 μm, 150 μm, 200 μm, and 300 μm) were fabricated by direct ink writing (DIW) with subsequent sintering. The interconnected pores of all scaffolds are highly precise and have no residual powders. The in-vitro degradation and compression tests were conducted on the scaffolds to evaluate the effect of pore size on degradation and mechanical behaviors, respectively. The results indicate that a decrease in pore size leads to an increase in degradation rate, from 0.0423 ± 0.0014 to 0.0433 ± 0.0035 mm/year, with the exception of the 50 μm scaffolds which exhibit the lowest value of 0.0052 ± 0.0018 mm/year due to clogging by degradation products; meanwhile, both elastic modulus and yield strength rise from 344.8 ± 18.6 MPa to 625.0 ± 52.5 MPa and from 9.5 ± 0.9 MPa to 15.1 ± 0.8 MPa, respectively. The pore size of 100 μm shows the most significant potential for use in Fe bone implants regarding its good degradation and mechanical behaviors. Our work provides new insight into the preferable pore size of metallic bone implants produced by additive manufacturing.