The effects of space holder size and volume fraction on the geometric characteristics and performance of absorbable iron scaffolds

Abstract

Although many studies have been performed on porous absorbable iron scaffolds created by the space holder method, no studies have considered the influence of variations in space holders on absorbable scaffolds. The present study aimed to clarify the effects of space holder particle size and volume fraction, on the pore structure, resultant mechanical properties and absorbability of iron scaffolds.
Spherical iron particles were used as the matrix powder and mixed with coarse and fine rectangular urea space holding particles in volume fractions of 20, 40 and 60%, to create six different scaffolds with the space holder method. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to observe the scaffolds surface. Thereafter x-ray microtomography and Archimedes’ tests were used to characterize the scaffolds topology. The absorbability of scaffolds was characterized by potentiodynamic polarization measurements and weight loss measurements after 3 and 11 of days immersion in modified simulated body fluid (m-SBF). Mechanical properties were determined by performing diametral compression of the scaffolds, both before and after immersion in m-SBF. SEM images showed that morphology of the final pores was similar to that of space holder particles. Pores at lower porosities are isolated and become more connective or fully connected at higher porosities. Archimedes’ data showed that space holder fraction was directly related to porosity. Pore connectivity, measured by 휇CT, is higher for fine space holding particles. Until at 60% space holder volume fraction, the high amount of space holder particles agglomerated. Furthermore, it was observed that space holding particles had a large preference to move in a plane perpendicular to the applied compaction pressure. Diametral compression showed that increasing porosity resulted in lower yield strength and elastic modulus of scaffolds. The effects of space holder particle size and immersion time on mechanical properties were less pronounced. PDP measurements showed that the corrosion mechanism of scaffolds did not change due to change in space holder particle size or volume fraction. Weight loss measurements after 3 and 11 day immersion in m-SBF showed that increasing porosity resulted in higher weight loss of scaffolds. Weight loss was marginally affected by space holder particle size. Space holder particle size affected the created pore size and surface area yet also changed the pore connectivity. Similarly, the space holder volume fraction affected the created porosity and pore connectivity yet also changed the pore size and shape. This means that the pore structure not independently changes due to space holder particle size or fraction, but rather as a combination of both. Furthermore, in this study it was mainly the created porosity that influenced the mechanical properties and absorption rate of the scaffold. Space holder particle size did not show the same differentiating results, although this might be caused by agglomerating space holder particles and its dependency on volume fraction. Differentiating the created pore structure more and using a longer immersion time should be the next objectives in the determination of the effect of space holder particles on the performance of absorbable iron scaffolds for orthopedic applications.