The impact of multimodal pore size considered independently from porosity on mechanical performance and osteogenic behaviour of titanium scaffolds

Titanium porous scaffolds comprising multimodal pore ranges (i.e., uni-, bi-, tri-modal and random) were studied to evaluate the effect of pore size on osteoblastogenesis. The scaffolds were manufactured using spaceholder-powder metallurgy and porosity and pore size were kept independent. Their mechanical and physical properties (i.e., stiffness, strength, total and open porosity) were determined. In a first step, unimodal porous samples were tested with a mouse osteoblastic clonal cell line to ascertain pore size and porosity effects on cellular behaviour. Their proliferation (via cell number and total protein content), differentiation (via ALP enzyme levels) and maturation potency were investigated with gene markers (Runx2, osteocalcin and cytoplasmatic calcium). In a second step informed by the previous results, multimodal scaffolds were shortlisted according to a set of criteria that included a stiffness equivalent to that of cortical or trabecular bone, high strength and high open porosity. Their bioactivity performance was then studied to assess the synergistic effects of mixed pore ranges. The study concludes that pre-osteoblasts cultivated in unimodal microstructures with a pore range 106–212 μm of 36% total (actual) porosity and 300–500 μm of 55% total (actual) porosity achieved the largest extent of maturation. Bimodal microstructures comprising small (106–212 μm) and large (300–500 μm) pore ranges, distinctively distributed within the volume, and 40% (actual) porosity outperformed others, including multimodal (i.e. three or more pore ranges) and non-porous samples. They displayed a synergistic effect over the unimodal distributions. This should be a consideration in the design of scaffolds for implantation and bioengineering applications.