Toward mimicking the bone structure: design of novel hierarchical scaffolds with a tailored radial porosity gradient

Guiding bone regeneration poses still unmet challenges due to several drawbacks of current standard treatments in the clinics. A possible solution may rely on the use of three-dimensional scaffolds with optimized structural properties in combination with human mesenchymal stem cells (hMSCs). Bone presents a radial gradient structure from the outside, where the cortical bone is more compact (porosity ranging from 5% to 10%), toward the inner part, where the cancellous bone is more porous (porosity ranging from 50% to 90%). Here, we present a new scaffold design with a built-in gradient in porosity, which approximate the radial bone structure. The pores of the outer ring were 500 μm, the ones in the middle zone were 750 μm and the inner part presented pores of 1000 μm. The porosity of each scaffold region resembled the gradient present in bone, with the outer ring having a porosity of 29.6% ± 5%, the middle and inner regions a porosity of 50.8% ± 8.1% and 77.6% ± 3.2% respectively. hMSCs behavior was analyzed in terms of growth, extracellular matrix deposition and differentiation toward the osteogenic lineage. A trend was displayed by the hMSCs residing in different zones of the gradient scaffolds after 7, 14 and 28 days of culture in mineralization medium. Osteogenic differentiation was influenced by pore size and location in scaffolds displaying a radial porosity gradient. Cell differentiation was confirmed by gene expression with upregulation of Runx2 and bone sialoprotein markers. Mineralization staining further confirmed the maturation of cell differentiation, as indicated by the presence of calcium and phosphate mineral deposits.