Bioreactor-based bone tissue engineering: The influence of dynamic flow on osteoblast phenotypic expression and matrix mineralization

Currently, osteoblast cells have been successfully seeded and cultured into 3D ceramic and biodegradable polymer scaffolds for bone substitute replacement, but the cells’ ingrowth into the scaffolds have been limited, possibly due to limited nutrient diffusion (200-800μm in PLAGA foam) in static culture conditions. Some have tried bypassing this diffusion limitation by utilizing a rotating bioreactor, which has low shear, is 3D, and most importantly, has a high mass transfer rate, imparting dynamic culture conditions. However, the densities of conventional scaffolds are generally greater than the density of the surrounding medium, causing the scaffold to collide with the bioreactor walls due to centrifugal force, inducing cell damage and disrupting cell growth and mineral deposition.

To address these issues, the investigators developed a system that utilized 3D degradable microcarrier scaffolds and a rotating bioreactor. The 3D scaffolds were composed of different mixtures of heavier-than-water (HTW) and lighter-than-water (LTW) PLAGA microspheres that were sintered together. These scaffolds, because of their varying densities, were proposed to have different interactions with the bioreactor walls than conventional scaffolds. The scaffolds were seeded with rat calvarial osteoblastic cells, which had been isolated from 2-day-old Sprague Dawley rats via the enzymatic digestive method at a density of 2E4 cells/ mL.

The cell cultures were characterized for cell proliferation, differentiation, mineralized matrix synthesis, bone marker protein expression, and morphological analysis. After 7 days of culture, cells were shown to grow mainly on the surface of the static control scaffolds unlike the scaffolds cultured in the rotating bioreactor, where the cells seemed to proliferate preferentially on the interior, as seen by SEM (scanning electron microscopy) and MTT assays. The osteoblasts also retained their characteristics and activity, as determined by alkaline phosphatase activity, osteocalcin, osteopontin, mineralized matrix formation, and calcium quantification. These results, in conjunction with video visualization, demonstrate that the mixed scaffold does indeed decrease the amount of collisions with the reactor walls, allowing the cells to proliferate and grow normally. Furthermore, by controlling the level of fluid flow, enhanced differentiation and mineral deposition could be achieved.

I chose this paper because bioreactor tissue culture has both economical and research implications: not only does it allow for better cell growth penetration into a scaffold, but the method also allows for faster and possibly more affordable methods of producing tissue engineered replacements for not only bone, but other tissues as well.