6 hMSCs cultured in gelatin-hyaluronic acid scaffolds also presented similar behavior.26 Particle dynamics studies show that the shear stress endured by a microcarrier increases with the density difference between the culture medium and the microcarrier. Most of the scaffolds used in these systems are denser than the surrounding medium in rotating vessels, thus they impart higher selleck shear stress and centrifugal forces cause them to collide with the walls of the culture chamber during rotation.7 This leads to cell damage and interferes with cell attachment and deposition of mineralized matrix.2 This could be the explanation behind the disappointing results obtained for the comparison of rotating wall reactors with spinner flasks.
Bearing this problem in mind, several groups developed lighter-than-water scaffolds or microcarriers that exhibit migration toward the center and avoid collisions with the walls.40 Using lighter-than-water microcarriers of PLGA seeded with human osteoblastic cells (Saos-2), ALP staining was positive on day 7 and was significantly higher for samples cultured in the rotating wall bioreactor compared with non-rotating 3D controls. Substantial amount of calcified matrix was also detected by alizarin histochemical staining.41 These cells, however, appear to migrate toward the inner region of the microcarriers as cell in-growth covered the entire depth of the 2.5 mm PLGA microcarriers.42 When rat calvarial osteoblastic cells were used, similar results were found for ALP expression and matrix mineralization and also the expression levels of osteopontin and osteocalcin significantly increased under rotating conditions.
2 Values of fluid shear were estimated and were in the range of 0.16�C32 N/m2 (1.6�C320 dyn/cm2), which are similar to the estimates of physiological level of shear stress on osteocytes under flow.43 hMSCs were cultured in silk scaffolds in rotating bioreactors for 36 d and the presence of mineralized matrix and collagen type I were visible by staining and microCT. Also the mineralized structures were distributed throughout the entire volume of the constructs.44 Hollow ceramic microspheres were also used as microcarriers for rotating bioreactors with rat MSCs and rat osteosarcoma cells (ROS 17/2.8). The formation of an aggregate was seen after 10 d of culture.
Most of the beads were entirely covered with cells and extensive production of extracellular matrix was also visible. Early stages of mineralization were identified by the presence of nodules in the matrix.39 Similar results were obtained using rat MSCs.45 A variation of the common rotating bioreactor was developed and it consists in a new rotational oxygen-permeable bioreactor system (ROBS). The objective of this system is to supply optimal oxygen levels and continuous hydrostatic pressure to biodegradable polymer scaffolds. It consists of a polypropylene centrifuge tube modified with a silicon elastomer Carfilzomib to allow gas exchange.