Abstract
Efficient use of different bioreactor designs to improve cell growth in three-dimensional scaffolds requires an understanding of their mechanism of action. To address this for rotating wall vessel bioreactors, fluid and scaffold motion were investigated experimentally at different rotation speeds and vessel fill volumes. Low cost bioreactors with single and dual axis rotation were developed to investigate the effect of these systems on human osteoblast proliferation in free floating and constrained collagen-glycosaminoglycan porous scaffolds. A range of scaffold motions (free fall, periodic oscillation, and orbital motion) were observed at the rotation speeds and vessel fluid/air ratios used, with 85% fluid fill and an outer vessel wall velocity of ∼14 mm s−1 producing a scaffold in a free fall state. The cell proliferation results showed that after 14 and 21 days of culture, this combination of fluid fill and speed of rotation produced significantly greater cell numbers in the scaffolds than when lower or higher rotation speeds (p < 0.002) or when the chamber was 60% or 100% full (p < 0.01). The fluid flow and scaffold motion experiments show that biaxial rotation would not improve the mass transfer of medium into the scaffold as the second axis of rotation can only transition the scaffold toward oscillatory or orbital motion and, hence, reduce mass transport to the scaffold. The cell culture results confirmed that there was no benefit to the second axis of rotation with no significant difference in cell proliferation either when the scaffolds were free floating or constrained (p > 0.05).
Highlights
Every year more than 1.5 million grafts are required for bone injuries[1] and it is anticipated that the market for bone repair in the United States alone will be worth $3.5 billion by 2017.2 Tissue engineering offers the production of cellular graft substitutes as promising alternatives to traditional bone graft materials
The scaffold is suspended within the fluid-filled vessel, and the angular velocity of the bioreactor is tailored to leave the scaffold in a state of ‘‘free fall.’’ Under free fall, the scaffold appears in a fixed position within the vessel as viewed by an external observer and experiences dynamic laminar flow of culture media past and through the scaffold.[2]
The fluid velocity field when the rotation axis is orthogonal to gravity is shown in Figures 4–6a at rotation speeds of 5, 10, and 15 rpm
Summary
Every year more than 1.5 million grafts are required for bone injuries[1] and it is anticipated that the market for bone repair in the United States alone will be worth $3.5 billion by 2017.2 Tissue engineering offers the production of cellular graft substitutes as promising alternatives to traditional bone graft materials. The maximum diameter of a cell spheroid is *1 mm, beyond which a necrotic core surrounded by healthy cell forms and tissue engineering constructs is limited by the ability to supply oxygen and nutrients and remove the products of cell metabolism.[3] To overcome this, the cell/scaffold construct can be grown in a bioreactor, which increases the mass transfer of nutrients between the cells seeded within the scaffolds and the culture media allowing larger constructs to be produced. One commonly used type of bioreactor is the rotating wall vessel (RWV) bioreactor, which exists in a variety of different forms. They generally consist of a circular culture vessel that rotates about its central axis. The scaffold is suspended within the fluid-filled vessel, and the angular velocity of the bioreactor is tailored to leave the scaffold in a state of ‘‘free fall.’’ Under free fall, the scaffold appears in a fixed position within the vessel as viewed by an external observer and experiences dynamic laminar flow of culture media past and through the scaffold.[2]
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