Effect of convection on osteoblastic cell growth and function in biodegradable polymer foam scaffolds
Introduction
For the repair of bone defects, the ideal biomaterial is one that has mechanical properties similar to bone, can be fabricated easily into a desired shape, supports cell attachment, contains factors to induce formation of new bone tissue, and biodegrades to permit natural bone formation and remodeling. To date many synthetic polymers have been examined that can meet many of these ideals (for a review see Behravesh et al. [1]), but a continuing challenge is incorporation of osteoinductive factors. One approach has been to incorporate growth factors into the material, and extensive research has focused on the use of bone morphogenic protein-2 [18], [19], [24]. (See Schmitt et al. [30] for a general review of bone morphogenic proteins.) An alternative approach is to culture osteoprogenitor cells in vitro within the biomaterial scaffold. In theory, these cells would proliferate and secrete an osteoinductive matrix onto the surfaces of the scaffold that would facilitate healing even if the cells did not survive implantation of the biomaterial.
Because bone defects that require surgical correction are large (typically many millimeters in size), the biomaterial scaffolds intended to bridge such defects would be sufficiently large to complicate in vitro culture. Diffusional rates of nutrients into the scaffold and metabolites out may not satisfy the metabolic requirements of seeded cells and result in suppression of cell growth and expression of phenotypic markers. Furthermore, gradients in nutrient and metabolite concentration may induce chemotaxis of the cells from the interior. These phenomena would produce an uneven distribution of cells and osteoinductive matrix favoring the exterior surfaces. Clearly, as the size of the scaffold is increased the problem would become more acute. A solution to this problem is to culture seeded scaffolds in a system that can enhance nutrient delivery.
A variety of different culturing systems have been documented, including perfusion [8], spinner flask [33], and rotary vessel [7], [26]. The spinner flask and rotary vessel use convection to ensure that the media surrounding the scaffolds is well-mixed. In addition, fluid flow across the rough external surfaces should form eddies in the superficial pores that will enhance nutrient transport into the pores. One key difference between the two culturing systems is that in the turning rotary vessel the scaffolds are allowed to tumble (i.e., their orientations to change continuously with respect to the direction of flow), whereas in the spinner flask they are fixed in place within the stirring media. The potential limitation with both of these systems is that mixing at the surface of the scaffolds may not be sufficient to deliver the necessary nutrients to the interior of the foam. A perfusion system, because it convects media through the interconnected pores of the scaffolds, should provide the best transport of nutrients. The limitation with the perfusion system is its relative complexity and difficulty in assembling and operating. Comparison of it with the other systems is necessary to justify its utility.
In this study, the growth and osteoblastic function of seeded pre-osteoblastic cells were examined for four different culturing methods: a flow system, a spinner flask, a rotary vessel, and static culture. Large porous foam scaffolds (12.7 mm diameter, 6.0 mm thick, 78.8% porous) of poly(dl-lactic-co-glycolic acid) (PLGA) were prepared by solvent casting followed by compression molding and particulate leaching and seeded with cells expanded from rat marrow stroma [9]. At the time of seeding, osteogenic factors (dexamethasone and l-ascorbic acid) were added to force primitive cells to differentiate into osteoblasts. This differentiation process is marked by sequential stages of proliferation, and expression of alkaline phosphatase (ALP) and osteocalcin (OC) [20]. To evaluate cell growth within the foams, total DNA and 3H-thymidine incorporation were measured. In addition, histological sections of cultured foams were examined at 7 days in culture to identify differences in cell distribution with culturing method. Finally, to evaluate osteoblastic function, ALP activity and OC expression were examined at 7 and 14 days in culture.
Section snippets
Materials
Foams were prepared using 75:25 poly(dl-lactic-co-glycolic acid) (PLGA) Birmingham Polymers (lot D98036, weight average molecular weight of 81 kDa, polydispersity index of 3.7 as determined by gel permeation chromatography) Birmingham, AL and NaCl (Acros, Pittsburgh, PA). 100% ethanol was purchased from Pharmco (Brookfield, CT). Trypsin/EDTA (ethylenediamine tetraacetic acid), phosphate-buffered saline (PBS) and high glucose Dulbecco's modified eagle's medium (DMEM) were purchased from Gibco BRL
Cell density and growth in foams
Cell densities per PLGA foam were determined by a total DNA assay. Cells maintained 1 day in static culture had a measured cell density of 8.6 (±1.2)×105 cells/foam. From the dimensions and porosity of the foam a void space per foam of 0.60 cm3 can be calculated. With a seeding cell suspension of 2×106 cells/cm3, optimal loading would be 1.2×106 cells/foam. For this study a 72% loading efficiency was achieved (neglecting cell growth). After 7 days in culture cell densities had increased roughly
Discussion
In this study four culturing systems were examined to ascertain the effect of fluid flow on cell growth and osteoblastic function in large porous scaffolds. Measurements of cell density and thymidine incorporation revealed similar cell growth in all culturing systems, indicating that nutrient diffusion did not limit cell growth in this study. However, histological sections demonstrated differences in cell distribution: cells were uniformly distributed in foams cultured in the rotary vessel and
Conclusions
In this study four different culturing schemes were examined for their ability to promote growth and osteoblastic function of cells seeded in porous PLGA foams. Although the techniques yielded similar cell densities, foams cultured in a rotary vessel or in a flow perfusion system had the most uniform distribution of cells. In contrast foams cultured in the rotary vessel had the lowest levels of ALP activity whereas those cultured in the perfusion system or in a spinner flask demonstrated
Acknowledgements
This work was funded by the National Institutes for Health (R29-AR42639) and the National Aeronautics and Space Administration (NAGW-5007, NAG5-4072). The authors would like to thank the assistance of Dr. John Olson with DNA measurements.
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Present address: Department of Chemical Engineering, Virginia Polytechnic Institute, Blacksburg, VA 24061, USA.