In their first test, the researchers found that stem cells changed into cartilage or bone cells in the directions of two opposing soluble growth factor gradients: one that promotes cartilage, called TGF-beta 1, and another that promotes bone, called BMP-2. The stem cells were placed in a solution of modified alginate, a material derived from seaweed that can form a jello-like material called a hydrogel when exposed to low level ultraviolet light.
The solution was divided between two computer-controlled syringe pumps, with BMP-2 in one syringe and TGF-beta 1 in the other. By controlling the rate of injection with the pumps and using a mixing unit, a hydrogel with a BMP-2 gradient starting with a large amount and tapering to nearly none and an opposing TGF-beta 1 gradient from low-to-high was formed.
The hydrogels were further modified in such a way that the growth factors were retained for a longer period of time. This enabled prolonged exposure of stem cells to the growth factors and further control over their differentiation into bone or cartilage cells.
The researchers then modified the hydrogel with a gradient of adhesion ligands, molecular strings that allow the stem cells to attach to the hydrogel itself. After two weeks of culturing the cells, they found the highest number of cells in the hydrogel region where the concentration of ligands was highest.
In a third test, they created a gradient of crosslink density within the hydrogels. Crosslinks provide structure to the gels. The lower the density, the more flexible the hydrogel; the higher, the stiffer the gel.
After two weeks, more cells were found in the most flexible gel regions within the gradient. The flexibility may allow for more free movement of nutrients and removal of waste products, Alsberg explained.
"This is exciting," Alsberg said. "We can look at this work as a proof of principle. Using this approach, you can use any growth fa
|Contact: Kevin Mayhood|
Case Western Reserve University