Mechanically Stimulated Photopolymerized Hydrogels for Cartilage Tissue Engineering
Stephanie J. Bryant, Idalis Villanueva, and Garret Nicodemus. Chemical and Biological Engineering, University of Colorado, Campus Box 424, Boulder, CO 80309
Photopolymerizable crosslinked hydrogels are attractive materials for tissue engineering for their ability to be gelled in situ with controlled architecture, a wide range of chemistries and desired macroscopic properties. Specifically, polyethylene glycol (PEG) hydrogels provide a neutral environment that is suitable for chondrocyte encapsulation. This study examines the influence of PEG gel properties, in the form of the crosslinking density (ρx), on chondrocyte response to loading during early culture times up to 48 hours. PEG gels were fabricated with a range of ρx's, 0.09 to 0.7 mol/l, by varying the PEG concentration to yield gels with the same chemistry but different network structures. An increase in ρx resulted in gels with increased compressive moduli from 55±2 to 870±30 kPa, respectively. When subjected to cyclic compressive strains (1 Hz, 15% amplitude strains), chondrocyte response was dependent on gel ρx. Nitric oxide, an inter- and intracellular signaling molecule, was inhibited (-41±8% compared to unloaded controls) in gels with low ρx (0.09 mol/l) and stimulated (+74±34% compared to unloaded controls) in gels with high ρx (0.7 mol/l) as measured by nitrite, the stable end product. In contrast, cell proliferation decreased with increasing ρx. A lower frequency (0.3 Hz, 15% amplitude strains) resulted in a similar, but opposite trend. An increase in ρx resulted in a decrease in NO production and an increase in cell proliferation. Dynamic loading (1 Hz) also affected gene expression as a function of gel ρx. Collagen type II mRNA expression increased, while aggrecan mRNA expression was unaffected, in gels with increased ρx as determined by real time PCR. Together, these results demonstrate that crosslinking density in PEG alters chondrocyte response in terms of cell signaling, cell proliferation and gene expression to applied dynamic compressive strains. Studies are currently underway to examine the effects of temporal changes in gel structure via degradation on chondrocyte response. Through careful manipulations in gel properties, chondrocyte response may be modulated to achieve a desired response.