Sliding Contact Loading Improves the Tensile Properties of MSC-Based Engineered Cartilage

Articular cartilage is a load-bearing surface whose mechanical function arises from its unique properties. The structural and mechanical properties of mature cartilage are inhomogeneous through the depth and anisotropic. Tissue maturation is directed by mechanical forces; loading induces remodeling of the immature matrix, leading to increases in compressive and tensile properties and the development of tissue anisotropy [1, 2]. Limitations in cartilage repair strategies have engendered numerous efforts to engineer functional replacements. As mesenchymal stem cells (MSCs) undergo chondrogenesis in 3D culture, this cell type has been increasingly utilized in these efforts [3]. Despite their initial promise however, generating MSC-based constructs with the mechanical complexity and integrity of cartilage remains a challenge; the properties of MSC-seeded hydrogels are consistently lower than those of the native tissue [4, 5]. As mechanical stimulation is critical to cartilage development and maturation, bioreactor systems that simulate the native mechanical environment of cartilage may bridge these functional disparities. Indeed, dynamic axial compression enhances the compressive properties of both chondrocyte- and MSC-based engineered cartilage, though collagen content remains low [6, 7]. While promising, these studies were not designed to generate either depth-dependence or constructs with improved tensile properties. We therefore developed a new sliding contact bioreactor system that can better recapitulate the mechanical stimuli arising from joint motion (two contacting cartilage layers). In previous experiments using this system, we demonstrated improved expression of chondrogenic genes with short-term sliding contact of MSC-seeded agarose; these changes in gene expression were dependent on both axial strain and TGF-β supplementation [8]. Furthermore, FEM analysis of sliding contact showed that tensile strains (parallel to the sliding direction) and fluid efflux/influx were depth-dependent and highest in the region closest to the construct surface [8]. In the current study, we applied long-term sliding contact to MSC-seeded agarose constructs using the optimized parameters previously determined. We hypothesized that sliding contact would improve tensile properties and direct depth-dependent matrix remodeling.