An optimal strategy for the functional tissue engineering of articular cartilage, particularly to accelerate construct development, may incorporate sequential application of different growth factors and applied deformational loading.

Deformational loading represents a primary component of the chondrocyte physical environment in vivo. This review summarizes our experience with physiologic deformational loading of chondrocyte-seeded agarose hydrogels to promote development of cartilage constructs having mechanical properties matching that of the parent calf tissue, which has a Young's modulus E(Y) = 277 kPa and unconfined dynamic modulus at 1 Hz G* = 7 MPa. Over an 8-week culture period, cartilage-like properties have been achieved for 60 x 10(6) cells/ml seeding density agarose constructs, with E(Y) = 186 kPa, G* = 1.64 MPa. For these constructs, the GAG content reached 1.74% ww and collagen content 2.64% ww compared to 2.4% ww and 21.5% ww for the parent tissue, respectively. Issues regarding the deformational loading protocol, cell-seeding density, nutrient supply, growth factor addition, and construct mechanical characterization are discussed. In anticipation of cartilage repair studies, we also describe early efforts to engineer cylindrical and anatomically shaped bilayered constructs of agarose hydrogel and bone (i.e., osteochondral constructs). The presence of a bony substrate may facilitate integration upon implantation. These efforts will provide an underlying framework from which a functional tissue-engineering approach, as described by Butler and coworkers (2000), may be applied to general cell-scaffold systems adopted for cartilage tissue engineering.

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Objective Chondrocyte-seeded agarose constructs of 4 mm diameter (2.34 mm thickness) develop spatially inhomogeneous material properties with stiffer outer edges and a softer central core suggesting nutrient diffusion limitations to the central construct region1. The effects of reducing construct thickness and creating channels running through the depth of the thick constructs were examined. Methods In Study 1, the properties of engineered cartilage of 0.78mm (thin) or 2.34mm (thick) thickness were compared. In Study 2, a single nutrient channel (1 mm diameter) was created in the middle of each thick construct. In Study 3, the effects of channels on larger 10 mm diameter, thick constructs was examined. Results Thin constructs developed superior mechanical and biochemical properties than thick constructs. The channeled constructs developed significantly higher mechanical properties versus control channel-free constructs while exhibiting similar GAG and collagen content. Collagen staining suggested that channels resulted in a more uniform fibrillar network. Improvements in constructs of 10mm diameter were similarly observed. Conclusions This study demonstrated that more homogeneous tissue engineered cartilage constructs with improved mechanical properties can be achieved by reducing their thickness or incorporating macroscopic nutrient channels. Our data further suggests that these macroscopic channels remain open long enough to promote this enhanced tissue development while exhibiting the potential to refill with cell elaborated matrix with additional culture time. Together with reports that <3 mm defects in cartilage heal in vivo and that irregular holes are associated with clinically used osteochondral graft procedures, we anticipate that a strategy of incorporating macroscopic channels may aid the development of clinically-relevant engineered cartilage with functional properties.