Development of modular, dual-perfused osteochondral constructs for cartilage repair.

Treatment of full-thickness osteochondral defects is challenging because of the need to integrate reparative materials with two separate tissue types. This study demonstrates a modular tissue engineering approach in which multiphase osteochondral tissues are built from hydrogel microbeads (50-150 µm in diameter) into which mesenchymal stromal cells (MSC) and specific matrix compositions were encapsulated. Chondrogenic microbeads consisted of an agarose base containing a polyelectrolyte complex of chondroitin sulfate and chitosan. Osteogenic microbeads consisted of an agarose base augmented with collagen type 1 and nanoparticulate hydroxyapatite. Modular microbeads were created and predifferentiated separately toward a tissue-specific phenotype, and were then combined to form biphasic osteochondral constructs. A simple, multi-port perfusion bioreactor chamber was developed to preferentially supply the appropriate medium type to each phase of the osteochondral construct, resulting in tissue development over time in vitro. Cell viability in microbeads was high and tissue-specific differentiation of MSC in the microbeads was demonstrated. Flow patterns and mixing between medium types in the bioreactor were characterized and used to design the perfusion culture protocol. Over time in culture, constructs showed evidence of both chondrogenic and osteogenic differentiation, with the development of a transition zone between phases that mirrored the structure of native tissue. This study therefore demonstrates a modular approach to creating and culturing multiphase tissues in way so as to maintain tissue-specific function and also allow development of composite engineered tissues.