Although they are generally considered biologically simple structures, musculoskelatal tissues consist of highly organized three-dimensional networks of cells and matrix, giving rise to tissue structures with remarkable mechanical properties. Although the field of cartilage and bone tissue engineering has progressed significantly in recent years, the development of structurally ordered tissues has not been accomplished.

Studies of the morphology and biology of articular cartilage reveal an elaborate, highly ordered structure, consisting principally of three zones: superficial, middle, and deep. The chondrocytes occupying each zone demonstrate differences in mitotic and metabolic activity and provide unique contributions to the overall mechanical properties of the tissue. Despite the importance of zonal organization on the physiological role of the tissue, few tissue engineering strategies have attempted to incorporate this feature into the design. To create higher order musculoskeletal structures, such a the recreation of the zonal organization of articular cartilage, or a complete joint with integrated cartilage and bone tissue, engineering designs may require controlling the spatial organization of multiple cell types in a three-dimensional system that encourages the production of the appropriate types and amounts of matrix molecules in the appropriate structural framework. Thus, multiple cell types, scaffold architectures and/or spatiotemporal growth factor presentation profiles must be combined in an innovative way to improve cell and matrix organization over current methods.

Photopolymerization is a method to form hydrogels that allows for temporal and spatial control over scaffold formation, enabling the creation of complex structures such as hydrogels consisting of multiple layers. We envision multi-layered hydrogels to be valuable tissue engineering scaffolds for

(i) encapsulating multiple cell types in a stratified organization, and
(ii) controlling spatial presentation of bioactive factors within the scaffold.

Recently, we reported the use of a PEO-based photopolymerizable hydrogel to develop a multi-layered scaffold capable of encapsulating chondrocytes from different zones of bovine articular cartilage in a stratified arrangement. The goal of this strategy is to encourage the production of the appropriate types and amounts of cartilage matrix in a stratified framework, leading to regeneration of the zonal organization in tissue engineered cartilage. In addition, this culture system can serve as a model to gain a better understanding of the influences of each zone on cell behavior and tissue regeneration.

We are using the multilayering technique to develop an injectable osteochondral implant. This may be accomplished by encapsulating different cell types capable of producing cartilage and bone in each layer of a bilayered hydrogel, or spatially controlling the presentation of cartilage and bone differentiating factors to a single population of undifferentiated cells in each layer.

We have demonstrated that photopolymerizable hydrogels may be useful in organizing cell types and bioactive factors, potentially leading to improvements in cartilage tissue engineering.