Title: Scaffold Design and Optimization for Osteochondral Interface Tissue Engineering
Description: A thin layer of calcified cartilage at the native cartilage-to-bone junction facilitates integration between deep zone articular cartilage and subchondral bone, while maintaining the integrity of the two distinct tissue regions. Regeneration of this interface remains a significant clinical challenge for long-term and functional cartilage repair. The strategy for osteochondral interface formation discussed in this thesis focuses on the design and optimization of a biomimetic scaffold for stable calcified cartilage formation. The ideal interface scaffold supports chondrocyte biosynthesis and the formation of calcified cartilage with physiologically-relevant mechanical properties. Furthermore, the interface scaffold allows for osteointegration and the maintenance of the calcified cartilage matrix. It is hypothesized that ceramic presence and zonal chondrocyte interactions regulate cell biosynthesis and mineralization, and these cell-matrix and cell-cell interactions are essential for calcified cartilage formation and maintenance.

Biomimetic design parameters for an interface scaffold were determined by characterizing the native interface in terms of mineral and matrix distribution. A composite hydrogel-hydroxyapatite scaffold was then designed to support formation of a functional calcified cartilage matrix. The hydrogel phase maintains the chondrocyte phenotype and allows for incorporation of ceramic particles, while the biomimetic ceramic phase is osteointegrative and decreases the need for cell-mediated mineralization. This scaffold was optimized <italic>in vitro</italic> based on hydrogel type, chondrocyte population, and ceramic particle size. The collective findings from these cell-ceramic interaction studies determined that hypertrophic chondrocytes, cultured in the presence of micron-sized hydroxyapatite particles, exhibit enhanced hypertrophy and matrix deposition. Scaffold ceramic dose and seeding density were also optimized for promoting calcified cartilage formation <italic>in vitro</italic>. In order to implement the scaffold for integrative cartilage repair, a scaffold was designed to regenerate both uncalcified and calcified cartilage on a bilayered hydrogel scaffold.

Furthermore, a polymer-ceramic nanofiber component was added to augment the original design for <italic>in vivo</italic> implementation. The hydrogel-nanofiber composite scaffold was evaluated <italic>in vivo</italic> and found to support mineralization and osteointegration within the bone region while preventing endochondral ossification within the repair tissue. Finally, inspired by the stratified organization of zonal chondrocyte populations above the calcified cartilage interface, the layered hydrogel model was used to determine the role of zonal chondrocyte organization on calcified cartilage stability. This thesis collectively explores cell-ceramic and cell-cell interactions, and their ramifications for calcified cartilage formation and maintenance. Specifically, ceramic presence promotes the deposition of a calcified cartilage matrix by hypertrophic chondrocytes in a dose-dependent manner, and furthermore, communication between surface zone and deep zone chondrocyte populations suppresses mineralization within articular cartilage above the calcified cartilage interface.

It is anticipated that the scaffold design strategy developed in this thesis can also be applied to the regeneration of other complex interfaces where there are transitions from soft-to-hard tissue.