Tailoring the Mechanical Properties of Tissue Engineering Scaffolds made from Decellularized Cartilage

Due to its limited regeneration capacity, articular cartilage defects are considered a frequent clinical problem. Initial cartilage defects, if left untreated, will progress in severity over time and can eventually lead to degenerative joint diseases such as osteoarthritis. Hence, orthopedic surgeons would like to aim to treat cartilage defects early on to prevent further damage. Healthy articular cartilage consists of hyaline cartilage which assures its proper function. However, the major drawback of current treatments such as Autologous Chondrocyte Implantation or Microfracture is that they cannot guide the formation of a pure hyaline cartilage. Following current treatments a fibrocartilage or a mixture of fibro- and hyaline cartilage fills the defect in the place of hyaline cartilage. Fibrocartilage has the disadvantage that it degrades over time due to its inferior mechanical properties compared to hyaline cartilage. To solve this issue, future treatments should focus on creating pure hyaline cartilage. Recently, it was hypothesized that one possibility to engineer pure hyaline cartilage is the production of scaffolds which mimic the mechanical properties and zonal structure of native cartilage.

The overall goal with my Ph.D. project is the development of a scaffold based on decellularised articular cartilage, which has zone-specific mechanical properties to induce zonal lineage commitment in chondro-progenitors. The Ph.D. project was divided into three major sections. In the first section, the zonal mechanical properties of human articular cartilage were measured by instrumented indentation which was information crucial to targeting the appropriate properties in scaffolds. This resulted in finding a depth-dependent mechanical property gradient. In the second section, a decellularisation method involving supercritical carbon dioxide in combination with a CO2-philic detergent was developed that could overcome the limitations of existing complex and time-consuming protocols to decellularise articular cartilage. Using this method, bovine articular cartilage was successfully decellularised while important cell adhesion molecules were maintained. The high matrix density of articular cartilage makes cell infiltration challenging. For that reason, the articular cartilage was processed into a porous scaffold, in the third section of this thesis. The porous scaffold was produced by pepsin-digestion of the decellularised cartilage, lyophilization and covalent crosslinking. It was demonstrated that the mechanical properties of these scaffolds could be tailored by changing the digest concentration prior to lyophilization. However, the developed scaffold fabrication procedure only enabled the achievement of the mechanical properties of the superficial zone, whereas the mechanical properties were too low to target the middle, deep and calcified zone. Further analysis was therefore only focused on superficial cartilage. The superficial zone-specific protein lubricin was evident on the surface of the scaffolds after 14 and 28 days of cell culture when seeded with human chondro-progenitors. This confirms that mimicking the zone-specific mechanical properties in these prepared scaffolds can produce zonal lineage commitment.

These results show a promising concept to induce zonal lineage commitment in chondro-progenitors, a valuable feature to engineer pure hyaline cartilage with natural structure in future cartilage treatments.