Articular cartilage is an avascular tissue type with very limited self-repair capacity, making it prone to degenerative diseases such as osteoarthritis (OA). The current therapeutic strategy for OA patients is predominantly directed towards pain relief rather than preventing degeneration and promoting the regeneration of cartilage tissue. Mesenchymal stromal cells (MSCs) have been proposed as a potential cell source for articular cartilage tissue engineering purposes. MSCs are mechanosensitive cells capable of sensing, transmitting, and responding to mechanical cues from their microenvironment through a process known as mechanotransduction. Utilizing the mechanotransductive behaviour of MSCs, cell differentiation can be directed towards a specific phenotype, in our case a chondrogenic cell. Considerable effort has been put into the identification of mechanotransductional regulators for chondrogenic differentiation of MSCs. However, due to the interdependency of the material properties by the crosslinking density, the effect of isolated material properties on cells remains unknown, and limits researchers to develop smart biomaterials for tissue engineering purposes through the concept of mechanobiology. This thesis will examine how material properties such as substrate stiffness and mesh size affect the chondrogenic differentiation potential of MSCs. A Unique approach based on the tunability of hydrogels to uncouple gel stiffness and mesh from each other will be used to assess the effect of isolated material properties. A hyaluronan based hydrogel with tyramine as a crosslinker (Ha-Tyr) enzymatically crosslinked with horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) will be used within this study. As tunable parameters for the Ha- Tyr hydrogel, the polymer and H2O2 concentration were altered independently from each other. The evidence from this study confirmed that the gel stiffness of Ha-Tyr based hydrogels is polymer concentration and H2O2 concentration dependent. The tunability of these two parameters independently from each other enabled the production of different gel conditions having matching bulk stiffness. Unfortunately, the mesh size determination was not precise enough for complete uncoupling of the gel stiffness and mesh size from each other. Despite its limitations, the findings indicate that the mesh size shows a trend towards larger mesh size with increasing polymer concentration and lower H2O2 concentration. Thereby, indicating that difference in crosslinking density may have been the driving force for the observed trend. Translation of the effect of material properties on the chondrogenic potential of MSCs indicated towards reduced cartilage-like matrix deposition with increasing crosslinking density. These findings suggest that in general the matrix deposition of cartilage-like tissue is driven by the crosslinking density rather than gel stiffness. Furthermore, gene expression levels for ECM remodelling genes like MMP1 and MMP3 showed increased expression patterns with higher crosslinking density, which hints towards the fact that these MMPs may have played a pivotal role in the observed matrix deposition. The information obtained helps to identify mechanotransductional regulators that could be used for the development of smart biomaterials for tissue engineering purposes. Ultimately, it brings us a step closer to the development of functional cartilage tissue that could be used as a possible therapeutic strategy for OA patients.

The osteochondral interface is known to smoothly mitigate the stress concentration over the surface area of the subchondral bone, with a structural transition from the cartilage to bone. These sites in the body are exposed to high forces, which makes them prone to failure. To improve the repair strategies, a better understanding of the mechanical property-structure relationship of osteochondral interface is crucial. The aim of this study was to observe the structure and map the stiffness of the equine osteochondral interface during maturation and detect a possible mechanical property-structure relationship. Osteochondral samples from the metacarpo- and metatarsophalangeal joint of 9 days, 12 days, 7 weeks, 13 months old and three mature horses were investigated in this study. Histology was performed to observe the structure. The local elastic modulus was determined using nanoindentation and the strain distribution with compression tests combined with digital image correlation (DIC). The cartilage thickness decreased with age and an emerging tidemark was detected in the 13 months old joint, indicating calcification of the cartilage starts at this age. The local elastic modulus showed a clear transitional region between low and high values for the mature interface, indicating the articular cartilage and subchondral bone region was indented, respectively. For the immature samples, a less clear transition was found and in the 9 days and 12 days old interface the entire matrix consisted of low elastic modulus values. The strain distribution from the compression tests over the interface confirmed the decreasing cartilage thickness with age. The transition location from high to low strain decreased with age. The mechanical transition from cartilage to bone showed significant changes within the interface and during maturation for the local elastic modulus and the strain distribution. These changes can be compared to the structural changes demonstrated in the histology images. The data obtained in this research provides new insights into the maturing osteochondral interface and the mechanical and structural changes within this complex biological tissue.

The solute transport across the osteochondral (OC) interface is of crucial metabolic importance for a normal function of articular cartilage, and, therefore, for the OC interface as a whole. A better understanding how the mechanical- and physiological properties of the OC interface affect the solute transport across the OC interface could lead to new insights in repair strategies. The aim of this master’s thesis was to investigate how graded mechanical- and/or physiological properties of the OC interface affect the solute transport across the OC interface. A combination of computational modelling and experimental diffusion tests on GelMA-based hydrogel plugs was used to approach the goal. Regarding the computational model, first, a multi-zone biphasic-solute finite element model that accurately replicates axial solute transport across the OC interface was designed and validated. Second, a power law function was used to apply several gradients on the initial values of the solid volume fraction (SVF), diffusion coefficient, and elastic modulus across the OC interface to study the effect of each parameter on the solute diffusion across the OC interface. On the experimental front, attempts were made to 3D-print GelMA-based hydrogel plugs but all failed. Alternatively, five groups (n = 3) of hydrogel plugs were created, each of which underwent different UV curing time, by casting GelMA into cylindrical plugs. Axial diffusion of an alizarin red solution through the hydrogel samples was recorded using a digital camera. The results of the computational model show that only the SVF plays a small role in the height of the equilibrium concentration reached in the subchondral bone layer. However, the influence of both the SVF and diffusion coefficient on the time when the equilibrium concentration is reached in the subchondral bone is considerably large. It is shown that the elastic modulus has a negligible influence on the solute transport. Regarding the experimental diffusion tests, air bubbles and/or sincere light reflections made all but six hydrogel plugs unusable for further analysis. A relationship between sample thickness and diffusion is observed in the remaining hydrogel samples. The results of the SVF computational model and experimental diffusion tests were compared, but sufficient experimental data was lacking to draw any solid conclusions from this comparison. This master’s thesis provides a new computational model of the OC interface which allows the implementation of graded parameters across the OC interface. It is concluded that the current experimental set-up is not suitable for obtaining consistent data on solute transport across hydrogel plugs. Suggestions to improve the experimental set-up are made.