Investigation of the biophysical role of the acoustic environment for cartilage tissue engineering

Osteoarthritis is characterized by degradation of the articular lining in the joints and surgical measures, such as microfracture, result in mechanically dissimilar repair tissue. An alternative repair strategy utilizes tissue engineering to generate tissue in vitro using a combination of cells, substrates, and bio-chemical/-mechanical cues. Ultrasound has been explored as a means of applying mechanical stimulation onto the tissue and, more recently, acoustofluidics has exploited the potential of levitating cells within a fluid channel to allow 3-dimensional culture in the absence of a physical scaffold. The purpose of this research is to identify the biophysical relevance of the acoustic trap on the cells during tissue culture. A layered resonator assembly was designed to culture cell aggregates in static fluid conditions. Pellet co-culture of human articular chondrocytes and skeletal stem cells was accomplished to identify an optimal chondrogenic population to use within the acoustofluidic bioreactor. The acoustic field interactions with the cells and streaming interactions with the fluid were visualized through high-speed imaging and quantified through particle tracking and analysed by finite element modelling. The acoustic field was manipulated by modifying the driving frequency range (sweep) and cycle speed through the sweep (sweep repetition rate). The long-term effects of modulating the acoustic field was determined through 21 day tissue culture with chondrocytes and quantification of the matrix composition using histology.The results from this thesis demonstrated more robust cartilage formation from human articular chondrocytes relative to skeletal stem cell pellets, therefore, long-term tissue culture experiments involved chondrocytes as the cell source of interest. The chondrocyte-acoustic interactions were quantified by mathematical modelling based on the experimental results at day 0. The fluid shear stress on the cells was found to oscillate and the stress amplitude varied in relation to the sweep repetition rate and frequency sweep. Following this, long-term bioreactor culture with the chondrocytes was performed at low and high stress conditions. The resulting histology demonstrated more robust cartilage development when the cells are supplied with a higher oscillatory shear and further modification of the culture environment to include parathyroid related hormone resulted in engineered cartilage structurally and mechanically similar to native cartilage. The conclusion from this thesis is that the acoustic field is a tunable system with biomechanical relevance that, combined with relevant biochemical factors, allows for the culture and development of hyaline-like cartilage and potentially other cell types