Development of multivalve-based bioprinting technology
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Pluripotent stem cells (PSCs) are the most favourable sources of cells for tissue engineering applications due to their unique potency and self-renewal characteristics however they are quite fragile and can be directed to differentiate erroneously by the application of external forces. A novel multi-nozzle valve-based bioprinting platform was developed that was able to position droplets of bio-ink – such as cells in suspension – with high spatial accuracy and low impact. Volumes as low as 2 nL were successfully dispensed. Several different versions of the machine were created before the final machine was made integrating improvements and solutions to problems encountered during development. A complete evaluation of cell compatibility was carried out in order to quantify the response of cells to the bioprinting process. In the first ever study of this kind, the viability and pluripotency of human embryonic and induced pluripotent stem cells was investigated post-printing and were found to be almost completely unaffected by the bioprinting process. Many cells require a 3D culture environment in order to maintain their in vivo functions. A hybrid bioprinted-hanging-droplet technique was used to create uniform spheroid aggregates of programmable sizes from PSCs which could be used to direct PSC differentiation or as building blocks for tissue generation. Hydrogels can also be used to recreate the 3D in vivo cellular environment using the bioprinter. Alginate and hybrid polypeptide-DNA hydrogels were used, the latter for the first time with a bioprinting platform. Complex 3D structures could be created in a layer-by-layer approach with programmable heterogeneous properties throughout. Cells were added to the hydrogel precursor solution and used to bioprint 3D structures. The cells were found to be functional and highly viable while being encapsulated throughout the 3D structure of the bioprinted hydrogel which will allow the future creation of more accurate human tissue models. PSCs were successfully directed to differentiate into hepatocyte-like cells. It was shown that the bioprinting process did not interrupt or alter the pre-programmed differentiation of the cells which means that these cells can be patterned in 3D using the bioprinter while differentiating, greatly speeding up the creation of mini-liver tissue. Hepatic stellates and HUVECs were co-cultured with the hepatocyte-like cells in various ratios in an attempt to improve their hepatic function. However, no clear improvement in cytochrome P450 activity was observed indicating that further optimisation is required in this area.