Abstract
In recent years, scaffold-based strategies adopting hydrogels for cartilage regeneration have received significant attention, offering manipulation of chemical, biological and physical parameters to match design criteria. However, our detailed understanding of cell-material interactions is still rather limited, and furthermore the library of hydrogel systems compatible with cartilage biofabrication remains exceedingly narrow. Designed constructs typically suffers from inadequate structural integrity and/or tissue compatibility (biofabrication window). The focus of this thesis was therefore to systematically investigate pro-chondrogenic structure-function relationships to engineer cell-instructive, photo-polymerisable hydrogels, and determine the effect of integrating chemical, biological and physical properties to synergistically enhance cartilage differentiation.
It was firstly investigated if the chemical modification necessary to covalently incorporate bioactive molecules, such as heparin, in photo-polymerisable hydrogels would affect the native bioactivity and subsequent tissue formation in commonly applied gelatin methacryloyl (GelMA) hydrogels. Results demonstrated that thiolation was superior for preserving anticoagulation and growth factor signaling capacity as compared to the more commonly applied methacrylation. Although low heparin retention was revealed, covalently incorporating thiolated heparin (HepSH) in chondrocyte-laden GelMA hydrogels yielded significantly greater differentiation and matrix deposition. With further concerns around photo-toxicity, application of a novel set of photoinitiators, ruthenium (Ru) combined with sodium persulfate (SPS), that absorbs in the visible light range (Vis) was investigated. Results demonstrated that chondrocytes exposed to Vis +Ru/SPS had significantly improved cell viability and metabolic activity as compared to cells exposed to widely applied ultraviolet light (UV) and Irgacure® 2959 (I2959). Interestingly, however, polymerisation of GelMA-HepSH hydrogels with UV + I2959 revealed an innovative strategy to introduce hydrolytic degradation of GelMA hydrogels with a thiol dose-dependent repose. It was successively demonstrated that HepSH covalently incorporated in GelMA hydrogels enhanced chondrogenesis as a direct effect of bioactivity as compared to the physical attributes of the hydrogels.
However, the widely applied GelMA platform demonstrated limited options to control the crosslinking process, mainly evident by poor HepSH retention and a narrow biofabrication window compatibility. To combine the powerful properties of thiol-ene crosslinking with the favorable biological properties of gelatin and HepSH, allyl glycidyl ether functionalised gelatin (GelAGE) was developed as a pure step-growth photo-polymerisable bioink for distinct biofabrication applications; digital light processing (DLP) and bioplotting. GelAGE was further copolymerised with HepSH, demonstrating low amounts of unreacted and leachable compounds, properly guiding the cells down a chondrogenic pathway with significantly enhanced matrix synthesis and compressive modulus following 5 weeks in vitro culture. In addition to exploring the application of single bioactive molecules to provide structural and biological support for embedded or invading cells, decellularised vitreous humor (VH), whose makeup resembles the natural composition of the cartilage matrix, was successfully isolated and favourably applied with cells to support chondrogenic differentiation in vitro.
In this thesis, we successfully demonstrated the design of cell-instructive hydrogels for promotion of functional cartilage tissue in vitro through the identification of controllable chemistry systems and cell-instructive matrix components. The combination of chemistry (thiol-ene), biological (heparin or VH) and physical (photo-initiating systems and macromolecules) elegantly widened the biofabrication window and enhanced cartilage differentiation.