Modular Enzyme-Responsive Polysaccharide-Based Hydrogels for Biofabrication
Abstract: Engineered human tissue and disease models can decrease the cost and time of developing new drugs and treatments, facilitate personalized medicine, and eliminate the need for animal models that poorly represent the human body and are ethically problematic. However, the current conventional cell expansion methods using 2D culture flasks cannot enable the development of such complex multi-cellular 3D models. In general, hydrogels are considered promising materials that can make the biofabrication of tissue models possible. Hydrogels are highly hydrated materials comprised of either synthetic or naturally derived polymers, or a combination of both, and can form an environment mimicking the biomacromolecular network surrounding cells in the body. This network of biopolymers, known as extracellular matrix (ECM), is comprised of proteins such as collagen, laminin, fibronectin, and polysaccharides such as hyaluronan (HA), heparan, keratan, and chondroitin sulfate. The design of hydrogels representing the physical and biochemical properties of the ECM and which can be used for biofabrication is challenging but of increasing interest due to the rapid progress in the development of 3D and 4D bioprinting techniques. As the ECM properties differ between various tissues and disease conditions and change over time, a dynamic modular hydrogel system is needed to that can be optimized for each cell and tissue type. This thesis aims to develop modular enzyme-responsive polysaccharide-based hydrogels for 3D cell culture and biofabrication. The natural polysaccharides, hyaluronic acid (HA) and alginate (Alg) were used as the main backbone in the hydrogels developed in this thesis. HA was modified by conjugating bicyclo[6.1.0]non-4-yne (BCN) to the backbone to form HA-BCN-based hydrogels by a bioorthogonal strain-promoted alkyne-azide cycloaddition. The click reaction between BCN and azide groups allowed for modulating the biochemical and mechanical properties of the HA-BCN hydrogels. HA-BCN was further decorated with peptides to explore peptide folding and dimerization-mediated dynamic cross-linking and biofunctionalization. This system was further used to explore possibilities to dynamically alter the properties of 3D bioprinted structures, mimicking the biomineralization process in bone tissue. In a different study, a tumor model comprising fibroblast and breast cancer cells (MCF7) was bioprinted using HA-BCN cross-linked by matrix metalloporotease (MMP) cleavable and PEG-diazide MMP-resistant cross-linkers, demonstrating the synergistic relationship between hydrogel degradability and cancer cell growth, intensified by the presence of fibroblasts. The possibility of incorporating a conductive module into this hydrogel system was explored using the enzyme-assisted polymerization of ETE-S to form an interpenetrating conductive network inside HA-BCN hydrogel. The in situ and user-triggered polymerization of conductive ETE-S was demonstrated after 3D printing HA-BCN bioink containing ETE-S monomers into a lattice shape structure. We also demonstrated that cellulose nanofibrils (CNF) improved the printability of HA-BCN bioinks, and this hybrid bioink was used for printing self-standing cell-laden 3D structures. Besides these studies, a novel enzymatically triggered thiol-based chemistry was developed to address the unwanted oxidation of thiol-containing hydrogels and decrease the off-target thiol reactions during hydrogel synthesis and formation. Alginate containing sulfhydryl moieties, protected by an enzyme-labile Phacm group (AlgCP), was treated with penicillin G acylase and subsequently formed a disulfide cross-linked hydrogel. We studied the gelation kinetics and rheological properties of AlgCP and different modes of cross-linking by reversible disulfide bonds, a thiol-maleimide Micheale-type addition reaction, and ionic interactions between alginate and Ca2+ ions. MCF7 breast cancer cells cultured in the AlgCP hydrogels formed spheroids that could be harvested by GSH dissolution of the hydrogels. Finally, this novel chemistry enabled bioprinting of multi-material 3D structures with control over the printed structure's physiochemical properties, including the type and density of cross-linkers. Bioprinted fibroblasts formed extended morphology, and MCF7 cells formed spheroids in the bioprinted lattice structures. The hydrogel systems developed and explored in this thesis are modular and exhibit dynamic and tunable properties, and are applicable for a wide range of 3D cell culture and bioprinting applications. The hydrogels were either formed in response to the activity of an enzyme or remodeled by enzymes. Both enzyme-responsive HA-BCN and AlgCP hydrogel systems are promising bioinks for generating more elaborate and spatially defined cell-laden 3D structures whose features can be altered post-printing by cell-secreted and extrinsic reagents. These hydrogel-based toolkits can play a vital role in developing tissue and disease models that can make the drug discovery process faster, cheaper, and animal-free.
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