Polyelectrolyte Complex/Covalent Interpenetrating Polymer Network Hydrogels
The objectives of this dissertation are to gain a fundamental understanding of the assembly and properties of polyelectrolyte complex (PEC) hydrogels, harness the self-assembly of PEC hydrogels to mitigate the shortcomings of existing photocrosslinkable materials, and create PEC/covalent interpenetrating polymer network (IPN) hydrogels with precisely tuned microstructure and material properties. PEC hydrogels self-assemble swiftly upon mixing oppositely charged triblock polyelectrolytes and feature tunable mechanical properties, microstructural diversity, as well as self-healing attributes and responsiveness to salt and pH changes in their environment. Moreover, the nanoscale PEC domains that constitute the three-dimensional (3D) network in PEC hydrogels spontaneously encapsulate charged macromolecules (e.g., protein, drug, and nucleic acids). However, a few drawbacks limit their widespread biomedical applications. The electrostatically assembled 3D network results in their moderate shear strength, poor tensile strength, and uncontrolled swelling. We demonstrate that interpenetration of the PEC network with a covalently-linked network not only addressed the limitations of the PEC hydrogels but also contribute to synergistic improvements in the mechanical performance of the resulting IPN hydrogels. The unique attributes of PEC hydrogels – swift self-assembly in aqueous surroundings, rapid moduli recovery upon cessation of flow, and interim insolubility in water – were further harnessed to employ them as scaffoldings for photocrosslinkable materials. PEC hydrogels were compatible with four representative precursors (linear and 4-arm poly(ethylene glycol) acrylate, acrylamide, and gelatin methacryloyl (GelMA)) and their corresponding networks, which featured different molecular weights, polymer origins, crosslinking mechanisms, and molecular structures. Mixing of oppositely charged bPEs with photocrosslinkable precursors resulted in precursor-encapsulating PEC hydrogels (PEC+precursor hydrogels) that exhibited significantly higher viscosity and shear strength as compared to precursors solutions. The PEC+precursor hydrogels did not suffer from issues such as dilution, precursor deactivation, and unwanted flows that affect in situ crosslinking of photocrosslinkable hydrogels in wet environments. Moreover, the PEC/precursor IPN hydrogels produced by in situ crosslinking of the PEC+precursor hydrogels exhibited improved shear and tensile properties. Consequently, PEC+GelMA hydrogels were demonstrated as robust bioinks for extrusion-based 3D bioprinting at physiological temperatures (37 �C). The PEC+GelMA inks avoided undesirable secondary flow and produced a higher printing resolution, enabling printing of intricate multilayered constructs.