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Year of Publication
Doctor of Philosophy (PhD)
Dr. Jon Pham
Dr. Gosia Chwatko
The exceptional mechanical and adhesive properties of mussel byssal threads come from supramolecular interactions of ligands found in the proteins comprising them. Ligands can form hydrophobic interactions, π-π stacking interactions, and hydrogen bonds, but the strongest supramolecular interaction they demonstrate is metal-ligand coordination. Specifically, Histidine (His-) and nitrodopamine (nDOPA) ligands coordinate to metals in a bidentate fashion and take on tris-, bis-, and mono- modalities (3, 2, or 1 ligand per ion, respectively). The distribution of these modalities is controlled by equilibrium thermodynamics. These same ligands can be used as crosslinks in supramolecular hydrogel networks. Supramolecular hydrogel networks are dissipative, compliant, and self-healing, making them viable candidates for damping materials, injectable adhesives, and hemostats. Coupling mussel-inspired ligands terminally to four-armed polyethylene glycol (4PEG) polymers in solution with metal ions as crosslinking sites can be used to create a supramolecular network with viscoelastic properties. Relationships between network crosslinking modalities and mechanical properties have been established in prior work, however the relationship of network binding modalities and interfacial properties is less established. Through spherical probe adhesion testing, we explore how network binding modalities are associated with gel adhesive strength. We investigate two mussel-inspired hydrogels: HIS functionalized 4PEG (4PEG-HIS) with nickel (II) ions, and nDOPA functionalized 4PEG (4PEG-nDOPA) with iron (III) ions. We find that interfacial strength increases with decreasing tris- bonding within the network, based on literature speciation calculations. This suggests that the stability of tris- bonds causes less migration of ligands from the network to the interface, where they may engage in supramolecular interactions with a surface. To increase network tunability, we expand from 4PEG macromolecules in solution to 6-arm PEG (6PEG) and 8-arm PEG (8PEG) end-functionalized with His- ligands to build supramolecular, mussel-inspired hydrogels. We then conduct new calculations for crosslinking modality speciation for Ni2+-His networks. By combining these calculations with probabilities and polymer mechanics calculations, we predict modulus of our hydrogels using a phantom network model. Here, we find that increasing the number of arms per polymer increases gel modulus, and that 4PEG and 6PEG gels show good agreement to a phantom network model, though 8PEG gels deviate from a phantom network model toward an affine network model. Finally, to decrease dehydration and increase stability in ambient environments, we introduce low volatility cosolvents (glycerol and dimethyl sulfoxide) to supramolecular hydrogels. Via mechanical testing, we demonstrate that these cosolvents slightly decrease network modulus. We further propose differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) experiments to ascertain the cosolvent effects on hydrogel thermal properties.
Digital Object Identifier (DOI)
We acknowledge financial support from the National Science Foundation (NSF, OIA-1832889), NSF KY-EPSCoR (OIA-1849213), and startup funds from the University of Kentucky.
Darby, Daniel, "Mechanical and adhesive properties of supramolecular, mussel-inspired hydrogels" (2023). Theses and Dissertations--Chemical and Materials Engineering. 150.
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