Mussel-inspired Biomaterials for the Enhancement of the Mechanical Properties of Soft Tissue Replacements

The marine mussel is often found anchoring itself to rocks in the intertidal zone. This environment is rich in nutrients and oxygen, but is mechanically very brutal. The mussel adheres to surfaces via a set of byssal threads that terminate in adhesive plaques. Both the adhesive plaques and threads possess outstanding mechanical properties, with the byssal thread said to possess self-healing properties. It is believed that metal-histidine coordination bonds within the thread serve as sacrificial bonds, breaking when a load is applied and reforming after removal of a load. The main focus of this thesis is to understand the impact that similar sorts of bonds can have on the bulk mechanical properties of hydrogels. It was hypothesized that reversible coordination bonds can toughen hydrogels, which are generally weak and brittle materials. The poor mechanical properties of most hydrogels have limited their use as structural biomaterials. While many hydrophobic engineering polymers possess mechanical properties suitable for structural biomaterials, these materials often lack good biocompatibility, inducing inflammation, coagulation or other negative outcomes. Hydrogels are mainly composed of water and are generally well tolerated by the body. Histidine, catechol and nitrocatechol coordination cross-linked hydrogels are studied in this thesis. A variety of techniques are employed in order to correlate the bulk mechanical response to the underlying molecular mechanics. Using histidine modified hydrogels we find a significant correlation between histidine-metal bond relaxation time and hydrogel relaxation time. We demonstrate that Fe3+ can induce covalent carbon-carbon cross-linking of catechols at acidic pH and be coordinated by catechols at basic pH. We use this discovery to produce dual coordination-covalent hydrogels that exhibit relaxation properties qualitatively similar to shock absorbing tissues. Inspired by the mussel, coordination bonds are shown to exhibit load-dissipating qualities that have the potential to enhance the robustness of soft biomaterials.