Abstract
Hierarchical biological materials such as bone, sea shells, and marine bioadhesives are providing inspiration for the assembly of synthetic molecules into complex structures. The adhesive system of marine mussels has been the focus of much attention in recent years. Several catechol-containing polymers are being developed to mimic the cross-linking of proteins containing 3,4-dihydroxyphenylalanine (DOPA) used by shellfish for sticking to rocks. Many of these biomimetic polymer systems have been shown to form surface coatings or hydrogels; however, bulk adhesion is demonstrated less often. Developing adhesives requires addressing design issues including finding a good balance between cohesive and adhesive bonding interactions. Despite the growing number of mussel-mimicking polymers, there has been little effort to generate structure-property relations and gain insights on what chemical traits give rise to the best glues. In this report, we examine the simplest of these biomimetic polymers, poly[(3,4-dihydroxystyrene)-co-styrene]. Pendant catechol groups (i.e., 3,4-dihydroxystyrene) are distributed throughout a polystyrene backbone. Several polymer derivatives were prepared, each with a different 3,4-dihyroxystyrene content. Bulk adhesion testing showed where the optimal middle ground of cohesive and adhesive bonding resides. Adhesive performance was benchmarked against commercial glues as well as the genuine material produced by live mussels. In the best case, bonding was similar to that obtained with cyanoacrylate "Krazy Glue". Performance was also examined using low- (e.g., plastics) and high-energy (e.g., metals, wood) surfaces. The adhesive bonding of poly[(3,4-dihydroxystyrene)-co-styrene] may be the strongest of reported mussel protein mimics. These insights should help us to design future biomimetic systems, thereby bringing us closer to development of bone cements, dental composites, and surgical glues.