Marine mussels clinging to rocks inspire the development of novel materials. Characterization of mussel adhesive plaques describes a matrix of proteins containing 3,4-dihydroxyphenylalanine (DOPA), which provides cross-linking chemistry that allows mussels to attach fi rmly. Several synthetic polymer systems have been developed based on this DOPA chemistry. High strength bonding has been achieved with poly[(3,4-dihydroxystyrene)-co -styrene], a simplifi ed mimic of mussel proteins in which 3,4-dihydroxystyrene provides the cross-linking and adhesion of DOPA. The poly(styrene) host polymer stands in for a protein backbone. Prior efforts showed that a monomer ratio of 1:2 3,4-dihydroxystyrene:styrene within the statistical copolymer poly[(3,4dihydroxystyrene)-co -styrene] yields the highest adhesion. To enhance adhesive performance of this biomimetic polymer, a systematic study is carried out in which a range of cross-linking agents, cure times, cure temperatures, polymer concentrations, and fi llers are examined. Lap shear adhesion testing revealed substantial increases in bond strength from each study. Consensus conditions are then determined and bonding performance is assessed on several substrates. Adhesion of this system turns out to be one of the strongest of all biomimetic polymers. These studies show that DOPA chemistry may be able to stand alongside of cyanoacrylate (e.g., Super Glue) and epoxy when it comes to high strength bonding.

Characterization of marine biological adhesives are teaching us how nature makes materials and providing new ideas for synthetic systems. One of the most widely studied adhering animals is the marine mussel. This mollusk bonds to wet rocks by producing an adhesive from cross-linked proteins. Several laboratories are now making synthetic mimics of mussel adhesive proteins, with 3,4-dihydroxyphenylalanine (DOPA) or similar molecules pendant from polymer chains. In select cases, appreciable bulk bonding results, with strengths as high as commercial glues. Polymer molecular weight is amongst several parameters that need to be examined in order to both understand biomimetic adhesion as well as to maximize performance. Experiments presented here explore how the bulk adhesion of a mussel mimetic polymer varies as a function of molecular weight. Systematic structure-function studies were carried out both with and without the presence of an oxidative cross-linker. Without cross-linking, higher molecular weights generally afforded higher adhesion. When a [N(C4H9)4](IO4) cross-linker was added, adhesion peaked at molecular weights of ~50,000-65,000 g/mol. These data help to illustrate how changes to the balance of cohesion versus adhesion influence bulk bonding. Mussel adhesive plaques achieve this balance by incorporating several proteins with molecular weights ranging from 6000 to 110,000 g/mol. To mimic these varied proteins we made a blend of polymers containing a range of molecular weights. Interestingly, this blend adhered more strongly than any of the individual polymers when cross-linked with [N(C4H9)4](IO4). These results are helping us to both understand the origins of biological materials as well as design high performance polymers.

High‐performance adhesives require mechanical properties tuned to demands of the surroundings. A mismatch in stiffness between substrate and adhesive leads to stress concentrations and fracture when the bonding is subjected to mechanical load. Balancing material strength versus ductility, as well as considering the relationship between adhesive modulus and substrate modulus, creates stronger joints. However, a detailed understanding of how these properties interplay is lacking. Here, a biomimetic terpolymer is altered systematically to identify regions of optimal bonding. Mechanical properties of these terpolymers are tailored by controlling the amount of a methyl methacrylate stiff monomer versus a similar monomer containing flexible poly(ethylene glycol) chains. Dopamine methacrylamide, the cross‐linking monomer, is a catechol moiety analogous to 3,4‐dihydroxyphenylalanine, a key component in the adhesive proteins of marine mussels. Bulk adhesion of this family of terpolymers is tested on metal and plastic substrates. Incorporating higher amounts of poly(ethylene glycol) into the terpolymer introduces flexibility and ductility. By taking a systematic approach to polymer design, the region in which material strength and ductility are balanced in relation to the substrate modulus is found, thereby yielding the most robust joints.

Marine mussels deposit adhesive proteins containing 3,4-dihydroxyphenylalanine (DOPA) to attach themselves to different surfaces. Isolating such proteins from biological sources for adhesion purposes tends to be challenging. Recently, a simplified synthetic adhesive polymer, poly[(3,4-dihydroxystyrene)-co-styrene] (PDHSS), was developed to mimic DOPA-containing proteins. The pendant catechol group in this polymer provides cross-linking and adhesion much like mussel proteins do. In this work, sum frequency generation (SFG) vibrational spectroscopy was applied to reveal the structures of this DOPA-inspired polymer at air, water, and polymer interfaces. SFG spectroscopy results showed that when underwater, the catechol rings and the quinone rings were ordered, ready to adhere to surfaces. At the hydrophobic polystyrene interface, benzene π-π stacking is likely the adhesive force, whereas at the hydrophilic poly(allylamine) interface, primary amines may form hydrogen bonds with catechol or react with quinones for adhesion.

Lotus leaves are well known for their extremely water repellent surfaces. Marine mussels are also a popular research topic when considering biological adhesives. Both organisms have inspired the development of several biomimetic materials. Herein we describe a two-sided film made almost entirely from polystyrene onto which the properties of both lotus leaves and mussel adhesive are incorporated. On one side of the film, imparting micrometer and nanometer scale hierarchical roughness yields superhydrophobicity and water repellency, which facilitates rapid fluid flow. The other side of the film is modified with a copolymer mimic of 3,4-dihydroxyphenylalanine (DOPA)-containing mussel adhesive proteins. This copolymer incorporates 3,4-dihydroxystyrene, to represent DOPA, randomly into a polystyrene host polymer. The flexibility of the polystyrene backing film enabled rolling of the assembly into a tubular shape. Inside the polystyrene tube was the superhydrophobic lotus mimic. The mussel adhesive mimic, on the outer layer, was used to glue the tube to itself, thus maintaining the tubular shape. The film was also successfully glued to a variety of flat substrates. These two-dimensional and three-dimensional assemblies can be used to direct and localize the flow of fluids, with partitioning between superhydrophobic and relatively hydrophilic regions. Such assemblies may facilitate the design of liquid transport for industrial and biomedical devices.