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<div>An estimated 20 million tons of adhesives are used globally each year, and the amount is continually increasing. Glues are used in nearly every economic sector but are largely consumed by key external drivers of the industry including construction and transportation equipment to replace mechanical fasteners. Many of these applications require specific functionality, like moisture resistance, desirable mechanical properties, or low toxicity. However, specific features usually occur at the expense of adhesive strength, and there is no “one size fits all” adhesive. The search for more practical and stronger glues has contributed to the development of biomimetic adhesives. Marine mussels and other sea creatures produce biological adhesives that stick well underwater. By using nature as an inspiration for better glues, we can combine stronger bonding and additional functionality into one adhesive system. Introducing the same catechol moiety used by marine organisms into synthetic polymers has allowed us to produce adhesives stronger than commercial glues in both dry and wet environments.</div><div><br></div><div>While many of these biomimetic polymer adhesives have been prepared, few have made it to market. Here, multiple biomimetic polymer adhesives are studied and optimized for different applications to provide the next step towards commercialization. The adhesives were tailored for use on different surfaces and conditions through formulation or polymer design. Structure-function studies have showed how surface energy influences optimal adhesion with catechol-containing polymers for applications in bonding dissimilar substrates while maintaining desired mechanical properties. Multiple adhesive systems were studied in mice to assess toxicity and determine viability as potential surgical glues. Underwater formulation and application methods were also pursued to improve product development strategies for offering a competitive advantage as an underwater glue. In addition to these practical-use modifications of the adhesives, industry research and market analysis was conducted to provide insight into further applications to pursue. A cost analysis led to creating new synthetic strategies for cost-reduction and scale-up, both of which are essential in the commercialization of a catechol-containing polymer adhesive.<br></div>

<p dir="ltr">Water molecules present an obstacle between most synthetic adhesives and surfaces, limiting molecular contact between the glue and substrates. Water can also hydrolyze or swell bulk adhesives, weakening cohesive strength. Nature has solved these challenges for millennia. Marine mussels’ ability to adhere well to wet surfaces stems from an uncommon amino acid, 3,4-dihydroxyphenylalanine (Dopa). The amino acid Dopa contains a catechol moiety that contributes to adhesion and cohesion through hydrogen bonding, metal coordination, and oxidative cross-linking. Hence, biomimetic systems often incorporate catechol groups to provide strong adhesion in both dry and wet environments. In addition to Dopa, mussel adhesive proteins are rich in cationic amino acids lysine and arginine. Previous studies have suggested that cations could displace surface-bound ions, enhancing surface adhesion. However, adhesion performance varied between systems, with no agreement on whether cations are advantageous or disadvantageous. A clear picture of how cations influence underwater adhesion has yet to emerge; therefore, this thesis aims to systematically study these effects.</p><p dir="ltr">In Chapter 2, the synthesis of catechol-containing biomimetic polymers with varying amounts of quaternary ammoniums is presented. Quaternary ammoniums, unlike protonated primary amines, contain non-reactive cations and were used to isolate effects from only charges on adhesion. In Chapter 3, differences between reactive primary amines and quaternary ammoniums were investigated. Structure-function studies have shown how cations influence bulk cohesion versus surface adhesion in dry, under deionized water, and under salt water. The roles of cations in adhesion were complex, with both cohesive and surface bonding relevant in different ways, sometimes even working in opposite directions.</p><p dir="ltr">Furthermore, a styrene-based catechol-containing polymer with excellent underwater adhesion performance is ready to enter the market, but several barriers hinder its industrial implementation. In Chapter 4, new synthetic strategies were developed to scale up and reduce the cost of producing p[vinylcatechol-<i>co-</i>styrene], which are essential for commercialization. This was achieved by selecting cheaper starting materials, switching from anionic to suspension polymerization, and optimizing deprotection reaction conditions. This change also improved adhesion in both dry and underwater conditions. This work is presented as part of our effort to advance the design of adhesives that function in challenging environments.</p>

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<p>The development of new materials has been inspired by lessons learned from natural systems. In the area of underwater adhesion and adhesives, inspiration has come from the complex protein adhesives generated by marine organism such as barnacle and mussels. These protein systems have a high incorporation of a unique amino acid, dihydroxyphenylalanine, and provides the unique adhesive qualities synthetic systems strive to emulate.</p> <p>By understanding how marine mussels stick to a variety of surfaces, new strategies can be explored for preventing the adhesion of biological organisms to various substrates. A continuous concern for marine vessels is the detrimental impact caused by biofouling on the hull of the ship. Fuel consumption can increase as the vessel’s drag increasing fuel consumption and non-native species can be introduced into new environments. Taking inspiration from catechol curing, new oxidative surfaces were investigated as potential antifouling coatings.</p> <p>Further insight into the marine mussels ability to apply and cure its adhesive on a variety of substrate has also inspired various synthetic polymers. The catechol moiety can be incorporated into a polymer backbone to give a new solvent based adhesive. Further investigation of the poly(styrene-co-(3,4-dihydroxystyrene)) adhesive system was done to formulate an underwater adhesive for unique use cases. A terpolymer was also explored as an ideal adhesive taking inspiration from the mussels by incorporating flexible, stiff, and sticky components to give a tunable adhesive.</p> <p>Having a strong bonding synthetic adhesive that can be used on a laboratory scale is good for academic investigation, but not of use outside the lab if it cannot easily be produced on a commercial scale. With the goal of large scale synthesis, a new polymerization method was introduced addressing some of the issues currently preventing commercial scale production.</p><br>