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High Strength Underwater Bonding with Polymer Mimics of Mussel Adhesive Proteins Michael North, Chelsey A. Del Grosso, and Jonathan James Wilker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00270 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017
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ACS Applied Materials & Interfaces
High Strength Underwater Bonding with Polymer Mimics of Mussel Adhesive Proteins Michael A. North†, Chelsey A. Del Grosso†, and Jonathan J. Wilker†,‡,* † ‡
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, USA. School of Materials Engineering, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907-2045, USA.
KEYWORDS: adhesive, biomimetic, mussel, polymers, underwater ABSTRACT: When it comes to underwater adhesion, shellfish are the true experts. Mussels, barnacles, and oysters attach to rocks with apparent ease. Yet our man-made glues often fail when trying to stick in wet environments. Results described herein focus on a copolymer mimic of mussel adhesive proteins, poly(catechol-styrene). Underwater bonding was examined as a function of parameters including polymer molecular weight and composition. In doing so, several surprising results emerged. Poly(catechol-styrene) may be the strongest underwater adhesive found to date. Bonding even exceeded that of the reference biological system, live mussels. Adhesion was also found to be stronger under salt water than deionized water. Such unexpected findings may contradict an earlier proposal in which charged amino acids were suggested to be key for mussel adhesive function. Taken together, these discoveries are helping us to both understand biological adhesion as well as develop new materials with properties not accessed previously.
INTRODUCTION Underwater adhesion presents several technical challenges.1 When applied to submerged substrates, glues interact with water instead of forming adhesive bonds atop the surface or cohesive bonds within the bulk. Although man-made adhesives do not work well underwater, nature has been addressing such design constraints for eons. A trip to the beach will show rocks covered by organisms including oysters, barnacles, sea grasses, and tube worms, each sticking with adhesives.1-3 The common blue mussel (Mytilus edulis) has gained a measure of fame by being the role model for our understanding of wet bonding (Figure 1).1-3 This shellfish attaches upon depositing a mixture of proteins containing an atypical amino acid, 3,4-dihydroxyphenylalanine (DOPA) (Figure 1).1-4 Cross-linking of these proteins generates cured glue. Despite such insights, we still do not understand how this system can work so well in an environment that tends to be particularly harsh toward adhesion. Furthermore, we have not yet been able to take what we have learned from biology and transition this knowledge into fully functioning biomimetic materials.5 In recent years there has been a blossoming of material systems that mimic various aspects of mussel adhesive proteins.1-3, 6-9 Quite often, synthetic polymers are used to substitute for the protein backbone and derivatives of catechol are appended to these chains for providing the cross-linking and adhesion chemistry of DOPA (Figure 1).7-8, 10-22 Notable findings have included hydrogels being developed with selfhealing properties.23 In terms of adhesion, dry bonding
Figure 1. Underwater bonding of a marine mussel and poly(catechol-styrene). The structures of 3,4dihydroxyphenylalanine (DOPA) in a protein and poly[(3,4dimethoxystyrene)-co-(styrene)] are shown. The mussel is attached to a piece of aluminum. The polymer is forming a lap shear joint between two aluminum substrates. Catecholcontaining components are highlighted in yellow.
strengths of mussel mimicking polymers have been able to exceed those of long established commercial products including Super Glue.20 Despite these advances, we are still lacking high strength bonding with synthetic materials when used underwater. We have also not developed much context for the performance of biomimetic materials in comparison to the biological counterparts. In work described here, a polymer system is shown to bond underwater with high strengths in bulk applications. Several aspects of polymer design were
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Figure 2. Polymer influences upon underwater bonding. a) Adhesion as a function of molecular weight for poly(catechol-styrene). b) Changes in adhesion with varying catechol content of the polymers. Data are reported with mean values ± 90% confidence intervals.
between 27% and 33% (Table S1). Representative proton nuclear magnetic resonance spectra and gel permeation chromatograms are provided in the Supporting Information section (Figures S1 and S2). To test underwater bonding, polished aluminum substrates were submerged into a tank of artificial sea water (Figure 1). Poly(catechol-styrene) was dissolved into chloroform and the solution deposited onto a substrate. The choice of chloroform was dictated by needing a solvent denser than water such that the adhesive formulation did not float up and off the substrate. Initial studies with acetone solvent, being miscible with water, brought about rapid precipitation and no surface bonding of the water insoluble polymer. Hydrophobic solvents such as chloroform are unable to dry a submerged surface.9, 28-30 Waters remain bound to high energy surfaces even when covered by hydrophobic liquids or after movement to ambient environments.9, 28-30 In other words, surface-bound waters are almost always present, creating a barrier to adhesive materials from making beneficial contacts. Furthermore, delivering polymers to surfaces in hydrophobic solvents is not a means of displacing these waters and fostering bonding. If achieving underwater bonding was a simple matter of placing polymers into hydrophobic solvents, several commercial products would likely already have demonstrated suitable performance. After deposition of the polymer solution onto one substrate, a second piece of aluminum was then placed atop the first to create a lap shear joint. Addition of a weight held the substrates together during a cure period, the assembly was removed from the tank, and then pulled apart immediately by a materials testing system to quantify bonding. Maximum force at failure was divided by substrate overlap area to provide adhesion values (in MPa). In all cases examined here bond failure was of a cohesive nature. Figure S3 shows that, after testing, there was polymer distributed evenly across both substrates. During mechanical testing, all bonds failed in a brittle fashion, indicating drying and insignificant amounts of solvent persisting within the bond. Data in Figure 2a show how adhesion changed as a function of molecular weight. Performance appeared to have peaked at ~85,000 grams/mole. To determine statistical differences in the mean values of adhesion for poly(catechol-styrene) as a function of molecular weight, data were analyzed by an
explored and, in the end, underwater bonding exceeded that achievable by the animals after which this material was modeled. In many regards the findings are surprising and serve to influence our understanding of the important chemical features used by biological adhesives.
RESULTS AND DISCUSSION Polymer Design Prior studies have shown that the random copolymer poly[(3,4-dihydroxystyrene)-co-styrene] (“poly(catecholstyrene)”) is a useful mimic of mussel adhesive proteins in terms of dry bonding performance (Figure 1).13, 20 Catechol groups pendant from a polystyrene host can represent, respectively, DOPA distributed throughout the polypeptide chains of mussel adhesive proteins.13, 20 The copolymers were synthesized on gram scales, thereby enabling bulk adhesion testing. Dry bonding of the polymer was appreciable, well into the Megapascal (MPa) range for lap shear joints between metal, plastic, and wood substrates.13, 20 How well might the dry bonding of this biomimetic system transfer to underwater applications? In terms of polymer composition and molecular weight, which derivatives should bring about the highest underwater bonding performance? Looking at the proteins found in a mussel’s adhesive plaque does not provide too much help with regard to design. The DOPA content can range from 3% to 30% of all amino acids.24 Molecular weights are as low as 6,000 and as high as 110,000 grams/mole.25 Adhesion Studies In order to explore underwater adhesion, a logical starting point was a polymer of ~33% 3,4-dihydroxystyrene and ~67% styrene, given prior data on this poly[(3,4dihydroxystyrene)33%-co-(styrene)67%] composition yielding maximum dry adhesion.13 Molecular weight can have a major impact upon adhesion, with shorter chains providing surface wetting, yet longer molecules being best at bringing about polymer-polymer interactions for cohesion.16, 26-27 Poly(catechol-styrene) of several different molecular weights was made here by changing the ratio of n-butyl lithium polymerization initiator to monomers in the reaction feed (Table S1). The catechol content of these polymers was held 2
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analysis of variance (ANOVA). Post hoc comparisons were performed using Tukey’s honest significant difference (HSD) test. The null hypothesis, that there is no difference in adhesion strength with changes to polymer molecular weights, was rejected at a confidence level of 95% (P
