Marine organisms such as mussels have mastered the challenges in underwater adhesion by incorporating post-translationally modified amino acids like l-3,4-dihydroxyphenylalanine (DOPA) in adhesive proteins. Here we designed a catechol containing elastomer adhesive to identify the role of catechol in interfacial adhesion in both dry and wet conditions. To decouple the adhesive contribution of catechol to the overall adhesion, the elastomer was designed to be cross-linked through [2 + 2] photo-cycloaddition of coumarin. The elastomer with catechol moieties displayed a higher adhesion strength than the catechol-protected elastomer. The contact interface was probed using interface-sensitive sum frequency generation spectroscopy to explore the question of whether catechol can displace water and bond with hydrophilic surfaces. The spectroscopy measurements reveal that the maximum binding energy of the catechol and protected-catechol elastomers to sapphire substrate is 7.0 ± 0.1 kJ/(mole of surface O–H), which is equivalent to 0.10 J/m2. The higher dry and wet adhesion observed in the macroscopic adhesion measurements for the catechol containing elastomer originates from multiple hydrogen bonds of the catechol dihydroxy groups to the surface. In addition, our results show that catechol by itself does not remove the confined interstitial water. In these elastomers, it is the hydrophobic groups that help in partially removing interstitial water. The observation of the synergy between catechol binding and hydrophobicity in enabling the mussel-inspired soft adhesive elastomer to stick underwater provides a framework for designing materials for applications in tissue adhesion and moist-skin wearable electronics.

In addition to DOPA, several other residues including lysine, tryptophan, histidine, arginine and phosphorylated serine are proposed to play key roles in adhesion of mussels and other underwater organisms. [5] In preparing synthetic mimics of mussel adhesives, incorporating DOPA or other hydrophilic moieties is a common theme with the aim that such moieties can potentially outcompete interactions of water with the surface. [6] Interestingly, the role of hydrophobic groups to drive water away has not been exploited in designing underwater adhesives. [7] The adhesive forces required to separate two hydrophobic surfaces apart in water are much higher than the adhesive forces reported for mussel foot proteins on model mica surfaces [8] (measured using surface force apparatus). The removal of water between two hydrophobic surfaces is much easier compared to two hydrophilic surfaces due to strongly hydrated water next to hydrophilic surfaces. [9] Therefore design of adhesives which include hydrophobic groups as a key component is likely to improve adhesion in wet environments. However, designing an adhesive solely based on hydrophobic interactions is not feasible because these surfaces are easily fouled by amphiphilic molecules and result in a rapid decrease in adhesion. [10] Here we show strong evidence that incorporation of hydrophobic and mussel-inspired components in the design of underwater adhesives can substantially improve the performance and durability of such synthetic adhesives.Designing underwater adhesives presents a dichotomy of design principles. To enable spreading of the adhesive without the use of any solvent, the molecular weight needs to be relatively low; however, this would decrease the cohesive strength. To improve cohesive strength, the molecular weight of the poly mer needs to be sufficiently high, but this makes the poly mer glue very viscous or even a solid at room temperature. This would necessitate the use of organic solvents for application of the adhesive, which can be toxic for biomedical applications. [11] Here, we present a unique design of synthetic adhesives by using a combination of soybean oil based hydrophobic and DOPA mimicking catechol pendant functional groups. The soybean oil based hydrophobic groups are important for several reasons. First, they help in lowering the glass transition temperature and the adhesive can be applied Recognizing the potential for synthetic adhesives that can function in wet environments, elements of mussel foot proteins such as L-3,4-dihydroxyphenylalanine (DOPA) and phosphoserine have been incorporated into synthetic adhesives. Such adhesives have corroborated the advantage of surface active groups like DOPA, but have not yet demonstrated superior performance in wet or underwater environments, without using organic solvents. What has been conspicuously absent from such designs is the effect of hydrophobic components in the performance of underwater adhesives. Herein it is shown that incorporation of hydrophobic groups in low modulus polyester adh...

To prepare chiral nanostructures coated with bioactive molecules, a side-chain amino acid containing macromolecular chain transfer agent (macroCTA), poly(Boc-L-alanine methacryloyloxyethyl ester) (PBLAEMA), has been used as the steric stabilizer for the reversible addition-fragmentation chain transfer (RAFT) mediated dispersion polymerization of benzyl methacrylate (BzMA) in methanol at 65°C. Gel permeation chromatography (GPC) analysis confirmed an efficient and well-controlled block copolymerization. A full spectrum of morphologies spanning spherical micelles, worm like micelles, fibres and vesicles could be attained by tuning (i) the length of the solvophobic block and (ii) the total solid content at which the block copolymerization is performed. Interestingly, a purely fibre phase morphology formed a thermoresponsive gel at room temperature above a critical fibre entanglement concentration, which underwent degelation upon heating because of the morphological transformation from anisotropic fibre to isotropic sphere. In actual fact, twisted nano-fibres have been formed through the hierarchical selfassembling of polymerization induced self-assembly (PISA) generated macromolecules in the gel state.Circular dichroism (CD) spectroscopy was used to elucidate chiroptical properties. Additionally, a high demanding wrinkle surface has been constructed preliminarily from this copolymer dispersion solution.Successful Boc-group expulsion facilitates the disassembly of vesicles to either worms or spheres with an appreciable cationic character below pH 7.0 as revealed by aqueous electrophoresis studies. Though the creation of nano-objects through PISA is well-known, fabricating chiral nanostructures with reactive handles and their hierarchical self-organization to have functional architectures is a less explored area. In this present work we were able to hybridize PISA, chirality and hierarchical self-assembling. † Electronic supplementary information (ESI) available: GPC chromatograms, CD spectra, 1 H NMR spectrum and DLS size distributions of various block copolymers. See

Water prevents adhesion by disrupting the interfacial interactions and weakening the cohesive network of the adhesive. This review summarizes the recent developments in the physical and chemical design principles of underwater adhesives.

To enable attachment to underwater surfaces, aquatic fauna such as mussels and sandcastle worms utilize the advantages of coacervation to deliver concentrated protein-rich adhesive cocktails in an aqueous environment onto underwater surfaces. Recently, a mussel adhesive protein Mfp-3s, was shown to exhibit a coacervation-based adhesion mechanism. Current synthetic strategies to mimic Mfp-3s often involve complexation of oppositely charged polymers. Such complex coacervates are more sensitive to changes in pH and salt, thereby limiting their utility to narrow ranges of pH and ionic strength. In this study, by taking advantage of the lower critical solution temperature-driven coacervation, we have created mussel foot protein-inspired, tropoelastin-like, bioabsorbable, nonionic, self-coacervating polyesters for the delivery of photo-cross-linkable adhesives underwater and to overcome the challenges of adhesion in wet or underwater environments. We describe the rationale for their design and the underwater adhesive properties of these nonionic adhesives. Compared to previously reported coacervate adhesives, these "charge-free" polyesters coacervate in wide ranges of pH (3−12) and ionic strength (0−1 M NaCl) and rapidly (<300 s) adhere to substrates submerged underwater. The study introduces smart materials that mimic the self-coacervation and environmental stability of Mfp-3s and demonstrate the potential for biological adhesive applications where high water content, salts, and pH changes can be expected.