Mussel‐Inspired Polyesters with Aliphatic Pendant Groups Demonstrate the Importance of Hydrophobicity in Underwater Adhesion

Abstract

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...

Mussel‐Inspired Alternating Copolymer as a High‐Performance Adhesive Material Both at Dry and Under‐Seawater Conditions

“…The CFPs are also capable of the adhesion at wet or underwater situations, although the bonding strength is much lower because of the formation of hydrated cation layers. [ 17 ] White and Wilker [ 18 ] and Joy and co‐workers [ 19,20 ] incorporated monomers with electrostatic charges or aliphatic pendants into CFPs, achieving underwater bonding strength of 0.4 MPa on aluminum and 0.65 MPa on glass, respectively. By adjusting the polarity [ 21 ] or the hydrophilicity [ 22 ] of CFP backbones, Wan et al reported improved bonding strength of 1.13 and 1.33 MPa on glass substrates.…”

Direct Observation of the Interplay of Catechol Binding and Polymer Hydrophobicity in a Mussel-Inspired Elastomeric Adhesive

“…We have reported recently that a hydrophilic adhesive with catechol does not adhere underwater compared to a relatively hydrophobic adhesive without catechol groups. 49 Therefore, for effective underwater adhesion, initially the polymer should make interfacial contact with the surface and remove bound water, which in this case is achieved by a conformable hydrophobic adhesive. Also, polar functional groups with strong interfacial interactions are essential to display significant adhesion underwater.…”

Bioinspired reversible hydrogel adhesives for wet and underwater surfaces

“…21,22 In order to overcome such difficulties, synthetic wet adhesives emulating natural adhesive materials of marine organisms have been extensively investigated. 23 Representative examples include wet adhesives mimicking the proteins present in biological system such as mussel adhesives, [23][24][25][26] barnacle cements, 27 and sandcastle-worm glues. 28 These marine organism-inspired synthetic wet adhesives typically have the use of 3,4-dihydroxyphenylalanine (DOPA) in common, and many different types of DOPA-based wet adhesives have demonstrated effective wet adhesion performance.…”