Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form.

Polyelectrolyte complexation is critical to the formation and properties of many biological and polymeric materials, and is typically initiated by aqueous mixing1 followed by fluid–fluid phase separation, such as coacervation2–5. Yet little to nothing is known about how coacervates evolve into intricate solid microarchitectures. Inspired by the chemical features of the cement proteins of the sandcastle worm, here we report a versatile and strong wet-contact microporous adhesive resulting from polyelectrolyte complexation triggered by solvent exchange. After premixing a catechol-functionalized weak polyanion with a polycation in dimethyl sulphoxide (DMSO), the solution was applied underwater to various substrates whereupon electrostatic complexation, phase inversion, and rapid setting were simultaneously actuated by water–DMSO solvent exchange. Spatial and temporal coordination of complexation, inversion and setting fostered rapid (~25 s) and robust underwater contact adhesion (Wad ≥ 2 J m−2) of complexed catecholic polyelectrolytes to all tested surfaces including plastics, glasses, metals and biological materials.

Polymeric materials that intrinsically heal at damage sites under wet or moist conditions are urgently needed for biomedical and environmental applications [1][2][3][4][5][6] . Although hydrogels with self-mending properties have been engineered by means of mussel-inspired metal-chelating catechol-functionalized polymer networks 7-10 , biological self-healing in wet conditions, as occurs in self-assembled holdfast proteins in mussels and other marine organisms 11,12 , is generally thought to involve more than reversible metal chelates. Here we demonstrate self-mending in metal-free water of synthetic polyacrylate and polymethacrylate materials that are surface-functionalized with mussel-inspired catechols. Wet self-mending of scission in these polymers is initiated and accelerated by hydrogen bonding between interfacial catechol moieties, and consolidated by the recruitment of other non-covalent interactions contributed by subsurface moieties. The repaired and pristine samples show similar mechanical properties, suggesting that the triggering of complete self-healing is enabled underwater by the formation of extensive catechol-mediated interfacial hydrogen bonds.All polymeric materials suffer damage in the course of their functional lifetimes. Few, if any, completely heal at damage sites. Despite recent progress in the design of self-mending polymeric materials based on crack-activated crosslinking 1 , light 2 , heat 3 or other external stimuli 4 , these remain less than perfectly healed, and, in the case of polymers in wet environments, self-healing technologies are even more limited than those engineered for dry conditions. Mussel adhesive holdfasts exhibit significant self-healing capabilities 11,12 , although the molecular mechanisms involved are poorly understood. Notwithstanding this, the selfmending adhesion and cohesion of isolated dopa (3,4-dihydroxyphenyl-L-alanine)-containing adhesive proteins were shown to rely critically on maintaining dopa in an acidic and reducing environment 13,14 . Significantly different conditions are required to recapitulate the self-healing cohesion of tris-dopa-Fe 3+ -mediated complexes in proteins and polymers [7][8][9]15 . Such results increasingly suggest the importance of dopa, but also its subtle and diverse interfacial reactivity vis-à-vis the traditional and still widely held view that dopa, and catechols generally, function primarily as crosslinkers after their 2-electron oxidation to quinones 16 . To better assess the contribution of catechol to polymer selfhealing in a reducing (pH 3), metal-free wet environment, we prepared a material from common, water-insoluble synthetic acrylic polymers having a catechol-functionalized surface. These materials are completely self-healing in a process initiated by catecholmediated interfacial hydrogen bonding, and consolidated by followup interactions (for example, hydrophobic and steric) after a brief compression (∼6 × 10 4 Pa). The crucial and robust roles played by

Nature employs sophisticated control of a structure's properties at multiple length scales to achieve its wet adhesion. However, the translation of such structures has very often been missing in biomimetic adhesives; in turn, their performance is significantly limited as compared to that of biological adhesion, e.g., from mussels. In this Perspective, we overview the major breakthroughs in this field, highlighting the recent advances that demonstrate that holistic multiscale translation is essential to biomimetic design. We argue that the multiscale coordination of numerous key elements in the natural adhesive system is essential to replicate the strong, instant, and durable wet adhesion of the marine sessile organism.

Despite the recent progress in and demand for wet adhesives, practical underwater adhesion remains limited or non-existent for diverse applications. Translation of mussel-inspired wet adhesion typically entails catechol functionalization of polymers and/or polyelectrolytes, and solution processing of many complex components and steps that require optimization and stabilization. Here we reduced the complexity of a wet adhesive primer to synthetic low-molecular-weight catecholic zwitterionic surfactants that show very strong adhesion (∼50 mJ m−2) and retain the ability to coacervate. This catecholic zwitterion adheres to diverse surfaces and self-assembles into a molecularly smooth, thin (<4 nm) and strong glue layer. The catecholic zwitterion holds particular promise as an adhesive for nanofabrication. This study significantly simplifies bio-inspired themes for wet adhesion by combining catechol with hydrophobic and electrostatic functional groups in a small molecule.