Thermoresponsive Complex Coacervate‐Based Underwater Adhesive

Abstract

Sandcastle worms have developed protein‐based adhesives, which they use to construct protective tubes from sand grains and shell bits. A key element in the adhesive delivery is the formation of a fluidic complex coacervate phase. After delivery, the adhesive transforms into a solid upon an external trigger. In this work, a fully synthetic in situ setting adhesive based on complex coacervation is reported by mimicking the main features of the sandcastle worm's glue. The adhesive consists of oppositely charged polyelectrolytes grafted with thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) chains and starts out as a fluid complex coacervate that can be injected at room temperature. Upon increasing the temperature above the lower critical solution temperature of PNIPAM, the complex coacervate transitions into a nonflowing hydrogel while preserving its volume—the water content in the material stays constant. The adhesive functions in the presence of water and bonds to different surfaces regardless of their charge. This type of adhesive avoids many of the problems of current underwater adhesives and may be useful to bond biological tissues.

Underwater Adhesion of Multiresponsive Complex Coacervates

“…However the values here recorded are much lower, indicating that most of the water is retained by the complex coacervate: as shown in similar work performed on polyelectrolyte complexes, the exposure to a lower I environment may lead to a sudden contraction of the polyelectrolyte matrix, leading to the formation of a kinetically arrested state, with most of the water remaining trapped in pores within the material (Figure B). The presence of a porous structure in this system has been already detected using optical microscopy in previous work and shown here as Figure S5 in the Supporting Information: both observations are in accordance with other reports about complex coacervates and polyelectrolyte complexes …”

Water‐Triggered Hyperbranched Polymer Universal Adhesives: From Strong Underwater Adhesion to Rapid Sealing Hemostasis

“…For instance, the widely used cyanoacrylate adhesives exhibit strong adhesion in air, but when applied in water environment, they are hardened quickly to form a layer of stiff plastics, eventually resulting in the loss of adhesion. [13,14] In nature, many organisms, such as mussels, barnacles, and castle worms, have evolved an unparalleled mechanism to perfectly tackle the underwater adhesion problem. Recently, host-guest chemistry strategy was reportedly employed to prepare underwater adhesives; however, the substrate surface needs to be modified in advance.…”

“…[5,12] In addition, electrostatic and hydrophobic interactions were also proved to contribute to enhanced underwater adhesion, but the adhesion strength was relatively poor. [13] In this process, phase separation and concurrently increased hydrophobicity induced by coacervation can dispel the hydrated water on the interface, leading to much enhanced interaction of adhesive groups with the adherent and thus stable underwater adhesion. [15][16][17] The finding of universality of catechol chemistry for wet adhesion has provided a valuable biomimetic source to develop diverse adhesives for use in aqueous environments.…”

“…[13,14] In nature, many organisms, such as mussels, barnacles, and castle worms, have evolved an unparalleled mechanism to perfectly tackle the underwater adhesion problem. Up to date, several complex coacervate adhesives with linear structure have been reported, but the occurrence of those coacervations in water needs external triggers, such as temperature, [13] pH, [20,23] and iron strength. However, several problems, such as the complexity of administration, release of harmful organic solvents, [18,19] long-term curing, [2] need for oxidant addition, [20,21] and low adhesion strength, [18,22] may hamper the actual applications of these bioinspired adhesives.…”

“…[13] In this process, phase separation and concurrently increased hydrophobicity induced by coacervation can dispel the hydrated water on the interface, leading to much enhanced interaction of adhesive groups with the adherent and thus stable underwater adhesion. [13] In this process, phase separation and concurrently increased hydrophobicity induced by coacervation can dispel the hydrated water on the interface, leading to much enhanced interaction of adhesive groups with the adherent and thus stable underwater adhesion.…”