A Near-Infrared Photothermal-Responsive Underwater Adhesive with Tough Adhesion and Antibacterial Properties

Developing tunable underwater adhesives that possess tough adhesion in service and easy detachment when required remains challenging. Herein, a strategy is proposed to design a near infrared (NIR) photothermal-responsive underwater adhesive by incorporating MXene (Ti3 C2 Tx )-based nanoparticles within isocyanate-modified polydimethylsiloxane (PDMS) polymer chains. The developed adhesive exhibits long-term and tough adhesion with an underwater adhesion strength reaching 5.478 MPa. Such strong adhesion is mainly attributed to the covalent bonds and hydrogen bonds at the adhesive-substrate interface. By making use of the photothermal-response of MXene-based nanoparticles and the thermal response of PDMS-based chains, the adhesive possesses photothermal-responsive performance, exhibiting sharply diminished adhesion under NIR irradiation. Such NIR-triggered tunable adhesion allows for easy and active detachment of the adhesive when needed. Moreover, the underwater adhesive exhibits photothermal antibacterial property, making it highly desirable for underwater applications. This work enhances the understanding of photothermal-responsive underwater adhesion, enabling the design of tunable underwater adhesives for biomedical and engineering applications.

A Visible Light Cross-Linked Underwater Hydrogel Adhesive with Biodegradation and Hemostatic Ability

Hydrogel adhesives with integrated functionalities are still required to match their ever-expanding practical applications in the field of tissue repair and regeneration. A simple and effective safety strategy is reported, involving an in situ injectable polymer precursor and visible light-induced cross-linking. This strategy enables the preparation of a hydrogel adhesive in a physiological environment, offering wet adhesion to tissue surfaces, molecular flexibility, biodegradability, biocompatibility, efficient hemostatic performance, and the ability to facilitate liver injury repair. The proposed one-step preparation process of this polymer precursor involves the mixing of gelatin methacryloyl (GelMA), poly(thioctic acid) [P(TA)], poly(acrylic acid)/amorphous calcium phosphate (PAAc/ACP, PA) and FDA-approved photoinitiator solution, and a subsequent visible light irradiation after in situ injection into target tissues that resulted in a chemically-physically cross-linked hybrid hydrogel adhesive. Such a combined strategy shows promise for medical scenarios, such as uncontrollable post-traumatic bleeding.

Rapid, Tough, and Trigger-Detachable Hydrogel Adhesion Enabled by Formation of Nanoparticles In Situ

Integrating hydrogel with other materials is always challenging due to the low mass content of hydrogels and the abundance of water at the interfaces. Adhesion through nanoparticles offers characteristics such as ease of use, reversibility, and universality, but still grapples with challenges like weak bonding. Here, a simple yet powerful strategy using the formation of nanoparticles in situ is reported, establishing strong interfacial adhesion between various hydrogels and substrates including elastomers, plastics, and biological tissue, even under wet conditions. The strong interfacial bonding can be formed in a short time (60 s), and gradually strengthened to 902 J m-2 adhesion energy within an hour. The interfacial layer's construction involves chain entanglement and other non-covalent interactions like coordination and hydrogen bonding. Unlike the permanent bonding seen in most synthetic adhesives, these nanoparticle adhesives can be efficiently triggered for removal by acidic solutions. The simplicity of the precursor diffusion and precipitation process in creating the interfacial layer ensures broad applicability to different substrates and nanoparticle adhesives without compromising robustness. The tough adhesion provided by nanoparticles allows the hydrogel-elastomer hybrid to function as a triboelectric nanogenerator (TENG), facilitating reliable electrical signal generation and output performance due to the robust interface.

Natural Underwater Bioadhesive Offering Cohesion Modulation via Hydrogen Bond Disruptor: A Highly Injectable and in Vivo Stable Remedy for Gastric Ulcer Resolution

Injectable bioadhesives are attractive for managing gastric ulcers through minimally invasive procedures. However, the formidable challenge is to develop bioadhesives that exhibit high injectability, rapidly adhere to lesion tissues with fast gelation, provide reliable protection in the harsh gastric environment, and simultaneously ensure stringent standards of biocompatibility. Here, a natural bioadhesive with tunable cohesion is developed based on the facile and controllable gelation between silk fibroin and tannic acid. By incorporating a hydrogen bond disruptor (urea or guanidine hydrochloride), the inherent network within the bioadhesive is disturbed, inducing a transition to a fluidic state for smooth injection (injection force <5 N). Upon injection, the fluidic bioadhesive thoroughly wets tissues, while the rapid diffusion of the disruptor triggers instantaneous in situ gelation. This orchestrated process fosters the formed bioadhesive with durable wet tissue affinity and mechanical properties that harmonize with gastric tissues, thereby bestowing long-lasting protection for ulcer healing, as evidenced through in vitro and in vivo verification. Moreover, it can be conveniently stored (≥3 m) postdehydration. This work presents a promising strategy for designing highly injectable bioadhesives utilizing natural feedstocks, avoiding any safety risks associated with synthetic materials or nonphysiological gelation conditions, and offering the potential for minimally invasive application.

Glycopolymer-Based Antiswelling, Conductive, and Underwater Adhesive Hydrogels for Flexible Strain Sensor Application

With the fast development of soft electronics, underwater adhesion has become a highly desired feature for various sensing uses. Currently, most adhesive hydrogels are based on catechol-based structures, such as polydopamine, pyrogallol, and tannic acid, with very limited structural variety. Herein, a new type of glycopolymer-based underwater adhesive hydrogel has been prepared straightforwardly by random copolymerization of acrylic acid, acetyl-protected/unprotected glucose, and methacrylic anhydride in dimethyl sulfoxide (DMSO). By employing a DMSO-water solvent exchange strategy, the underwater adhesion was skillfully induced by the synergetic effects of hydrophobic aggregation and hydrogen bonding, leading to excellent adhesion behaviors on various surfaces, including pig skins, glasses, plastics, and metals, even after 5 days of storage in water. In addition, the underwater adhesive hydrogels with simple and low-cost protected/unprotected carbohydrate compositions showed good mechanical and rheological properties, together with cytocompatibility and antiswelling behavior in water, all of which are beneficial for underwater adhesions. In application as a flexible strain sensor, the adhesive hydrogel exhibited stable and reliable sensing ability for monitoring human motion in real time, suggesting great potential for intelligent equipment design.

Water-Induced Phase Separation for Anti-Swelling Hydrogel Adhesives in Underwater Soft Electronics

The development of hydrogel-based underwater electronics has gained significant attention due to their flexibility and portability compared to conventional rigid devices. However, common hydrogels face challenges such as swelling and poor underwater adhesion, limiting their practicality in water environments. Here, a water-induced phase separation strategy to fabricate hydrogels with enhanced anti-swelling properties and underwater adhesion is presented. By leveraging the contrasting affinity of different polymer chains to water, a phase-separated structure with rich hydrophobic and dilute hydrophilic polymer phases is achieved. This dual-phase structure, meticulously characterized from the macroscopic to the nanoscale, confers the hydrogel network with augmented retractive elastic forces and facilitates efficient water drainage at the gel-substrate interface. As a result, the hydrogel exhibits remarkable swelling resistance and long-lasting adhesion to diverse substrates. Additionally, the integration of carboxylic multiwalled carbon nanotubes into the hydrogel system preserves its anti-swelling and adhesion properties while imparting superior conductivity. The conductive phase-separated hydrogel exhibited great potential in diverse underwater applications, including sensing, communication, and energy harvesting. This study elucidates a facile strategy for designing anti-swelling underwater adhesives by leveraging the ambient solvent effect, which is expected to offer some insights for the development of next-generation adhesive soft materials tailored for aqueous environments.

Uncover rock-climbing fish's secret of balancing tight adhesion and fast sliding for bioinspired robots

The underlying principle of the unique dynamic adaptive adhesion capability of a rock-climbing fish (Beaufortia kweichowensis) that can resist a pull-off force of 1000 times its weight while achieving simultaneous fast sliding (7.83 body lengths per second (BL/S)) remains a mystery in the literature. This adhesion-sliding ability has long been sought for underwater robots. However, strong surface adhesion and fast sliding appear to contradict each other due to the need for high surface contact stress. The skillfully balanced mechanism of the tight surface adhesion and fast sliding of the rock-climbing fish is disclosed in this work. The Stefan force (0.1 mN/mm2) generated by micro-setae on pectoral fins and ventral fins leads to a 70 N/m2 adhesion force by conforming the overall body of the fish to a surface to form a sealing chamber. The pull-off force is neutralized simultaneously due to the negative pressure caused by the volumetric change of the chamber. The rock-climbing fish's micro-setae hydrodynamic interaction and sealing suction cup work cohesively to contribute to low friction and high pull-off-force resistance and can therefore slide rapidly while clinging to the surface. Inspired by this unique mechanism, an underwater robot is developed with incorporated structures that mimic the functionality of the rock-climbing fish via a micro-setae array attached to a soft self-adaptive chamber, a setup which demonstrates superiority over conventional structures in terms of balancing tight underwater adhesion and fast sliding.

Water-assisted strong underwater adhesion via interfacial water removal and self-adaptive gelation

In nearly all cases of underwater adhesion, water molecules typically act as a destroyer. Thus, removing interfacial water from the substrate surfaces is essential for forming super-strong underwater adhesion. However, current methods mainly rely on physical means to dislodge interfacial water, such as absorption, hydrophobic repulsion, or extrusion, which are inefficient in removing obstinate hydrated water at contact interface, resulting in poor adhesion. Herein, we present a unique means of reversing the role of water to assist in realizing a self-strengthening liquid underwater adhesive (SLU-adhesive) that can effectively remove water at contact interface. This is achieved through multiscale physical-chemical coupling methods across millimeter to molecular levels and self-adaptive strengthening of the cohesion during underwater operations. As a result, strong adhesion over 1,600 kPa (compared to ~100 to 1,000 kPa in current state of the art) can be achieved on various materials, including inorganic metal and organic plastic materials, without preloading in different environments such as pure water, a wide range of pH solutions (pH = 3 to 11), and seawater. Intriguingly, SLU-adhesive/photothermal nanoparticles (carbon nanotubes) hybrid materials can significantly reduce the time required for complete curing from 24 h to 40 min using near-infrared laser radiation due to unique thermal-response of the chemical reaction rate. The excellent adhesion property and self-adaptive adhesion procedure allow SLU-adhesive materials to demonstrate great potential for broad applications in underwater sand stabilization, underwater repair, and even adhesion failure detection as a self-reporting adhesive. This concept of "water helper" has potential to advance underwater adhesion and manufacturing strategies.

Polyacrylic Acid Hydrogel Coating for Underwater Adhesion: Preparation and Characterization

Underwater adhesion involves bonding substrates in aqueous environments or wet surfaces, with applications in wound dressing, underwater repairs, and underwater soft robotics. In this study, we investigate the underwater adhesion properties of a polyacrylic acid hydrogel coated substrate. The underwater adhesion is facilitated through hydrogen bonds formed at the interface. Our experimental results, obtained through probe-pull tests, demonstrate that the underwater adhesion is rapid and remains unaffected by contact pressure and pH levels ranging from 2.5 to 7.0. However, it shows a slight increase with a larger adhesion area. Additionally, we simulate the debonding process and observe that the high-stress region originates from the outermost bonding region and propagates towards the center, spanning the thickness of the target substrate. Furthermore, we showcase the potential of using the underwater adhesive hydrogel coating to achieve in-situ underwater bonding between a flexible electronic demonstration device and a hydrogel contact lens. This work highlights the advantages of employing hydrogel coatings in underwater adhesion applications and serves as inspiration for the advancement of underwater adhesive hydrogel coatings capable of interacting with a wide range of substrates through diverse chemical and physical interactions at the interface.

Cooperative Tridentate Hydrogen-Bonding Interactions Enable Strong Underwater Adhesion

Multidentate hydrogen-bonding interactions are a promising strategy to improve underwater adhesion. Molecular and macroscale experiments have revealed an increase in underwater adhesion by incorporating multidentate H-bonding groups, but quantitatively relating the macroscale adhesive strength to cooperative hydrogen-bonding interactions remains challenging. Here, we investigate whether tridentate alcohol moieties incorporated in a model epoxy act cooperatively to enhance adhesion. We first demonstrate that incorporation of tridentate alcohol moieties leads to comparable adhesive strength with mica and aluminum in air and in water. We then show that the presence of tridentate groups leads to energy release rates that increase with an increase in crack velocity in air and in water, while materials lacking these groups do not display rate-dependent adhesion. We model the rate-dependent adhesion to estimate the activation energy of the interfacial bonds. Based on our data, we estimate the lifetime of these bonds to be between 2 ms and 6 s, corresponding to an equilibrium activation energy between 23k_BT and 31k_BT. These values are consistent with tridentate hydrogen bonding, suggesting that the three alcohol groups in the Tris moiety bond cooperatively form a robust adhesive interaction underwater.

Underwater Self-Healing and Recyclable Ionogel Sensor for Physiological Signal Monitoring

Ionogels with self-healing properties have become more and more desirable because they can improve the reliability, safety, and fatigue-resistant performance of flexible devices. However, the self-healing property of ionogels is usually susceptible to water molecules, and the application of ionogel sensors is limited to the atmospheric environment. Inspired by gelatinous jellyfish, herein, an underwater self-healing ionogel was prepared via one-step photoinitiated polymerization of acrylic acid 2,2,2-trifluoroethyl ester and N-isopropylacrylamide (NIPAm) in a hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]). The dynamic physical interactions (hydrogen bonding and ion-dipole interactions) endowed the ionogel with remarkable transparency, underwater self-healing (up to 96%), toughness (3.93 MJ m^-3), and underwater adhesion. And the cross-linking ionogel could be green recycled by ethanol for further application. Especially, the ionogel-based sensor presented excellent strain and pressure sensing sensitivity, rapid responsiveness (140 ms), and ultrastability. The ionogel could be further assembled into an optical camouflage sensor to detect and distinguish different human motions in real time with high sensitivity, stability, and repeatability, as well as for underwater electrocardiography monitoring wirelessly. This ionogel provides a promising strategy for the development of underwater self-healing sensors.

Robust Underwater Adhesion of Catechol-Functionalized Polymer Triggered by Water Exchange

Adhesives with strong and stable underwater adhesion performance play a critical role in industrial and biomedical fields. However, achieving strong underwater adhesion, especially in flowing aqueous and blood environments, remains challenging. In this work, a novel solvent-exchange-triggered adhesive of catechol-functionalized polyethylenimine ethoxylated is presented. The authors show that the dimethyl sulfoxide (DMSO) solution of the catechol-functionalized polymer can be directly applied to various substrates and exhibits robust dry/underwater adhesion performance induced through in situ liquid-to-solid phase transition triggered by water-DMSO solvent exchange. The adhesive can even strongly bond low-surface-energy substrates (e.g., > 86 kPa for polytetrafluoroethylene) in diverse environments, including deionized water, air, phosphate-buffered saline solution, seawater, and aqueous conditions with different pH values. Moreover, the adhesive exhibits strong adhesion to biological tissues and can be used as a hemostatic sealant to prevent bleeding from arteries and severe trauma to the viscera. The adhesives developed in this study with strong dry/underwater adhesion performance and excellent hemostatic capabilities display enormous application prospects in the biomedical fields.

Hydrogel with Robust Adhesion in Various Liquid Environments by Electrostatic-Induced Hydrophilic and Hydrophobic Polymer Chains Migration and Rearrangement

Hydrogels with wet adhesion are promising interfacial adhesive materials; however, their adhesion in water, oil, or organic solvents remains a major challenge. To address this, a pressure-sensitive P(AAm-co-C₁₈ )/PTA-Fe hydrogel is fabricated, which exhibits robust adhesion to various substrates in both aqueous solutions and oil environments. It is demonstrated that the key to wet adhesion under liquid conditions is the removal of the interfacial liquid, which can be achieved through rational molecular composition regulation. By complexing with hydrophilic polymer networks, phosphotungstic acid (PTA) is introduced into the hydrogel network as a physical cross-linker and anchor point to improve the cohesion strength and drive the migration of polymer chains. The migration and rearrangement of hydrophilic and hydrophobic polymer chains on the hydrogel surface are induced by the electrostatic interactions of Fe³⁺ , which create a surface with interfacial water- and oil-removing properties. By co-regulating the hydrophilic and hydrophobic polymer chains, the P(AAm-co-C₁₈ )/PTA-Fe hydrogel is able to act as a pressure-sensitive adhesive under water and oils with adhesion strength of 92.6 and 90.0 kPa, respectively. It is anticipated that this regulation strategy for polymer chains will promote the development of wet adhesion hydrogels, which can have a wide range of applications.

Multifunctional Underwater Adhesive Film Enabled by a Single-Component Poly(ionic liquid)

Tremendous efforts have been devoted to exploiting synthetic wet adhesives for real-life applications. However, developing low-cost, robust, and multifunctional wet adhesive materials remains a considerable challenge. Herein, a wet adhesive composed of a single-component poly(ionic liquid) (PIL) that enables fast and robust underwater adhesion is reported. The PIL adhesive film possesses excellent stretchability and flexibility, enabling its anchoring on target substrates regardless of deformation and water scouring. Surface force measurements show the PIL can achieve a maximum adhesion of 56.7 mN·m^-1 on diverse substrates (both hydrophilic and hydrophobic substrates) in aqueous media, within ∼30 s after being applied. The adhesion mechanisms of the PIL were revealed via the force measurements, and its robust wet adhesive capacity was ascribed to the synergy of different non-covalent interactions, such as of hydrogen bonding, cation-π, electrostatic, and van der Waals interactions. Surprisingly, this PIL adhesive film exhibited impressive underwater sound absorption capacity. The absorption coefficient of a 0.7 mm-thick PIL film to 4-30 kHz sound waves could be as high as 0.80-0.92. This work reports a multifunctional PIL wet adhesive that has promising applications in many areas and provides deep insights into interfacial interaction mechanisms underlying the wet adhesion capability of PILs.

Collagen-Based Organohydrogel Strain Sensor with Self-Healing and Adhesive Properties for Detecting Human Motion

Conductive hydrogels are ideal for flexible sensors, but it is still a challenge to produce such hydrogels with combined toughness, self-adhesion, self-healing, anti-freezing, moisturizing, and biocompatibility properties. Herein, inspired by natural skin, a highly stretchable, strain-sensitive, and multi-environmental stable collagen-based conductive organohydrogel was constructed by using collagen (Col), acrylic acid, dialdehyde carboxymethyl cellulose, 1,3-propylene glycol, and AlCl₃. The resulting organohydrogel exhibited excellent tensile (strain >800%), repeatable adhesion (>10 times), self-healing [self-healing efficiency (SHE) ≈ 100%], anti-freezing (-60 °C), moisturizing (>20 d), and biocompatible properties. This organohydrogel also possessed good electrical conductivity (σ = 3.4 S/m) and strain-sensitive properties [GF (gauge factor) = 13.65 with the maximal strain of 400%]. Notably, the organohydrogel had a considerable low-temperature self-healing performance (SHE = 88% at -24 °C) and rapid underwater self-healing property (SHE = 92%, self-healing time <20 min). This type of strain sensor could not only accurately and continuously monitor the large-scale motions of the human body but also provide an accurate response to the human tiny motions. This work not only proposes a development strategy for a multifunctional conductive organohydrogel with multiple environmental stability but also provides potential research value for the construction of biomimetic electronic skin.

Coacervate-Derived Hydrogel with Effective Water Repulsion and Robust Underwater Bioadhesion Promotes Wound Healing

Achieving robust underwater adhesion by bioadhesives remains a challenge due to interfacial water. Herein a coacervate-to-hydrogel strategy to enhance interfacial water repulsion and bulk adhesion of bioadhesives is reported. The polyethyleneimine/thioctic acid (PEI/TA) coacervate is deposited onto underwater substrates, which can effectively repel interfacial water and completely spread into substrate surface irregularities due to its liquid and water-immiscible nature. The physical interactions between coacervate and substrate can further enhance interfacial adhesion. Furthermore, driven by the spontaneous hydrophobic aggregation of TA molecules and strong electrostatic interaction between PEI and TA, the coacervate can turn into a hydrogel in situ within minutes without additional stimuli to develop enhanced matrix cohesion and robust bulk adhesion on diverse underwater substrates. Molecular dynamics simulations further reveal atomistic details of the formation and wet adhesion of the PEI/TA coacervate via multimode physical interactions. Lastly, it is demonstrated that the PEI/TA coacervate-derived hydrogel can effectively repel blood and therefore efficiently deliver the carried growth factors at wound sites, thereby enhancing wound healing in an animal model. The advantages of the PEI/TA coacervate-derived hydrogel including body fluid-immiscibility, strong underwater adhesion, adaptability to fit irregular target sites, and excellent biocompatibility make it a promising bioadhesive for diverse biomedical applications.

Ultrasonic-Induced Synthesis of Underwater Adhesive and Antiswelling Hydrogel for Strain Sensor

To perceive the human body's multienvironmental mobility, intelligent flexible electronic equipment with an underwater motion monitoring function has potential research value in the field of intelligent detection. Hydrogels are widely used in the field of flexible electronics for their unique three-dimensional polymer networks. Due to the instinctive hydrophilicity of hydrogels, the swelling of hydrogels underwater and the formation of hydration coating on the surface become the primary obstacles to underwater applications. Herein, a hydrogel sensor that can achieve underwater utilization was prepared through copolymerization between hydrophobic and hydrophilic polymer monomers. The synergistic impact of electrostatic interaction, metal coordination, and hydrogen bonding ensured the hydrogel's remarkable underwater adhesive ability to a variety of substrates. The hydrophobic micelles and self-hydrophobization process induced from ultrasonic dispersion in the polymer matrix gave an outstanding hydrophobic performance (water contact angle of 130.4°) and antiswelling property (swelling ratio of 26% after 72 h of immersion), presenting unprecedented underwater adaptability. The above-mentioned hydrogel could be assembled into a flexible hydrogel sensor with satisfactory sensitivity (gauge factor of 0.44), ultrafast response rate (106 ms), and excellent cyclic stability, demonstrating accurate monitoring of complex human motions in water and air.

Bio-Inspired Instant Underwater Adhesive Hydrogel Sensors

Underwater adhesion plays an essential role in soft electronics for the underwater interface. Although hydrogel-based electronics are of great interest, because of their versatility, water molecules prevent hydrogels from adhering to substrates, thus bottlenecking further applications. Herein, inspired by the barnacle proteins, MXene/PHMP hydrogels with strong repeatable underwater adhesion are developed through the random copolymerization of 2-phenoxyethyl acrylate, 2-methoxyethyl acrylate, and N-(2-hydroxyethyl) acrylamide with the presence of MXene nanosheets. The hydrogels are mechanically tough (elastic modulus of 32 kPa, fracture stress of 0.11 MPa), and 2-phenoxyethyl acrylate (PEA) with aromatic groups endows the hydrogel with nonswelling property and prevents water molecules from invading the adhesive interface, rendering the hydrogels an outstanding adhesive behavior toward various substrates (including glass, iron, polyethylene terephthalate (PET), porcine). Besides, dynamic physical interactions allow for instant and repeatable underwater adhesion. Furthermore, the MXene/PHMP hydrogels exhibit a high conductivity (0.016 S/m), fast responsiveness, and superior sensitivity as a strain sensor (gauge factor = 7.17 at 200%-500% strain) and pressure sensor (0.63 kPa^-1 at 0-70 kPa). The underwater applications of bionic hydrogel-based sensors have been demonstrated, such as human motion, pressure sensing, and holding objects. It is anticipated that the instant and repeatable underwater adhesive hydrogel-based sensors extend the underwater applications of hydrogel electronics.

A Transparent, Highly Stretchable, Solvent-Resistant, Recyclable Multifunctional Ionogel with Underwater Self-Healing and Adhesion for Reliable Strain Sensors

Ionogels have gained increasing attentions as a flexible conductive material. However, it remains a big challenge to integrate multiple functions into one gel that can be widely applied in various complex scenes. Herein, a kind of multifunctional ionogels with a combination of desirable properties, including transparency, high stretchability, solvent and temperature resistance, recyclability, high conductivity, underwater self-healing ability, and underwater adhesiveness is reported. The ionogels are prepared via one-step photoinitiated polymerization of 2,2,2-trifluoroethyl acrylate and acrylamide in a hydrophobic ionic liquid. The abundant noncovalent interactions including hydrogen bonding and ion-dipole interactions endow the ionogels with excellent mechanical strength, resilience, and rapid self-healing capability at room temperature, while the fluorine-rich polymeric matrix brings in high tolerance against water and various organic solvents, as well as tough underwater adhesion on different substrates. Wearable strain sensors based on the ionogels can sensitively detect and differentiate large body motions, such as bending of limbs, walking and jumping, as well as subtle muscle movements, such as pronunciation and pulse. It is believed that the designed ionogels will show great promises in wearable devices and ionotronics.

Extreme suction attachment performance from specialised insects living in mountain streams (Diptera: Blephariceridae)

Suction is widely used by animals for strong controllable underwater adhesion but is less well understood than adhesion of terrestrial climbing animals. Here we investigate the attachment of aquatic insect larvae (Blephariceridae), which cling to rocks in torrential streams using the only known muscle-actuated suction organs in insects. We measured their attachment forces on well-defined rough substrates and found that their adhesion was less reduced by micro-roughness than that of terrestrial climbing insects. In vivo visualisation of the suction organs in contact with microstructured substrates revealed that they can mould around large asperities to form a seal. We have shown that the ventral surface of the suction disc is covered by dense arrays of microtrichia, which are stiff spine-like cuticular structures that only make tip contact. Our results demonstrate the impressive performance and versatility of blepharicerid suction organs and highlight their potential as a study system to explore biological suction mechanisms.

Bioinspired Underwater Adhesives

Underwater adhesives are in high demand in both commercial and industrial sectors. Compared with adhesives used in dry (air) environments, adhesives used for wet or submerged surfaces in aqueous environments have specific challenges in development and performance. In this review, focus is on adhesives demonstrating macroscopic adhesion to wet/underwater substrates. The current strategies are first introduced for different types of underwater adhesives, and then an overview is provided of the development and performance of underwater adhesives based on different mechanisms and strategies. Finally, the possible research directions and prospects of underwater adhesives are discussed.

Pressure-Sensitive Adhesive with Enhanced and Phototunable Underwater Adhesion

Pressure-sensitive adhesives (PSAs) are extensively used in diverse applications such as semiconductor manufacturing, labeling, and healthcare because of their quick and viscoelasticity-driven physical adhesion to dry surfaces. However, most of the existing PSAs normally fail to maintain or even establish adhesion under harsh conditions, particularly underwater, due to the lack of robust chemical functionalities for chemistry-based adhesion. Meanwhile, these PSAs are incapable of altering the adhesion in response to external stimuli, limiting their employment in applications requiring dynamic adhesion. Here, we develop a chemically functionalized PSA (f-PSA) with enhanced and phototunable underwater adhesion by incorporating an underwater adhesion enhancer (i.e., mussel-inspired catechol) and a photoresponsive functionality (i.e., anthracene) into a standard acrylic PSA matrix. The synergistic coupling of viscoelasticity-driven physical adhesion originating from the matrix with catechol-enabled chemical adhesion secures sufficient interfacial molecular interactions and leads to enhanced underwater adhesion. The efficient dimerization of anthracene via light-triggered cycloaddition facilely mediates the viscoelastic property of f-PSA, rendering the phototunable adhesion. We envision that this f-PSA can open up more opportunities for applications such as underwater manipulation, transfer printing, and medical adhesives.

Air Bubble Bridge-Based Bioinspired Underwater Adhesion

Wet adhesion is greatly demanded in fields of wearable devices, wound dressings, and smart robotics. However, reusable, noninvasive and convenient adhesive pads in the liquid environment have remained a challenge. Here, a novel concept of underwater adhesion inspired by the diving beetle, which utilizes the air bubbles as an adhesive to realize nondestructive and repeatable adhesion working across a wide range of scales is shown. The mechanism of underwater bubble adhesion is revealed by the capillarity of air-bubble bridge, of which the property depends on the dynamic bubble contact angles and the gap distance. The design principle of the air bubble-based underwater adhesion is proposed and validated to tune the interfacial acting force by theoretical and experimental results. Finally, a strong, reusable surface adhesive based on air bubble bridges is demonstrated from macro- to microscales in applications of particle manipulation and particle self-assembly. This unique view of underwater bubble adhesion provides new ideas for broader applications.

Robust Antiwater and Anti-oil-fouling Double-Sided Tape Enabled by SiO2 Reinforcement and a Liquefied Surface

Realizing simultaneous antiwater and anti-oil-fouling adhesion is extremely challenging owing to the solvated overlayer on the surface of substrates. Herein, we develop a supertough polyacrylate-based tape bearing SiO₂ as a reinforcing filler and a solvent to liquefy the surface. The SiO₂ reinforcement enhances the cohesion strength, while the liquefied surface not only expels the solvated overlayer but also improves the interfacial wettability and interaction. This material design imparts the double-sided tape with admirable antiwater and anti-oil-fouling adhesion performance, which far exceeds that of commercial tapes, as well as high transparency and long-term stability. In addition, we carry out an in-depth study on the adhesive mechanism for the tape and clarify the role of the solvent and the interaction between SiO₂ and a polymer matrix. This work provides a novel strategy for designing antiwater and anti-oil-fouling adhesives with wide applications in various fields such as leakage repair, antiseep, underwater adhesion, building materials, and biological adhesives.

Novel Bionic Soy Protein-Based Adhesive with Excellent Prepressing Adhesion, Flame Retardancy, and Mildew Resistance

Soy protein (SP)-based adhesives can replace traditional aldehyde-based adhesives for the manufacturing of wood-based panels. However, developing a SP-based adhesive with excellent prepressing bonding strength, flame retardancy, and mildew resistance remains a challenge. Herein, an inorganic crystal cross-linked hybrid SP adhesive was developed inspired by the "secreting-hardening" process of the mussel adhesive protein and the organic-inorganic hybrid adhesive system of the oyster. Calcium sulfoaluminate (CSA) was introduced into the adhesive mixture of SP and acrylic acid to induce the in situ polymerization of acrylic acid to achieve adhesive gelation. The generation of the inorganic crystals by hydration of CSA not only contributed to the formation of a stable cross-linked hybrid adhesive system for strong cohesion but also provided strong interfacial adhesion between the adhesive layers and the plywood veneers. As anticipated, the prepared plywood sample bonded with the hybrid adhesive gel had an excellent prepressing bonding strength of 544 kPa, representing a significant increase compared to that of the pure SP adhesive (19 kPa). Moreover, the generated inorganic crystals endowed the adhesive with excellent mildew resistance and flame retardancy. This study provides a novel and effective strategy for the preparation of high-performance SP-based adhesives.

Light-Activated Adhesion and Debonding of Underwater Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSAs) such as sticky notes and labels are a ubiquitous part of modern society. PSAs with a wide range of peel adhesion strength are designed by tailoring the bulk and surface properties of the adhesive. However, designing an adhesive with strong initial adhesion but showing an on-demand decrease in adhesion has been an enduring challenge in the design of PSAs. To address this challenge, we designed alkoxyphenacyl-based polyurethane (APPU) PSAs that show a photoactivated increase and decrease in peel strength. With increasing time of light exposure, the failure mode of our PSAs shifted from cohesive to adhesive failure, providing residue-free removal with up to 83% decrease in peel strength. The APPU-PSAs also adhere to substrates submerged underwater and show a similar photoinduced decrease in adhesion strength.

Environmentally Compatible Wearable Electronics Based on Ionically Conductive Organohydrogels for Health Monitoring with Thermal Compatibility, Anti-Dehydration, and Underwater Adhesion

Hydrogel-based electronics have found widespread applications in soft sensing and health monitoring because of their remarkable biocompatibility and mechanical features similar to human skin. However, they are subjected to potential challenges like structural failure, functional degradation, and device delamination in practical applications, especially facing extreme environmental conditions (e.g., abnormal temperature and humidity). To address these, ionically conductive organohydrogel-based soft electronics are developed, which can perform at subzero and elevated temperatures (thermal compatibility) as well as at dehydrated and hydrated environments (hydration compatibility) for extended applications. More specifically, gelatin/poly(acrylic acid-N-hydrosuccinimide ester) (PAA-NHS ester)-based ionic-conductive organohydrogel is synthesized. By introducing a glycerol-water binary solvent system, the gel can maintain mechanical softness in a wide temperature range (from -80 to 60 °C). Besides, excellent conductivity is achieved under various conditions by soaking the gel into lithium chloride anhydrous (LiCl) solution. Strong adhesion with skin, even under water, can be realized by covalent bonds between NHS ester from gel and amino groups from human skin. The excellent performances of LiCl-loaded PAA-based organohydrogel (L-PAA-OH)-based electronics are further demonstrated under freezing and high temperatures as well as underwater conditions, unveiling their promising prospects in wearable health monitoring in various conditions.

Proteogenomic Characterization of the Cement and Adhesive Gland of the Pelagic Gooseneck Barnacle Lepas anatifera

We focus on the stalked goose barnacle L. anatifera adhesive system, an opportunistic less selective species for the substrate, found attached to a variety of floating objects at seas. Adhesion is an adaptative character in barnacles, ensuring adequate positioning in the habitat for feeding and reproduction. The protein composition of the cement multicomplex and adhesive gland was quantitatively studied using shotgun proteomic analysis. Overall, 11,795 peptide sequences were identified in the gland and 2206 in the cement, clustered in 1689 and 217 proteinGroups, respectively. Cement specific adhesive proteins (CPs), proteases, protease inhibitors, cuticular and structural proteins, chemical cues, and many unannotated proteins were found, among others. In the cement, CPs were the most abundant (80.5%), being the bulk proteins CP100k and -52k the most expressed of all, and CP43k-like the most expressed interfacial protein. Unannotated proteins comprised 4.7% of the cement proteome, ranking several of them among the most highly expressed. Eight of these proteins showed similar physicochemical properties and amino acid composition to known CPs and classified through Principal Components Analysis (PCA) as new CPs. The importance of PCA on the identification of unannotated non-conserved adhesive proteins, whose selective pressure is on their relative amino acid abundance, was demonstrated.

An All-in-One Tannic Acid-Containing Hydrogel Adhesive with High Toughness, Notch Insensitivity, Self-Healability, Tailorable Topography, and Strong, Instant, and On-Demand Underwater Adhesion

Hydrogels that are mechanically tough and capable of strong underwater adhesion can lead to a paradigm shift in the design of adhesives for a variety of biomedical applications. We hereby innovatively develop a facile but efficient strategy to prepare hydrogel adhesives with strong and instant underwater adhesion, on-demand detachment, high toughness, notch-insensitivity, self-healability, low swelling index, and tailorable surface topography. Specifically, a polymerization lyophilization conjugation fabrication method was proposed to introduce tannic acid (TA) into the covalent network consisting of polyethylene glycol diacrylate (PEGDA) of substantially high molecular weight. The presence of TA facilitated wet adhesion to various substrates by forming collectively strong noncovalent bonds and offering hydrophobicity to allow water repellence and also provided a reversible cross-link within the binary network to improve the mechanical performance of the gels. The long-chain PEGDA enhanced the efficacy and stability of TA conjugation and contributed to gel mechanics and adhesion by allowing chain diffusion and entanglement formation. Moreover, PEGDA/TA hydrogels were demonstrated to be biocompatible and capable of accelerating wound healing in a skin wound animal model as compared to commercial tissue adhesives and can be applied for the treatment of both epidermal and intracorporeal wounds. Our study provides new, critical insight into the design principle of all-in-one hydrogels with outstanding mechanical and adhesive properties and can potentially enhance the efficacy of hydrogel adhesives for wound healing.

A Solvent-Exchange Strategy to Regulate Noncovalent Interactions for Strong and Antiswelling Hydrogels

Physical hydrogels from existing polymers consisting of noncovalent interacting networks are highly desired due to their well-controlled compositions and environmental friendliness; and therefore, applied as adhesives, artificial tissues, and soft machines. Nevertheless, these gels have suffered from weak mechanical strength and low water resistance. Current methodologies used to fabricate these hydrogels mainly involve the freezing-thawing process (cryogels), which are complicated in preparation and short in adjustment of polymer conformation. Here, taking the merits of noncovalent bonds in adjustability and reversibility, a solvent-exchange strategy is developed to construct a class of exogels. Based on the exchange from a good solvent subsequently to a poor one, the intra- and interpolymer interactions are initially suppressed and then recovered, resulting in dissolving and cross-linking to polymers, respectively. Key to this approach is the good solvent, which favors of a stretched polymer conformation to homogenize the network, forming cross-linked hydrogel networks with remarkable stiffness, toughness, antiswelling properties, and thus underwater adhesive performance. The exogels highlight a facile but highly effective strategy of turning the solvent and consequently the noncovalent interactions to achieve the rational design of enhanced hydrogels and hydrogel-based soft materials.

Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish

The clingfish attaches to rough surfaces with considerable strength using an intricate suction disc, which displays complex surface geometries from structures called papillae. However, the exact role of these structures in adhesion is poorly understood. To investigate the relationship between papillae geometry and adhesive performance, we developed an image processing tool that analyzed the surface and structural complexity of papillae, which we then used to model hydrodynamic adhesion. Our tool allowed for the automated analysis of thousands of papillae in specimens across a range of body sizes. The results led us to identify spatial trends in papillae across the complex geometry of the suction disc and to establish fundamental structure-function relationships used in hydrodynamic adhesion. We found that the surface area of papillae changed within a suction disc and with fish size, but that the aspect ratios and channel width between papillae did not. Using a mathematical model, we found that the surface structures can adhere considerably when subjected to disturbances of moderate to high velocities. We concluded that a predominant role of the papillae is to leverage hydrodynamic adhesion and wet friction to reinforce the seal of the suction disc. Overall, the trends in papillae characteristics provided insights into bioinspired designs of surface microstructures for future applications in which adhesion is necessary to attach to diverse surfaces (in terrestrial or aquatic environments), even when subjected to disturbance forces of randomized directionality.

Water-Resistant Conformal Hybrid Electrodes for Aquatic Endurable Electrocardiographic Monitoring

Underwater vital signs monitoring of respiratory rate, blood pressure, and the heart's status is essential for healthcare and sports management. Real-time electrocardiography (ECG) monitoring underwater can be one solution for this. However, the current electrodes used for ECGs are not suitable for aquatic applications since they may lose their adhesiveness to skin, stable conductivity, or/and structural stability when immersed into water. Here, the design and fabrication of water-resistant electrodes to repurpose stretchable electrodes for applications in an aquatic environment are reported. The electrodes are composed of stretchable metal-polymer composite film as the substrate and dopamine-containing polymer as a coating. The polymer is designed to possess underwater adhesiveness from the dopamine motif, water stability from the main scaffold, and ionic conductivity from the carboxyl groups for signal transmission. Stable underwater conductivity and firm adhesion to skin allow the electrodes to collect reliable ECG signals under various conditions in water. It is shown that wearable devices incorporated with the water-resistant electrodes can acquire real-time ECG signals during swimming, which can be used for revealing the heart condition. These water-resistant electrodes realize underwater detection of ECG signals and can be used for health monitoring and sports management during aquatic activities.

The Quantitative Proteome of the Cement and Adhesive Gland of the Pedunculate Barnacle, Pollicipes pollicipes

Adhesive secretion has a fundamental role in barnacles’ survival, keeping them in an adequate position on the substrate under a variety of hydrologic regimes. It arouses special interest for industrial applications, such as antifouling strategies, underwater industrial and surgical glues, and dental composites. This study was focused on the goose barnacle Pollicipes pollicipes adhesion system, a species that lives in the Eastern Atlantic strongly exposed intertidal rocky shores and cliffs. The protein composition of P. pollicipes cement multicomplex and cement gland was quantitatively studied using a label-free LC-MS high-throughput proteomic analysis, searched against a custom transcriptome-derived database. Overall, 11,755 peptide sequences were identified in the gland while 2880 peptide sequences were detected in the cement, clustered in 1616 and 1568 protein groups, respectively. The gland proteome was dominated by proteins of the muscle, cytoskeleton, and some uncharacterized proteins, while the cement was, for the first time, reported to be composed by nearly 50% of proteins that are not canonical cement proteins, mainly unannotated proteins, chemical cues, and protease inhibitors, among others. Bulk adhesive proteins accounted for one-third of the cement proteome, with CP52k being the most abundant. Some unannotated proteins highly expressed in the proteomes, as well as at the transcriptomic level, showed similar physicochemical properties to the known surface-coupling barnacle adhesive proteins while the function of the others remains to be discovered. New quantitative and qualitative clues are provided to understand the diversity and function of proteins in the cement of stalked barnacles, contributing to the whole adhesion model in Cirripedia.

Hybrid Complex Coacervate

Underwater adhesion represents a huge technological challenge as the presence of water compromises the performance of most commercially available adhesives. Inspired by natural organisms, we have designed an adhesive based on complex coacervation, a liquid–liquid phase separation phenomenon. A complex coacervate adhesive is formed by mixing oppositely charged polyelectrolytes bearing pendant thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains. The material fully sets underwater due to a change in the environmental conditions, namely temperature and ionic strength. In this work, we incorporate silica nanoparticles forming a hybrid complex coacervate and investigate the resulting mechanical properties. An enhancement of the mechanical properties is observed below the PNIPAM lower critical solution temperature (LCST): this is due to the formation of PNIPAM–silica junctions, which, after setting, contribute to a moderate increase in the moduli and in the adhesive properties only when applying an ionic strength gradient. By contrast, when raising the temperature above the LCST, the mechanical properties are dominated by the association of PNIPAM chains and the nanofiller incorporation leads to an increased heterogeneity with the formation of fracture planes at the interface between areas of different concentrations of nanoparticles, promoting earlier failure of the network—an unexpected and noteworthy consequence of this hybrid system.

Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives

In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liquid to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphology, the final mechanical properties strongly depend on the preparation conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheological and adhesive properties. This study enables the optimization of polymer composition and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m² was found for the complex coacervates prepared at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide additional dissipation, without softening the material so much that it becomes too weak to resist detaching stress.

Thermo-Responsive Polymer Brushes with Side Graft Chains: Relationship Between Molecular Architecture and Underwater Adherence

During the last few decades, wet adhesives have been developed for applications in various fields. Nonetheless, key questions such as the most suitable polymer architecture as well as the most suitable chemical composition remain open. In this article, we investigate the underwater adhesion properties of novel responsive polymer brushes with side graft chain architecture prepared using “grafting through” approach on flat surfaces. The incorporation in the backbone of thermo-responsive poly(N-isopropylacrylamide) (PNIPAm) allowed us to obtain LCST behavior in the final layers. PNIPAm is co-polymerized with poly(methyl ethylene phosphate) (PMEP), a poloyphosphoester. The final materials are characterized studying the surface-grafted polymer as well as the polymer from the bulk solution, and pure PNIPAm brush is used as reference. PNIPAm-g-PMEP copolymers retain the responsive behavior of PNIPAm: when T > LCST, a clear switching of properties is observed. More specifically, all layers above the critical temperature show collapse of the chains, increased hydrophobicity and variation of the surface charge even if no ionizable groups are present. Secondly, effect of adhesion parameters such as debonding rate and contact time is studied. Thirdly, the reversibility of the adhesive properties is confirmed by performing adhesion cycles. Finally, the adhesive properties of the layers are studied below and above the LCST against hydrophilic and hydrophobic substrates.

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

Despite recent advance in bioinspired adhesives, achieving strong adhesion and sealing hemostasis in aqueous and blood environments is challenging. A hyperbranched polymer (HBP) with a hydrophobic backbone and hydrophilic adhesive catechol side branches is designed and synthesized based on Michael addition reaction of multi-vinyl monomers with dopamine. It is demonstrated that upon contacting water, the hydrophobic chains self-aggregate to form coacervates quickly, displacing water molecules on the adherent surface to trigger increased exposure of catechol groups and thus rapidly strong adhesion to diverse materials from low surface energy to high energy in various environments, such as deionized water, sea water, PBS, and a wide range of pH solutions (pH = 3 to 11) without use of any oxidant. Also, this HBP adhesive (HBPA) exhibits a robust adhesion to fractured bone, precluding the problem of mismatched surface energy and mechanical properties. The HBPA's adhesion is repeatable in a wet condition. Intriguingly, the HBPA is capable of gluing dissimilar materials with distinct properties. Importantly, introducing long alkylamine into this modular hyperbranched architecture contributes to formation of an injectable hemostatic sealant that can rapidly stop visceral bleeding, especially hemorrhage from deep wound.

Thermoresponsive Complex Coacervate-Based Underwater Adhesive

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.

Bioinspired Nucleobase-Driven Nonswellable Adhesive and Tough Gel with Excellent Underwater Adhesion

Underwater adhesives have drawn much attention in the areas of industrial and biomedical fields. However, it is still demanding to construct a tough underwater gel-based adhesive completely based on chemical constitution. Herein, a nonswellable and high-strength underwater adhesive gel is successfully fabricated through the random copolymerization of acrylic acid, butyl acrylate, and acrylated adenine in dimethyl sulfoxide (DMSO). The underwater adhesive behavior is skillfully regulated through hydrophobic aggregation induced by water-DMSO solvent exchange. The adhesive gels exhibit an excellent adhesive behavior for polytetrafluoroethylene, plastics, metals, rubber, and glasses in air and various aqueous solutions, including deionized water, seawater, and acid and alkali solutions (pH = 3 and 10, respectively). Moreover, the adhesive gels exhibited robust mechanical performance and remarkable nonswellable behavior, which were particularly important for applications of gel-based adhesives in water. It is anticipated that the strategy of bioinspired nucleobase-assisted underwater adhesive gel via hydrophobic aggregation induced by solvent exchange would provide an inspiration for the development of underwater adhesives.

Hypoxia weakens mussel attachment by interrupting DOPA cross-linking during adhesive plaque curing

Marine mussels (Mytilus spp.) attach to a wide variety of surfaces underwater using a network of byssal threads, each tipped with a protein-based adhesive plaque that uses the surrounding seawater environment as a curing agent. Plaques undergo environmental post-processing, requiring a basic seawater pH be maintained for up to 8 days for the adhesive to strengthen completely. Given the sensitivity of plaques to local pH conditions long after deposition, we investigated the effect of other aspects of the seawater environment that are known to vary in nearshore habitats on plaque curing. The effect of seawater temperature, salinity and dissolved oxygen concentration were investigated using tensile testing, atomic force microscopy and amino acid compositional analysis. High temperature (30°C) and hyposalinity (1 PSU) had no effect on adhesion strength, while incubation in hypoxia (0.9 mg l-1) caused plaques to have a mottled coloration and prematurely peel from substrates, leading to a 51% decrease in adhesion strength. AFM imaging of the plaque cuticle found that plaques cured in hypoxia had regions of lower stiffness throughout, indicative of reductions in DOPA cross-linking between adhesive proteins. A better understanding of the dynamics of plaque curing could aid in the design of better synthetic adhesives, particularly in medicine where adhesion must take place within wet body cavities.

Self-Assembled Nanofibers for Strong Underwater Adhesion: The Trick of Barnacles

Developing adhesives that can function underwater remains a major challenge for bioengineering, yet many marine creatures, exemplified as mussels and barnacles, have evolved their unique proteinaceous adhesives for strong wet adhesion. The mechanisms underlying the strong adhesion of these natural adhesive proteins provide rich information for biomimetic efforts. Here, combining atomic force microscopy (AFM) imaging and force spectroscopy, we examine the effects of pH on the self-assembly and adhesive properties of cp19k, a key barnacle underwater adhesive protein. For the first time, we confirm that the bacterial recombinant Balanus albicostatus cp19k (rBalcp19k), which contains no 3,4-dihydroxyphenylalanine (DOPA) or any other amino acids with post-translational modifications, can self-assemble into aggregated nanofibers at acidic pHs. Under moderately acidic conditions, the adhesion strength of unassembled monomeric rBalcp19k on mica is only slightly lower than that of a commercially available mussel adhesive protein mixture, yet the adhesion ability of rBalcp19k monomers decreases significantly at increased pH. In contrast, upon preassembly at acidic and low-salinity conditions, rBalcp19k nanofibers keep stable in basic and high-salinity seawater and display much stronger adhesion and thus show resistance to its adverse impacts. Besides, we find that the adhesion ability of Balcp19k is not impaired when it is combined with an N-terminal Thioredoxin (Trx) tag, yet whether the self-assembly property will be disrupted is not determined. Collectively, the self-assembly-enhanced adhesion presents a previously unexplored mechanism for the strong wet adhesion of barnacle cement proteins and may lead to the design of barnacle-inspired adhesive materials.

Tough Hydrogels with Fast, Strong, and Reversible Underwater Adhesion Based on a Multiscale Design

Hydrogels have promising applications in diverse areas, especially wet environments including tissue engineering, wound dressing, biomedical devices, and underwater soft robotics. Despite strong demands in such applications and great progress in irreversible bonding of robust hydrogels to diverse synthetic and biological surfaces, tough hydrogels with fast, strong, and reversible underwater adhesion are still not available. Herein, a strategy to develop hydrogels demonstrating such characteristics by combining macroscale surface engineering and nanoscale dynamic bonds is proposed. Based on this strategy, excellent underwater adhesion performance of tough hydrogels with dynamic ionic and hydrogen bonds, on diverse substrates, including hard glasses, soft hydrogels, and biological tissues is obtained. The proposed strategy can be generalized to develop other soft materials with underwater adhesion.

Environmental post-processing increases the adhesion strength of mussel byssus adhesive

Marine mussels (Mytilus trossulus) attach to a wide variety of surfaces underwater using a protein adhesive that is cured by the surrounding seawater environment. In this study, the influence of environmental post-processing on adhesion strength was investigated by aging adhesive plaques in a range of seawater pH conditions. Plaques took 8-12 days to achieve full strength at pH 8, nearly doubling in adhesion strength (+94%) and increasing the work required to dislodge (+59%). Holding plaques in low pH conditions prevented strengthening, causing the material to tear more frequently under tension. The timescale of strengthening is consistent with the conversion of DOPA to DOPA-quinone, a pH dependent process that promotes cross-linking between adhesive proteins. The precise arrangement of DOPA containing proteins away from the adhesive-substratum interface emphasizes the role that structural organization can have on function, an insight that could lead to the design of better synthetic adhesives and metal-coordinating hydrogels.

Texture-Dependent Adhesion in Polydopamine Nanomembranes

The conformal nature of in situ polymerization of adhesive dopamine molecules permits the strong underwater adhesion between polydopamine (PDA) nanomembranes and the target substrates. However, the adhesive interaction between the postdeposit PDA nanomembranes and other macrobodies is strongly influenced by the texture of PDA nanomembranes. Here we report the texture-dependent adhesion of PDA nanomembranes both in air and aqueous environments. Despite the nanometer-scale roughness of PDA nanomembranes, interfacial adhesion between PDA nanomembranes and elastomeric bodies are the strong function of the root-mean-square roughness of PDA nanomembranes, root-mean-square gradient of PDA nanomembranes, and the elasticity of the bulk materials. Reduced adhesion due to increased texture is intensified in hydrated conditions, possibly hinting that the conventional explanation of the negative effect of water to adhesion from a molecular level needs to be revisited. These findings can inform the role of adhesive interaction in conformal coatings and provide an explanation for the differential adhesion observed in freestanding PDA nanomembranes.

Importance of Substrate Functionality on the Adhesion and Debonding of a Pressure-Sensitive Adhesive under Water

We investigate the effect of an aqueous environment on the adhesion of a model acrylic pressure-sensitive adhesive (PSA) composed of 2-ethylhexyl acrylate-co-acrylic acid. We use probe-tack adhesion measurements accompanied by in situ imaging of the contact region during bonding and debonding. Within the probe-tack tests, we use both hydrophilic (piranha and plasma treatment) and hydrophobic (C₁₈-silanization) surface treatments to investigate the contribution of the probe's surface energy on the underwater adhesion. In examining contact formation in air and underwater, we find that the presence of water when contact is made leads to different modes of PSA relaxation and contact formation. For all probes investigated, the adhesive strength between the PSA and the probe decreases when measured underwater. Additionally, we observe that the presence of water during debonding has a more pronounced effect on the adhesive strength of the PSA when probed by a hydrophilic surface as opposed to a hydrophobic surface. Using fingering wavelength analysis, we estimate the surface energy of the PSA in situ and find that when submerged in water, the PSA has a significantly higher surface energy compared to in air. Therefore, combining the observation of different modes of contact formation, the increase in surface energy, and the importance of the surface energy of the probe, we suggest that the decrease in adhesive strength in water can be explained by the hydration of the PSA and by trapped water defects between the PSA and the probe.