Underwater Adhesion Research Articles

Underwater Adhesion and Anti‐Swelling Hydrogels

AbstractWearable devices, biomaterials, and tissue engineering have all gained from the development of flexible materials. Hydrogels are made up of the polymer matrix in an amount of water and are now being explored extensively. Ascribed to their high hydrophilicity, conventional hydrogels are difficult to deal with high humidity, being easily swell and even decompose. The utilization of hydrogel underwater is influenced by two key factors: the hydrogel absorbs water and swells easily to depress the mechanical characteristics, and the hydration layer at the contact interface also prevents the hydrogel from making close contact with the target surface, preventing conformal and accurate smart perception. So far, many researchers have proposed satisfying improvements for underwater applications from these two perspectives. This review first summarizes the general mechanisms of underwater adhesion hydrogels. Then, various anti‐swelling strategies of the reported hydrogel are described. Finally, the existing challenges and prospects of underwater adhesion and anti‐swelling hydrogels are provided.

Eco-friendly silk fibroin/tannic acid coacervates for humid and underwater wood adhesives

Bioadhesives derived from biomass are steadily gaining spotlight as substitutes for formaldehyde-based resins in the adhesive industry. However, there is a need to develop novel water-resistant bioadhesives with high adhesive and cohesive strengths because the currently available biomaterial-based adhesives have low mechanical strength. In this study, a complex coacervate was prepared easily by mixing silk fibroin and tannic acid to produce a bioadhesive with high adhesive and cohesive strengths as well as water resistance. The silk fibroin–tannic acid coacervate adhered well to various substrates, and its adhesive strength according to the type of substrate and water contact angle were evaluated comparatively. In particular, the adhesive strength of this adhesive on a wood substrate was systematically analyzed by varying different experimental parameters (relative humidity, surface roughness of the substrate, water stability, and pH). This cost-effective coacervate is applicable as an eco-friendly wood adhesive.

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.

Rational design of adhesives for effective underwater bonding.

Underwater adhesives hold great promises in our daily life, biomedical fields and industrial engineering. Appropriate underwater bonding can reduce the huge cost from removing the target substance from water, and greatly lift working efficiency. However, different from bonding in air, underwater bonding is quite challenging. The existence of interfacial water prevents the intimate contact between the adhesives and the submerged surfaces, and water environment makes it difficult to achieve high cohesiveness. Even so, in recent years, various underwater adhesives with macroscopic adhesion abilities were emerged. These smart adhesives can ingeniously remove the interfacial water, and enhance cohesion by utilizing their special physicochemical properties or functional groups. In this mini review, we first give a detail introduction of the difficulties in underwater bonding. Further, we overview the recent strategies that are used to construct underwater adhesives, with the emphasis on how to overcome the difficulties of interfacial water and achieve high cohesiveness underwater. In addition, future perspectives of underwater adhesives from the view of practical applications are also discussed. We believe the review will provide inspirations for the discovery of new strategies to overcome the obstacles in underwater bonding, and therefore may contribute to designing effective underwater adhesives.

Stuck on mussel adhesion

A better understanding of the molecules and interactions involved in the underwater adhesion of marine organisms could help develop new biomimetic adhesives.

Small Amphiphile-Based Coacervation.

Coacervation plays an important role in the molecular assembly towards soft materials with a diversity of function (e. g., underwater adhesives of mussels and membraneless organelles). Coacervation is observed when one homogenous solution spontaneously separates into two immiscible liquid phases of low and high solute concentration, also known as liquid-liquid phase separation (LLPS), which enables spatiotemporally local concentration of specific molecules. LLPS is a common physical phenomenon in aqueous solutions of polyelectrolytes, surfactants and biomolecules, which has been extensively explored for applications in the fields of environmental remediation, cosmetic formulation, protein purification, extractive fermentation and pharmaceutical microencapsulation. This review summarizes the development of LLPS with low molecular weight amphiphiles to construct simple and complex coacervates using conventional surfactants and novel amphiphiles such as azobenzene-derivatives and peptides. We also highlight the applications of these amphiphile coacervates in the extraction of biomolecules, construction of protocell models and drug delivery.

Topologically Enhanced Cation–π Interactions for Developing High‐Performance Underwater Adhesive

AbstractCation–π interactions provide an excellent way to develop high‐performance underwater adhesives. Although numerous underwater adhesive systems are designed based on cation–π interactions, they still lack precise design of the adhesive polymer backbone and topological control of the adhesive polymer. In this study, a novel short‐chain adhesive polymer with excellent underwater adhesion is developed based on the topologically enhanced cation–π interactions. Dihydrocaffeic acid containing catechol is incorporated into a short‐chain cationic polymer of polycarbodiimides through the nucleophilic reactions between carboxyl and carbodiimide groups to produce a catechol‐decorated cationic adhesive polymer (CCAP). The unique molecular structure of the CCAP confers the adhesive system strong cohesion and topologically enhanced cation–π interactions, leading to excellent underwater adhesion. Fe3+ is introduced into the adhesive system to promote the coordination of catechol and the oxidative crosslinking of catechol to further improve the cohesion of the CCAP. In addition, based on the synergy between reversible noncovalent bonds and the irreversible oxidative crosslinking system, the prepared adhesive can also act as a high‐strength hot‐melt adhesive, which maintains nearly 90% of its adhesion strength after 10 attachment–detachment cycles, showing its great potential for use as a sustainable reusable dismountable adhesive. This study provides a novel strategy to develop high‐performance underwater adhesive with temperature sensitivity.

An essential role of disulfide bonds for the hierarchical self-assembly and underwater affinity of CP20-derived peptides.

Barnacles are typical fouling organisms strongly adhere to immersed solid substrates by secreting proteinaceous adhesives called cement proteins (CPs). The self-assembly of the CPs forms a permanently bonded layer that binds barnacles to foreign surfaces. However, it is difficult to determine their natural structure and describe their self-assembly properties due to the abundance of cysteines in whole-length CP20. A putative functional motif of Balanus albicostatus CP20 (BalCP20) was identified to present distinctive self-assembly and wet-binding characteristics. Atomic-force microscopy (AFM) and transmission electron microscope (TEM) investigations showed that wildtype BalCP20-P3 formed grain-like spindles, which assembled into fractal-like structures like ears of wheat. SDS-PAGE, AFM, and LSCM showed that DTT treatment opened up disulfide bonds between cysteines and disrupted fractal-like structures. Additionally, these morphologies were abolished when one of the BalCP20-P3 four cysteines was mutated by alanine. Circular dichroism (CD) results suggested that the morphological diversity among BalCP20-P3 and its mutations was related to the proportion of α-helices. Finally, quartz crystal microbalance with dissipation (QCM-D) detected that BalCP20-P3 and its mutations with diverse self-assemblies occupied different affinities. The above results demonstrated that cysteines and disulfide bonds played a crucial role in the self-assembly and wet binding of BalCP20-P3. The work provides new ideas for the underwater bonding of BalCP20 and developing new bionic underwater adhesives.

Mussel foot inspired bionic adhesive material enhanced by a reconstructed in vitro system for interfacial adhesion

The strong adhesion abilities of mussel adhesive protein in wet environment rely on complicated solidified processes. Among these processes, several are found to be key factors affecting underwater interfacial adhesion, including post-translational modification for tyrosine catalyzed by tyrosinase in gland cells, protein storage and DOPA antioxidation in vesicles after the modification reaction, delayed oxidation at the interface and pH change after protein secretion in seawater. In view of this, an in vitro system simulating the natural underwater adhesion process is reconstructed in this study to prepare the mussel inspired bionic material. First, a mussel foot protein (Mfp) −3/5 mimetic precursor (PGA-Tyr-Arg) is prepared as substrate in the reaction. Second, different reductants are compared to prevent DOPA-catechol groups oxidation. Tannic acid proves to have minimal inhibition of tyrosinase activity while functioning properly as the suitable antioxidant during tyrosinase catalyzed modification reaction. And Mfp6 biomimetic protein (PGA-Cys) is used as antioxidation for storage and adhesion phases after modification reaction, because it has more negative effects on tyrosinase activity and better oxidation resistance. Third, the new immobilized tyrosinase, IEAMNP-Cu2+-Tyrosinase overcomes the deficiencies of free tyrosinase and other immobilized tyrosinases in catalyzing large molecule substrates and achieves enhanced activity recovery (∼150% to 300%), rapid separation from the catalyzed system, and high conversion of tyrosine to DOPA (93%). The bionic adhesive material prepared by this reconstructed in vitro system can achieve up to 9.5 MPa of lap shear strength, showing it can improve the interfacial adhesion of mussel inspired bionic materials.

Water‐Triggered Spontaneously Solidified Adhesive: From Instant and Strong Underwater Adhesion to In Situ Signal Transmission (Adv. Funct. Mater. 44/2022)

Underwater Adhesives In article number 2205597, Si Yu Zheng, Jimin Fu, Jintao Yang, and co-workers design a polymeric glue based on triazole-beared macromolecules and ionic liquids, which can flow freely to fill the crack with various morphology and solidify rapidly to achieve fast, robust, long-lasting adhesion under seawater. Furthermore, the glue possesses additional functions of underwater sensing and fluorescent labeling, which exhibits great potentials in underwater repair and follow-up crack monitoring.

Hydrogen-bonds mediate liquid-liquid phase separation of mussel derived adhesive peptides

Marine mussels achieve strong underwater adhesion by depositing mussel foot proteins (Mfps) that form coacervates during the protein secretion. However, the molecular mechanisms that govern the phase separation behaviors of the Mfps are still not fully understood. Here, we report that GK-16*, a peptide derived from the primary adhesive protein Mfp-5, forms coacervate in seawater conditions. Molecular dynamics simulations combined with point mutation experiments demonstrate that Dopa- and Gly- mediated hydrogen-bonding interactions are essential in the coacervation process. The properties of GK-16* coacervates could be controlled by tuning the strength of the electrostatic and Dopa-mediated hydrogen bond interactions via controlling the pH and salt concentration of the solution. The GK-16* coacervate undergoes a pH induced liquid-to-gel transition, which can be utilized for the underwater delivery and curing of the adhesives. Our study provides useful molecular design principles for the development of mussel-inspired peptidyl coacervate adhesives with tunable properties.

Development and Control of a Bioinspired Robotic Remora for Hitchhiking

Remora, a bony fish, is well known for the remarkable hitchhiking behavior, by which it can transport itself over large distances without too much effort. In this article, a novel robotic remora with a stable adhesion system and its motion control methods are proposed for transferring the hitchhiking behavior to the engineered system. In terms of the mechatronic design, a robotic prototype with the uncoupled planar and vertical motion mechanisms is created to realize the precise and agile movement. Furthermore, a reliable adhesion system possessing low preload demand, considerable adhesive force ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim \text{250}\text{ N}$</tex-math></inline-formula> ), and reversible adhesion ability is developed, and then, the design principle of the adhesive disc is investigated. In the aspect of motion control, a mode-switch-based planar pose (position and orientation) control method and an antidisturbance depth control method are designed. The combination of these two controllers endows the robotic remora with the ability to precisely arrive at any position in a 3-D space, which paves the way for the hitchhiking task. Extensive experimental results are presented to support the performance benefits of the proposed robotic remora and control methods. Finally, the hitchhiking behavior is implemented successfully by robotic remora. To the best of our knowledge, this is the first engineered implementation of hitchhiking for a fin-actuated underwater robot. The results obtained will promote the design and control of the future underwater adhesion robot and offer valuable insights into the long-endurance robotic fish and mother–son multirobot system.

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.

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.

Underwater instant adhesion mechanism of self-assembled amphiphilic hemostatic granular hydrogel from Andrias davidianus skin secretion

SummaryThe widespread use of biological tissue adhesives for tissue repair is limited by their weak adhesion in a wet environment. Herein, we report the wet adhesion mechanism of a dry granular natural bioadhesive from Andrias davidianus skin secretion (ADS). Once contacting water, ADS granules self-assemble to form a hydrophobic hydrogel strongly bonding to wet substrates in seconds. ADS showed higher shear adhesion than current commercial tissue adhesives and an impressive 72-h underwater adhesion strength of ∼47kPa on porcine skin tissue. The assembled hydrogel in water maintained a dissipation energy of ∼8 kJ/m3, comparable to the work density of muscle, exhibiting its robustness. Unlike catechol adhesion mechanism, ADS wet adhesion mechanism is attributed to water absorption by granules, and the unique equilibrium of protein hydrophobicity, hydrogen bonding, and ionic complexation. The in vivo adhesion study demonstrated its excellent wet adhesion and hemostasis performance in a rat hepatic and cardiac hemorrhage model.

Mussel adhesion: A fundamental perspective on factors governing strong underwater adhesion.

Protein-based underwater adhesives of marine organisms exhibit extraordinary binding strength in high salinity based on utilizing a variety of molecular interaction mechanisms. These include acid-base interactions, bidentate bindings or complex hydrogen bonding interactions, and electrochemical manipulation of interfacial bonding. In this Perspective, we briefly review recent progress in the field, and we discuss how interfacial electrochemistry can vary interfacial forces by concerted tuning of surface charging, hydration forces, and tuning of the interfacial ion concentration. We further discuss open questions, controversial findings, and new paths into understanding and utilizing redox-proteins and derived polymers for enhancing underwater adhesion in a complex salt environment.

Molecular functional analyses of larval adhesion in a highly fouling invasive model ascidian

Successful papillary adhesion is the first essential step for serious ascidian biofouling in marine ecosystems; however, the mechanisms of papillary adhesion have not been fully elucidated. Here, we employed transcriptome sequencing to assess the differentially expressed genes (DEGs) between papillae and remaining body in the larvae of the highly fouling invasive model ascidian Ciona robusta. Enrichment results for DEGs showed that papillae were adhesive structures with the combined functions of substrate recognition, environmental perception, and adhesive protein synthesis and secretion. Two candidate ascidian papilla adhesive proteins (APAPs), APAP-1 and APAP-2, were identified, expressed, and purified in vitro. Surface coating tests showed that APAP-1 was a cohesive protein while APAP-2 was an interfacial protein involved in the adhesion between papillae and material surface. Collectively, the obtained DEGs and adhesive proteins provide candidates to deeply understand molecular mechanisms of underwater adhesion and further develop anti-fouling strategies in marine ecosystems.

Abalone-Inspired Adhesive and Photo-Responsive Microparticle Delivery Systems for Periodontal Drug Therapy.

Antibiotics provide promising strategies for treating periodontitis, while their delivery and controllable release with desired oral retention remain challenging. Here, inspired by the unique suction‐cup structures of abalones, a novel adhesive and photo‐responsive microparticle (MP) delivery system is developed to treat periodontitis through microfluidic electrospray technology. Such MPs are generated by quickly ionic cross‐linking of sodium alginate together with photo‐curing of poly(ethylene glycol) diacrylate of the distorted microfluidic droplets during their high‐speed dropping into calcium chloride solution. Attributing to their unique concave structures, the abalone‐inspired MPs exhibit desired underwater adhesion ability and stability under running water. In addition, due to the loading of antibiotics minocycline hydrochloride and near‐infrared (NIR)‐responsive black phosphorus during their fabrication, the resultant MPs can not only eradicate bacteria directly, but also realize a controllable and effective drug release upon NIR irradiation. Based on these features, it is demonstrated from in vivo periodontitis that the abalone‐inspired MPs are firmly adhesive and can controlled‐release drugs on the tooth, and thus have outstanding antibacterial efficacy against Porphyromonas gingivalis. These results indicate the particular values of the abalone‐inspired MPs for oral‐related disease treatment.

A highly stretchable, UV resistant and underwater adhesive hydrogel based on xylan derivative for sensing

Hydrogels with fascinating adhesion have been demonstrated great potential in various applications. However, most hydrogels lose their adhesion in wet or underwater environments due to the influence of interfacial water. Inspired by mussel, an underwater adhesive hydrogel was facilely fabricated by introducing electrostatic interactions, which consisted of poly (acrylic acid) (PAA), quaternized xylan (QAX) and tannic acid (TA). In this hydrogel, -COO− from PAA, -N+(CH3)3 from QAX and catechol group from TA resembled amino acids with negative and positive charges and 3,4-dihydroxyphenylalanine units in mussel, which endowed the hydrogels with great underwater adhesion through multiple interactions. Notably, acrylic acid (AA) played a key role in the dispersion of the system. QAX, a biomass derived from plants with excellent properties, worked with PAA to construct hydrogel networks. The resultant hydrogels exhibited excellent mechanical properties including remarkable stretchability (>4000 %) and compressibility. Moreover, the hydrogels had superior UV-blocking (~99.96 %), and showed good adhesion both in air and underwater. The hydrogels can be exploited as a wearable sensor to monitor human motions and even subtle motions, which have the potential to be explored in human health monitoring.

Electrically switched underwater capillary adhesion

Developing underwater adhesives that can rapidly and reversibly switch the adhesion in wet conditions is important in various industrial and biomedical applications. Despite extensive progresses, the manifestation of underwater adhesion with rapid reversibility remains a big challenge. Here, we report a simple strategy that achieves strong underwater adhesion between two surfaces as well as rapid and reversible detachment in on-demand manner. Our approach leverages on the design of patterned hybrid wettability on surfaces that selectively creates a spatially confined integral air shell to preserve the water bridge in underwater environment. The overall adhesion strength can be multiplied by introducing multiple air shells and rapidly broken by disturbing the integrity of the protective air shell in response to the applied voltage on two surfaces. Our design can be constructed on the flexible substrate with hybrid wettability, which can be applied to non-conductive substrates and adapted to more complicated morphologies, extending the choice of underlying materials.