Underwater Adhesion Research Articles

Environment-tolerant, inherently conductive and self-adhesive gelatin-based supramolecular eutectogel for flexible sensor

Although hydrogels have attracted increasing attention in the stretchable devices, the low adhesion properties and poor environmental adaptation still seriously restrict their development and application. Herein, we focused on the interaction between polymer networks with disperse media and their resultant influence on gel performance, and constructed self-adhesive and environment-tolerant gelatin/polyacrylamide supramolecular-polymer double-network (Gelatin/PAM SP-DN) eutectogels using multiple supramolecular interactions between natural macromolecule and well-designed deep eutectic solvent (DES). The dual networks of Gelatin/PAM SP-DN eutectogels produced significant supramolecular forces with DES, including hydrogen bonding and electrostatic interaction, contributing to enhance the energy dissipation capacity. Additionally, the Gelatin-PAM SP-DN eutectogels were more prone to generate strong bonding force to various substrates, showcasing both in-situ and ex-situ adhesion performance, and even being used for wet and underwater adhesion. The eutectogels revealed excellent environmental tolerance to maintain excellent mechanical flexibility, conductivity and adhesion at high and low temperatures, ensuring the constructed sensor to sensitively and reliably perceive strain, pressure and human motions over a wide temperature range. Also, the eutectogel demonstrated great potential as a temperature sensor. This work opens up a new horizon in the design of multifunctional and environment-tolerant natural macromolecule-based gel materials for flexible electronics, human-machine interaction and health diagnosis.

Powdered Medical Adhesive with Long Lasting Adhesion in Water Environment.

Medical adhesives have been used under surgical conditions. However, it is always a big challenge to maintain long-term adhesion in a water environment. Besides, it usually takes a long time to complete the adhesion, and the operation might be complicated. In this study, tannic acid and gelatin solution under acidic conditions were mixed, flocculated, lyophilized, and crushed; thus, a powdered medical adhesive (POWDER) was prepared with long-lasting adhesion in a water environment, convenience, and low price. Tannic acid bound gelatin and maintained adhesive force primarily through hydrogen bonding and reacted with amino sulfhydryl and other amino acid residues after oxidation into aldehyde, exhibiting excellent underwater adhesion. Oxidized dextran (ODex) powder rich in an aldehyde group was introduced to provide covalent binding in the adhesive. In vitro and in vivo studies showed that POWDER could quickly adhere to various tissues in the water environment. In vitro skin adhesion experiments demonstrated that it could achieve effective adhesion in a water environment for up to 60 days. Its blood compatibility, low cytotoxicity, and biodegradability were also verified. The POWDER developed in this study is of great significance for patients who need rapid wound treatments.

Strong, Spontaneous, and Self-Healing Poly(Ionic Liquid) Elastomer Underwater Adhesive with Borate Ester Dynamic Crosslinking.

Adhesion in aqueous environments is often hindered by the water layer on the surface of the substrate due to the water sensitivity of the adhesive, greatly limiting the application environment. Here, a borate ester dynamically crosslinked poly(ionic liquid) elastomer adhesive (PIEA) with high strength, toughness, self-healing abilities, and ionic conductivity is synthesized by copolymerizing hydrophobic ionic liquid monomer ([HPVIm][TFSI]) and 2-methoxyethyl acrylate (MEA). The adhesion strength of PIEA can increase spontaneously from almost no adhesion to 314 kPa after 12 h without any external preloading due to the dissociation of the borate ester in water, leading to noncovalent interactions between the hydroxyl groups of PIEA and the substrate. Additionally, PIEA can be developed for soft sensors or ion electrodes to enable underwater detection and communication. This strategy offers broad application potential for the development of novel underwater smart adhesives.

Robust, Switchable and Printable Underwater Adhesives Based on a Temperature‐Deactivated Design

AbstractConventional adhesives generally suffer from diminished adhesion in aqueous environments, posing significant challenges for their application in wet and submerged conditions. While extensive research efforts are directed toward enhancing the interfacial bonding, cohesive strength, and durability of adhesives in such environments, most existing underwater adhesives remain static and irreversible. This limitation hinders their reusability and often results in undesirable residues. Addressing the challenge of achieving strong underwater adhesion with on‐demand detachment remains crucial. This study introduces the development of temperature‐responsive underwater adhesives that demonstrate outstanding bonding strength, reversibility, and durability. Through the strategic integration of interfacial bonding, reinforced cross‐linking networks, dynamic hydrogen bonds, and upper critical solution temperature (UCST)‐driven phase transitions, one of the few temperature‐deactivated adhesives is created. The optimized adhesive is distinguished by its performance to achieve underwater adhesion strengths exceeding 1.4 MPa, nearly 100% switching efficiency and residue‐free adherend under mild thermal cycling. Beyond the template‐assisted fabrication of adhesive patches, 3D printable adhesives are achieved with programmable architectures via direct ink‐writing. This work highlights the development of temperature‐deactivated underwater adhesives activated by UCST‐type phase transitions, not only boosting adhesion strengths and switching efficiencies across various water conditions but also broadening their applicability with user‐defined and application‐specific functions.

Mussel-inspired thermo-switchable underwater adhesive based on a Janus hydrogel

On-demand underwater adhesives with excellent adhesive and gentle detachment properties enable stable connections to various biomedical devices and biointerfaces and avoid the risk of harmful tissue damage upon detachment. Herein, we present a Janus hydrogel adhesive that can reversibly switch its adhesion strength, which is controlled by temperature, using a thermoresponsive polymer and mussel-inspired molecules. This thermoswitchable adhesive (TSA) hydrogel displays both strong adhesion and gentle detachment with an over 1000-fold gap in underwater adhesion strength onto glass, titanium, aluminum, and Teflon substrates when exposed to temperatures above and below the lower critical solution temperature (LCST). The adhesion switch is possibly caused by the change in toughness of the TSA hydrogels with temperature because the Janus hydrogel possesses gradient crosslinked structures. Moreover, the lowermost surface is sufficiently soft to gently detach from the substrate below the LCST. The electrode-integrated hydrogel remains on human skin, and electrical signals are continuous over 10 min above the LCST. In contrast, commercially available hydrogel electrodes quickly swell and detach from the skin. The thermoswitchability of the TSA hydrogel, with its robust adhesion and gentle detachment, offers significant potential for biomedical applications characterized by minimally invasive procedures.

Sticky Science: Using Complex Coacervate Adhesives for Biomedical Applications.

Tissue adhesives are used for various medical applications, including wound closure, bleeding control, and bone healing. Currently available options often show weak adhesion or cause adverse effects. Recently, there has been an increasing interest in complex coacervates as medical adhesives. Complex coacervates are formed by mixing oppositely charged macromolecules that associate and undergo liquid-liquid phase separation, in which the dense bottom phase is the complex coacervate. Complex coacervates are strong and often biocompatible, and show strong underwater adhesion. The properties of the resulting materials are tunable by intrinsic factors such as polymer chemistry, molecular weight, charge density, and topology of the macromolecules, as well as extrinsic factors such as temperature, pH, and salt concentration. Therefore, complex coacervates are interesting new candidates for medical adhesives. In this review, it is described how complex coacervates form and how different factors influence their behavior. Next, an overview of recent studies on complex coacervates in the context of medical adhesives is presented. The application of complex coacervates as hemostatic or embolic agents, skin or bone repair adhesives, and soft tissue sealants is discussed. Lastly, additional possibilities for utilizing these materials in the future are discussed.

Lignin‐Based Macromolecular Photoinitiator for Non‐Migration, Self‐Adhesive, and Water‐Resistant Eutectogels toward Underwater Reliable Communication

AbstractHydrophobic conductive gels have attracted much attention in underwater sensing as the soft robots and wearable sensors. Among the various methods for preparing conductive gels, UV‐initiated polymerization shows advantages of rapid curing and significant energy savings. However, traditional commonly used photoinitiators in UV‐initiated polymerization often migrate or diffuse within conductive gels, especially with prolonged use or underwater, posing potential environmental and health risks. Herein, a novel lignin‐based macromolecular photoinitiator (L‐BF) is first designed by incorporating benzoylformate into the lignin backbone, demonstrating excellent UV absorption, high radical initiation efficiency, and hydrophobicity. Subsequently, hydrophobic eutectogels (HEG) are prepared via UV‐initiated polymerization using L‐BF as photoinitiator. The HEG shows excellent mechanical properties (toughness of 5.67 MJ m−3 and strain of 941%) and scarcely L‐BF migration underwater. Moreover, the hydrophobic network of L‐BF endowed HEG with hydrophobicity (water contact angle exceeds 110°; swelling ratio is <5.8%), superior underwater adhesion (255 kPa), even maintained excellent sensing performance (gauge factor of 4.69), and long‐term reproducibility (20 000 cycles) in air and underwater. Validation confirms the potential of HEG as an effective wearable sensor for underwater posture monitoring and communication, advancing the use of lignin and low‐migration conductive gels in marine exploration.

Highly Stretchable, Transparent, Solvent-Resistant Multifunctional Ionogel with Underwater Self-Healing and Adhesion for Wearable Strain Sensors and Barrier-Free Information Transfer.

Ionogels with excellent deformability, high ionic conductivity, and a sensitive stimulus response have been widely used and rapidly developed in flexible wearable systems. However, previously reported ionogels are mainly limited to atmospheric environments applications and have difficulty meeting the requirements of solvent-resistant, self-healing, and adhesion properties in underwater environments. Herein, a multifunctional ionogel capable of underwater applications is prepared by one-step photoinitiated polymerization of a fluorine-containing monomer (2,2,3,4,4,4-hexafluorobutyl acrylate, HFBA) and acrylic acid (AA) in a hydrophobic ionic liquid ([EMIM][TFSI]). The dynamic physical interactions of hydrogen bonds and ionic dipoles endow the ionogel with remarkable transparency, tunable mechanical properties, and underwater self-healing properties. Moreover, the fluoropolymer matrix offers high resistance to water and various solvents and exhibits strong underwater adhesion on different substrates. Thus, the sensor based on the ionogel exhibits excellent sensing properties, including high sensitivity, fast response, and superior durability. In particular, the ionogel can be used as a wearable underwater sensor to perform barrier-free information transfer. This study provides a design idea for the development of underwater flexible strain sensors.

Nucleotide‐Tackified Degradable and Closed‐Loop Recyclable Underwater Adhesive

Nucleotide‐Tackified Degradable and Closed‐Loop Recyclable Underwater Adhesive

Eutectogel adhesives with underwater-enhanced adhesion to hydrophilic surfaces and strong adhesion in harsh environments

It has been a challenge to achieve strong adhesion strength for gel adhesives in water. In this study, a novel eutectogel adhesive is synthesized via one-step photoinitiated polymerization of N-vinylpyrrolidone (NVP) in deep eutectic solvent (DES). The abundant hydrogen bonding interactions and the hydrophilic polyvinylpyrrolidone network structure endow the eutectogel with excellent adhesion to both hydrophilic and hydrophobic surfaces in air and underwater. Moreover, for hydrophilic substrates, the underwater adhesion strength of the eutectogel adhesives is 1.3 times larger than that in air, demonstrating an underwater-enhanced adhesion effect. In contrast, for hydrophobic substrates, the underwater adhesion strength is lower than in air. Furthermore, strong adhesion performance is maintained even in boiling water, strong acidic medium, strong basic medium, and organic solvents. The experimental and simulation results have unveiled the underwater adhesion enhancement mechanism of the eutectogel. It is believed that the designed eutectogel will show great promise in the development of the next-generation of gel adhesives with underwater adhesion capabilities and high environmental adaptability.

Tannic Acid-Enabled Antioxidant and Stretchable MXene/Silk Strain Sensors for Diving Training Healthcare.

MXene-based conductive hydrogels hold significant promise as epidermal sensors, yet their susceptibility to oxidation represents a formidable limitation. This study addresses this challenge by incorporating MXene into a tannic acid (TA) cross-linked silk fibroin matrix. The resulting conductive hydrogel (denoted as e-dive) exhibits favorable characteristics such as adjustable mechanical properties, self-healing capabilities (both mechanically and electrically), and strong underwater adhesion. The existence of a percolation network of MXene within the nanocomposites guarantees good electrical conductivity. Importantly, the surface interaction of MXene nanosheets with the hydrophobic moiety from TA substantially reduced moisture and oxygen interactions with MXene, thereby effectively mitigating MXene oxidation within hydrogel matrices. This preservation of the electrical characteristics ensures prolonged functional stability. Furthermore, the e-dive demonstrates inherent antibacterial properties, making it suitable for use in underwater environments where bacterial contamination is a concern. The utilization of this advanced e-dive system extends to the correction of diving postures and the facilitation of underwater healthcare and security alerts. Our study presents a robust methodology for enhancing the stability of MXene-based conductive hydrogel electronics, thereby expanding their scope of potential applications.

Bioinspired Asymmetric-Adhesion Janus Hydrogel Patch Regulating by Zwitterionic Polymers for Wet Tissues Adhesion and Postoperative Adhesion Prevention.

Asymmetrically adhesive hydrogel patch with robust wet tissue adhesion simultaneously anti-postoperative adhesion is essential for clinical applications in internal soft-tissue repair and postoperative anti-adhesion. Herein, inspired by the lubricative role of serosa and the underwater adhesion mechanism of mussels, an asymmetrically adhesive hydrogel Janus patch is developed with adhesion layer (AL) and anti-adhesion layer (anti-AL) through an in situ step-by-step polymerization process in the mold. The AL exhibits excellent adhesion to internal soft-tissues. In contrast, the anti-AL demonstrated ultralow fouling property against protein and fibroblasts, which hinders the early and advanced stages of development of the adhesion. Moreover, the Janus patch simultaneously promotes tissue regeneration via ROS clearance capability of catechol moieties in the AL. Results from in vivo experiments with rabbits and rats demonstrate that the AL strongly adheres to traumatized tissue, while the anti-AL surface demonstrate efficacy in preventing of post-abdominal surgery adhesions in contrast to clinical patches. Considering the advantages in terms of therapeutic efficacy and off the shelf, the Janus patch developed in this work presents a promise for preventing postoperative adhesions and promoting regeneration of internal tissue defects.

Multi-Modal Sensing Ionogels with Tunable Mechanical Properties and Environmental Stability for Aquatic and Atmospheric Environments.

Ionogels have garnered significant interest due to their great potential in flexible iontronic devices. However, their limited mechanical tunability and environmental intolerance have posed significant challenges for their integration into next-generation flexible electronics in different scenarios. Herein, the synergistic effect of cation-oxygen coordination interaction and hydrogen bonding is leveraged to construct a 3D supramolecular network, resulting in ionogels with tunable modulus, stretchability, and strength, achieving an unprecedented elongation at break of 10800%. Moreover, the supramolecular network endows the ionogels with extremely high fracture energy, crack insensitivity, and high elasticity. Meanwhile, the high environmental stability and hydrophobic network of the ionogels further shield them from the unfavorable effects of temperature variations and water molecules, enabling them to operate within a broad temperature range and exhibit robust underwater adhesion. Then, the ionogel is assembled into a wearable sensor, demonstrating its great potential in flexible sensing (temperature, pressure, and strain) and underwater signal transmission. This work can inspire the applications of ionogels in multifunctional sensing and wearable fields.

Water-resistant adhesive tackified by dynamic adhesion factor of nucleotide

Underwater adhesives have gained much attention across diverse fields including tissue adhesives, wearable electronics, and underwater electronics. However, it remains challenging to design a tough and stable underwater adhesion interface only relying on physical adhesion mechanisms, because the adhesion factors of the physical adhesives are limited and bound by molecular networks. Inspired by the toughening theory of “sacrificial bonds”, we present a strategy of dynamic adhesion factor to fabricate a tough and stable underwater adhesion interface. The adhesives tackified by nucleotide of dynamic adhesion factor are successfully developed and demonstrate excellent adhesion behavior in aquatic environments. The adhesion factor of nucleotide featuring reversible dynamic structural characteristics not only facilitates easier interaction with the substrate but also effectively dissipates failure stress and mitigates stress concentration when exposed to destructive stress. Moreover, the adhesives exhibit excellent recyclability, on-demand detachability, and long-term stability. Importantly, this strategy demonstrated a universality for variable monomers to construct tough underwater adhesion. It is anticipated that the dynamic adhesion factor strategy will inspire a new paradigm in the advancement of next-generation water-resistant adhesives.

Coacervate Dense Phase Displaces Surface-Established Pseudomonas aeruginosa Biofilms.

For millions of years, barnacles and mussels have successfully adhered to wet rocks near tide-swept seashores. While the chemistry and mechanics of their underwater adhesives are being thoroughly investigated, an overlooked aspect of marine organismal adhesion is their ability to remove underlying biofilms from rocks and prepare clean surfaces before the deposition of adhesive anchors. Herein, we demonstrate that nonionic, coacervating synthetic polymers that mimic the physicochemical features of marine underwater adhesives remove ∼99% of Pseudomonas aeruginosa (P. aeruginosa) biofilm biomass from underwater surfaces. The efficiency of biofilm removal appears to align with the compositional differences between various bacterial biofilms. In addition, the surface energy influences the ability of the polymer to displace the biofilm, with biofilm removal efficiency decreasing for surfaces with lower surface energies. These synthetic polymers weaken the biofilm-surface interactions and exert shear stress to fracture the biofilms grown on surfaces with diverse surface energies. Since bacterial biofilms are 1000-fold more tolerant to common antimicrobial agents and pose immense health and economic risks, we anticipate that our unconventional approach inspired by marine underwater adhesion will open a new paradigm in creating antibiofilm agents that target the interfacial and viscoelastic properties of established bacterial biofilms.

Effect of thermal processing on the integrity of polyurea microcapsules for underwater adhesives

Adhesives that rely on a curing mechanism struggle to function in underwater environments due to detrimental interactions with water. One strategy to overcome this involves protecting sensitive components in mechanically responsive microcapsules. In this way, a two-part adhesive can be achieved via a single mixture, applied as one would apply a pressure sensitive adhesive. This work focused on improving polyurea microcapsule shell integrity, specifically focusing on the effect of additional thermal processing. The improved microcapsule shells displayed limited payload leakage with only an 8 % loss in pot life compared to the base isocyanate resin alone. A mixture of optimized microcapsules and isocyanate resin was evaluated against a wide range of substrates and conditions, including dry and wet environments. When microcapsules were intentionally ruptured to release the catalyst and crosslinker payload, the microcapsule-isocyanate mixture had improved adhesive strength (506 %) compared to the base isocyanate resin without microcapsules.

Endocytosis-Inspired Zwitterionic Gel Tape for High-Efficient and Sustainable Underoil Adhesion.

Marine oil exploration is important yet greatly increases the risk of oil leakage, which will result in severe environment pollution and economic losses. It is an urgent need to develop effective underoil adhesives. However, realizing underoil adhesion is even harder than those underwater, due to the stubborn attachment of a highly viscous oil layer on target surface. Here, inspired by endocytosis, a tough gel tape composed of zwitterionic polymer network and zwitterionic surfactants is developed. The amphiphilic surfactants can form micelle to capture the oil droplets and transport them from the interface to gel via electrostatic attraction of polymer backbone, mimicking the endocytosis and achieving robust underoil adhesion. Benefiting from the oil-resistance of polymer backbone, the gel further realizes a combination of i) long-term adhesion with high durability, ii) repeated adhesion in oil, and iii) renewable adhesion efficiency after exhausted use. The tape exhibits an ultra-high adhesive toughness of 2446.86 J m-2 to stainless steel in silicone oil after 30 days' oil-exposure; such value of adhesive toughness surpasses many of those achieved in underwater adhesion and is greater than underoil adhesion performance of commercial tape. The strategy illustrated here will motivate the design of sustainable and efficient adhesives for wet environments.

Low modulus hydrogel-like elastomer sensors with ultra-fast self-healing, underwater self-adhesion, high durability/stability and recyclability for bioelectronics

Flexible sensors simultaneously with property of hydrogel-like low modulus/room temperature ultra-fast self-healing and with elastomer-like durability /environmental stability and underwater adhesion for bioelectronics has not been reported. A low modulus hydrogel-like elastomer that achieves ultrafast self-healing through molecular chain entanglement at room temperature was prepared based on furfuryl alcohol-modified poly(sebacate glyceride) (PGS) prepolymer, furfuryl alcohol-modified poly(ionic liquid) and bismaleimide by Diels-Alder (DA) reaction. The conductive elastomer-based flexible sensors exhibit hydrogel-like properties of low modulus (6.41 kPa) and ultra-fast self-healing (98 % self-healing efficiency within 5 s). The elastomer also possesses rapid subzero and underwater self-healing properties within 5 s. Moreover, PGS-0.2DA-0.2PIL exhibits pressure sensitive adhesive properties and can be adhered/re-adhered in water. The flexible sensor shows elastomer-like high durability, high environmental stability, multiple recyclability and reusability, and it exhibits wide detection ranges, fast response time, low hysteresis, anti-freezing, anti-bacterial and good biocompatibility. The flexible sensors can accurately identify micro-expressions/eye rotation, monitor human movement/health, detect ECG/EMG signals and control robotic arm movements. In conclusion, a new strategy for design of hydrogel-like conductive elastomers via molecular structure design is proposed, and the elastomers-based flexible sensors with low modulus, rapid self-healing and durability/environmental stability show great promising for bioelectronic applications.

Bioinspired Green Underwater Adhesive Gelatin‐Tannic Acid Hydrogel With Wide Range Adjustable Adhesion Strength and Multiple Environmental Adaptability

Bioinspired Green Underwater Adhesive Gelatin‐Tannic Acid Hydrogel With Wide Range Adjustable Adhesion Strength and Multiple Environmental Adaptability

Barnacle-inspired amphipathic high strength adhesives under-water/oil

Adhesives designed for bonding in liquid environments have a broad application prospect. In general, amphiphilic polymer structures are important research objects for underwater bonding. And in the study of underwater adhesives, it was found that they can quickly dispel or absorb interfacial water and form reliable bulk strength and adhesion strength. However, achieving strong adhesion in both water-based and oily environments poses a significant challenge. In this work, an adhesive inspired by barnacles that can meet the bonding requirements in both water-based and oily environments has been prepared. Hydrophilic (polyethylene glycol (PEG)) main chains and hydrophobic (1-Octadecyl thiol (ODT)) side chains are simultaneously integrated into the polyurethane structure. Different from the past, hydrophobic and hydrophilic segments are included in the hard segment and soft segment microregion, respectively; At the same time, both hydrophilic and hydrophobic chain segments have strong crystallization, which greatly strengthens the driving force of molecular segments migration and phase separation compared with ordinary hydrophilic/hydrophobic structures. Water-based and oil-based solvents drive hydrophilic and hydrophobic molecular chains to diffuse to the surface, respectively, which enhances the adhesive strength to hydrophilic and hydrophilic surfaces, respectively. In comparison to the adhesion strength observed in air, the adhesion strength of the adhesive to polymethyl methacrylate (PMMA) increases by 1.72 times in oil environments (from 1.02 to 1.75 MPa). Similarly, the adhesion strength to aluminum (Al) increases by 2.23 times under water (from 1.82 to 4.04 MPa). These findings suggest that the regulation strategy for polymer chains employed in this study holds great potential for advancing adhesive performance in diverse environments.