On-Site Supramolecular Adhesion to Wet and Soft Surfaces via Solvent Exchange

Supramolecular/Dynamic Covalent Design of High-Performance Pressure-Sensitive Adhesive from Natural Low-Molecular-Weight Compounds

Adhesive materials have played an essential role in the history of humanity. Natural adhesives composed of low-molecular-weight monomers have been overshadowed by modern petroleum-based glues. With the development of green economy, the demand for eco-friendly materials has increased. Herein, two natural biocompatible compounds, namely thioctic acid (TA) and malic acid (MA), are selected to prepare a high-performance pressure-sensitive adhesive poly[TA-MA]. This adhesive can be quantitatively obtained via a simple mixing and heating process. Poly[TA-MA] shows interesting and useful properties, including reversible flexibility, high elongation, and good self-healing, owing to its dynamic polymerization pattern and reversible cross-linking behavior. Poly[TA-MA] exhibits excellent adhesion performance under various extreme conditions, such as at low temperatures and in hot water. High values of shear strength (3.86 MPa), peel strength (7.90 N cm-1), loop tack (10.60 N cm-1), tensile strength (1.02 MPa), and shear resistance (1628 h) demonstrate the strong adhesive effect of poly[TA-MA]. Additionally, TA can be regenerated in the monomer forms from poly[TA-MA] with high recovery rate (>90%). Meanwhile, strong anti-bacterial behavior of poly[TA-MA] is recorded. This study not only reported a new pressure-sensitive adhesive but also fully displayed the feasibility of using natural small molecules to achieve robust surface adhesion.

Concentration-induced spontaneous polymerization of protic ionic liquids for efficient in situ adhesion

The advancement of contemporary adhesives is often limited by the balancing act between cohesion and interfacial adhesion strength. This study explores an approach to overcome this trade-off by utilizing the spontaneous polymerization of a protic ionic liquid-based monomer obtained through the neutralization of 2-acrylamide-2-methyl propane sulfonic acid and hydroxylamine. The initiator-free polymerization process is carried out through a gradual increase in monomer concentration in aqueous solutions caused by solvent evaporation upon heating, which results in the in-situ formation of a tough and thin adhesive layer with a highly entangled polymeric network and an intimate interface contact between the adhesive and substrate. The abundance of internal and external non-covalent interactions also contributes to both cohesion and interfacial adhesion. Consequently, the produced protic poly(ionic liquid)s exhibit considerable adhesion strength on a variety of substrates. This method also allows for the creation of advanced adhesive composites with electrical conductivity or visualized sensing functionality by incorporating commercially available fillers into the ionic liquid adhesive. This study provides a strategy for creating high-performance ionic liquid-based adhesives and highlights the importance of in-situ polymerization for constructing adhesive composites.

Wet environment-induced adhesion and softening of coenzyme-based polymer elastic patch for treating periodontitis

Periodontitis, a common chronic inflammatory disease caused by pathogenic bacteria, can be treated with diverse biomaterials by loading drugs, cytokines or proteins. However, these biomaterials often show unsatisfactory therapeutic efficiency due to their poor adhesion, short residence time in the wet and dynamic oral cavity and emerging drug resistance. Here we report a wet-responsive methacrylated gelatin (GelMA)-stabilized co-enzyme polymer poly(α-lipoic acid) (PolyLA)-based elastic patch with water-induced adhesion and softening features. In PolyLA-GelMA, the multiple covalent and hydrogen-bonding crosslinking between PolyLA and GelMA prevent PolyLA depolymerization and slow down the dissociation of PolyLA in water, allowing durable adhesion to oral periodontal tissue and continuous release of LA-based bioactive small molecule in periodontitis wound without resorting external drugs. Compared with the undifferentiated adhesion behavior of traditional adhesives, this wet-responsive patch demonstrates a favorable periodontal pocket insertion ability due to its non-adhesion and rigidity in dry environment. In vitro studies reveal that PolyLA-GelMA patch exhibits satisfactory wet tissue adhesion, antibacterial, blood compatibility and ROS scavenging abilities. In the model of rat periodontitis, the PolyLA-GelMA patch inhibits alveolar bone resorption and accelerates the periodontitis healing by regulating the inflammatory microenvironment. This biomacromolecule-stabilized coenzyme polymer patch provides a new option to promote periodontitis treatment.

A supramolecular approach for converting renewable biomass into functional materials

The rational transformation and utilization of biomass have attracted increasing attention because of its high importance in sustainable development and green economy. In this study, we used a supramolecular approach to convert biomass into functional materials. Six biomass raw materials with distinct chemical structures and physical properties were copolymerized with thioctic acid (TA) to afford poly[TA-biomass]s. The solvent-free copolymerization leads to the convenient and quantitative fabrication of biomass-based versatile materials. The non-covalent bonding and reversible solid-liquid transitions in poly[TA-biomass]s endow them with diversified features, including thermal processability, 3D printing, wet and dry adhesion, recyclability, impact resistance, and antimicrobial activity. Benefiting from their good biocompatibility and nontoxicity, these biomass-based materials are promising candidates for biological applications.

Solvent-Exchange Triggered Solidification of Peptide/POM Coacervates for Enhancing the On-Site Underwater Adhesion

Peptide-based biomimetic underwater adhesives are emerging candidates for understanding the adhesion mechanism of natural proteins secreted by sessile organisms. However, there is a grand challenge in the functional recapitulation of the on-site interfacial spreading, adhesion and spontaneous solidification of native proteins in water using peptide adhesives without applied compressing pressure. Here, a solvent-exchange strategy was utilized to exert the underwater injection, on-site spreading, adhesion and sequential solidification of a series of peptide/polyoxometalate coacervates. The coacervates were first prepared in a mixed solution of water and organic solvents by rationally suppressing the non-covalent interactions. After switching to a water environment, the solvent exchange between bulk water and the organic solvent embedded in the matrix of the peptide/polyoxometalate coacervates recovered the hydrophobic effect by increasing the dielectric constant, resulting in a phase transition from soft coacervates to hard solid with enhanced bulk cohesion and thus compelling underwater adhesive performance. The key to this approach is the introduction of suitable organic solvents, which facilitate the control of the intermolecular interactions and the cross-linking density of the peptide/polyoxometalate adhesives in the course of solidification under the water line. The solvent-exchange method displays fascinating universality and compatibility with different peptide segments.

Water-Triggered Segment Orientation of Long-Lasting Anti-Biofouling Polyurethane Coatings on Biomedical Catheters via Solvent Exchange Strategy

The formation of biofilm and thrombus on medical catheters poses a significant life-threatening concern. Hydrophilic anti-biofouling coatings upon catheter surfaces with complex shapes and narrow lumens are demonstrated to have the potential in reducing complications. However, their effectiveness is constrained by poor mechanical stability and weak substrate adhesion. Herein, a novel zwitterionic polyurethane (SUPU) with strong mechanical stability and long-term anti-biofouling is developed by controlling the ratio of sulfobetaine-diol and ureido-pyrimidinone. Once immersed in water, as-synthesized zwitterionic coating (SUPU3 SE) would undergo a water-driven segment reorientation to obtain much higher durability than its direct drying one, even under various extreme treatments, including acidic solution, abrasion, ultrasonication, flushing, and shearing, in PBS at 37 °C for 14 days. Moreover, SUPU3 SE coating could achieve a 97.1% of exceptional reducing protein fouling, complete prevention of cell adhesion, and long-lasting anti-biofilm performance even after 30 days. Finally, the good anti-thrombogenic formations of SUPU3 SE coating with bacterial treatment are validated in blood circulation through an ex vivo rabbit arteriovenous shunt model. This work provides a facile approach to fabricating stable hydrophilic coating through a simple solvent exchange to reduce thrombosis and infection of biomedical catheters.

Zwitterionic Eutectogel-Based Wearable Strain Sensor with Superior Stretchability, Self-Healing, Self-Adhesion, and Wide Temperature Tolerance

Ionic conductive eutectogels have great application prospects in wearable strain sensors owing to their temperature tolerance, simplicity, and low cost. Eutectogels prepared by cross-linking polymers have good tensile properties, strong self-healing capacities, and excellent surface-adaptive adhesion. Herein, we emphasize for the first time the potential of zwitterionic deep eutectic solvents (DESs), in which betaine is a hydrogen bond acceptor. Polymeric zwitterionic eutectogels were prepared by directly polymerizing acrylamide in zwitterionic DESs. The obtained eutectogels owned excellent ionic conductivity (0.23 mS cm^-1), superior stretchability (approximately 1400% elongation), self-healing (82.01%), self-adhesion, and wide temperature tolerance. Accordingly, the zwitterionic eutectogel was successfully applied in wearable self-adhesive strain sensors, which can adhere to skins and monitor body motions with high sensitivity and excellent cyclic stability over a wide temperature range (-80 to 80 °C). Moreover, this strain sensor owned an appealing sensing function on bidirectional monitoring. The findings in this work can pave the way for the design of soft materials with versatility and environmental adaptation.

Organic-inorganic covalent-ionic molecules for elastic ceramic plastic

Although organic-inorganic hybrid materials have played indispensable roles as mechanical^1-4, optical^5,6, electronic^7,8 and biomedical materials^9-11, isolated organic-inorganic hybrid molecules (at present limited to covalent compounds^12,13) are seldom used to prepare hybrid materials, owing to the distinct behaviours of organic covalent bonds¹⁴ and inorganic ionic bonds¹⁵ in molecular construction. Here we integrate typical covalent and ionic bonds within one molecule to create an organic-inorganic hybrid molecule, which can be used for bottom-up syntheses of hybrid materials. A combination of the organic covalent thioctic acid (TA) and the inorganic ionic calcium carbonate oligomer (CCO) through an acid-base reaction provides a TA-CCO hybrid molecule with the representative molecular formula TA₂Ca(CaCO₃)₂. Its dual reactivity involving copolymerization of the organic TA segment and inorganic CCO segment generates the respective covalent and ionic networks. The two networks are interconnected through TA-CCO complexes to form a covalent-ionic bicontinuous structure within the resulting hybrid material, poly(TA-CCO), which unifies paradoxical mechanical properties. The reversible binding of Ca²⁺-CO₃^2- bonds in the ionic network and S-S bonds in the covalent network ensures material reprocessability with plastic-like mouldability while preserving thermal stability. The coexistence of ceramic-like, rubber-like and plastic-like behaviours within poly(TA-CCO) goes beyond current classifications of materials to generate an 'elastic ceramic plastic'. The bottom-up creation of organic-inorganic hybrid molecules provides a feasible pathway for the molecular engineering of hybrid materials, thereby supplementing the classical methodology used for the manufacture of organic-inorganic hybrid materials.

Achieving strong, stable, and durable underwater adhesives based on a simple and generic amino-acid-resembling design

Developing underwater adhesives is important in many applications. Despite extensive progress, achieving strong, stable, and durable underwater adhesion via a simple and effective way is still challenging, mainly due to the conflict between the interfacial and bulk properties. Here, we report a unique bio-inspired strategy to facilely construct superior underwater adhesives with desirable interfacial and bulk properties. For adhesive design, a hydrophilic backbone is utilized to quickly absorb water for effective dehydration, and a novel amino acid-resembling functional block is developed to provide versatile molecular interactions for high interfacial adhesion. Moreover, the conjunction of these two components enables the generation of abundant covalent crosslinks for robust bulk cohesion. Such a rational design allows the adhesive to present a boosted underwater adhesion (3.92 MPa to glass), remarkable durability (maintaining high strength after one month), and good stability in various harsh environments (pH, salt, high temperature, and organic solvents). This strategy is generic, allowing the derivation of more similar adhesive designs easily and triggering new thinking for designing bio-inspired adhesives and beyond.

Mussel-Inspired, Underwater Self-Healing Ionoelastomers Based on α-Lipoic Acid for Iontronics

Weak adhesion and lack of underwater self-healability hinder advancing soft iontronics particularly in wet environments like sweaty skin and biological fluids. Mussel-inspired, liquid-free ionoelastomers are reported based on seminal thermal ring-opening polymerization of a biomass molecule of α-lipoic acid (LA), followed by sequentially incorporating dopamine methacrylamide as a chain extender, N,N'-bis(acryloyl) cystamine, and lithium bis(trifluoromethanesulphonyl) imide (LiTFSI). The ionoelastomers exhibit universal adhesion to 12 substrates in both dry and wet states, superfast self-healing underwater, sensing capability for monitoring human motion, and flame retardancy. The underwater self-repairabilitiy prolongs over three months without deterioration, and sustains even when mechanical properties greatly increase. The unprecedented underwater self-mendability benefits synergistically from the maximized availability of dynamic disulfide bonds and diverse reversible noncovalent interactions endowed by carboxylic groups, catechols, and LiTFSI, along with the prevented depolymerization by LiTFSI and tunability in mechanical strength. The ionic conductivity reaches 1.4 × 10^-6 -2.7 × 10^-5 S m^-1 because of partial dissociation of LiTFSI. The design rationale offers a new route for creating a wide range of LA- and sulfur-derived supramolecular (bio)polymers with superior adhesion, healability, and other functionalities, and thus has technological implications for coatings, adhesives, binders and sealants, biomedical engineering and drug delivery, wearable and flexible electronics, and human-machine interfaces.

Tough Wet Adhesion of Hydrogen-Bond-Based Hydrogel with On-Demand Debonding and Efficient Hemostasis

Hydrogels have been widely used in wet tissues. However, the insufficient adhesion of hydrogels for wound hemostasis remains a grand challenge. Herein, a facile yet effective strategy is developed to fabricate tough wet adhesion of hydrogen-bond-based hydrogel (PAAcVI hydrogel) using copolymerization of acrylic acid and 1-vinylimidazole in dimethyl sulfoxide followed by solvent exchange with water. The PAAcVI hydrogel shows equally robust adhesion (>400 J m^-2) to both wet and dry tissues. Moreover, the PAAcVI hydrogel also exhibits strong long-term stable adhesion underwater and in various wet environments. Meanwhile, the adhesion of PAAcVI hydrogel can be adjusted through Zn²⁺-ion-mediated on-demand debonding, which makes it easy to peel off from the tissue reducing pain during dressing removal and avoiding secondary injury. The PAAcVI hydrogel displays efficient hemostasis in the mice-tail docking model and mice-liver bleeding model. This hydrogen-bond-based hydrogel shows tough wet adhesion, and its adhesion is controllable, demonstrating its promising application in moisture-resistant adhesives, medical adhesives, and hemostatic materials.

Underwater luminescent labeling materials constructed from a supramolecular approach

Underwater labeling under complicated conditions is challenging for modern adhesive materials. In this work, a series of supramolecular polymer adhesives were successfully prepared via the non-covalent copolymerization of low-molecular-weight monomers (thioctic acid (TA) and tetraphenylethene derivatives (TPEs)). Strong adhesion effects were observed under various conditions. The poly(TA-TPE)s showed long-term stability in underwater labeling. Due to the aggregation-induced emission (AIE) behavior of TPEs, poly(TA-TPE)s showed great potential as fluorescent labeling materials in water. Complicated and cryptographic information can be stored in labeling structures, and analyzed under ultraviolet (UV) irradiation. Supramolecular labeling showed excellent distinguishability in complex backgrounds. Meanwhile, fluorescent adhesives exhibited a number of advantages over visible colored labels.