Nanomechanics of Cation-π Interactions in Aqueous Solution

CO2-Responsive Smart Wood Scaffold for Natural Organic Matter Removal without Secondary Pollution

Ensuring drinking water safety remains a critical challenge, particularly when treating complex water sources, due to secondary pollution caused by active chemical additives. Herein, a novel CO2-responsive smart wood scaffold that leverages non-toxic CO2 activation is developed to achieve highly efficient removal of carcinogenic natural organic matter (NOM) and broad-spectrum microbial disinfection without requiring additional chemical agents. Unlike conventional water purification techniques that face a safety-efficacy trade-off, the multi-stage CO2-responsive wood scaffold offers exceptional tunability in NOM abatement across diverse environmental conditions, including variable water chemistry, NOM composition, high salinity, and real-world water sources. The purified water meets stringent drinking water standards (e.g., UV254 reduction, dissolved organic carbon removal, and bacterial elimination). It is found that the highly efficient NOM adsorption mainly originates from the strong and stable CO2-triggered cation-π interaction between the scaffold surface and aromatic NOM groups, as revealed via high-resolution mass spectrometry and direct intermolecular force measurements. This ecofriendly and contamination-free CO2-responsive strategy provides a transformative approach to overcoming secondary pollution challenges in water purification, offering a scalable and sustainable platform for environmental applications and beyond.

Engineering Protein-Based Lipid-Binding Nanovesicles via Catechol-Amine-Derived Coacervation with Their Underlying Interfacial Mechanisms

The development of nonphospholipid nanovesicles has garnered tremendous attention as a viable alternative to traditional liposomal nanovesicles. Protein/peptide-based nanovesicles have demonstrated their potential to reduce immunogenicity while enhancing bioactivity. However, a fundamental understanding of how proteinaceous vesicles interact with lipids and cell membranes remains elusive. In this study, we engineered a series of protamine-based nonphospholipid nanovesicles by modulating intramolecular catechol-amine interactions. By grafting trihydroxybenzene (GA) and catechol (CA) groups onto the protamine (Prot), a salt-triggered coacervation was observed in an alkaline environment with the size of as-prepared vesicles ranging from 200 to 1200 nm. The bonding affinity to lipid interfaces followed the order of Prot-CA-Fe3+(25 μM) > Prot-CA-Fe3+(10 μM) > Prot-CA > original Prot with the underlying nanomechanics investigated by the lipid bubble force measurement. Direct quantification of interactions between the nanovesicles and living human gingival fibroblasts was performed by using surface charge difference mapping. Introducing trace amounts of Fe3+ (at 10 and 25 μM) enhanced vesicle-lipid interactions via the synergy of catechol-amine interactions and Fe3+-induced complexation. This work provides improved valuable insights into the interactions between nanovesicles and cell membranes, offering an energetic paradigm for modulating cell-target delivery processes via intramolecular short-range interactions.

Junctional Role of Anionic Domain of Mussel Foot Protein Type 4 in Underwater Mussel Adhesion

Mussel byssi form a robust underwater adhesive system, anchoring to various surfaces in harsh marine environments. Central to byssus is foot protein type 4 (fp-4), a junction protein connecting collagenous threads to proteinaceous plaque. This study investigated an anionic plaque-binding domain of fp-4 (fp-4a) and its interactions with cationic foot proteins (fp-1, fp-5, and fp-151 as model substitutes for fp-2) and metal ions (Ca2+, Fe3+, and V3+). Aggregation, a liquid-solid phase transition, was confirmed for recombinant fp-4a (rfp-4a) with rfp-5, rfp-151, and metal ions using turbidity measurements and microscopy. Molecular cohesion forces were measured by the surface forces apparatus, while dynamic light scattering, circular dichroism spectroscopy, and chaotropic agent assay clarified the aggregation mechanisms. Collectively, we discovered that rfp-4a formed aggregates with cationic rfps through electrostatic interactions and hydrogen bonding, further stabilized by metal ion incorporation, emphasizing its critical role in mussel adhesion systems and its potential for bioadhesive applications.

Strong Hydrophobic Interaction of High Molecular Weight Chitosan in Aqueous Solution

Chitosan is a versatile bioactive polysaccharide in various industries, such as pharmaceuticals and environmental applications, owing to its abundance, biodegradability, biocompatibility, and antibacterial properties. To effectively harness its potential for various purposes, it is crucial to understand the mechanisms of its interaction in water. This study investigates the interactions between high molecular weight (HMW, >150 kDa) chitosan and four different functionalized self-assembled monolayers (SAMs) at three different pHs (3.0, 6.5, and 8.5) using a surface forces apparatus (SFA). We report that HMW chitosan exhibits the strongest adhesion to methyl-terminated SAM (CH3-SAM) at all pHs, showing potential for strong hydrophobic interactions against other molecules containing hydrophobic moieties. Noting that hydrogen bonding has been considered the dominating interaction mechanism of chitosan, the consequence of this study provides valuable insights into its applications in developing chitosan-based eco-friendly materials.

Formulation, Characterization, and Antioxidant Properties of Chitosan Nanoparticles Containing Phenolic Compounds from Olive Pomace

Olive phenolic compounds like hydroxytyrosol (OH-Tyr), tyrosol (Tyr), and their precursors have different health-promoting properties, mainly based on their strong antioxidant capacity. However, their presence in extra-virgin olive oil (EVOO) is scarce since they are primarily contained in the by-products of oil production, such as olive pomace (OP). The aim of this work was to extract and encapsulate OP phenolic compounds into chitosan-tripolyphosphate nanoparticles (NPs) using an ionotropic gelation lyophilization approach to increase their resistance to environmental and chemical stress. NPs resulted in a monodisperse (PDI: 0.21) population of cationic NPs (ζ-potential: 33 mV, size: 229 nm) with an encapsulation efficiency (EE%), expressed as total phenolic content (TPC) and total OH-Tyr + Tyr content, of 64-65%. Mannitol and maltodextrin DE 19 (MD-19) were evaluated as lyoprotectants to counteract irreversible NP aggregation during lyophilization. The NP powder freeze dried using 0.7% of MD-19 showed the best performance, returning a monodispersed population of particles after rehydration. The antioxidant capacity of the obtained NPs was confirmed both in cell-free assays and in a THP-1 cell model of oxidative stress. This method represents a promising way to deliver health-promoting olive phenols for nutraceutical purposes and, hence, to valorize otherwise wasted by-products.

Gallic acid: design of a pyrogallol-containing hydrogel and its biomedical applications

Polyphenol hydrogels have garnered widespread attention due to their excellent adhesion, antioxidant, and antibacterial properties. Gallic acid (GA) is a typical derivative of pyrogallol that is used as a hydrogel crosslinker or bioactive additive and can be used to make multifunctional hydrogels with properties superior to those of widely studied catechol hydrogels. Furthermore, compared to polymeric tannic acid, gallic acid is more suitable for chemical modification, thus broadening its range of applications. This review focuses on multifunctional hydrogels containing GA, aiming to inspire researchers in future biomaterial design. We first revealed the interaction mechanisms between GA molecules and between GA and polymers, analyzed the characteristics GA imparts to hydrogels and compared GA hydrogels with hydrogels containing catechol. Subsequently, in this paper, various methods of integrating GA into hydrogels and the applications of GA in biomedicine are discussed, finally assessing the current limitations and future development potential of GA. In summary, GA, a natural small molecule polyphenol with excellent functionality and diverse interaction modes, has great potential in the field of biomedical hydrogels.

Progress of polysaccharide-based tissue adhesives

Recently, polymer-based tissue adhesives (TAs) have gained the attention of scientists and industries as alternatives to sutures for sealing and closing wounds or incisions because of their ease of use, low cost, minimal tissue damage, and short application time. However, poor mechanical properties and weak adhesion strength limit the application of TAs, although numerous studies have attempted to develop new TAs with enhanced performance. Therefore, next-generation TAs with improved multifunctional properties are required. In this review, we address the requirements of polymeric TAs, adhesive characteristics, adhesion strength assessment methods, adhesion mechanisms, applications, advantages and disadvantages, and commercial products of polysaccharide (PS)-based TAs, including chitosan (CS), alginate (AL), dextran (DE), and hyaluronic acid (HA). Additionally, future perspectives are discussed.

Interaction mechanism between low molecular weight chitosan nanofilm and functionalized surfaces in aqueous solutions

Low-molecular-weight chitosan (LMW chitosan, <10 kDa) have a significant potential for biomedical applications (e.g., antimicrobial and gene/drug delivery) because of their higher water solubility at pH values ranging from 3.0 to 8.5, compared to that of the high-molecular-weight (>100 kDa) chitosan. A comprehensive understanding of the LMW interaction mechanism with specific functional groups is necessary to predict their binding efficiency to other molecules for effectively utilizing their potential within biological systems. In this study, we used a surface forces apparatus (SFA) to investigate molecular interactions between LMW chitosan and four different functionalized self-assembled monolayers (SAMs) in aqueous solutions at pH values of 3.0, 6.5, and 8.5. Chitosan exhibited the strongest interaction energy with methyl-terminated SAM (CH3-SAM), indicating the significance of hydrophobic interaction. Many chitin/chitosan fibers in nature bind polyphenols (e.g., eumelanin) to form robust composites, which can be attributed to the strong attraction between chitosan and phenyl-SAM, presumably caused by cation-π interactions. These findings demonstrate the potential of modulating the magnitude of the interaction energy by controlling the solution pH and types of targeted functional groups to realize the optimal design of chitosan-based hybrid composites with other biomolecules or synthetic materials.

Recent advances in fabricating injectable hydrogels via tunable molecular interactions for bio-applications

Hydrogels with three-dimensional structures have been widely applied in various applications because of their tunable structures, which can be easily tailored with desired functionalities. However, the application of hydrogel materials in bioengineering is still constrained by their limited dosage flexibility and the requirement of invasive surgical procedures. Compared to traditional hydrogels, injectable hydrogels, with shear-thinning and/or in situ formation properties, simplify the implantation process and reduce tissue invasion, which can be directly delivered to target sites using a syringe injection, offering distinct advantages over traditional hydrogels. These injectable hydrogels incorporate physically non-covalent and/or dynamic covalent bonds, granting them self-healing abilities to recover their structural integrity after injection. This review summarizes our recent progress in preparing injectable hydrogels and discusses their performance in various bioengineering applications. Moreover, the underlying molecular interaction mechanisms that govern the injectable and functional properties of hydrogels were characterized by using nanomechanical techniques such as surface forces apparatus (SFA) and atomic force microscopy (AFM). The remaining challenges and future perspectives on the design and application of injectable hydrogels are also discussed. This work provides useful insights and guides future research directions in the field of injectable hydrogels for bioengineering.

Mussel-Inspired Cation-π Interactions: Wet Adhesion and Biomimetic Materials

Cation-π interaction is one of the most important noncovalent interactions identified in biosystems, which has been proven to play an essential role in the strong adhesion of marine mussels. In addition to the well-known catecholic amino acid, l-3,4-dihydroxyphenylalanine, mussel foot proteins are rich in various aromatic moieties (e.g., tyrosine, phenylalanine, and tryptophan) and cationic residues (e.g., lysine, arginine, and histidine), which favor a series of short-range cation-π interactions with adjustable strengths, serving as a prototype for the development of high-performance underwater adhesives. This work highlights our recent advances in understanding and utilizing cation-π interactions in underwater adhesives, focusing on three aspects: (1) the investigation of the cation-π interaction mechanisms in mussel foot proteins via force-measuring techniques; (2) the modulation of cation-π interactions in mussel mimetic polymers with the variation of cations, anions, and aromatic groups; (3) the design of wet adhesives based on these revealed principles, leading to functional materials in the form of films, coacervates, and hydrogels with biomedical and engineering applications. This review provides valuable insights into the development and optimization of smart materials based on cation-π interactions.

Designing Bio-Inspired Wet Adhesives through Tunable Molecular Interactions

Marine organisms, such as mussels and sandcastle worms, can master rapid and robust adhesion in turbulent seawater, becoming leading archetypes for the design of underwater adhesives. The adhesive proteins secreted by the organisms are rich in catecholic amino acids along with ionic and amphiphilic moieties, which mediate the adaptive adhesion mainly through catechol chemistry and coacervation process. Catechol allows a broad range of molecular interactions both at the adhesive-substrate interface and within the adhesive matrix, while coacervation promotes the delivery and surface spreading of the adhesive proteins. These natural design principles have been translated to synthetic systems toward the development of biomimetic adhesives with water-resist adhesion and cohesion. This review provides an overview of the recent progress in bio-inspired wet adhesives, focusing on two aspects: (1) the elucidation of the versatile molecular interactions (e.g., electrostatic interactions, metal coordination, hydrogen bonding, and cation-π/anion-π interactions) used by natural adhesives, mainly through nanomechanical characterizations; and (2) the rational designs of wet adhesives based on these biomimetic strategies, which involve catechol-functionalized, coacervation-induced, and hydrogen bond-based approaches. The emerging applications (e.g., tissue glues, surgical implants, electrode binders) of the developed biomimetic adhesives in biomedical, energy, and environmental fields are also discussed, with future research directions proposed.

Comparison between nanocellulose-polyethylenimine composites synthesis methods towards multiple water pollutants removal: A review

Nanocellulose/polyethylenimine composites have attracted growing attention due to their versatility as new materials for application in different fields. Water remediation is one of the traditional applications of these composites and their investigation as adsorbents for single water pollutants is well established. However, most water resources such as rivers, lakes, and even oceans contain complex mixtures of pollutants. Despite several recently published reviews on water purification technology, they only focused on these material as single pollutant removers and hardly mentioned their capacity to simultaneously recover multiple pollutants. Therefore, there is still a gap in the archived literature considering nanocellulose/polyethylenimine composites targeting water remediation with multiple water pollutants. In this review, methods for synthesizing such composites are classified and compared according to the mechanism of reactions, such as chemical crosslinking and physical adsorption, while outlining advantages and limitations. Then, the water pollutants mainly targeted by those composites are discussed in detail to expound the relationship between the synthesis method and the type and adsorption capacity. Finally, the last section presents challenges and opportunities of these nanocellulose/polyethylenimine composites as emerging sorbents for sustainable multiple water pollutants purification technologies. This review aims to lay out the basis for future developments of these composites for multiple water pollutants.

Mussel-Inspired Reversible Molecular Adhesion for Fabricating Self-Healing Materials

Nature offers inspiration for the development of high-performance synthetic materials. Extensive studies on the universal adhesion and self-healing behavior of mussel byssus reveal that a series of reversible molecular interactions occurring in byssal plaques and threads play an essential role, and the mussel-inspired chemistry can serve as a versatile platform for the design of self-healing materials. In this Perspective, we provide an overview of the recent progress in the detection, quantification, and utilization of mussel-inspired reversible molecular interactions, which includes the elucidation of their binding mechanisms via force-measuring techniques and the development of self-healing materials based on these dynamic interactions. Both conventional catechol-medicated interactions and newly discovered chemistry beyond the catechol groups are discussed, providing insights into the design strategies of advanced self-healing materials via mussel-inspired chemistry.

Sum-Frequency Vibrational Spectroscopic Study of the Cation-π Interaction: Amine and Guanidine

The cation-π interaction is an interaction between a positively charged cation and π electrons in an aromatic group of a molecule. It is considered to play key roles in signal transduction, stabilization of the protein structure, enzyme catalysis in biology, and wet adhesion and biomolecular condensation. In this study, octadecylguanidine hydrochloride (ODG) and octadecylamine (ODA) having guanidine and amine headgroups, respectively, are found to interact with π molecules (phenol or indole) as investigated by sum-frequency vibrational spectroscopy. ODG is unstable and does not form a neat monolayer on the water surface. However, after adding π molecules into subphase water, it becomes more stable against dissolution as evidenced by the appearance of its CH_x peaks and a CH peak of the aromatic ring in the sum-frequency spectrum. Unlike ODG, ODA forms a stable monolayer on the water surface at a neutral pH. After adding π molecules into the solution, the amine-π interaction promotes the protonation of the amine headgroup and the penetration of the π molecules makes the ODA monolayer more disordered. Indole is found to be more effective in binding with the ODG as compared to phenol.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Bilayer Hydrogels for Wound Dressing and Tissue Engineering

A large number of different skin diseases such as hits, acute, and chronic wounds dictate the search for alternative and effective treatment options. The wound healing process requires a complex approach, the key step of which is the choice of a dressing with controlled properties. Hydrogel-based scaffolds can serve as a unique class of wound dressings. Presented on the commercial market, hydrogel wound dressings are not found among proposals for specific cases and have a number of disadvantages—toxicity, allergenicity, and mechanical instability. Bilayer dressings are attracting great attention, which can be combined with multifunctional properties, high criteria for an ideal wound dressing (antimicrobial properties, adhesion and hemostasis, anti-inflammatory and antioxidant effects), drug delivery, self-healing, stimulus manifestation, and conductivity, depending on the preparation and purpose. In addition, advances in stem cell biology and biomaterials have enabled the design of hydrogel materials for skin tissue engineering. To improve the heterogeneity of the cell environment, it is possible to use two-layer functional gradient hydrogels. This review summarizes the methods and application advantages of bilayer dressings in wound treatment and skin tissue regeneration. Bilayered hydrogels based on natural as well as synthetic polymers are presented. The results of the in vitro and in vivo experiments and drug release are also discussed.

Probing Anion - π interactions between fluoroarene and carboxylate anion in aqueous solutions

Despite the much progress in developing π-conjugated fluoroarene moieties based functional materials in which anion - π interactions are commonly involved, it remains challenging to quantitatively characterize the nanomechanical interaction mechanism of these anion - π systems, particularly in aqueous solutions. In this study, we reported the first experimental quantification of the nanomechanics of anion - π interactions between π-conjugated fluoroarene moieties and carboxylate anions in aqueous solutions through direct molecular force measurements, with a special focus on the impact of the anion species, concentration and of the substitution effect of aromatic side group. The results using surface forces apparatus (SFA) and single-molecule force spectroscopy (SMFS) provide complementary evidences to demonstrate that the robust and reversible adhesion measured between the fluoroarene π systems and carboxylate anions was mainly attributed to anion - π interaction. Moreover, their nanomechanical properties were also systematically scrutinized, with the interaction strength being found to be significantly determined by the contact time, the type of fluoroarene systems (PFST > DFST) and the type of anions and ion concentration (HPO₄^2- > CO₃^2- > I^- > Cl^- ≈ NO₃^- > F^-).

Quantitative determination of cation–π interactions between metal ions and aromatic groups in aqueous media by a hydrogel Donnan potential method

The binding ratios of various metal ions to aromatic groups by cation–π interactions in aqueous media have been quantitatively calculated by using Donnan potential measurements.

Ultrastretchable, Adhesive, and Antibacterial Hydrogel with Robust Spinnability for Manufacturing Strong Hydrogel Micro/Nanofibers

The ultrastretchable (over 12 400%) hydrogel with long-lasting adhesion, strong antibacterial activity, and robust spinnability is developed based on the oxidative decarboxylation and quinone-catechol reversible redox reaction induced by Ag-lignin nanoparticles in a precursor solution containing citric acid (CA), acrylic acid (AA), and poly (acrylamide-co-acrylic acid) (P(AAm-co-AA)). With massive reversible interactions including hydrogen bonds and electrostatic forces, such hydrogel exhibits promising injectability and is facilely spun via manual drawing, draw-spinning, and electrospinning for manufacturing strong hydrogel micro/nanofibers. The resulting fibers exhibit excellent mechanical properties, including tensile stress of 422.0 MPa, strain of 86.5%, Young's modulus of 8.7 GPa, and toughness of 281.6 MJ m^-3 . The hydrogel microfibers obtained from a house-built spinner are scaled-up fabricated while retaining promising mechanical properties, as evidenced by lifting a load (317.2 g) using the spun fibers of ≈33 000 times lighter weight (9.5 mg), indicating their great potentials in the applications such as net and safety cord which require robust mechanical properties. Moreover, assisted by a commercial electrospinning machine, nanosized hydrogel fibers are facilely spun on personal protective equipment such as a mask to offer an antiseptic coating with near 100% killing efficiency against airborne bacteria aerosols, demonstrating the capability of spun hydrogel fibers on disinfection-related applications.

Coacervate-Based Instant and Repeatable Underwater Adhesive with Anticancer and Antibacterial Properties

Underwater adhesion is a great challenge for the development of adhesives as the attractive interfacial intermolecular interactions are usually weakened by the surface hydration layer. The coacervation process of sessile organisms like marine mussels and sandcastle worms has inspired substantial research interest in the fabrication of long-lasting underwater adhesives, but they generally suffer from time-consuming curing triggered by surrounding environmental changes and cannot reserve the adhesiveness once damaged. Herein, an instant and repeatable underwater adhesive was developed based on the coacervation of tannic acid (TA) and poly(ethylene glycol)₇₇-b-poly(propylene glycol)₂₉-b-poly(ethylene glycol)₇₇ (PEG-PPG-PEG, F68), which was driven by hydrogen-bonding interaction, and the hydrophobic cores of F68 micelles offered an additional cross-linking to enhance the mechanical properties. The TA-F68 coacervates could be facilely painted on different substrates, exhibiting robust and instant underwater adhesion (with adhesion strength up to 1.1 MPa on porcine skin) and excellent repeatability (at least 1000 cycles), superior to the previously reported coacervates. Due to the biological activities of TA, the underwater adhesive displayed innate anticancer and antibacterial properties against different types of cancer cells and bacteria, showing great potential for diverse biomedical applications, such as injectable drug carriers, tissue glues, and wound dressings.

Tissue adhesive hydrogel bioelectronics

Flexible bioelectronics have promising applications in electronic skin, wearable devices, biomedical electronics, etc. Hydrogels have unique advantages for bioelectronics due to their tissue-like mechanical properties and excellent biocompatibility. Particularly, conductive and tissue adhesive hydrogels can self-adhere to bio-tissues and have great potential in implantable wearable bioelectronics. This review focuses on the recent progress in tissue adhesive hydrogel bioelectronics, including the mechanism and preparation of tissue adhesive hydrogels, the fabrication strategies of conductive hydrogels, and tissue adhesive hydrogel bioelectronics and applications. Some perspectives on tissue adhesive hydrogel bioelectronics are provided at the end of the review.

Interfacial ion specificity modulates hydrophobic interaction

HYPOTHESIS

Ion specificity is crucial in assembly and aggregation of polymers in water driven by hydrophobic interaction. An increasing number of studies have suggested that specific ion adsorption and consequent impact on interfacial water molecules should play an important role in modulating hydrophobic interaction.

EXPERIMENTS

Here, bubble probe atomic force microscopy (AFM) combined with theoretical modeling analysis was applied to quantify hydrophobic interactions involving three model polymers in solutions containing different ions.

FINDINGS

For polystyrene, the hydrophobic interaction's decay length D₀ was reduced from 0.75 nm to 0.60 nm by introducing weakly hydrated cations (e.g., K⁺ and NH₄⁺), while varying anion type had little effect. For poly(methyl methacrylate) and polydimethylsiloxane, anion specificity was demonstrated more evident in shortening the hydrophobic interaction range, with D₀ decreasing from 0.63 nm to 0.50 nm and from 0.72 nm to 0.58 nm respectively when strongly hydrated F^- or Cl^- was replaced by weakly hydrated I^-. Such results could arise from specific ion adsorption at water/polymer interface which disrupts the water structuring effect. From the nanomechanical perspective, this work has revealed the importance of interfacial ion specificity in modulating hydrophobic interaction, which offers novel implications for tuning assembly behavior of macromolecules in relevant engineering applications such as micelle formation and foam stabilization.

Adjacent cationic-aromatic sequences yield strong electrostatic adhesion of hydrogels in seawater

Electrostatic interaction is strong but usually diminishes in high ionic-strength environments. Biosystems can use this interaction through adjacent cationic-aromatic amino acids sequence of proteins even in a saline medium. Application of such specific sequence to the development of cationic polymer materials adhesive to negatively charged surfaces in saline environments is challenging due to the difficulty in controlling the copolymer sequences. Here, we discover that copolymers with adjacent cation-aromatic sequences can be synthesized through cation-π complex-aided free-radical polymerization. Sequence controlled hydrogels from diverse cation/aromatic monomers exhibit fast, strong but reversible adhesion to negatively charged surfaces in seawater. Aromatics on copolymers are found to enhance the electrostatic interactions of their adjacent cationic residues to the counter surfaces, even in a high ionic-strength medium that screens the electrostatic interaction for common polyelectrolytes. This work opens a pathway to develop adhesives using saline water.

Bio-inspired swellable hydrogel-forming double-layered adhesive microneedle protein patch for regenerative internal/external surgical closure

Significant tissue damage, scarring, and an intense inflammatory response remain the greatest concerns for conventional wound closure options, including sutures and staples. In particular, wound closure in internal organs poses major clinical challenges due to air/fluid leakage, local ischemia, and subsequent impairment of healing. Herein, to overcome these limitations, inspired by endoparasites that swell their proboscis to anchor to host's intestines, we developed a hydrogel-forming double-layered adhesive microneedle (MN) patch consisting of a swellable mussel adhesive protein (MAP)-based shell and a non-swellable silk fibroin (SF)-based core. By possessing tissue insertion capability (7-times greater than the force for porcine skin penetration), MAP-derived surface adhesion, and selective swelling-mediated physical entanglement, our hydrogel-forming adhesive MN patch achieved ex vivo superior wound sealing capacity against luminal leaks (139.7 ± 14.1 mmHg), which was comparable to suture (151.0 ± 23.3 mmHg), as well as in vivo excellent performance for wet and/or dynamic external and internal tissues. Collectively, our bioinspired adhesive MN patch can be successfully used in diverse practical applications ranging from vascular and gastrointestinal wound healing to transdermal delivery for pro-regenerative or anti-inflammatory agents to target tissues.

The Renewable and Sustainable Conversion of Chitin into a Chiral Nitrogen-Doped Carbon-Sheath Nanofiber for Enantioselective Adsorption

Well-known hard-template methods for nitrogen (N)-doped chiral carbon nanomaterials require complicated construction and removal of the template, high-temperature pyrolysis, harsh chemical treatments, and additional N-doping processes. If naturally occurring chiral nematic chitin nanostructures [(C₈ H₁₃ NO₅ )_n ] in exoskeletons were wholly transformed into an N-doped carbon, this would be an efficient and sustainable method to obtain a useful chiral nanomaterial. Here, a simple, sacrificial-template-free, and environmentally mild method was developed to produce an N-doped chiral nematic carbon-sheath nanofibril hydrogel with a surface area >300 m²  g^-1 and enantioselective properties from renewable chitin biomass. Calcium-saturated methanol physically exfoliated bulk chitin and produced a chiral nematic nanofibril hydrogel. Hydrothermal treatment of the chiral chitin hydrogel at 190 °C produced an N-doped chiral carbon-sheath nanofibril hydrogel without N-doping. This material preferentially adsorbed d-lactic acid over l-lactic acid and produced 16.3 % enantiomeric excess of l-lactic acid from a racemic mixture.

Mussel-Inspired Catechol-Functionalized Hydrogels and Their Medical Applications

Mussel adhesive proteins (MAPs) have a unique ability to firmly adhere to different surfaces in aqueous environments via the special amino acid, 3,4-dihydroxyphenylalanine (DOPA). The catechol groups in DOPA are a key group for adhesive proteins, which is highly informative for the biomedical domain. By simulating MAPs, medical products can be developed for tissue adhesion, drug delivery, and wound healing. Hydrogel is a common formulation that is highly adaptable to numerous medical applications. Based on a discussion of the adhesion mechanism of MAPs, this paper reviews the formation and adhesion mechanism of catechol-functionalized hydrogels, types of hydrogels and main factors affecting adhesion, and medical applications of hydrogels, and future the development of catechol-functionalized hydrogels.

Polydopamine-Based Interfacial Engineering of Extracellular Matrix Hydrogels for the Construction and Long-Term Maintenance of Living Three-Dimensional Tissues

Diverse biological processes in the body rely on the ability of cells to exert contractile forces on their extracellular matrix (ECM). In three-dimensional (3D) cell culture, however, this intrinsic cellular property can cause unregulated contraction of ECM hydrogel scaffolds, leading to a loss of surface anchorage and the resultant structural failure of in vitro tissue constructs. Despite advances in the 3D culture technology, this issue remains a significant challenge in the development and long-term maintenance of physiological 3D in vitro models. Here, we present a simple yet highly effective and accessible solution to this problem. We leveraged a single-step surface functionalization technique based on polydopamine to drastically increase the strength of adhesion between hydrogel scaffolds and cell culture substrates. Our method is compatible with different types of ECM and polymeric surfaces and also permits prolonged shelf storage of functionalized culture substrates. The proof-of-principle of this technique was demonstrated by the stable long-term (1 month) 3D culture of human lung fibroblasts. Furthermore, we showed the robustness and advanced application of the method by constructing a dynamic cell stretching system and performing over 100 000 cycles of mechanical loading on 3D multicellular constructs for visualization and quantitative analysis of stretch-induced tissue alignment. Finally, we demonstrated the potential of our technique for the development of microphysiological in vitro models by establishing microfluidic 3D co-culture of vascular endothelial cells and fibroblasts to engineer self-assembled, perfusable 3D microvascular beds.

Tough and Alkaline-Resistant Mussel-Inspired Wet Adhesion with Surface Salt Displacement via Polydopamine/Amine Synergy

The mussel-inspired catechol-based strategy has been well recognized as a promising alternative to design and exploit new generation adhesive materials applicable in many fields, ranging from biomedical adhesives to coatings of biomedical devices and engineering applications. However, in situ achievement of tough adhesion capability to substrate surfaces (e.g., minerals) is severely limited under the physiological environment or seawater condition (namely, relatively high salinity and mild alkalinity). In this work, a facile and versatile approach is proposed to in situ achieve robust wet adhesion in aqueous solutions of high salinity and mild alkalinity, via integrating primary amines into mussel-inspired polydopamine (PDA). By using a surface forces apparatus (SFA), the corresponding interaction behaviors have been systematically investigated. The strong wet adhesion was demonstrated and achieved via a synergetic effect of amine and PDA to the wet surfaces, including the surface salt displacement assisted by primary amine, strong adhesion to substrates facilitated by the catechol groups on PDA moieties, and enhanced cohesion through their cation-π interactions. Our results provide useful insights into the design and development of high-performance underwater adhesives and water-resistance materials.

Polymerizing Like Mussels Do: Toward Synthetic Mussel Foot Proteins and Resistant Glues

A novel strategy to generate adhesive protein analogues by enzyme-induced polymerization of peptides is reported. Peptide polymerization relies on tyrosinase oxidation of tyrosine residues to Dopaquinones, which rapidly form cysteinyldopa-moieties with free thiols from cysteine residues, thereby linking unimers and generating adhesive polymers. The resulting artificial protein analogues show strong adsorption to different surfaces, even resisting hypersaline conditions. Remarkable adhesion energies of up to 10.9 mJ m-2 are found in single adhesion events and average values are superior to those reported for mussel foot proteins that constitute the gluing interfaces.

Five different chitin nanomaterials from identical source with different advantageous functions and performances

Chitin is a renewable and sustainable biomass material that can be converted into various one-dimensional crystalline nanomaterials different in 1) length, 2) diameter, 3) charge density, 4) type of charge, and 5) crystallinity via diverse top-down synthetic methods. These nanomaterials have great potential as sustainable reinforcing and biologically functional materials. The proper design of chitin nanomaterials maximizes their performances in specific applications. Extensive efforts are devoted to understanding each type of chitin nanomaterial produced from different chitin sources; however, few studies have compared different chitin nanomaterials. Herein, we synthesize five different types of chitin nanomaterials from identical sources and compare their physical and chemical properties, including suitability for assorted purposes. Factors 1)-5) are discussed regarding their dominance in determining functionality depending on the specific goals of a) gas barriers, b) mechanical reinforcements, c) dispersibility in various pH aqueous buffers, d) thermal dimensional stability, and e) antibacterial activity. This study gives insights to design new chitin nanomaterial-based materials.

Tuning underwater adhesion with cation-π interactions

Cation-π interactions drive the self-assembly and cohesion of many biological molecules, including the adhesion proteins of several marine organisms. Although the origin of cation-π bonds in isolated pairs has been extensively studied, the energetics of cation-π-driven self-assembly in molecular films remains uncharted. Here we use nanoscale force measurements in combination with solid-state NMR spectroscopy to show that the cohesive properties of simple aromatic- and lysine-rich peptides rival those of the strong reversible intermolecular cohesion exhibited by adhesion proteins of marine mussel. In particular, we show that peptides incorporating the amino acid phenylalanine, a functional group that is conspicuously sparing in the sequences of mussel proteins, exhibit reversible adhesion interactions significantly exceeding that of analogous mussel-mimetic peptides. More broadly, we demonstrate that interfacial confinement fundamentally alters the energetics of cation-π-mediated assembly: an insight that should prove relevant for diverse areas, which range from rationalizing biological assembly to engineering peptide-based biomaterials.

Molecular Weight Dependence of Synthetic Glycopolymers on Flocculation and Dewatering of Fine Particles

In this study, poly(2-lactobionamidoethyl methacrylamide) of various molecular weights (MWs) was synthesized using conventional free-radical polymerization. The effect of MW and polymer dosage on the settling rate of kaolin particles, turbidity of the supernatant, mud-line position, and solid content was investigated to determine the flocculation performance. The interaction forces, polymer conformation, particle sizes, and MWs were determined using several techniques, including surface forces apparatus, atomic force microscopy (AFM), dynamic light scattering, and gel permeation chromatography. Our results reveal that the initial settling rate of kaolin particles and the clarity of supernatants increase with increasing MW of the glycopolymers. Surface force measurements and AFM imaging of the adsorbed polymer surfaces show strong polymer-particle adhesion and bridging attraction between the glycopolymers and clay surfaces, which increase with increasing MW of the glycopolymer. The strengthened bridging attraction with the polymer MW is attributed to the formation of stronger adhesion (e.g., via hydrogen bonding) between the fine particles and the abundant hydroxyl groups in the glycopolymers of higher MW, thus contributing to enhanced flocculation behaviors. Our results provide new insights into the development of eco-friendly polymer flocculants based on glycopolymers for an efficient solid-liquid separation in tailing treatment and into the fundamental understanding of associated intermolecular interactions and flocculation mechanisms.