Surface force measurements and simulations of mussel-derived peptide adhesives on wet organic surfaces

Recent Advances in Dopamine-Based Membrane Surface Modification and Its Membrane Distillation Applications

Surface modification of membranes is essential for improving flux and resistance to contamination for membranes. This is of great significance for membrane distillation, which relies on the vapor pressure difference across the membrane as the driving force. In recent years, biomimetic mussel-inspired substances have become the research hotspots. Among them, dopamine serves as surface modifiers that would achieve highly desirable and effective membrane applications owing to their unique physicochemical properties, such as universal adhesion, enhanced hydrophilicity, tunable reducibility, and excellent thermal conductivity. The incorporation of a hydrophilic layer, along with the utilization of photothermal properties and post-functionalization capabilities in modified membranes, effectively addresses challenges such as low flux, contamination susceptibility, and temperature polarization during membrane distillation. However, to the best of our knowledge, there is still a lack of comprehensive and in-depth discussions. Therefore, this paper systematically compiles the modification method of dopamine on the membrane surface and summarizes its application and mechanism in membrane distillation for the first time. It is believed that this paper would provide a reference for dopamine-assisted membrane separation during production, and further promote its practical application.

Nature-Inspired Wet Drug Delivery Platforms

Nature has created various organisms with unique chemical components and multi-scale structures (e.g., foot proteins, toe pads, suckers, setose gill lamellae) to achieve wet adhesion functions to adapt to their complex living environments. These organisms can provide inspirations for designing wet adhesives with mediated drug release behaviors in target locations of biological surfaces. They exhibit conformal and enhanced wet adhesion, addressing the bottleneck of weaker tissue interface adhesion in the presence of body fluids. Herein, it is focused on the research progress of different wet adhesion and bioinspired fabrications, including adhesive protein-based adhesion and inspired adhesives (e.g., mussel adhesion); capillarity and Stefan adhesion and inspired adhesive surfaces (e.g., tree frog adhesion); suction-based adhesion and inspired suckers (e.g., octopus' adhesion); interlocking and friction-based adhesion and potential inspirations (e.g., mayfly larva and teleost adhesion). Other secreted protein-induced wet adhesion is also reviewed and various suckers for other organisms and their inspirations. Notably, one representative application scenario of these bioinspired wet adhesives is highlighted, where they function as efficient drug delivery platforms on target tissues and/or organs with requirements of both controllable wet adhesion and optimized drug release. Finally, the challenges of these bioinspired wet drug delivery platforms in the future is presented.

Self-adhesive, ionic-conductive, mechanically robust cellulose-based organogels with anti-freezing and rapid recovery properties for flexible sensors

Cellulose-based functional gels have received considerable attention because of their good mechanical properties, biocompatibility, and low cost. However, the preparation of cellulose gels with self-adhesion, mechanical robustness, ionic conductivity, anti-freezing ability, and environmental stability remains a challenge. Here, gallic acid esterified microcrystalline cellulose (MCC-GA) was obtained by grafting gallic acid (GA) onto the macromolecular chains of microcrystalline cellulose (MCC) through a one-step esterification method. Then the prepared MCC-GA was dissolved in Lithium chloride/dimethyl sulfoxide (LiCl/DMSO) system and polymerized with acrylic acid (AA) to prepare a multi-functional cellulose-based organogel. The prepared MCC-GA/polyacrylic acid (PAA) organogels exhibited enhanced interfacial adhesion through hydrogen bonding, π-π interactions, and electrostatic interactions. Additionally, the MCC-GA/PAA organogels could withstand 95 % of the compressive deformation and rapidly self-recover owing to chemical cross-linking and dynamic non-covalent interactions. The organogels also exhibited excellent anti-freezing properties (up to -80 °C), solvent retention, and ionic conductivity. Considering its excellent overall performance, the MCC-GA/PAA organogel was used as an effective flexible sensor for human motion detection and is expected to play an important role in the future development of flexible bioelectronics.

Molecular cloning, expression, mRNA secondary structure and immunological characterization of mussel foot proteins (Mfps) (Mollusca: Bivalvia)

The macroscale production of mussel foot proteins (Mfps) in the expression system has not succeeded to date. The principal reasons for this are low levels of expression and yield of Mfps, lack of post-translational modifications (PTMs), and immunological toxic effects on the host system. Identification of post-translational modification sites, suitable expression hosts, and immunological responses through an experimental approach is very costly and time-consuming. However, in the present study, in silico post-translation modification, antigenicity, allergenicity, and the immunological reaction of all available Mfps were characterized. Furthermore, all Mfps were codon optimized in three different expression systems to determine the best expression host. Finally, we performed the in-silico cloning of all codon-optimized Mfps in a suitable host (E. coli K12, pET28a(+) vector) and analyzed the secondary structure of mRNA and its structural stability. Among the 78 Mfps, six fps are considered potential allergenic proteins, six fps are considered non-allergenic proteins, and all other fps are probably allergenic. High antigenicity was observed in bacterial cells as compared to yeast and tumor cells. Nevertheless, the predicted expression of Mfps in a bacterial host is higher than in other expression hosts. Important to note that all Mfps showed significant immunological activity in the human system, and we concluded that these antigenic, allergenic, and immunological properties are directly correlated with their amino acid composition. The study's major goal is to provide a comprehensive understanding of Mfps and aid in the future genetic engineering and expression of Mfps and its diverse applications in different fields.Communicated by Ramaswamy H. Sarma.

Two-liquid wetting and phase electrowetting as a probe for surface chemistry of hydrophobic materials

Two-liquid contact angle measurement and electrowetting under low frequency are two measurement methods extremely sensitive to surface polarity and charge density. The theory underlying these two methods are described and have been applied to hydrophobic coatings. By combining these two methods, we successfully demonstrated the variations of the surface properties of polymer-based hydrophobic coatings when exposed to different chemical environments (pH modification or molecular grafting) with real-time phase electrowetting measurements.

Wetting behavior of polyelectrolyte complex coacervates on solid surfaces

The wetting behavior of complex coacervates underpins their use in many emerging applications of surface science, particularly wet adhesives and coatings. Many factors dictate if a coacervate phase will condense on a solid surface, including solution conditions, the nature of the polymer-substrate interaction, and the underlying supernatant-coacervate bulk phase behavior. In this work, we use a simple inhomogeneous mean-field theory to study the wetting behavior of complex coacervates on solid surfaces both off-coexistence (wetting transitions) and on-coexistence (contact angles). We focus on the effects of salt concentration, the polycation/polyanion surface affinity, and the applied electrostatic potential on the wettability. We find that the coacervate generally wets the surface via a first order wetting transition with second order transitions possible above a surface critical point. Applying an electrostatic potential to a solid surface always improves the surface wettability when the polycation/polyanion-substrate interaction is symmetric. For asymmetric surface affinity, the wettability has a nonmonotonic dependence with the applied potential. We use simple scaling and thermodynamic arguments to explain our results.

Bioinspired Bottlebrush Polymers for Aqueous Boundary Lubrication

An extremely efficient lubrication system is achieved in synovial joints by means of bio-lubricants and sophisticated nanostructured surfaces that work together. Molecular bottlebrush structures play crucial roles for this superior tribosystem. For example, lubricin is an important bio-lubricant, and aggrecan associated with hyaluronan is important for the mechanical response of cartilage. Inspired by nature, synthetic bottlebrush polymers have been developed and excellent aqueous boundary lubrication has been achieved. In this review, we summarize recent experimental investigations of the interfacial lubrication properties of surfaces coated with bottlebrush bio-lubricants and bioinspired bottlebrush polymers. We also discuss recent advances in understanding intermolecular synergy in aqueous lubrication including natural and synthetic polymers. Finally, opportunities and challenges in developing efficient aqueous boundary lubrication systems are outlined.

BioAdhere: tailor-made bioadhesives for epiretinal visual prostheses

Introduction: Visual prostheses, i.e. epiretinal stimulating arrays, are a promising therapy in treating retinal dystrophies and degenerations. In the wake of a new generation of devices, an innovative method for epiretinal fixation of stimulator arrays is required. We present the development of tailor-made bioadhesive peptides (peptesives) for fixating epiretinal stimulating arrays omitting the use of traumatic retinal tacks. Materials and methods: Binding motifs on the stimulating array (poly[chloro-p-xylylene] (Parylene C)) and in the extracellular matrix of the retinal surface (collagens I and IV, laminin, fibronectin) were identified. The anchor peptides cecropin A (CecA), KH1, KH2 (author's initials) and osteopontin (OPN) were genetically fused to reporter proteins to assess their binding behavior to coated microtiter plates via fluorescence-based assays. Domain Z (DZ) of staphylococcal protein A was used as a separator to generate a bioadhesive peptide. Following ISO 10993 "biological evaluation of medical materials", direct and non-direct cytotoxicity testing (L-929 and R28 retinal progenitor cells) was performed. Lastly, the fixating capabilities of the peptesives were tested in proof-of-principle experiments. Results: The generation of the bioadhesive peptide required evaluation of the N- and C-anchoring of investigated APs. The YmPh-CecA construct showed the highest activity on Parylene C in comparison with the wildtype phytase without the anchor peptide. eGFP-OPN was binding to all four investigated ECM proteins (collagen I, laminin > collagen IV, fibronectin). The strongest binding to collagen I was observed for eGFP-KH1, while the strongest binding to fibronectin was observed for eGFP-KH2. The selectivity of binding was checked by incubating eGFP-CecA and eGFP-OPN on ECM proteins and on Parylene C, respectively. Direct and non-direct cytotoxicity testing of the peptide cecropin-A-DZ-OPN using L-929 and R28 cells showed good biocompatibility properties. Proof-of-concept experiments in post-mortem rabbit eyes suggested an increased adhesion of CecA-DZ-OPN-coated stimulating arrays. Conclusion: This is the first study to prove the applicability and biocompatibility of peptesives for the fixation of macroscopic objects.

Attractive forces slow contact formation between deformable bodies underwater

Thermodynamics tells us to expect underwater contact between two hydrophobic surfaces to result in stronger adhesion compared to two hydrophilic surfaces. However, the presence of water changes not only energetics but also the dynamic process of reaching a final state, which couples solid deformation and liquid evacuation. These dynamics can create challenges for achieving strong underwater adhesion/friction, which affects diverse fields including soft robotics, biolocomotion, and tire traction. Closer investigation, requiring sufficiently precise resolution of film evacuation while simultaneously controlling surface wettability, has been lacking. We perform high-resolution in situ frustrated total internal reflection imaging to track underwater contact evolution between soft-elastic hemispheres of varying stiffness and smooth-hard surfaces of varying wettability. Surprisingly, we find the exponential rate of water evacuation from hydrophobic-hydrophobic (adhesive) contact is three orders of magnitude lower than that from hydrophobic-hydrophilic (nonadhesive) contact. The trend of decreasing rate with decreasing wettability of glass sharply changes about a point where thermodynamic adhesion crosses zero, suggesting a transition in mode of evacuation, which is illuminated by three-dimensional spatiotemporal height maps. Adhesive contact is characterized by the early localization of sealed puddles, whereas nonadhesive contact remains smooth, with film-wise evacuation from one central puddle. Measurements with a human thumb and alternatively hydrophobic/hydrophilic glass surface demonstrate practical consequences of the same dynamics: adhesive interactions cause instability in valleys and lead to a state of more trapped water and less intimate solid-solid contact. These findings offer interpretation of patterned texture seen in underwater biolocomotive adaptations as well as insight toward technological implementation.

Engineered Bio-inspired Multifunctional Peptide- and Protein-based Therapeutic Biomolecules for Better Wound Care

Developing non-immunogenic therapeutic biomolecules for facilitating blood clotting followed by wound healing via therapeutic angiogenesis, still remains a formidable challenge. Excessive blood loss of accident victims and battalions cause a huge number of deaths worldwide. Patients with inherited bleeding disorders face acute complications during injury and post-surgery. Biologically-inspired peptide-based hemostat can act as a potential therapeutic for handling coagulopathy. Additionally, non-healing wounds for patients having ischemic diseases can cause severe clinical complications. Advancement in stabilized growth-factor-based proangiogenic therapy may offer effective possibilities for the treatment of ischemic pathology. This review will discuss nature-inspired biocompatible stabilized peptide- and protein-based molecular medicines to serve unmet medical challenges for handling traumatic coagulopathy and impaired wound healing.

Theoretical and Experimental Elucidation of the Adsorption Process of a Bioinspired Peptide on Mineral Surfaces

Inorganic materials used for biomedical applications such as implants generally induce the adsorption of proteins on their surface. To control this phenomenon, the bioinspired peptidomimetic polymer 1 (PMP1), which aims to reproduce the adhesion of mussel foot proteins, is commonly used to graft specific proteins on various surfaces and to regulate the interfacial mechanism. To date and despite its wide application, the elucidation at the atomic scale of the PMP1 mechanism of adsorption on surfaces is still unknown. The purpose of the present work was thus to unravel this process through experimental and computational investigations of adsorption of PMP1 on gold, TiO₂, and SiO₂ surfaces. A common mechanism of adsorption is identified for the adsorption of PMP1 which emphasizes the role of electrostatics to approach the peptide onto the surface followed by a full adhesion process where the entropic desolvation step plays a key role. Besides, according to the fact that mussel naturally controls the oxidation states of its proteins, further investigations were performed for two distinct redox states of PMP1, and we conclude that even if both states are able to allow interaction of PMP1 with the surfaces, the oxidation of PMP1 leads to a stronger interaction.

Molecular Context of Dopa Influences Adhesion of Mussel-Inspired Peptides

Improving adhesives for wet surfaces is an ongoing challenge. While the adhesive proteins of marine mussels have inspired many synthetic wet adhesives, the mechanisms of mussel adhesion are still not fully understood. Using surface forces apparatus (SFA) measurements and replica-exchange and umbrella-sampling molecular dynamics simulations, we probed the relationships between the sequence, structure, and adhesion of mussel-inspired peptides. Experimental and computational results reveal that peptides derived from mussel foot protein 3 slow (mfp-3s) containing 3,4-dihydroxyphenylalanine (Dopa), a post-translationally modified variant of tyrosine commonly found in mussel foot proteins, form adhesive monolayers on mica. In contrast, peptides with tyrosine adsorb as weakly adhesive clusters. We further considered simulations of mfp-3s derivatives on a range of hydrophobic and hydrophilic organic and inorganic surfaces (including silica, self-assembled monolayers, and a lipid bilayer) and demonstrated that the chemical character of the target surface and proximity of cationic and hydrophobic residues to Dopa affect peptide adsorption and adhesion. Collectively, our results suggest that conversion of tyrosine to Dopa in hydrophobic, sparsely charged peptides influences peptide self-association and ultimately dictates their adhesive performance.

Facile fabrication of superhydrophobic polyester fabric based on rapid oxidation polymerization of dopamine for oil-water separation

Through the special chemical structure of dopamine (DA), superhydrophobic polyester (PET) fabric was fabricated by introducing the low surface energy substance hexadecyltrimethoxysilane (HDS) into the PET fabric and chelating Fe ions with phenolic hydroxyl groups of polydopamine (PDA) to form a rough surface. The water contact angle (WCA) of the prepared PDA/Fe/HDS PET fabric was higher than 160° and the scrolling angle (SA) was lower than 2.09°. The excellent adhesion property of polydopamine (PDA) on the substrate is helpful to improve the stability of superhydrophobic PDA/Fe/HDS PET fabric. The tests results showed that the modified PET fabric maintains excellent mechanical properties. Its superhydrophobic property had good stability and durability in the harsh environment of washing, mechanical friction, UV irradiation, seawater immersion, acid–base and organic reagents erosion. The PDA/Fe/HDS PET fabric also had good self-cleaning and oil–water separation properties. It still had good oil–water separation performance after repeated use for 25 times, and the separation efficiency was more than 95%. The preparation method was facile, the treatment time can be shortened, the cost of the modified substrate was low, and fluorine-free substances were used in the process. This work provides a new way to expand the added value of PET fabrics and develop durable superhydrophobic fabrics in practical application.

Through the special chemical structure of dopamine, superhydrophobic polyester fabric was fabricated by introducing the low surface energy substance hexadecyltrimethoxysilane into the PET fabric and chelating Fe ions to form rough surface.

Hydrophobic and Hydrophilic Effects in a Mussel-Inspired Citrate-Based Adhesive

The citrate-based tissue adhesive, synthesized by citric acid, diol, and dopamine, is a kind of mussel-inspired adhesive. The adhesion of mussel-inspired adhesive is not completely dependent on 3, 4-dihydroxyphenylalanine (Dopa) groups. The backbone structure of the adhesive also greatly affects the adhesion. In this study, to explore the effects of hydrophobicity and hydrophilicity of the backbone structure on adhesion, we prepared a series of citrate-based tissue adhesives (POEC-d) by changing the molar ratio of two diols, 1, 8-octanediol (O) and poly(ethylene oxide) (E), which formed hydrophobic segment units and hydrophilic segment units, respectively, in the molecule structure. The properties of cured adhesives showed that the adhesive with high E units had high swelling, rapid degradation, and low cohesion. In the adhesion strength measurement on the porcine skin, the adhesive with higher hydrophobicity was more likely to perform better. For the interfacial adhesion, hydrophilicity was conducive to the diffusion and penetration on the skin surface, but hydrophobic interaction showed a stronger effect to adhere with skin and hydrophobic association increased the adhesive concentration on the interface; for the bulk cohesion, hydrophobicity led to coacervation, promoting the Dopa-quinone coupling for cross-linking. In this amphipathic, citrate-based, soft-tissue adhesive system, when the feed ratio of hydrophilic segment was lower than 0.7, the coacervation could be formed through hydrophobic interaction, forming an efficient underwater adhesion system similar to that of mussels.

Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute-surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute-surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil-water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute-surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity-suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute-surface interactions.

Design and Synthesis of Bio-Inspired Polyurethane Films with High Performance

In the present work, the synthesis of segmented polyurethanes functionalized with catechol moieties within the hard or the soft segment is presented. For this purpose, a synthetic route of a new catechol diol was designed. The direct insertion of this catechol-free derivative into the rigid phase led to segmented polyurethanes with low performance (σ_max ≈ 4.5 MPa). Nevertheless, when the derivative was formally located within the soft segment, the mechanical properties of the corresponding functionalized polyurethane improved considerably (σ_max ≈ 16.3 MPa), owing to a significant increase in the degree of polymerization. It is proposed that this difference in reactivity could probably be attributed to a hampering effect of this catecholic ring during the polyaddition reaction. To corroborate this hypothesis, a protection of the aromatic ring was carried out, blocking the hampering effect and avoiding secondary reactions. The polyurethane bearing the protected catechol showed the highest molecular weight and the highest stress at break described to date (σ_max ≈ 66.1 MPa) for these kind of catechol-functionalized polyurethanes. Therefore, this new approach allows for the obtention of high-performance polyurethane films and can be applied in different sectors, benefiting from the molecular adhesion introduced by the catechol ring.

Bioinspired Non-Immunogenic Multifunctional Sealant for Efficient Blood Clotting and Suture-Free Wound Closure

Engineering bioinspired peptide-based molecular medicine is an emerging paradigm for the management of traumatic coagulopathies and inherent bleeding disorder. A hemostat-based strategy in managing uncontrolled bleeding is limited due to the lack of adequate efficacy and clinical noncompliance. In this study, we report an engineered adhesive peptide-based hybrid regenerative medicine, sealant 5, which is designed integrating the structural and functional features of fibrin and mussel foot-pad protein. AFM studies have revealed that sealant 5 (55.8 ± 6.8 nN adhesive force) has higher adhesive force than fibrin (46.4 ± 7.3 nN adhesive force). SEM data confirms that sealant 5 retains its network-like morphology both at 37 and 60 °C, inferring its thermal stability. Both sealant 5 and fibrin exhibit biodegradability in the presence of trypsin, and sealant 5 also showed biocompatibility in the presence of fibroblast cells. Engineered sealant 5 efficiently promotes hemostasis with enhanced adhesiveness and less blood-loss than fibrin. In vivo data suggests that in heparinized conditions, sealant 5 ceases bleeding at 212.3 ± 15.1 s, whereas fibrin halts bleeding at 294.3 ± 21.4 s and blood-loss is ∼4-fold less in sealant 5 than in fibrin. In a heparinized system, sealant 5 facilitates faster blood-clotting than fibrin (∼82 s faster) and RADA-16, a reported peptide-based sealant (∼113 s faster). Additionally, in the case of sealant 5, the process of clotting mimicry-like fibrin is independent of the body's own coagulation system. Sealant 5 efficiently halts bleeding for both external and internal wounds, even for a heparinized system overcoming the bacterial infection. ELISA data and PMBC cell proliferation data support the non-immunogenic feature of sealant 5. Though fibrin and sealant 5 have exhibited comparable efficacy in suture-free wound closure, in vivo H&E staining images have revealed infiltration of very few immune cells as well as the presence of abundant collagen formation in the case of sealant 5-treated wound. Such nature-inspired non-immunogenic sealants offer exciting possibilities for the treatment of uncontrolled bleeding vis-à-vis wound closure.

Structure and sequence features of mussel adhesive protein lead to its salt-tolerant adhesion ability

Mussels can strongly adhere to hydrophilic minerals in sea habitats by secreting adhesive proteins. The adhesion ability of these proteins is often attributed to the presence of Dopa derived from posttranslational modification of Tyr, whereas the contribution of structural feature is overlooked. It remains largely unknown how adhesive proteins overcome the surface-bound water layer to establish underwater adhesion. Here, we use molecular dynamics simulations to probe the conformations of adhesive protein Pvfp-5β and its salt-tolerant underwater adhesion on superhydrophilic mica. Dopa and positively charged basic residues form pairs, in this intrinsically disordered protein, and these residue pairs can lead to firm surface binding. Our simulations further suggest that the unmodified Tyr shows similar functions on surface adhesion by forming pairing structure with a positively charged residue. We confirm the presence of these residue pairs and verify the strong binding ability of unmodified proteins using nuclear magnetic resonance spectroscopy and lap shear tests.

Polyphenol-Mediated Assembly for Particle Engineering

Polyphenols are naturally occurring compounds that are ubiquitous in plants and display a spectrum of physical, chemical, and biological properties. For example, they are antioxidants, have therapeutic properties, absorb UV radiation, and complex with metal ions. Additionally, polyphenols display high adherence, which has been exploited for assembling nanostructured materials. We previously reviewed the assembly of different phenolic materials and their applications (Angew. Chem. Int. Ed. 2019, 58, 1904-1927); however, there is a need for a summary of the fundamental interactions that govern the assembly, stability, and function of polyphenol-based materials. A detailed understanding of interactions between polyphenols and various other building blocks will facilitate the rational design and assembly of advanced polyphenol particles for specific applications. This Account discusses how different interactions and bonding (i.e., hydrogen, π, hydrophobic, metal coordination, covalent, and electrostatic) can be leveraged to assemble and stabilize polyphenol-based particles for diverse applications. In polyphenol-mediated assembly strategies, the polyphenols typically exert more than one type of stabilizing attractive force. However, one interaction often dominates the assembly process and dictates the physicochemical behavior of the particles, which in turn influences potential applications. This Account is thus divided into sections that each focus on a key interaction with relevant examples of applications to highlight structure-function relationships. For example, metal coordination generally becomes weaker at lower pH, which makes it possible to engineer metal-phenolic materials with a pH-responsive disassembly profile suitable for drug delivery. Engineered particles, such as hollow capsules, mesoporous and core-shell particles, and self-assembled nanoparticles are some of the systems that are covered to highlight how polyphenols interact with other building blocks and therefore make up the major focus of this Account. Some of the applications of these materials exemplified in this Account include drug delivery, catalysis, environmental remediation, and forensics. Finally, a perspective is provided on the current challenges and trends in polyphenol-mediated particle assembly, and viable near-term strategies for further elucidating the interplay of various competing interactions in particle formation are discussed. This Account is also expected to serve as a reference to guide fundamental research and facilitate the rational design of polyphenol-based materials for diverse emerging applications.

An Antifreezing/Antiheating Hydrogel Containing Catechol Derivative Urushiol for Strong Wet Adhesion to Various Substrates

Tough adhesive hydrogels that can tightly bond to wet tissue/polymer/ceramic/metal surfaces have great potentials in various fields. However, conventional adhesive hydrogels usually show short-term and nonreversible adhesion ability, as the water component in a hydrogel readily transforms to vapor or ice in response to fluctuation of environment temperature, hindering their applications in extreme conditions such as in freezing Arctic and roasting Africa. For the first time, urushiol (UH), a natural catechol derivative with a long alkyl side chain, is used as a starting material to copolymerize with acrylamide for fabricating adhesive hydrogels, which contain hydrophobic/hydrophilic moieties, antifreezing agent, and adhesive catechol groups. The antifreezer/moisturizer glycerol/water binary solvent dispersed in the hydrogel endows it with antifreezing/antiheating property. The hydrophobic association and π-π interaction from UH moieties of the copolymer greatly improve its mechanical strength (tensile stress: ∼0.12 MPa with strain of ∼1100%, toughness: ∼72 kJ/m³, compression stress: ∼6.72 MPa at strain of 90%). The hydrogel can strongly adhere to various dry/wet biological/polymeric/ceramic/metallic substrates at temperatures ranging from -45 to 50 °C. Under ambient conditions, its adhesion force to porcine skin, glass, and tinplate may reach up to 160, 425, and 275 N/m, respectively. Even stored at -45 or 50 °C for 30 d, the hydrogel still maintains good flexibility and robust adhesion force. It also shows repeatable underwater adhesion to biological tissue, glass, ceramic, plastic, and rubber. This novel antifreezing/antiheating adhesive hydrogel may be applied in extremely cold or hot environments and in underwater conditions.

Computational modelling of wet adhesive mussel foot proteins (Bivalvia): Insights into the evolutionary convolution in diverse perspectives

Underwater adhesion in mussels (Bivalvia) is an extreme adaptation to achieve robust and firm wet adhesion in the freshwater/brackish/ocean, which biochemically shaped through millions of years. The protein-based adhesion has huge prospective in various fields like industry, medical, etc. Currently, no comprehensive records related to the systematic documentation of structural and functional properties of Mussel foot proteins (Mfps). In this study, we identified the nine species of bivalves in which the complete sequence of at least one adhesive protein is known. The insilico characterization revealed the specific physio-chemical structural and functional characters of each Mfps. The evolutionary analyses of selected bivalves are mainly based on Mfps, Mitogenome, and TimeTree. The outcome of the works has great applications for designing biomimetic materials in future.

The physico-chemistry of adhesions of protein resistant and weak polyelectrolyte brushes to cells and tissues

The non-specific adhesion of polymers and soft tissues is of great interest to the field of biomedical engineering, as it will shed light on some of the processes that regulate interactions between scaffolds, implants and nanoparticles with surrounding tissues after implantation or delivery. In order to promote adhesion to soft tissues, a greater understanding of the relationship between polymer chemistry and nanoscale adhesion mechanisms is required. In this work, we grew poly(dimethylaminoethyl methacrylate) (PDMAEMA), poly(acrylic acid) (PAA) and poly(oligoethylene glycol methacrylate) (POEGMA) brushes from the surface of silica beads, and investigated their adhesion to a variety of substrates via colloidal probe-based atomic force microscopy (AFM). We first characterised adhesion to a range of substrates with defined surface chemistry (self-assembled monolayers (SAMs) with a range of hydrophilicities, charge and hydrogen bonding), before studying the adhesion of brushes to epithelial cell monolayers (primary keratinocytes and HaCaT cells) and soft tissues (porcine epicardium and keratinized gingiva). Adhesion assays to SAMs reveal the complex balance of interactions (electrostatic, van der Waals interactions and hydrogen bonding) regulating the adhesion of weak polyelectrolyte brushes. This resulted in particularly strong adhesion of PAA brushes to a wide range of surface chemistries. In turn, colloidal probe microscopy on cell monolayers highlighted the importance of the glycocalyx in regulating non-specific adhesions. This was also reflected by the adhesive properties of soft tissues, in combination with their mechanical properties. Overall, this work clearly demonstrates the complex nature of interactions between polymeric biomaterials and biological samples and highlights the need for relatively elaborate models to predict these interactions.

Mussel-inspired hydrogels: from design principles to promising applications

This review presents the recent progress of mussel-inspired hydrogels from fundamental interaction mechanisms and design principles to promising applications.

Fluctuation-induced free energy of thin peptide films

We apply the Lifshitz theory of dispersion forces to find a contribution to the free energy of peptide films that is caused by the zero-point and thermal fluctuations of the electromagnetic field. For this purpose, using available information about the imaginary parts of the dielectric permittivity of peptides, an analytic representation for permittivity of a typical peptide along the imaginary frequency axis is devised. Numerical computations of the fluctuation-induced free energy are performed at room temperature for freestanding peptide films containing different fractions of water, and for similar films deposited on dielectric (SiO_{2}) and metallic (Au) substrates. It is shown that the free energy of a freestanding peptide film is negative and thus contributes to its stability. The magnitude of the free energy increases with increasing fraction of water and decreases with increasing thickness of a film. For peptide films deposited on a dielectric substrate, the free energy is nonmonotonous. It is negative for films thicker than 100nm, reaches the maximum value at some film thickness, but vanishes and changes its sign for films thinner than 100nm. The fluctuation-induced free energy of peptide films deposited on metallic substrate is found to be positive, which makes films less stable. In all three cases, simple analytic expressions for the free energy of sufficiently thick films are found. The obtained results may be useful to attain film stability in the next generation of organic microdevices with further reduced dimensions.

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

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

Direct Observation of the Interplay of Catechol Binding and Polymer Hydrophobicity in a Mussel-Inspired Elastomeric Adhesive

Marine organisms such as mussels have mastered the challenges in underwater adhesion by incorporating post-translationally modified amino acids like l-3,4-dihydroxyphenylalanine (DOPA) in adhesive proteins. Here we designed a catechol containing elastomer adhesive to identify the role of catechol in interfacial adhesion in both dry and wet conditions. To decouple the adhesive contribution of catechol to the overall adhesion, the elastomer was designed to be cross-linked through [2 + 2] photo-cycloaddition of coumarin. The elastomer with catechol moieties displayed a higher adhesion strength than the catechol-protected elastomer. The contact interface was probed using interface-sensitive sum frequency generation spectroscopy to explore the question of whether catechol can displace water and bond with hydrophilic surfaces. The spectroscopy measurements reveal that the maximum binding energy of the catechol and protected-catechol elastomers to sapphire substrate is 7.0 ± 0.1 kJ/(mole of surface O-H), which is equivalent to 0.10 J/m2. The higher dry and wet adhesion observed in the macroscopic adhesion measurements for the catechol containing elastomer originates from multiple hydrogen bonds of the catechol dihydroxy groups to the surface. In addition, our results show that catechol by itself does not remove the confined interstitial water. In these elastomers, it is the hydrophobic groups that help in partially removing interstitial water. The observation of the synergy between catechol binding and hydrophobicity in enabling the mussel-inspired soft adhesive elastomer to stick underwater provides a framework for designing materials for applications in tissue adhesion and moist-skin wearable electronics.

Morphology of soft and rough contact via fluid drainage

The dynamic of contact formation between soft materials immersed in a fluid is accompanied by fluid drainage and elastic deformation. As a result, controlling the coupling between lubrication pressure and elasticity provides strategies to design materials with reversible and dynamic adhesion to wet or flooded surfaces. We characterize the elastic deformation of a soft coating with nanometer-scale roughness as it approaches and contacts a rigid surface in a fluid environment. The lubrication pressure during the approach causes elastic deformation and prevents contact formation. We observe deformation profiles that are drastically different from those observed for elastic half-space when the thickness of the soft coating is comparable to the hydrodynamic radius. In contrast, we show that surface roughness favors fluid drainage without altering the elastic deformation. As a result, the coupling between elasticity and slip (caused by surface roughness) can lead to trapped fluid pockets in the contact region.

Computational discovery of chemically patterned surfaces that effect unique hydration water dynamics

The interactions of water with solid surfaces govern their apparent hydrophobicity/hydrophilicity, influenced at the molecular scale by surface coverage of chemical groups of varied nonpolar/polar character. Recently, it has become clear that the precise patterning of surface groups, and not simply average surface coverage, has a significant impact on the structure and thermodynamics of hydration layer water, and, in turn, on macroscopic interfacial properties. Here we show that patterning also controls the dynamics of hydration water, a behavior frequently thought to be leveraged by biomolecules to influence functional dynamics, but yet to be generalized. To uncover the role of surface heterogeneities, we couple a genetic algorithm to iterative molecular dynamics simulations to design the patterning of surface functional groups, at fixed coverage, to either minimize or maximize proximal water diffusivity. Optimized surface configurations reveal that clustering of hydrophilic groups increases hydration water mobility, while dispersing them decreases it, but only if hydrophilic moieties interact with water through directional, hydrogen-bonding interactions. Remarkably, we find that, across different surfaces, coverages, and patterns, both the chemical potential for inserting a methane-sized hydrophobe near the interface and, in particular, the hydration water orientational entropy serve as strong predictors for hydration water diffusivity, suggesting that these simple thermodynamic quantities encode the way surfaces control water dynamics. These results suggest a deep and intriguing connection between hydration water thermodynamics and dynamics, demonstrating that subnanometer chemical surface patterning is an important design parameter for engineering solid-water interfaces with applications spanning separations to catalysis.

Complex coacervates based on recombinant mussel adhesive proteins: their characterization and applications

Complex coacervates are a dense liquid phase of oppositely charged polyions formed by the associative separation of a mixture of polyions. Coacervates have been widely employed in many fields including the pharmaceutical, cosmetic, and food industries due to their intriguing interfacial and bulk material properties. More recently, attempts to develop an effective underwater adhesive have been made using complex coacervates that are based on recombinant mussel adhesive proteins (MAPs) due to the water immiscibility of complex coacervates and the adhesiveness of MAPs. MAP-based complex coacervates contribute to our understanding of the physical nature of complex coacervates and they provide a promising alternative to conventional invasive surgical repairs. Here, this review provides an overview of recombinant MAP-based complex coacervations, with an emphasis on their characterization and the uses of such materials for applications in the fields of biomedicine and tissue engineering.

Deposition and Adhesion of Polydopamine on the Surfaces of Varying Wettability

Mussel-inspired chemistry, particularly the versatile coating capability of polydopamine (PDA), has received much research interest as a promising strategy for fabricating functional coatings in numerous fields. However, the understanding of deposition mechanisms and adhesion behaviors of PDA on different substrates still remains incomplete, significantly limiting the related fundamental research and its practical applications. In this work, a colloidal probe atomic force spectroscopy technique was employed to quantify the interaction forces and adhesion between the PDA coatings and the substrate surfaces with different wettabilities. The surface force measurements and thermodynamic analysis of interaction energy indicate that the surface wettability has a significant influence on the adhesion, deposition behaviors, and morphologies of PDA coatings. Compared with the hydrophilic surfaces, the hydrophobic surfaces exhibit stronger adhesion with the PDA coatings. Furthermore, for the first time, this work demonstrates that ethanol has the capability of effectively displacing the trapped air/vapor layer or the so-called "hydrophobic depletion layer" on the hydrophobic substrate to allow the intimate contact between PDA and the substrate, thus enhancing the adhesion and facilitating the PDA deposition. This work provides new insights into the fundamental PDA deposition mechanism as well as the design and development of versatile mussel-inspired coatings on the substrates of varying hydrophobicity.

Molecular interactions between DOPA and surfaces with different functional groups: a chemical force microscopy study

The adhesion of mussel foot proteins (Mfps) to a variety of surfaces has been widely investigated, but the mechanisms behind the mussel adhesion to surfaces with different properties are far from being understood.