Toward Artificial Mussel-Glue Proteins: Differentiating Sequence Modules for Adhesion and Switchable Cohesion

Aqueous Graphene Dispersion and Biofunctionalization via Enzymatic Oxidation of Tripeptides

Graphene, a 2D carbon material, possesses extraordinary mechanical, electrical, and thermal properties, making it highly attractive for various biological applications such as biosensing, biotherapeutics, and tissue engineering. However, the tendency of graphene sheets to aggregate and restack hinders its dispersion in water, limiting these applications. Peptides, with their defined amino acid sequences and versatile functionalities, are compelling molecules with which to modify graphene-aromatic amino acids can strengthen interactions through π-stacking and charged groups can be chosen to make the sheets dispersible and stable in water. Here, a facile and green method for covalently functionalizing and dispersing graphene using amphiphilic tripeptides, facilitated by a tyrosine phenol side chain, through an aqueous enzymatic oxidation process is demonstrated. The presence of a second aromatic side chain group enhances this interaction through non-covalent support via π-π stacking with the graphene surface. Futhermore, the addition of charged moieties originating from either ionizable amino acids or terminal groups facilitates profound interactions with water, resulting in the dispersion of the newly functionalized graphene in aqueous solutions. This biofunctionalization method resulted in ≈56% peptide loading on the graphene surface, leading to graphene dispersions that remain stable for months in aqueous solutions outperforming currently used surfactants.

Enhancing Adhesion Properties of Commodity Polymers through Thiol-Catechol Connectivities: A Case Study on Polymerizing Polystyrene-Telechelics via Thiol-Quinone Michael-Polyaddition

Segmented block copolymers with adhesive functionality bridges in between are synthesized through the combination of controlled radical polymerization (CRP) and thiol-quinone Michael-polyaddition. CRP provides a set of α,ω-dithiol polystyrenes (PS), which react as telechelics with a low molecular weight bisquinone, resulting in thiol-catechol connectivities (TCCs). By introducing as little as 3 mol % of TCC functionalities, the bonding of the polymer on dry and wet aluminum surfaces is significantly improved while keeping the integrity of the PS segments undisturbed to constitute favorable bulk properties. This improvement is evidenced by reaching up to 3.8 MPa adhesive strength, representing a 600% increase compared to nonfunctional PS.

Femtosecond Laser Direct Writing of Gecko-Inspired Switchable Adhesion Interfaces on a Flexible Substrate

Biomimetic switchable adhesion interfaces (BSAIs) with dynamic adhesion states have demonstrated significant advantages in micro-manipulation and bio-detection. Among them, gecko-inspired adhesives have garnered considerable attention due to their exceptional adaptability to extreme environments. However, their high adhesion strength poses challenges in achieving flexible control. Herein, we propose an elegant and efficient approach by fabricating three-dimensional mushroom-shaped polydimethylsiloxane (PDMS) micropillars on a flexible PDMS substrate to mimic the bending and stretching of gecko footpads. The fabrication process that employs two-photon polymerization ensures high spatial resolution, resulting in micropillars with exquisite structures and ultra-smooth surfaces, even for tip/stem ratios exceeding 2 (a critical factor for maintaining adhesion strength). Furthermore, these adhesive structures display outstanding resilience, enduring 175% deformation and severe bending without collapse, ascribing to the excellent compatibility of the micropillar's composition and physical properties with the substrate. Our BSAIs can achieve highly controllable adhesion force and rapid manipulation of liquid droplets through mechanical bending and stretching of the PDMS substrate. By adjusting the spacing between the micropillars, precise control of adhesion strength is achieved. These intriguing properties make them promising candidates for various applications in the fields of microfluidics, micro-assembly, flexible electronics, and beyond.

Modification of Low-Energy Surfaces Using Bicyclic Peptides Discovered by Phage Display

Solid-binding peptides are a simple and versatile tool for the non-covalent modification of solid material surfaces, and a variety of peptides have been developed by reference to natural proteins or de novo design. Here, for the first time, we report the discovery of a bicyclic peptide targeting the heterogeneous material polypropylene by combining phage display technology and next-generation sequencing. We find that the enrichment properties of bicyclic peptides capable of binding to polypropylene are distinct from linear peptides, as reflected in amino acid abundance and a trend toward negative net charges and high hydrophobicity. The selected bicyclic peptide has a higher binding affinity for polypropylene compared with a previously reported linear peptide, enabling the hydrophilic and adhesive properties of the polypropylene to be more effectively enhanced. Our work paves the way for the exploration and utilization of conformational-restricted cyclic peptides as a new family of functionally evolvable agents for material surface modification.

Biospecific cation-π interaction by modulating molecular hydration and supramolecular structure of short peptides

This study presents novel adhesive materials that use cation-π interactions to achieve highly specific cohesive interaction under water. The materials are short length peptides based on the FKF motif flanked by different side groups. Using the surface forces apparatus, we show that the composition of the side group allows to finely tune the strength of the cohesive and adhesive energies of the peptide and its specificity, meaning its capacity to bind strongly only to substrates bearing the same peptide. The interfacial properties of these adhesive peptides are shown to strongly depend on the composition of the deposition solvent, with DMSO being the solvent of choice to achieve high cohesive and adhesive energies. This result was correlated with the supramolecular structure of the peptide film and confirmed that needle-like structures can significantly enhance the adhesion of the material. Altogether, we showed that cation-π interaction can be used efficiently to create adhesive materials that incorporate features already known for underwater adhesives such as activation via solvent displacement, as well as new ones such as specificity and supramolecular structure enhanced adhesion.

Exploring Self-Healing and Switchable Adhesives based on Multi-Level Dynamic Stable Structure

It is a challenge to develop adhesives simultaneously capable of strong adhesion and efficient switchable ability. Herein, the authors report multifunctional switchable adhesives named Cu²⁺ -curcumin-imidazole-polyurethane (CIPUs:Cu²⁺ ) by introducing 1-(3-aminopropyl) imidazole and curcumin into polyurethane system crossed by Cu²⁺ forming dynamic metal-ligand bonds. This CIPUs:Cu²⁺ has strong adhesion (up to 2.46 MPa) on various material surfaces due to their specially designed functional groups alike the secretions from mussels. It can achieve fast switching speed (30 s) and high switch efficiency through multiple contactless remote stimulations. Importantly, density functional theory (DFT) calculation reveals that such metal-ligand bonds consisting of two components: stronger Cu²⁺ -curcumin complexes and weaker Cu²⁺ -imidazole complexes can aggregate to form multi-level dynamic stable structure . The special structure can not only be acted as sacrificial sites for easily broken and reformed, allowing efficient switchable adhesion and enormous energy dissipation but also acted as firm sites to maintain the cohesion of the adhesive and the reversible reconstruction network. Intriguingly, the CIPUs:Cu²⁺ can achieve self-healing at room temperature without needing external stimuli. Overall, this strategy can further broaden the design of switchable adhesives in the fields of intelligent gadgets, wearable bio-monitoring devices, etc.

Critical Analysis of Adhesion Work Measurements from AFM-based Techniques for Soft Contact

HYPOTHESIS

The work of adhesion is a thermodynamic quantity that is frequently measured by atomic force microscopy (AFM). Determination of the work of adhesion requires quasi-equilibrium measurements, where we address the question of to what extent atomic force microscopy qualifies for quasi-equilibrium measurements.

EXPERIMENT

To measure the work of adhesion, we combined soft colloidal probe AFM (SCP AFM) with reflection interference contrast microscopy (RICM). This allowed us to extract the work of adhesion either from the pull-off force or from the contact radius. With these methods, we investigated the adhesion behavior of poly(N-isopropylacrylamide) (PNIPAM) polymer brushes in the swollen and solvent-induced collapsed state by systematically analyzing contact radii and adhesive forces.

FINDINGS

In the swollen state, the adhesion to the PNIPAM brush was fivefold larger and exhibited significant time dependencies when measured with SCP AFM. A strong rate dependence of the pull-off force method was indicative of a non-equilibrium process. In order to reliably determine the equilibrium work of adhesion, the contact radius method was found to be the better because it is not rate dependent. Our work highlights the important benefits of using optical measurements to determine the contact radius when deriving the works of adhesion between colloidal probes and polymer brush surfaces.

Ternary Synergy of Lys, Dopa, and Phe Results in Strong Cohesion of Peptide Films

Synergistic interactions between 3,4-dihydroxyphenylalanine (Dopa, Y*), cationic residues, and the aromatic rings have been recently highlighted as influential factors that enhance the underwater adhesion strength of mussel foot proteins and their derivatives. In this study, we report the first ever evidence of a cation-catechol-benzene ternary synergy between Y*, lysine (Lys, K), and phenylalanine (Phe, F) in adhesive peptides. We synthesized three hexapeptides containing a different combination of Y*, K, and F, i.e., (KY*)₃, (KF)₃, and (KY*F)₂, respectively, exploring the relationship between the cohesive performance and molecular architecture of peptides. The peptide with the (KY*F)₂ sequence displays the strongest underwater cohesion energy of 10.3 ± 0.3 mJ m^-2 from direct nanoscale surface force measurements. Combined with molecular dynamics simulation, we demonstrated that there are more bonding interactions (including cation-π, π-π, and hydrogen bond interactions) in (KY*F)₂ compared to the other two peptides. In addition, peptide (KY*F)₂ still shows the strongest cohesive energies of 7.6 ± 0.7 and 3.7 ± 0.5 mJ m^-2 in acidic and high-ionic strength environments, respectively, although the cohesive energy decreases compared to the value in pure water. Our results further explain the underwater cohesion mechanisms combining multiple interactions and offer insights on designing Dopa containing underwater adhesives.

Antifouling Surface Coating on Various Substrates by Inducing Tyrosinase-Mediated Oxidation of a Tyrosine-Conjugated Sulfobetaine Derivative

Inspired by the melanogenesis occurring in nature, we report tyrosinase-mediated antifouling surface coating by synthesizing a tyrosine-conjugated sulfobetaine derivative (Tyr-SB). Synthetic Tyr-SB contains zwitterionic sulfobetaine and tyrosine, whose phenolic amine group acts as a dormant coating precursor. In contrast to catecholamine derivatives, tyrosine derivatives are stable against auto-oxidation and are enzymatically oxidized only in the presence of tyrosinase to initiate melanin-like oxidation. When the surface of interest was applied during the course of Tyr-SB oxidation, a superhydrophilic poly(Tyr-SB) film was coated on the surfaces, thereby showing antifouling performance against proteins or adherent cells. Because the oxidation of Tyr-SB occurred under mild aqueous conditions (pH 6-7) without the use of any chemical oxidants, such as sodium periodate or ammonium persulfate, we anticipate that the coating method described herein will serve as a biocompatible tool in the field of biosensors, cell surface engineering, and medical devices, whose interfaces differ in chemistry.

Switchable Adhesion: On-Demand Bonding and Debonding

Adhesives have a long and illustrious history throughout human history. The development of synthetic polymers has highly improved adhesions in terms of their strength and environmental tolerance. As soft robotics, flexible electronics, and intelligent gadgets become more prevalent, adhesives with changeable adhesion capabilities will become more necessary. These adhesives should be programmable and switchable, with the ability to respond to light, electromagnetic fields, thermal, and other stimuli. These requirements necessitate novel concepts in adhesion engineering and material science. Considerable studies have been carried out to develop a wide range of adhesives. This review focuses on stimuli-responsive material-based adhesives, outlining current research on switchable and controlled adhesives, including design and manufacturing techniques. Finally, the potential for smart adhesives in applications, and the development of future adhesive forms are critically suggested.

Development of Universal and Clickable Film by Mimicking Melanogenesis: On-Demand Oxidation of Tyrosine-Based Azido Derivative by Tyrosinase

In this study, we synthesized a tyrosine-based azido derivative (TBAD) that permits both substrate-independent surface coating and clickable film functionalization by mimicking natural melanogenesis. In contrast to catechol derivatives, which are generally susceptible to oxidation by air under ambient conditions, the monophenol-based TBAD remains stable under alkaline and neutral conditions, and is activated to oxidized quinone in situ by tyrosinase to initiate melanin-like polymerization. The resulting poly(TBAD) film can be formed on various substrates including noble metals, metal oxides, and synthetic polymers, which can undergo click reaction with terminal alkyne moieties on the entire surface or a specific region through Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The enzyme-mediated coating can rapidly form thin films (∼10 nm) and produce a uniform film morphology, which are important aspects in surface chemistry. This on-demand, clickable coating may become a significant tool for bioconjugation, soft lithography, and labeling techniques. This article is protected by copyright. All rights reserved.

Bio-based and bio-inspired adhesives from animals and plants for biomedical applications

With the "many-headed" slime mold Physarum polycelphalum having been voted the unicellular organism of the year 2021 by the German Society of Protozoology, we are reminded that a large part of nature's huge variety of life forms is easily overlooked - both by the general public and researchers alike. Indeed, whereas several animals such as mussels or spiders have already inspired many scientists to create novel materials with glue-like properties, there is much more to discover in the flora and fauna. Here, we provide an overview of naturally occurring slimy substances with adhesive properties and categorize them in terms of the main chemical motifs that convey their stickiness, i.e., carbohydrate-, protein-, and glycoprotein-based biological glues. Furthermore, we highlight selected recent developments in the area of material design and functionalization that aim at making use of such biological compounds for novel applications in medicine - either by conjugating adhesive motifs found in nature to biological or synthetic macromolecules or by synthetically creating (multi-)functional materials, which combine adhesive properties with additional, problem-specific (and sometimes tunable) features.

Toward Activatable Collagen Mimics: Combining DEPSI "Switch" Defects and Template-Guided Self-Organization to Control Collagen Mimetic Peptides

Collagen mimetic peptides (CMPs), which imitate various structural or functional features of natural collagen, constitute advanced models illuminating the folding aspects of the collagen triple helix (CTH) motif. In this study, the CMPs of repeating Gly-Pro-Pro (GPP) triplets are tethered to an organic scaffold based on a tris(2-aminoethyl) amine (TREN) derivative (TREN(sucOH)₃ ). These three templated peptide strands are further expanded via native chemical ligation to increase the number of GPP triplets and lead to a TREN(sucGPPGPPG(Ψ)SPGPPCPP[GPP]₄ )₃ construct. The incorporation of an ester switch segment, G(Ψ)S, as a positional O-acyl isopeptide (DEPSI) defect into the peptide strands allows the pH-controlled acceleration of CTH formation. The strand assembly process is monitored by circular dichroism (CD) spectroscopy. The results of pH jump experiments and thermal denaturation studies provide new insights into the contributions of structural DEPSI defects to the template-guided self-assembly of the CTH motif. While the organic scaffold drives the CTH formation, the switch defects act as temporary opponents and slow down the folding. CD spectroscopy data confirm that the switch defects contribute to the formation of a more stable CTH motif by enhancing the structural dynamics at the early stage of the folding process.

A Superstrong and Reversible Ionic Crystal-Based Adhesive Inspired by Ice Adhesion

In this study, we developed a superstrong and reversible adhesive, which can possess a high bonding strength in the "adhesive" state and detach with the application of heating. An ionic crystal (IC) gel, in which an IC was immobilized within a soft-polymer matrix, were synthesized via in situ photo-crosslinking of a precursor solution composed of N, N-dimethyl acrylamide (DMAA) and a melted IC. The obtained IC gel is homogenous and transparent at melt point. When cooled to the phase transition temperature of the IC, the gel turns into the adhesive with the adhesion strength of 5.82 MPa (on glasses), due to the excellent wetting of melted gel and a thin layer of crystalline IC with high cohesive strength formed on the substrates. The synergistic effects between IC, polymer networks and substrates were investigated by solid state ¹ H NMR and molecular dynamics simulation. Such an adhesive layer is reversable and can be detached by heating and subsequent re-adhesion via cooling. This study proposed the new design of removable adhesives, which can be used in dynamic and complex environments.

Accessing the Next Generation of Synthetic Mussel-Glue Polymers via Mussel-Inspired Polymerization

The formation of cysteinyldopa as biogenic connectivity in proteins is used to inspire a chemical pathway toward mussel-adhesive mimics. The mussel-inspired polymerization (MIPoly) exploits the chemically diverse family of bisphenol monomers that is oxidizable with 2-iodoxybenzoic acid to give bisquinones. Those react at room temperature with dithiols in Michael-type polyadditions, which leads to polymers with thiol-catechol connectivities (TCC). A set of TCC polymers proved adhesive behavior even on challenging poly(propylene) substrates, where they compete with commercial epoxy resins in dry adhesive strength. MIPoly promises facile scale up and exhibits high modularity to tailor adhesives, as proven on a small library where one candidate showed wet adhesion on aluminum substrates in both water and sea water models.

Implementing Zn2+ ion and pH-value control into artificial mussel glue proteins by abstracting a His-rich domain from preCollagen

A His-rich domain of preCollagen-D found in byssal threads is derivatized with Cys and Dopa flanks to allow for mussel-inspired polymerization. Artificial mussel glue proteins are accessed that combine cysteinyldopa for adhesion with sequences for pH or Zn2+ induced β-sheet formation. The artificial constructs show strong adsorption to Al2O3, the resulting coatings tolerate hypersaline conditions and cohesion is improved by activating the β-sheet formation, that enhances E-modulus up to 60%.

Information-Based Design of Polymeric Drug Formulation Additives

Tailor-made copolymers are designed based on a peptide-poly(ethylene glycol) (QFFLFFQ-PEG) conjugate as a blueprint, to solubilize the photosensitizer meta-tetra(hydroxyphenyl)chlorin (m-THPC). The relevant functionalities of the parent peptide-PEG are mimicked by employing monomer pairs that copolymerize in a strictly alternating manner. While styrene (S) or 4-vinylbenzyl-phthalimide (VBP) provide aromatic moieties like Phe, the aliphatic isobutyl side chain of Leu4 is mimicked by maleic anhydride (MA) that reacts after polymerization with isobutylamine to give the isobutylamide-carboxyl functional unit (iBuMA). A set of copolymer-PEG solubilizers is synthesized by controlled radical polymerization, systematically altering the length of the functional segment (DP_n = 2, 4, 6) and the side chain functionalization (iBuMA, iPrMA, MeMA). The m-THPC hosting and release properties of P[S-alt-iBuMA]₆-PEG reached higher payload capacities and more favored release rates than the parent peptide-PEG conjugate. Interestingly, P[S-alt-RMA]_n-PEG mimics the sensitivity of the peptide-PEG solubilizer well, where the exchange of Leu4 residue by Val and Ala significantly reduces the drug loading by 92%. A similar trend is found with P[S-alt-RMA]_n-PEG as the exchange of iBu → iPr → Me reduces the payload capacity up to 78%.