Резюме. L-Dopa-containing proteins with adhesive property exist within the mussel feet, commonly called mussel foot proteins (fps). These bioadhesive proteins adhered tightly to all kinds of surfaces and, more interesting, under wet environment. Catechol functionality of Dopa plays crucial role for such adhesion. Mussel-inspired catechol-containing polymers were conceived and their adhesion forces were compared to the commercial glues. Applications in biomedicals, hydrogels, antifoulings, and nanotechnology are claimed. One promising outlook is the surgical suture with such bioadhesive and the engineering of self-healing related materials.

Ключови думи: adhesive, bioadhesive, catechol, dopa, mussel, polymer

Introduction

Background in chemistry, as with other scientific disciplines, is better acquired in schools, colleges, and universities, providing two essential elements: a chemistry teacher and a laboratory. Observed phenomena in chemical experiments have led to set laws, structures, and properties. The advent of science, in general, has allowed delving into a deeper pit of the matter, developing hence the nanoscience of which emerged the nanochemistry. All the sofar-made discoveries and the upcoming ones are but the already-created phenomena, natural ones, which man has cleverly deciphered. Nature must be understood as the man surroundings including himself (man, animals, plants, trees, oceans, sky, earth, planets, etc.). Chemist has reached a level of scientific maturity to conceive and make molecules of his own, either for a mere curiosity or for a targeted objective. However, natural molecules or commonly called natural products stand superior by far vis-à-vis the synthetic or man-made ones. For example, it has been demonstrated that the natural vitamins are safer, more effective than the synthetic analogues, and their absorption by the metabolism is somewhat different, although the molecular architectures are wholly identical (Thiel, 2000). Yet, the good side of this event is the daring of chemist to accept the challenge of making the natural molecules of intricate architectures. Examples of such synthesized natural molecules are innumerable and the multistep routes of making them are excitingly onerous (Nicolau et al., 2000). An ever-leading one is the synthesis of vitamin B12 whose designer or builder was crowned with a Nobel Prize, for achieving such a robust challenge (Moulay, 2007). Total biosynthesis of natural products remains unique, defying, and undecipherable. And, to reproduce and comprehend such natural phenomena may be mere efforts. Natural rubber, a hydrocarbon polymer, is secreted in a form of an emulsion consisting of this polymer in spherical form finely dispersed in aqueous medium that contains several other components (proteins, phospholipids, fats, waxes, aminoacids and other organic acids such ascorbic acid, carbohydrates, sterols, ….etc) in small portions (0.002 to 1%) (Jacob et al., 1993). Synthetic rubber, however, requires a lesser number of different ingredients. Nowadays, even the organic synthesis is being oriented to biological systems, so-called Green Chemistry, that is to handle the work-up in environmentally friendly aqueous media, avoiding the use of the toxic organic solvents.

Other spellbinding and impressive species are spiders. Indeed, spiders secrete different natural silks (like dragline silk) for different purposes, whose mechanical properties are rivaling those of engineering polymers such as aramid and Kevlar (Vollrath, 2000; Hinman et al., 2000; Vollrath et al., 2011), and whose adhesive properties are but stunning (Sahni et al., 2011); the spiders’ silks are among the toughest materials ever known.

Mussel bioadhesive

Adhesives are materials of a paramount importance in our modern life, facilitating the firm assembly of objects for various applications. The variety of chemical natures of adhesives is closely linked to the different natures of the substrata (organic and inorganic), a fact imposed by the sizing extent. Classical or conventional adhesives are based on natural resins and natural/synthetic polymers, as their solutions stand as colloidal systems: emulsions of some acrylic polymers and poly(vinyl acetate) (Elmer’s glue, for paper and wood sizing), ethylcyanoacrylate glue (Krazy glue, for objects of all kinds), epoxy resins/hardener system (Quick-set), and urea- and phenol- formaldehyde resins. The adhesiveness of these materials to items occurred usually under dry (wet out) environment. Their adhesion on water-wetted objects does not happen for water molecules will prevent the attractive forces between the adhesive and bonding surface from taking place. Thus, an adhesive with a property of sticking to a wet substrate would be of a great value and its realization would be certainly of a great accomplishment. While man perceives this adhesive not to be possible, nature masterly approves it. Indeed, and to man’s surprise, there exist mollusks called “Mussels” (Fig. 1) whose feet secrete a sticky liquid that allows them to attach to rocks in seawaters. The mussels family is composed of three members, distinct by their colors: Mytilus edulis (blue), Perna viridis (green), and Dreissena polymorpha (zebra). Bioadhesives are also secreted from other animals (insects and reptiles) and plants (Von Byern & Grunwald, 2010; Favi et al., 2014).

To alleviate such a surprise and quench one’s curiosity, several workers succeeded in unraveling the secret behind this gluing phenomenon of mussel in water environment (Waite & Tanzer, 1981; Waite, 1999; 2002; Lin et al., 2007). The outcome is that this bioadhesive substance is of a proteinaceous nature, commonly coined either “mussel adhesive protein” (MAP) or “mussel foot protein” (Mfp). A feature of this protein is that its chemical structure bears 3,4-dihydroxypheny-L-alanine (Dopa), a natural molecule formed by posttranslational modification of tyrosine, as pictured in Fig.2. The catechol functionality in Dopa, being catecholamine, was found to be primarily responsible for water-resistant adhesion to all substrates (Lee et al., 2006a; Lee et al., 2006b). Five Mfp’s were extracted from Mytilus edulis species with different molecular weights and different Dopa contents as gathered in Table 1 (Rzepecki & Waite, 1991).

As expected, the actual adhesion on different substrata is better assured by Mfp’s of higher Dopa contents. Indeed, while Mfp-2 and Mfp-4 that are of the lowest Dopa contents and of moderate molecular weights do not show appreciable contribution, Mfp-3 and Mfp-5 that are of highest DOPA contents and of the lowest molecular weights serve as the adhesive primers, and Mfp-1 that is of moderate Dopa content and of the highest molecular weight acts as the coating. This observation is a reminder of the paint coating which requires a primer layer for a better adhesion to the substrate and a coating layer for its protection. Such different roles seem to be linked to the spatial structures of the Mfp’s. The mechanism of the adhesion of Mfp’s onto wet surfaces was revealed to involve the oxidation of catechol units of Dopa to a certain extent (Rzepecki & Waite, 1991; Petrone, 2013; Bandara et al., 2013) leading to crosslinking by a coupling phenomenon, and the reaction of free amines of the Mfp’s with carbonyl groups of the dopaquinones via Michael reaction (Fig.3). However, a balance between cohesive and adhesive bonding interactions is requisite for an optimum adhesion. Lower Dopa oxidation and higher crosslinking extent secure this balance. Besides, polar surfaces undergo greater adhesion than the non-polar ones. In vivo, catecholase enzyme promoted the dopaquinone formation in the course of underwater adhesion. Yet, naked catechol functionality, that is the reduced form, implies an adhesion enhancing. Hence, as may be remarked, a clear-cut mechanism in this wet bioadhesiveness seems to be far-reached as several species and sites are involved.

The force or energy of adhesion of mussel foot proteins to several adherends including mica, glass, silica, metals (Au, Ag, Pt, and Pd), metal oxides (Al2O3, Cr2O3, TiO 2, Ta2O5, Nb2O5, ZrO2, Fe 2O3), quartz, slate, ceramic, wood, skin, and synthetic polymers (PVC, PS, PMMA, polyurethane, PET, teflon, silicone rubber, etc.) can be experimentally assessed (Moulay, 2014). For example, adhesion energy Wadh for joining two mica surfaces with Mfp-3 was in the range of 0.03-14mJ/m2, depending on the pH. The adhesion force, that is the interaction extent between the Mfp’s and the surfaces, is tightly linked to the backbone flexibility of proteins and to the chemical nature of the surfaces; the differences in the interaction were imputed to several mechanism including electrostatic, hydrogen bonding, hydrophobic interactions, cation- interaction, - stacking, and metal-complexation.

In vitro, crosslinking of the Mfp’s shown in Fig.3 during adhesion process could be achieved using several kinds of crosslinkers or crosslinking inducers such as periodate ion (IO4-) and cupric ion (Cu2+). The oxidation of catechol unit of Dopa, a transformation that occurs to a certain extent during the adhesion, could occur either by raising the pH of the medium (auto-oxidation) or by using periodate ion acting as an oxidizing agent; the strength of adhesion Mfp-3 to mica surface decreased either with increasing pH or when periodate solution was added. Clearly, the dopaquinone formation would lessen the adhesion significantly. Mfp-1 proteins, highly positively charged polyelectrolytes and devoid of cohesive tendency, were useless in adhering two mica surfaces but effective when experimented in the presence of low Fe(III) aqueous solution concentrations; this was imputed to the bridging formation between Mfp-1 layers, resulting from iron(III)-catechol units complexation (chelation). In fact, the adhesion energy Wadh was measured in the range of 4.3 mJ/m2 , a value greater than that for Mfp-3/mica (0.6 mJ/ m2) (Zeng et al., 2010).

Mussel-bioinspired adhesives

As a routine step in research, synthetic chemists put forward the lessons from nature and, without further delay, undertake the conception of Mfp-like materials for a better understanding of the Dopa-related adhesiveness. To this end, the basic strategy was either to attach covalently the catechol-containing molecule to a premade polymeric matrix (Fig.4), or to polymerize/copolymerize catechol-bearing monomers or catechol precursor monomers (Fig.5). Yet, stringent requirements for polymeric matrixes and for the latter monomers are to be in tune with coveted adhesive properties and biomimetic Mfp’s. For example, PEG is a versatile candidate for catechol functionalization because of its biocompatibility fulfillment, biodegradability, nontoxicity and non-immunogenicity, as required for its bioapplication.

Several polymers, synthetic and natural ones, were subjected to functionalization with catechol-containing molecules. Of these polymers there are hyaluronic acid, chitosan, alginate, cellulose (cotton), poly(ethylene glycol) (PEG), polyethylenimine (PEI), polysiloxanes, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, butadiene and maleic anhydride copolymers, poly(ethylene glycol)-b-poly(2-methyl-2-carboxyl-propylene carbonate)-b-poly(Llactide), poly(methyl methacrylate)-poly(methacrylic acid)-poly(methyl methacrylate) (PMMA-PMAA-PMMA), poly(acrylic acid), and multi-walled carbon nanotubes (MWCNT). The grafting catechol-containing molecules involved in such functionalization were: Dopa, α-methylDopa, dopamine, α-methyldopamine, norepinephrine, 3,4-dihydroxyphenylpropionic acid, and 3,4-dihydroxyphenylacetic acid. Most importantly is the polymerization/copolymerization of a monomer having catechol/catechol precursor units as pictured in Fig.5. The catecholic monomers and co-monomers comprise the vinylics such as dopamine methacrylamide, N-(3,4-dihydroxyphenethyl) methacrylamide, N-2-(3′,4′-di-triethylsilyloxyphenyl)ethyl methacrylamide, N-dopamine methacrylamide, 3,4-di-trimethylsilyloxystyrene, 3,4-dimethoxystyrene, N-methacrylated Dopa, Dopa/ catechol-containing macromonomer, and the polyfunctional ones such as Dopa and derivatives, dopamine, norepinephrine, 3,4-dihydroxycinnamic acid, 3,4-dihydroxyhydrocinnamic acid.

For illustrative purpose, some related catechol-containing materials are herein cited. Dopamine-functionalized hyaluronic acid (Fig.6) proved to be efficient in building layer-by-layer (LBL) assembly on several synthetic polymer surfaces (PTFE, PET, PE, PC) (Lee et al., 2008; Messershmith et al., 2012); such assembly was ensured through the adhesivity of catechol groups. Dopa-functionalized poly(ethylene glycol) (Fig.7) showed good adhesive and mechanical properties upon treatment with Fe(III) solutions (Statz et al., 2006; Holten-Andersen et al., 2011); different catecholato-Fe3+ complexes were formed depending on the pH. LBL assembly could be built by chitosan and the conjugate dopamine-modified hyaluronic acid; the adhesion strength of the multilayer film obtained was in the range of 2.30 MPa, a high strength when compared to that for multilayer film made from chitosan and unmodified hyaluronic acid (0.75 MPa) (Neto et al., 2014).

A bioinspired adhesive could be made by mixing the demethylated lignin (after cellulose, lignin is ranked as a second component of plants) and polyethylenimine (PEI) in aqueous medium at pH = 10 (Liu & Li, 2006); the lap shear strength of the adhesive applied between two maple veneers attained ~1.5-2 MPa in wet experimental conditions. Materials made by polycondensation of caffeic acid (3,4-dihydroxycinnamic acid) and coumaric acid (4-hydroxycinnamic acid) exhibited an adhesion force, when applied to carbon and glass fibers, of about 7 MPa, a value matching that for superglue (Kaneko et al., 2011). Surface coating could be realized by polymerizing Dopa or dopamine, simply by immersion of the object into buffer solutions (pH = 8.5) of these two monomers, either in Tris-HCl (Tris = 2-amino-2-(hydroxymethyl)propane-1,3-diol) or phosphate buffer solution (PBS) (Na2HPO4 + KH2PO4) (Xi et al., 2009; Yu et al., 2010; Ku et al., 2010). The polyDopa and polydopamine were believed to have the structure shown in Fig.8, elucidating the catechol functionality that is responsible for coating formation. Poly(3,4-dihydroxystyrene-co-styrene) and poly(3,4-dihydroxystyrene-co-styrene-co-p-OEGstyrene) (OEG = oligo(ethylene glycol) ) were conceived as biomimetic mussel adhesives and were made by radical copolymerization of 3,4-dimethoxystyrene with styrene (Fig.9) (Westwood et al., 2007; Matos-Pérez et al., 2012; Matos-Pérez & Wilker, 2012; Jenkins et al., 2013; Meredith et al., 2014; Neto et al., 2013). Adhesion of poly(3,4-dihydroxystyrene-co-styrene) cured by treatment with oxidants such as Fe3+, IO4-, and Cr2O72-, onto aluminum substrate was estimated by lap-shear testing and was dependent on the curing agent; that is, for a composition of 3.4:96.6 of 3,4-dihydroxystyrene:styrene copolymer (average molecular weight Mn = 16 150 g/mol), the adhesion strength was 0.7, 0.9, and 1.2 MPa, respectively. Besides, the adhesion propensity is linked to the molecular weight of the copolymer. For a copolymer with molecular weight of Mn = 60 000 g/mol, the lap shear on Al substrate was about 11MPa, and those for commercial glues such as Elmer’s glue (polyvinyl acetate), superglue (ethylcyanoacrylate), and quickset glue (epoxy) were 3.8, 5 and 18 MPa, respectively.

Applications and outlook

In parallel with the advent of synthesis of the mussel-mimetic materials, a number of uses and applications have been shortly sighted and prospected, exploiting the valuable adhesiveness property of catechol functionality (Moulay, 2014), profitably in biomedicals (Mehdizadeh & Yang, 2013; Bouten et al., 2014).

Antifouling of microorganisms on wet surfaces such as ship hulls has been always of a primary concern. Thus, the undesirable biofouling could be prevented by applying a mussel-mimicking polymer (Statz et al., 2005; Dalsin & Messersnith, 2005; Yang et al., 2014), such as the one shown in Fig.10, as a coating onto the naked surfaces. The Dopa/catechol-modified PEG materials such as that shown in Fig.7, have an antifouling capacity against marine algae and bacterium when coated on Ti, Si, Au, Al, silicone surfaces, and show a resistance to protein adsorption such as human serum and fibrinogen and to mammalian cells.

Needless is to recall the merits of the hydrogels in our today’s items such as the biomedicals. Henceforth, the use of mussel-bioinspired polymers in hydrogels making is of an evitable consideration and highly appraised. Dopa/catechol-functionalized PEG’s form hydrogels upon their treatment with oxidizing agents such as sodium periodate; the hydrogel is the result of the o-quinones formation, generating the crosslinking therefrom. Although the robustness of hydrogel is usually sought for, Dopa groups in the formed hydrogels prove to be the key for setting the required mechanical moduli for biomedical uses; that is, storage modulus G’ of the order of 10 kPa and loss modulus G’’ of within 1 kPa. Some of these hydrogels are promising tissue sealants such as a fetal membrane sealant for iatrogenic preterm premature rupture of membranes (Haller et al., 2011; Brubaker & Messershmith, 2011). The injectable hydrogel engineered by crosslinking the polymer made by polycondensation of citric acid, PEG, and Dopa with sodium periodate in PBS buffer solution, is characterized by a suture propensity, a prompt healing of created incision (Mehdizadeh et al., 2012; Wilker, 2014). Another injectable hydrogel for healing the human tissue consists of catechol-conjugated chitosan and thiol-terminated Pluronic (Pluronic: copolymer composed of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) (Ryu et al., 2011).The hydrogel based on dopamine-functionalized alginate showed a capacity of healing the arterial incisions, and a drug-eluting for blood vessels and atherosclerotic plaques (Kastrup et al., 2012).

Dopa-grafted copolymers of butadiene and maleic anhydride were designed for itraconozale delivery (Schestopol et al., 2005). Catechol-tethered PEG stands as a cancer drug releasing propensity. Indeed, Bortezomib, a cancer drug, can be carried by this material and be delivered to enhance cytotoxicity against breast cancer cells (Su et al., 2011). Doxorubicin, another effective cancer drug, can be loaded in core-crosslinked micelles formed by dopamine-grafted triblock copolymer (monomethoxypoly(ethylene glycol)-b-poly(2-methyl-2-carboxyl-propylene carbonate)-b-poly(L-lactide)) (Wu et al., 2012). Polydopamine-coated substrates and catechol-modified ones could serve as carriers or immobilizers of biomolecules such as trypsin, heparin, bovine serum albumin (BSA), proteins, and DNA. The immobilization of polylysine on polydopamine-coated substrates helps culturing neurons that show good growth and formation of neuronal networks (Kang et al., 2011). Molecules bearing alkyl quaternary ammonium that can ensure an antimicorbial activity or a corrosion inhibition could be embedded as a co-monomer in copolymers of dopamine-based monomers such as dopamine methacrylamide; obviously, the latter secures the coating on substrate.

Adhesive property of the mussel catechol-grafted polymers has been valorized in nanoscience includingoptics, electronics, photonics, and medicine. Assembling nanoparticles (gold, silver, iron oxide, quantum dots) through catechol units can be accomplished by using catechol-modified polymers such as cotton and hyaluronic acid (Lee et al., 2010; Xu et al., 2011). The fact that the inhibitionof Escherichia coli is almost complete and its reduced rate using the polydopamine-cotton/Ag nanoparticles would hint at the potency of mussel-adhesive property of polydopamine. Peptide/hydroxyapatite nanocomposites, bone-like materials, can be assembled by coating them with polydopamine (Ryu et al. 2011). Electrodes of silicon nanoparticles bound with dopamine-grafted poly(acrylic acid) and dopamine-grafted alginate showed good galvanostatic properties (Ryou et al., 2013). It is possible to coat the nanotube-Ru(bpy) 32+ , an electrochemiluminescent sensor immobilized on glassy carbon electrode, with polydopamine to end up with hydrophilic sensing electrode (Xing & Yin, 2009)Uptake and release of dyes such as rhodamine 6G can be achieved using polydopamine nanocapsules (Yu et al., 2009). Encapsulation of gold and silver nanoparticles in Dopa-functionalized polymers shells is believed to occur via the reduction of the metal ions (Au3+, Ag+) by the redox property of catechol units of the Dopa groups (Black et al., 2011).

As far as polymer synthesis is concerned, research has been pointed to the use of Mfp-inspired materials in surface-initiated polymerization (SIP). First, the polymerization initiating species is chemically attached to catechol-containing molecule, and the thus-obtained system is anchored to a surface through the catechol functionality, and the initiation may start in the presence of a suitable monomer. Few examples are herein cited. SI-ATRP technique (SIP method coupled with atom transfer radical polymerization) was applied to the polymerization of methyl methacrylate (MMA) (Fan et al., 2006); to this end, the ATRP initiator was conceived as the one shown in Fig.11A and immobilized on TiO2 nanoparticles. Poly(methyl methacrylate)-grafted nanoparticle (Fig. 11B), a polymer functionalized metal oxide nanoparticle, was obtained. PMMA could be subsequently exfoliated from the metal oxide nanoparticle as desired. On the other hand, PMMA brushes were skillfully designed and elegantly made via the SI-ATRP method on macroscopic planar substrates and nanoscaled ones such as graphene oxide and carbon nanotubes, using a catechol-containing macro-initiator (Wei et al., 2012).

Another strategy is to apply catechol-bearing film on a surface and affix the initiating species. For example, anodic aluminum oxide membrane (AAO) as surface was first coated with polydopamine film, then, this film reacts with 2-bromoisobutyryl bromide, the initiator precursor for the SI-ATRP polymerization of acrylic acid as pictured in Fig. 12 (Wang et al., 2010).

NOTES

1. Reproduced by permission of Oxford University Press.

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