Advanced Surface Engineering Materials

By Ashutosh Tiwari and Rui Wang

Advanced surfaces enriches the high-throughput engineering of physical and chemical phenomenon in relatin to electrical, magnetic, electronics, thermal and optical controls, as well as large surface areas, protective coatings against water loss and excessive gas exchange. A more sophisticated example could be a highly selective surface permeability allowing passive diffusion and selective transport of molecules in the water or gases. The smart surface technology provides an interlayer model which prevents the entry of substances without affecting the properties of neighboring layers. A number of methods have been developed for coatings, which are essential building blocks for the top-down and/or bottom-up design of numerous functional materials. Advanced Surface Engineering Materials offers a detailed up-to-date review chapters on the functional coatings and adhesives, engineering of nanosurfaces, high-tech surface, characterization and new applications. 

The 13 chapters in this book are divided into 3 parts (Functional coatings and adhesives; Engineering of nanosurfaces; High-tech surface, characterization and new applications) and are all written by worldwide subject matter specialists.

The book is written for readers from diverse backgrounds across chemistry, physics, materials science and engineering, medical science, environmental, bio- and nano- technologies and biomedical engineering. It offers a comprehensive view of cutting-edge research on surface engineering materials and their technological importance.

• Language: English
• Publisher: Wiley
• Release date: Sep 6, 2016
• ISBN: 9781119314189

Book preview

Preface

Advanced surfaces enrich the high-throughput engineering of physical and chemical phenomenon (e.g., electrical, magnetic, electronics, thermal and optical controls), large surface area, protective coatings against water loss and excessive gas exchange, etc. A more sophisticated example could be a highly selective surface permeability allowing passive diffusion and selective transport of molecules in the water or gases. Smart surface technology provides an interlayer model which prevents the entry of substances without affecting the properties of neighboring layers. A number of methods have been developed for coatings, which are essential building blocks for the top-down and/or bottom-up design of numerous functional materials. They own exclusive surface features in terms of new age device applications. This book, Advanced Surface Engineering Materials, offers detailed up-to-date chapters on functional coatings and adhesives, engineering of nanosurfaces, high-tech surface, characterization and new applications.

With the increasingly deep integration of present and emerging surface engineering technologies for new materials exploration, the last decade has witnessed an inspiring growth in research activities involving multidisciplinary knowledge innovation, some of which have already activated further market demand. Advanced Surface Engineering Materials, part of the Advanced Materials Series, provides a wide spectrum of readers with an overview and systematic knowledge of the aforementioned categories. With such a book in hand, one can easily figure out the methodology and essential rationales underlying every aspect of material innovations—from bio-inspired coating or polymer films to biosynthesis of nanomaterials and graphene, from carbon structures growth to deep-blue organic light-emitting diodes, or from latent biosensor application to high efficiency devices assembly—and the topic can be extended even to the modulation of enzymes or the assessment of plasma-material interactions for process safety. As a whole, this book constitutes a state-of–the-art review of the advances in surface engineering materials science and technology.

This book is written for readers from diverse backgrounds across the fields of chemistry, physics, materials science and engineering, medical science, environmental, bio- and nanotechnologies, and biomedical engineering. It offers a comprehensive view of cutting-edge research on surface engineering materials and their technological importance.

We would like to express our gratitude to all the contributors for their collective and fruitful work. It is their efforts and expertise that have made the monograph comprehensive, valuable and unique. We are also grateful to Drs. Sachin Mishra and Sophie Thompson, managing editors of the Advanced Materials Series, for their help and useful suggestions in preparing this book.

Editors

Ashutosh Tiwari, PhD, DSc

Rui Wang, PhD

Bingqing Wei, PhD

July 2016

Part 1

FUNCTIONAL COATINGS AND ADHESIVES

Chapter 1

Bio-inspired Coatings and Adhesives

Saurabh Das1,2,3 and B. Kollbe Ahn1,3*

1Marine Science Institute, University of California, Santa Barbara, CA 93106, USA

2Chemical Engineering, University of California, Santa Barbara, CA 93106, USA

3Materials Research Science and Engineering Center, University of California, Santa Barbara, CA 93106, USA

*Corresponding author: [e-mail]

Abstract

Biological organisms such as marine mussels have attracted attention as a paradigm of strong and versatile adhesion to hard surfaces under the severe chemical and physical environments of the wave-swept shores. Recent studies to understand the molecular mechanisms and mechanochemical aspects of mussel foot protein adhesion to different substrates have inspired the development of a variety of underwater adhesives, strain-resistant materials, hydrogels, self-healing polymers, and surfactants for tissue repair, drug delivery, anti-fouling coatings, and medical adhesives applications. In this chapter, we start to systematically discuss the physicochemical process at the molecular level during the attachment of mussel plaque to a substrate followed by the role of different amino acid residues in the attachment process. We then provide fundamental insights into the molecular architecture–function relationship for synthetic bio-inspired adhesives as well as begin to develop design principles for bio-inspired wet adhesives. This is followed by a thorough review of the recent development in mussel-inspired underwater polymer adhesive coatings and surfactant nano-adhesives that emphasizes the importance of the balance between electrostatic and hydrophobic interactions for wet adhesion and coacervation in addition to catecholic interactions, e.g., oxidative cross-linking, metal coordination, and intermolecular hydrogen bonding. We also shed light on intermolecular hydrogen bonding for surface-initiated underwater self-healing of polymers and metal-mediated cross-linking inspired from the mussel threads that provide sacrificial and reversible bonds at interfaces for strain-resistant materials.

Keywords: Mussel, wet, underwater, adhesives, coatings

1.1 Introduction

Nature has developed surprisingly elegant and diversified adaptations for the survival of the fittest organisms by a smart control of the interfacial forces and regulating surface interactions with the surroundings. For instance, geckos can cling and run with impeccable dexterity on most surfaces regardless of its roughness by controlling the frictional adhesion [1, 2] between its hierarchical fibrillar structures on the footpad and the surface. They avoid slip [3] during sticking and shearing of the nanosized spatula on a surface and employ van der Waals’ forces [4] to adhere to dry surfaces. Similarly, tree frogs that reside in the arboreal habitat of the wet rainforests take advantage of the capillary and viscous forces to prevent it from falling while running on surfaces [5, 6]. Currently, researchers in the wet adhesion community are spearheaded to solve the engineering challenge of wet underwater adhesion through mimicking techniques employed by the marine organisms such as barnacles [7, 8], pearl oysters [9, 10], minicollagens from sea anemones [11], sandcastle worms [12, 13], and the marine mussels [14, 15].

Harsh intertidal oceanic waves are no match for the mighty mussel that produces strong, flexible threads and cling to the surfaces of rocks, piers, and boats and even to other mussels without getting washed by the impact of water. This extraordinary ability of the mussels to adhere to any surface underwater has been baffling researchers for the past few decades. The adhesion mechanism used by the marine mussels has been extensively explored recently, and efforts have been made to develop coatings and adhesives for a variety of applications ranging from dental adhesives [16], self-assembled bilayer nano-adhesives [17], antifouling surfaces [18], self-healing polymers [19], drug delivery chaperons [20], medical glues [21], etc. Understanding the technique used by the mussels to prepare the surface for adhesion and the molecular mechanism underpinning the adhesive strength of the mussel glue (i.e., the mussel foot proteins or mfps) is fundamental to design synthetic mimics of the biological system.

1.2 The Interfacial Biochemistry of a Mussel Adhesive

Marine mussels are experts at ‘wet’ adhesion, achieving strong and durable attachment to a variety of surfaces in their marine habitat. Adhesion is mediated by a byssus, essentially a bundle of leathery threads that emerge from living mussel tissue at one end and tipped by flat adhesive plaques at the other (Figure 1.1). The byssal plaques consist of a complex array of proteins (mostlysix different mussel foot proteins, mfps 1–6), each of which has a distinct localization and function in the structure, but all share the unusual amino acid 3,4-dihydroxyphenylalanine (Dopa), a post-translational modification from tyrosine (Tyr or Y), that features prominently in mfps, ranging from less than 5 mol% in mfp-4 to 30 mol% in mfp-5 [24–29]. Mussels use its foot to make a snug contact with a target surface prior to depositing adhesive mfps in a fashion resembling injection molding [30]. The dimpled distal depression of the foot is positioned over a surface like an inverted rubber cup and compressed, thereby pushing out bulk water. Mfps are then secreted into the remaining gap from 8 to 10 pores in the depression ceiling [31].

Figure 1.1 Dr. Nadine Martinez (former graduate student at the University of California Santa Barbara, currently postdoc at the Stanford School of Medicines), picking up mussel from the wave-swept beach shore at UCSB campus point during low tide (Left, Photo credits: Saurabh Das). A mussel secured to a mineral surface (Right inset). Adhesive mfps such as mfp-3 (blue circles) and mfp-5 (green circles) binds the plaque to a mineral surface. In mussel byssal threads, collagens known as preCOLs mediate the transfer of load between the mussel plaque and the thread [22]. PreCOLs come within a few nm of the mica surface and thus may bind directly to adhesive mfps such as mfp-3 and mfp-5. The preCOLs are protected by a coating protein, mfp-1, that can accommodate high strains while simultaneously contributing to its disparate stiffness. (Adapted from Thesis [23]: Das, S. Bio-Inspired Adhesion, Friction and Lubrication, University of California, Santa Barbara, 2014, 226 pages; 3682889.)

Strong and durable adhesion is achieved despite the surrounding seawater at pH 8.2, high salt and saturating levels of dissolved O2. The interfacial pH at which mussels buffer the local environment during mfp deposition was determined using a pH-sensitive surface (e.g., mica functionalized with a fluorescent bilayer) to range from 2.2 to 3.3, which is well below the seawater pH of 8 [14]. The mussel foot significantly acidifies the interface during initial protein deposition (Figure 1.2). The role of acidification isto retard the oxidation of Dopa residues in the mfps for the formation of hydrogen bonds, metal–catechol coordination, or cation–π interactions with the surface to secure the proteins/plaque to the substrate. Deposition of adhesive proteins at acidic pH has important implications for mussel-inspired technology. The acidic pH allows delivery of the mfps to a surface as complex coacervate fluids; together with antioxidants [32], stabilizes the catecholic residue in the protein enabling the formation of electrostatic bonds with a mineral surface or coordination bonds with the surface oxides; favors the formation of cationic functionalities, e.g., Lys, Arg, and His for long-range attraction to electronegative surfaces. The acidic pH in combination with seawater (pH 8.2) serves as a switch for initiating protein insolubility, quinone-based cross-linking and catechol-mediated metal chelation (Figure 1.3).

Figure 1.2 Fluorescent images and intensities of the plaque substrate interface during plaque formation by juvenile mussels (<10 mm). Transmitted light images taken at t = 0 and t = 11.5 min, respectively, of an Oregon Green DHPE/DMPC-labeled mica surface during foot contact (a) and following foot disengagement (b) from the new plaques. Corresponding fluorescence images are in (c) and (d). Distal depression of the foot is highlighted by a red circle (A = 2.7 × 10⁴ µm², diameter ~209 µm). (e) Change in normalized fluorescence intensity (I) after disengagement of foot from plaque and direct equilibration with seawater. (f) Change in normalized fluorescent intensity (right axis) and pH (left axis) during actual mussel foot–surface contacts (shaded gray area) which typically lasted ≤180 s in juvenile mussels. This figure has been adapted from Ref. [14].

Figure 1.3 Mussels adhesive plaque formation on a pH-sensitive mica surface depicting chemistry under reducing (acidic pH) and oxidizing (neutral to slightly alkaline pH) conditions. (a) Mussel Mytilus californianus with extended foot and a single completed plaque and thread. (b) Foot contact with a mica surface evicts seawater from the distal depression and lowers the pH to ~2.2. (c) The foot disengages from the surface, and a plaque is deposited. The uncross-linked proteins at low pH interact with the mineral surface through bidentate catechol-mediated interactions. (d) Foot has disengaged from plaque allowing its equilibration with the ambient seawater. pH increase to pH 8 is linked directly and indirectly (via catechol oxidase) to formation of cross-links within the plaque. This figure has been adapted from Ref. [14].

The oxidation of Dopa to quinones and related products is highly favorable under high-pH conditions of the seawater and undermines the strength of protein adhesion to mineral surfaces [32, 33]. The acidic pH micro environment used by the mussels was proposed to limit mfp-Dopa oxidation, thereby enabling the catecholic functionalities to adsorb to surface oxides and provide a solubility switch for mfps, most of which aggregate at pH ≥ 7–8 [14] (Figure 1.3).

The mussel foot proteins, mfp-1, a coating protein [34] and, adhesive mussel foot proteins, mfp-3 [22] and mfp-5 [35], have been shown to exhibit remarkable binding to mineral surfaces such as mica, TiO2 [34, 36], and collagen [22], a biological protein that is abundant in bones, muscles, skin, and tendons, where it forms a scaffold to provide strength and structure. The versatility of mussel adhesion to surfaces with wide-ranging chemical and physical properties has inspired much research dedicated to understanding the mechanism of mussel adhesion as well as developing biomimetic coatings and adhesives for wide-ranging industrial and biomedical applications, the latter including paints for coronary arteries [37], fetal membrane sealants [38], cell encapsulants [39], and for securing transplants for diabetics [40].

Several studies with Dopa-functionalized polymers have demonstrated a strong positive linear correlation between Dopa content and adhesion to different surfaces [41–45]. In fact, the binding ability of the mfps to different substrates thus has been mostly attributed to the Dopa functionality in the protein, and the role of the other peptide residues in the adhesive properties of the protein remains elusive. Mfps such as mfp-3 [22] and mfp-5 [35] contain 20–30 mol% Dopa and are highly adhesive (e.g., adhesion energy, Wad ~ 8 mJ/m² for mfp-3 to ~14 mJ/m² for mfp-5 on mica) within narrowly defined solution conditions, e.g., low salt <150 mM ionic strength and pH~3–5. However, more recently, it was demonstrated that the adhesion (interaction of the protein with a different substrate) and cohesion (self-interaction between the proteins) of mfps is independent of the Dopa residues in the protein [46]. It was proposed that the mfps adhere to mineral surfaces through cation–π interactions between the aromatic residues in the protein and cations (e.g., potassium ions) adsorbed to the mineral surface rather than bidentate hydrogen bonding (Figure 1.4) [46]. This is a paradigm shift in our understanding of the molecular mechanisms underlying the adhesive properties of mfps and calls for further inquiry into the effects of peptide residues beyond Dopa chemistry.

A strongly bound stable hydration layer to a surface and/or adhesive polymer (or molecule) is thought to be a potential barrier that averts reliable adhesion of the polymer to the surface. Recently, dynamic nuclear polarization technique was used to demonstrate that hydrophobicity in the mfps mediates dehydration at substrate protein interface to allow force-free adhesion of the protein to a substrate mediated by Dopa [47]. It was also proposed that Dopa expedites the kinetics of bonding interactions between proteins and surfaces [46] or between polymers under wet environment [19]; however, the eventual strength of adhesion to a surface is more dependent on an optimal balance between the hydrophobic and electrostatic residues in the material [17, 48]. For instance, common amino acids, for example, cationic residues (lysine, K; histidine, H), anionic residues (aspartic acid, D), nonionic polar residues (asparagine, N), aromatic residues (tyrosine, Y; tryptophan, W), and nonpolar residues (alanine, A) are relevant to mfp adhesion and their role in wet adhesion of proteins and synthetic polymers are under increasing scrutiny. Synthetic polymers mimicking the properties of these other aromatic residues with an optimal balance between these different functional residues along with Dopa have recently been incorporated into polymers to achieve the strongest underwater adhesion to date for mussel-inspired polymer adhesives [48].

Figure 1.4 Representative force vs. distance plot showing the effect of contact time, tc, on the adhesion between (a) Dopa-less and (b) Dopa-containing recombinant mussel foot protein-1 (rmfp-1) films and a mica surface, respectively. The similar adhesion energies of Dopa-modified and Dopa-unmodified protein to mica suggest that cation-π interaction between the aromatic residues of the peptides in the protein and the K+ in the mica crystal lattice could possibly cause enhanced interaction between the protein and the surface, and bidentate bonds between Dopa and the polysiloxane lattice of mica might play a minor role in the adhesion [46]. This figure has been adapted from Ref. [46].

The adhesion of the natural and recombinant mussel foot proteins has been studied onto substrates other than mica such as collagen [22], silica [23, 49], silicones [23], and titania [41, 50], which are more relevant to biomedical applications. Single-molecule tensile tests using an atomic force microscope (AFM) where Dopa was tethered to a cantilever tip showed that Dopa contributes to nano-Newton adhesion on iron oxide, titania, and amine-functionalized surfaces [51]. The force–distance profiles and adhesion energies of mussel foot protein 3 (mfp-3) to TiO2 surfaces were measured at three different pHs (3, 5.5, and 7.5) using a surface forces apparatus (SFA) [50]. At low pH (3), mfp-3 showed the strongest adhesion force on TiO2, with adhesion energy of ~7.0 mJ/m². Increasing the pH gives rise totwo opposing effects: (1) increased oxidation of Dopa, thus decreasing availability for the Dopa-mediated adhesion, and (2) increased bidentate Dopa–Ti coordination (Figure 1.5), leading to the further stabilization of the Dopa group and, thus an increase inadhesion force. Both effects are reflected in the resonance-enhanced Raman spectra obtained at different deposition pHs. The two competing effects give rise to a higher adhesion force of mfp-3 on the TiO2 surface at pH 7.5 than at pH 5.5.

Figure 1.5 Possible modes of Dopa (catechol) binding to non-hydrated TiO2 surfaces. The catechol group can form molecular adsorption with (a) two hydrogen bonds, (b) monodentate adsorption with one hydrogen bond and one coordination bond, and (c) bidentate adsorption with two coordination bonds, although which form the Dopa binds to a TiO2 surface depends is pH dependent: at lower pH (<5.5), the molecular adsorption is preferred, and at higher pH (>7), the coordination charge transfer is more favorable. As marked, the red atoms are oxygen and the blue ones are titanium. (d) A summary of the adhesion energies of mfp-3 on TiO2 in different pH buffers. The adsorption of Dopa on TiO2 surface is highly pH dependent. At low pH, the protonated Dopa predominates, whereas at pH 7.5, there exists a mixture of both half- and fully deprotonated catecholates. This figure has been adapted from Ref. [50].

Mfps have an intriguing potential for repair of collagenous tissues and serves as an inspiration for the design of medical glues. In the byssal attachment, mfp-3 mediates adhesion between the collagens of the thread on one side and a foreign surface on the other (Figure 1.1). The adhesive mfps also show a strong intermolecular adhesive interaction collagen. The binding tendency of collagen to mfp-3 was investigated for the first time by preparing a molecular ‘sandwich’ consisting of mfp-3/collagen/mfp-3 system in an SFA study [22]. Collagen type 1α is effective at reversibly bridging the two mfp-3 films regardless of the oxidation state of Dopa. Extensive H-bonding, cation–π, and electrostatic interactions between mfp-3 and collagen were proposed for the strong interaction between the mussel foot proteins and collagen (Figure 1.6). This demonstrated the impeccable potential of the adhesive mfps to serve as an inspiration for medical glues and suture less medical glues for surgical applications. Similarly, silicone is among the most widely used medical implant material and biocompatible, understanding the binding mechanism of mfp mimetic polymers and proteins to silicone surfaces is of considerable theoretical and practical interests. Mfp mimetic peptides were also shown to bind strongly (Wad = 8 ± 1 mJ/m²) to silicone surfaces (Figure 1.6) through hydrophobic interactions [23] at a wide range of pHs through hydrophobic and specific columbic interactions [23]. The recent development of polymers and molecules inspired from the mussel foot protein will be discussed later in this chapter.

Figure 1.6 (a) H-bonding and cation–π interactions between collagen and mfp-3 mediate the strong but reversible binding between these molecules (adapted form Ref. [22]). Scheme of the surfaces analyzed by the SFA. (b) Rmfp-1 with or without Dopa is adsorbed as a thin film on one or both mica surfaces. (c) PDMS is grafted to an amino-functionalized SAM layer on one mica surface, and rmfp-1 with or without Dopa is adsorbed to the other mica surface. (d) Schematics of the electrostatic interactions, e.g., cation–π and series hydrogen bonding between the protein and mica surface (adapted from Ref. [23]).

1.3 Tough Coating Proteins in the Mussel Thread

Metal complexes function as reversible sacrificial bonds in several biological structures. For example, spider silk doped with transition metal ions demonstrates a significant increase in toughness through the formation of coordination complexes [52]. Similarly, the collagenous core of the marine mussel byssal threads exhibits remarkable toughness and self-healing and is stabilized by Dopa–Fe³+ complexes as opposed to typical collagen crosslinking chemistry [53]. All exposed portions of the byssus (plaque and the thread) that marine mussels use to attach to intertidal rocks are covered by a protective cuticle. Mussel foot protein-1 (mfp-1) is a natural coating protein found in the cuticle of the mussel byssus threads that makes the coating hard, yet extensible. The naturally occurring polymeric coatings of mussel byssus have a modulus of 2 GPa and strains of about 75% and 120% in Mytilus galloprovincialis (Mg) [54] and Mytilus californianus (Mc) [55, 56], respectively, making them among the most energy tolerant coatings known [57]. Natural composite coatings of marine mussels involve prefabrication of granules and matrix in secretory cells (Figure 1.7) of the accessory gland located in the foot.

Figure 1.7 Schematics of a mussel (left) with the byssus (thread and plaque) securing the mussel shell to a mineral surface. Scanning electron microscope (SEM) image of a thread (right) showing the probable failure sites due to frictional stresses at the respective interface. Site 1: Sand-cuticle interface, Site 2: Granule-matrix interface, and Site 3: thread interior (collagenous core)-cuticle matrix interface. The granules are the regions of high concentration of mfp-1 iron coordination complex. The figure has been adapted from Ref. [23].

During thread formation, the granule/matrix blend is released over the collagenous thread core in a process similar to injection molding. Upon equilibration with seawater, the blend matures into a hard coating crosslinked by coordination with Fe³+ complexes [54] (Figures 1.8 and 1.9). The reversible complexation of Fe³+ by the Dopa sidechains in mfp-1 in Mytilus californianus (Mc) and Mytilus edulis (Me) cuticle is responsible for its high extensibility as well as its stiffness due to the formation of sacrificial bonds that help to dissipate energy and avoid accumulation of stresses in the material [34, 46, 56]. The oxidation of catechol and reduction of Fe³+ occur at similar potentials (0.75 V) [58], and hence Fe³+ interactions with DOPA proteins could be via chelation, redox chemistry, or a combination of both [59].

Figure 1.8 (a) Molecular schematics of mfp-1 (Me) and mfp-1 (Mc) films on mica showing the interaction of the Dopa side chain with Fe³+. The multivalent Fe³+–Dopa complex is indicative of bis and/or tris mode of catecholato–Fe³+ coordination. The contribution of Lys and other amino acid residues is not shown in this figure. It should be noted that each mfp-1 molecule has ~100 Dopa residues, and we present only a few of them to demonstrate the mechanism of metal chelation for the sake of clarity. Representative force vs. distance plots showing the effect of contact time, tc, on the cohesion between two symmetric recombinant mfp-1 (rmfp-1) analog that contains (b) 2 and (c) 12 tandem repeats of the decapeptide sequence AKPSYPPTYK, which is the approximate decapeptide repeats in native mfp-1. Ferric cation does not induce Fe³+-mediated cohesion between the rmfp-1 films for the short decapeptide dimer of AKPSYPPTYK, since the cohesive force between [AKPSYPPTYK]2 does not change in Fe³+ environment unlike [AKPSYPPTYK]12. This figure has been adapted from Refs. [34, 46].

Figure 1.9 Basic model illustrating the cohesive role of Dopa–Fe complexes in the byssus cuticle. Granules contain a higher cross-link density than matrix. When the cuticle is stretched to less than 30% strain, the randomly coiled mfp-1 chains begin to unravel, and the granule and matrix deform equivalently. However, when stretched beyond 30% strain mfp-1 chains are largely unraveled, and microcracks form outside the granules because of the difference in cross-link density. When relaxed, the granule returns to its initial shape, whereas microcracks do not exhibit immediate recovery. This figure has been adapted from Ref. [56].

The mechanism of Fe³+ bridging of mfp-1 in the mussel thread coating from two homologous mussel species was recently determined through surface and bulk sensitive experimental techniques [34]. It was shown that, in the presence of Fe³+, mfp-1 from Mc molecule is inclined to collapse, whereas mfp-1 (intra-molecular chelation Fe³+) from Me reaches out to share Fe³+ with other mfp-1 (intermolecular chelation Fe³+) [34] (Figure 1.8). cDNA-deduced protein sequences of mfp-1 (Mc) [60] and mfp-1 (Me) [61] show that there is a subtle difference in the decapeptide repeat in the two proteins. An intriguing biological consequence of this is that the granules in the Mc mussel byssal cuticle are much smaller (~80%) than those in Me mussels (Me and its subspecies Mg) [55, 62] and able to withstand almost twice the strain of those in M. edulis [54, 55].

It was also demonstrated that Dopa in a peptide sequence does not necessarily lead to the formation of cross-links between protein films through metal chelation, and the length of the peptide is a crucial parameter for enabling metal-ion-mediated bridging between surfaces [46]. There is a critical number for the mfp-1 repeating decapeptide units between 2 and 12 necessary to trigger metal chelation between the protein films (Figure 1.8). The cohesion between the Dopa-modified mfp-1 films measured is comparable with biotin–avidin interfacial bond energy (Wad ~ 10 mJ/m²) [63], the strongest known non-covalent interaction between a protein and a ligand. Two to three Dopa residues of mfp-1 in the cuticle of the marine mussels complex with a single Fe³+ [56], thereby creating a stable complex that can, in principle, be translated to cross-link other structural proteins (Figure 1.9). These iron–protein complexes have a breaking force nearly half that of covalent bonds (as measured in our experiments), but unlike covalent bonds they can form and break reversibly, making them ideal for creating sacrificial cross-links to prevent catastrophic failure of a material. Thus, the coating protein on mussel threads provides an inspiration to fabricate polymeric coatings that are hard yet extensible to ‘iron-clad’ compatible surfaces.

1.4 Mussel-inspired Coatings and Adhesives

Metal chelation with the right molecular architecture of polymers can be used as a potential strategy to exploit mfp mimetic biomacromolecules at physiological pH for wet adhesive and coating applications. Recently, researchers have developed coatings, self-healing polymers, and adhesive materials based on metal coordination, hydrogen-bonding, and oxidative cross-linking properties of the natural mussel foot proteins. Surfaceadherent polydopamine films were deposited on noble metals, oxides, polymers, semiconductors, and ceramics surfaces using dopamine self-polymerization and were demonstrated to be a versatile platform to create a variety of adhesive layers, including self-assembled monolayers through deposition of long-chain molecular building blocks, metal films by electro-less metallization, and bio-inert and bioactive surfaces via grafting of macromolecules [64]. Similar strategies were used to prepare biomolecule-immobilized mussel-inspired polydopamine (PDA) coatings to improve the blood compatibility of broad ranges of substrates, e.g., nylon, cellulose, and polyethersulfone membrane surfaces [65]. Mussel-inspired PDA was coated onto differentsubstrates by pH-induced polymerization followed by the immobilization of Lys, Glu, and bovine serum albumin (BSA) onto the PDA layer via the Michael addition or the Schiff base reactions. This technique improves the blood compatibilityon surfaces and might be considered as a universal coating to modify the materials of body implant devices. Mussel-inspired PDA ad-layer on electrospun nanofibers was also shown to be promising and effective strategy for vascular tissue engineering that requires efficient endothelialization of graft surfaces [66] as well as for encapsulating individual living cell, e.g., yeast cells for application in cell based sensors and devices [39].

Acidity-dependent ferric ion–protein complexation provides essential properties for improving stiffness and self-healing of the material and is thought to be used by the mussels for fabricating the byssal threads as described previously in this chapter. Supramolecular polymers, i.e., polymers with small building blocks, inspired from the coating protein, mfp-1, of the marine mussels were cross-linked by metal ions and shown to exhibit strong mechanical properties and self-healing capabilities [67]. The Fe³+ polymer complexationwas shown to be fully reversible with mono Dopa–Fe³+ complex at low pH~3 to a tris complex at high pH~10. The ferric-polymer gel lost all of its mechanical strength (elasticity) upon lowering pH, while it rapidly recovered back its elasticity after increasing pH. Dopa-modified polyethylene glycol polymer (PEG-Dopa4) was tested to determine the catechol–Fe³+ interpolymer cross-linking induced by changing pH [68] for the development of mussel coating inspired self-healing coatings. Resonance Raman signature of catechol–Fe³+ cross-linked synthetic polymer gels at high pH was shown to be similar to that from native mussel thread cuticle. The iron-chelated polymer gels displayed elastic moduli that approach covalently cross-linked gels with self-healing properties. More recently, brick-and-mortar structure of nacre and catechol–ferric ion complexes in marine mussel adhesive fibers inspired the development of PDA nanocomposite films that (Figure 1.10) exhibited excellent fire-shielding properties [69]. PDA was used as super glue and clay nanosheets as bricks to fabricate nacre-like polydopamine-coated clay (D-clay) films using a vacuum filtration-assisted assembly method. Fe³+ ions were used to cross-link the nanosheets through diffusion of Fe³+ ions into the D-clay films inducing morphological rearrangement of the D-clay platelets. This resulted in a dense packing structure and strong cross-linking coordination bonds significantly enhancing mechanical properties for the D-clay/Fe³+ films. This novel material was demonstrated to be used as a polymer surface to provide fire-shielding applications. Thus, we see that the current development in the understanding and mimicking of the mussel-inspired metal-mediated cross-linking of polymers has led to the advancement in fabricating materials for strain-tolerant composite coatings for a variety of applications for engineering and biomedical usage.

Figure 1.10 Schematics showing conformation rearrangement of D-clay nanosheets upon Fe³+ ions diffusing into the interlayer of a D-clay film. This figure has been adapted from Ref. [69].

Mussels are known to adhere to surfaces by pH-induced curing of the proteins and intermolecular hydrogen bonding [14]. Self-healing hydrogels have been engineered by means of mussel-inspired metal-chelating catechol-functionalized polymers through reversible coordinate bond rupture formation as described in the previous section. Similarly, biological self-healing in wet conditions mediated via hydrogen bonding is also thought to be a plausible mechanism for self-healing in the mussel byssal cuticle [70]. This served as an inspiration for the synthesis of hydrogen-bond-mediated underwater self-healing synthetic polymers (Figure 1.11).

Figure 1.11 Schematic diagram of the steps entailed in polymer-rod healing studies. (a) Polymer rods (1; semi-rigid, blue rectangle; rigid, red rectangle) were processed as follows: bisected (2), immersed in H2O (pH 3 buffer) at room temperature (3), brought into contact (4), and pulled in tension (5). The blue (semi-rigid) and red (rigid) arrows denote the location of the healed incisions. (b) Self-healing scheme. (c) Average tensile strength of the samples (error bars indicate standard deviation, n = 3). Adapted from Ref. [19].

The self-mending of synthetic polyacrylate and polymethacrylate polymers in metal-free water that are surface-functionalized with mussel-inspired catechol was recently synthesized [19]. Wet self-mending of scission in these polymers is initiated and accelerated by the formation of extensive catechol-mediated interfacial hydrogen bonds and consolidated by the recruitment of other non-covalent interactions contributed by subsurface moieties through (Figure 1.11). High density of catechols at the exposed polymer interface was shown as compelling evidence for hydrogen-bond-initiated self-healing. It was also demonstrated that every catechol donor on a non-oxidized exposed polymer surface could hydrogen bond to a quinone (or oxidized catechols) acceptor on an apposing polymer surface (Figure 1.12). This configuration resulted in adhesion forces between the polymer interfaces comparable to or higher than those of symmetric catechol–catechol surfaces, where the catechols are both acceptors and donors (Figure 1.12).

Figure 1.12 Adhesion force between various polymeric surfaces with a contact time of 5 s and 250 mN of applied load. (a) Catechol surfaces are partially (I) or fully (II) oxidized to quinone surfaces by adding different concentrations (0.01–100 mM) of periodate. Error bars indicate standard deviation, n = 5. (b) Proposed interfacial chemistry for I, II, III, and IV.

Mussel wet adhesion has been a model for synthetic adhesives, but most research has considered the incorporation of Dopa or catechol functionalities into synthetic polymers. The specific role of other residues in the mussel protein sequence is yet to be considered for its role in the super wet adhesive properties of the mussel glue. Coacervation has been proposed as a biological process to compensate for the Dopa instability to oxidation, and mussel foot protein 3 slow (mfp-3s) has shown its capability of coacervation (liquid–liquid micro-phase separation) and was comparatively stable to oxidation [71]. Mfp-3s mimetic copolyampholytes, which – in addition to the catechol functionality – also comprise amphiphilic and ionic features characteristic of mfp-3s were synthesized (Figure 1.13) and the wet adhesive properties of the resulting polymer glue were characterized [48]. Aqueous solutions of two of the four mfp-3s-inspired copolymers were shown to coacervate-like spherical microdroplets (φ ~ 1–5 μm at pH ~4). The coacervates were stable to oxidation, coated mica substrates effectively, and strongly bonded mineral surfaces (pull-off strength: ~17.0 mJ/m²). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ~32.9 mJ/m² without oxidative cross-linking, and is ~8.8, ~2.4, and ~1.6 times greater than mfp-3s [22], mfp-5 (the most adhesive natural protein) [35], and the recently engineered mfp-amyloid protein [72], respectively. The mfp-3s-mimetic copolyampholyte has potential as a high-performance wet adhesive/coating with its very strong wet adhesion and stable coacervation. This study emphasized the importance of the balance between electrostatic and hydrophobic interactions for coacervation and wet adhesion in addition to catecholic interactions, e.g., oxidative cross-linking [14], metal coordination [34], and intermolecular hydrogen bonding [17].

Figure 1.13 Key features of mfps and synthetic homologs. (a) Primary sequence of mfp-3s. (b) Pie chart of key functionalities in mfp-3s and synthetic analogues: copolymer 1 (P1) to copolymer 4 (P4). (c) Chemical composition of a copolyacrylate with randomly distributed mfp-3s-mimetic functionalities. Polymer P2 showed the most superior coacervation and underwater adhesion properties due to a fair balance between the different functionalities, e.g., anionic, cationic, polar, non-polar, and the aromatic residues, in the polymer. Adapted from Ref. [48].

In the past decade, researchers have applied various synthetic chemistry approaches to form mussel-inspired adhesives by the incorporation of DOPA and other catechol derivatives into natural and synthetic polymers. The modification of polymers with catechol groups draws significant attention beyond adhesion due to the abilities of these groups to act as antioxidant agents, radical trappers, metal chelators, oxidizable reducing agents, etc. Although this approach has been successfully used for various important applications [73], one major challenge remains: the ability to present a high density of catechol functional groups in a defined ultrastructural organization and architecture at the nanoscale. A solution to meet this challenge was recently presented by harnessing the molecular self-assembly process to form well-ordered structures with a dense exposure of a Dopa interface [74, 75] (Figure 1.14).

Figure 1.14 Schematics of the self-assembly of the DOPA-containing small organic building blocks showing electron microscopy micrographs of the formed nanostructures. Modified from Ref. [74].

Inspired by the ability of short aromatic motifs to self-assemble into nanostructures, short aromatic DOPA-functionalized peptides were designed (Table 1.1) [74]. Peptide rational design was based on the substitution of phenylalanine (Phe) residues in known peptide motifs with DOPA, since the aromatic catechol group of DOPA will allow π-stacking interactions between DOPA groups, which contribute to molecular recognition and self-assembly. The novel DOPA-containing peptides self-assembled into ordered nanostructures. On a macroscopic level, one of the peptides formed a self-supporting hydrogel. The resulting hydrogel was characterized by a wide storage modulus (G’) range that could be dynamically modulated according to the final concentration of the peptide.

Table 1.1 Designed DOPA-containing peptides.

A key property of the self-assembled nanostructure is the repetitive and dense display of DOPA units. The nanostructures formed through self-assembly could reduce ionic silver to metallic silver resulting in a seamless metallic coating. The properties of this metal deposition were unique compared to any known electroless metal coating of biological or polymer nano-assemblies and should prove very useful in the templating of inorganic materials on organic surfaces at the nanoscale for various applications.

Another advancement in translating mussel’s wet adhesive capability into nano-adhesives for wet underwater applications was achieved by designing single-molecular low-molecular-weight zwitterionic molecules [17] (Figure 1.15). These molecules assembled into bilayers onto mica, silica, and metal oxide surfaces exposing the catechol residues to bind to an opposing surface. It reduced the complexity in synthetic low-molecular-weight catecholic zwitterionic molecules and showed very strong wet adhesion (~47 mJ/m², the highest value to date for mussel-inspired molecules) and coacervation. The catecholic zwitterions were adaptable for diverse surfaces as adhesives and at multiple length scales, for instance, it was remarkable in adhering nano-sized silica beads onto silicon surface effectively, a property essential for the fabrication of nickel ion batteries.

Figure 1.15 The key features of natural and translated mussel adhesion. (a) A mussel anchored by byssal threads and plaques to a rock in the intertidal zone. (b) Schematic of the distribution of different mussel foot proteins (mfp) in a plaque. (c) Primary sequence of mfp-5, S (green) denotes phosphoserine. (d) Pie chart of key functionalities in mfp-5. (e) One example (Z-Cat-C10) of a zwitterionic surfactant inspired by mfp-5. (f) Light micrograph image of liquid-phase separated Z-Cat-C10 at 100 mg ml–1 concentration. This figure has been adapted from Ref. [17].

The macro-length-scale adhesion application of the nano-zwitterionic adhesives were also demonstrated by measuring the adhesion of a lap joint, i.e., by gluing two steel plates immersed in water using the lower-dense coacervate (Figure 1.16). The lap joint (cross-sectional area = 2.54 cm × 2.54 cm) made in seawater and periodate solution, respectively, prevented the rupture of the joint for a load up to 3 N under water. On the other hand, the lap joint prepared in DI water held only 1 N. Standard three-point bending peel strength of the lap joint(cross-sectional area = 2.54 cm × 1.27 cm) was also measured (Figure 1.16). The peel strength of the joint made by gluing with Z-Cat-C10 in the aqueous periodate solution exhibited 20.5 N/cm (standard deviation = 0.9, n = 4) and >41.3 N/cm (standard deviation, n = 4) after dried 12 and 24 h, respectively, which are much stronger than the joint made in ambient dry condition by 3M double-side Scotch tape®, 12.0 N/cm (standard deviation = 0.1, n = 4). The results suggest the oxidative cross-linking is necessary to obtain high adhesion as shown in the SFAstudy [17]. Given their atomically smooth, thin (<4 nm), and strong glue layers, these zwitterions hold particular promise for nanofabrication. The nano-adhesive molecules significantly expand the scope of translation, particularly of combining Dopa with hydrophobic and electrostatic functionalities for tuning the performance of both coacervation and adhesion.

Figure 1.16 A macro-scale lab adhesion test: the phase-separated fluid at the bottom was collected with a syringe (a) and injected on to a top of steel-plated underwater (b), then glued onto the other plate (c), and the lab adhesion test (d, e, and f) after 12 h. (g) Demonstration of lap joint fracture test (apparent cross section = 2.54 cm × 2.54 cm). (h) Demonstration of standard three-point bending peel test (apparent cross section = 1.27 cm × 2.54 cm). Adapted from Ref. [17].

1.5 Conclusions and Future Research Avenues for Bio-inspired Adhesives and Coatings

A framework for understanding the conditions under which mussels deposit adhesive proteins was developed recently [14], and this will greatly aid the translation of mussel-inspired wet adhesives into functional synthetic materials. Of particular importance is the discovery that the mussels reduce the auto-oxidation of catechols by imposing a reduced pH at the site of protein deposition. It provides an increased understanding of the ways marine mussels tailor the local environment of the distal depression during plaque formation to prevent the auto-oxidation of Dopa residues. The insights gained from the mussels will aid in the development of strategies for deploying Dopa-based wet adhesives while retaining the adhesive functionality of redox-sensitive chemical groups. Dopa and Dopa analogues are redox-active chemical residues that lose their adhesive functionality following oxidation in hostile environments. The development of strategies for controlling this redox activity would greatly aid the implementation of Dopa-based moieties in wet adhesives. For instance, the adhesive and cohesive performances of Dopa-containing peptides can be controlled reversibly by electrochemically altering the redox state of Dopa. Similarly, the role of coacervation and hydrophobic shielding in increasing the oxidative stability of Dopa and catechol residues in synthetic molecules [17, 48] were inspired from the mfps [23, 71].

The novel findings showing that the adhesive and metal ion chelating performance of mfp peptides are dependent not only on the Dopa moieties but also on the peptide lengths and architectures [34, 46, 79] provides important design rules for the development of synthetic mimics of the natural protein. The model proposing series hydrogen bonding and/or cation–π interaction over bidentate hydrogen-bonding responsible for the adhesive strength of mussel mimetic wet adhesives [46] opens up new avenues for the design of bio-inspired underwater adhesives, paints, coatings, sealants, and so forth. Comprehensive characterization of the natural/recombinant proteins and their synthetic analogues will contribute to the development of a tunable system for applications in protective coatings, wet adhesives, and drug delivery.

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Chapter 2

Advancement of Surface by Applying a Seemingly Simple Sol–gel Oxide Materials

Justyna Krzak1*, Beata Borak1, Anna Łukowiak2, Anna Donesz-Sikorska1, Bartosz Babiarczuk1, Krzysztof Marycz3 and Anna Szczurek1

1Department of Mechanics, Materials Science and Engineering, Wrocław University of Science and Technology, Wrocław, Poland

2Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland

3Electron Microscopy Laboratory, University of Environmental and Life Sciences Wroclaw, Poland

*Corresponding author: [e-mail]

Abstract

The growing possibilities of surface modification which manifests in newly emerging technologies, newly-formed materials and possibility of combination of technologies and materials, gives scientists new tools for providing the ever increasing requirements of global markets. Advanced surface achieved by sol-gel coating technology gives possibilities to exploit surfaces in better ways. This chapter discusses sol-gel oxides such as TiO2, SiO2, ZrO2, ZnO2, Al2O3, hybrid coating materials, functionalized oxide coatings, coatings for cells and sol-gel materials as Interface Materials.

Keywords: Surface modification, functionalization, thin films, coatings, interlayers, hybrids, biomaterials, sealing coatings

2.1 Introduction

Achieving the proper adaptation of the material with the environment through enriching the top layer with suitable properties has been known for millennia. Looking for such adjustments, we found quite basic functions in the nature. There we can find matches such as tree bark providing mechanical, chemical, and thermal protection; leaves with large surface area providing absorption of light, and thus proper nutrition, or with a wax layer, acting as a protective coat against water loss and excessive gas exchange; or other examples of protective coatings essential for life – the cell wall, skin, carapace, etc. A more sophisticated example could be a blood–brain barrier (BBB) formed by brain endothelial cells which work as an interlayer between the circulating blood and the brain extracellular fluid in the central nervous system. Such an interlayer is highly selective permeability barrier allowing passive diffusion of, e.g., water or gases and selective transport of chosen molecules, e.g. glucose and amino acids. On the other hand, the interlayer prevents the entry of lipophilic substances that may act as potential neurotoxins. A simpler example of an interlayer is a glue layer that joins two elements with predetermined resistance and without affecting the properties of both materials.

Thus, relying on and following the very primary nature examples, coating of materials by properly selected layers has become a matching method without the necessity to change the whole structure of the final product. Coatings may increase stability or, moreover, improve the physical including mechanical, chemical, and biological features of well-known or entirely new structural materials. Thinking about possible changes introduced by a dedicated external layer, one should mention at least increasing thermal, mechanical, or chemical stability and wear protection, and also enhancing lifetime and durability, decreasing friction, or inhibiting corrosion. Moreover, nowadays thin layers have to possess additional functions, e.g. optical, biological, and electrical, which did not characterize a substrate material.

Different coating methods are used for materials in engineering laboratories and industrial laboratories. Some frequently used and well-known methods of changing surface layer properties are, among others, chemical or physical vapor deposition (CVD, PVD, and their modification), electrochemical deposition, pulsed laser deposition, magnetron sputtering, self-assembly, layer-by-layer coating, tubes by fiber templating (TUFT) process, or mechanical milling, chemical reduction, and also the sol–gel technique.

In most of the above cases, the limitation is the temperature of process (technologies commonly used in industry) and the shape of the coated substrate (preferably planar). A good solution for such problems is provided by the sol–gel method which gives the opportunity to work with complex shapes and allows to obtain a solid material at room temperature, so the temperature is safe even for compounds, molecules, and temperature-sensitive corpuscle, e.g. living cells [1].

Application of the sol–gel processes, which are mostly used for oxides fabrication, meets both the advanced requirements of emerging technologies and the expansion of demand for modern materials. Metal oxides, with a general formula MexOy, find very broad applications mainly due to their diverse properties that depend, among others, on metal ions, oxide structure, size of the particles (grains), and materials porosity. These materials have metal–oxygen bonds established, which are building oxides structure and undoubtedly affect the desirable thermal, optical, magnetic, electrical, biological, and other properties [2].

The following sections discuss the nature of sol–gel oxides, their usefulness, possibilities of application in the key branches of industry, and examples showing the sophistication of the sol–gel technology.