The formation of mussel byssus: anatomy of a natural manufacturing process

Coacervate or precipitate? Formation of non-equilibrium microstructures in coacervate emulsions

Non-equilibrium processing of aqueous polyelectrolyte complex (PEC) coacervates is critical to many applications. In particular, many coacervate-forming systems are known to become trapped in out-of-equilibrium states (e.g., precipitation). The mechanism and conditions under which these states form, and whether they age, is not clearly understood. Here, we elucidate the influence of processing on the PEC coarsening mechanism as it varies with flow during mixing for a model system of poly(allylamine hydrochloride) and poly(acrylic acid sodium salt) in water. We demonstrate that flow conditions can be used to toggle the formation of rough, precipitate-like aggregates of micron-scale PEC structures. These structures form at compositions with viscous-dominant equilibrium rheology, and observations of their formation via optical microscopy suggest that they comprise colloidal aggregates of PEC coacervate droplets. We further show that these aggregates exhibit micron-scale coarsening, with a mixing time-dependent characteristic aging time scale. The results show that the formation of precipitate-like structures is not solely determined by composition, but is instead highly sensitive to mass transport and colloidal instability effects. Our observations suggest that the details of mixing flow can provide non-equilibrium structural control of a broad range of PEC coacervate materials orthogonally to structure-property inspired polymeric design. We anticipate that these findings will open the door for future studies on the control of non-equilibrium PEC formation and structure.

A strong quick-release biointerface in mussels mediated by serotonergic cilia-based adhesion

The mussel byssus stem provides a strong and compact mechanically mismatched biointerface between living tissue and a nonliving biopolymer. Yet, in a poorly understood process, mussels can simply jettison their entire byssus, rebuilding a new one in just hours. We characterized the structure and composition of the byssus biointerface using histology, confocal Raman mapping, phase contrast-enhanced microcomputed tomography, and advanced electron microscopy, revealing a sophisticated junction consisting of abiotic biopolymer sheets interdigitated between living extracellular matrix. The sheet surfaces are in intimate adhesive contact with billions of motile epithelial cilia that control biointerface strength and stem release through their collective movement, which is regulated neurochemically. We posit that this may involve a complex sensory pathway by which sessile mussels respond to environmental stresses to release and relocate.

Dipeptide-Based Photoreactive Instant Glue for Environmental and Biomedical Applications

Nature-inspired smart materials offer numerous advantages over environmental friendliness and efficiency. Emulating the excellent adhesive properties of mussels foot proteins, where the lysine is in close proximity with the 3,4-dihydroxy-l-phenylalanine (DOPA), we report the synthesis of a novel photocurable peptide-based adhesive consisting exclusively of these two amino acids. Our adhesive is a highly concentrated aqueous solution of a monomer, a cross-linker, and a photoinitiator. Lap-shear adhesion measurements on plastic and glass surfaces and comparison with different types of commercial adhesives showed that the adhesive strength of our glue is comparable when applied in air and superior when used underwater. No toxicity of our adhesive was observed when the cytocompatibility on human dermal fibroblast cells was assessed. Preliminary experiments with various tissues and coral fragments showed that our adhesive could be applied to wound healing and coral reef restoration. Given the convenience of the facile synthesis, biocompatibility, ease of application underwater, and high adhesive strength, we expect that our adhesive may find application, but not limited, to the biomedical and environmental field.

Increase in Charge and Density Improves the Strength and Toughness of Mussel Foot Protein 5 Inspired Protein Materials

Mussel foot protein 5 (fp5) found in the adhesive byssal plaque of Mediterranean mussel Mytilus galloprovincialis exhibits exceptional underwater adhesion to diverse surfaces to the extent that adhesion strength typically exceeds the cohesive strength of the plaque. While sequence effects such as presence of charged residues, metal ion coordination, and high catechol content have been identified to govern fp5's interaction with surfaces, molecular contributors to its cohesive strength remain to be fully understood. Addressing this issue is critical for designing mussel-inspired sequences for new adhesives and biomaterials enabled by synthetic biology. Here we carry out all-atom molecular dynamics simulations on hydrated model fp5 biopolymer melts to understand how sequence features such as tyrosine and charge content affect packing density and inter-residue and ionic interaction strengths and consequently influence the cohesive strength and toughness. Systematic serine (S) substitutions for lysine (K), arginine (R) and tyrosine (Y) residues reveal that Y to S substitution surprisingly results in improvement of cohesive strength due to densification of the material by removal of steric hindrances, whereas the removal of charge in K and R to S substitutions has a detrimental impact on strength and toughness as it reduces cohesive interactions facilitated by electrostatic interactions. Additionally, melts formed from split fp5 sequences with only C or N terminal halves show distinct mechanical responses that further illustrate the role of charge. Our findings provide new insights for designing materials that could potentially surpass the performance of existing biomolecular and bioinspired adhesives, specifically by tailoring sequences for balancing charge and excluded volume effects.

Dynamic Crosslinked Injectable Mussel-Inspired Hydrogels with Adhesive, Self-Healing, and Biodegradation Properties

The non-invasive tissue adhesives with strong tissue adhesion and good biocompatibility are ideal for replacing traditional wound treatment methods such as sutures and needles. The self-healing hydrogels based on dynamic reversible crosslinking can recover their structure and function after damage, which is suitable for the application scenario of tissue adhesives. Herein, inspired by mussel adhesive proteins, we propose a facile strategy to achieve an injectable hydrogel (DACS hydrogel) by grafting dopamine (DOPA) onto hyaluronic acid (HA) and mixing it with carboxymethyl chitosan (CMCS) solution. The gelation time and rheological and swelling properties of the hydrogel can be controlled conveniently by adjusting the substitution degree of the catechol group and the concentration of raw materials. More importantly, the hydrogel exhibited rapid and highly efficient self-healing ability and excellent biodegradation and biocompatibility in vitro. Meanwhile, the hydrogel exhibited ~4-fold enhanced wet tissue adhesion strength (21.41 kPa) over the commercial fibrin glue. This kind of HA-based mussel biomimetic self-healing hydrogel is expected to be used as a multifunctional tissue adhesive material.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

Fabrication of Tunable Mechanical Gradients by Mussels via Bottom-Up Self-Assembly of Collagenous Precursors

Functionally graded interfaces are prominent in biological tissues and are used to mitigate stress concentrations at junctions between mechanically dissimilar components. Biological mechanical gradients serve as important role models for bioinspired design in technically and biomedically relevant applications. However, this necessitates elucidating exactly how natural gradients mitigate mechanical mismatch and how such gradients are fabricated. Here, we applied a cross-disciplinary experimental approach to understand structure, function, and formation of mechanical gradients in byssal threads─collagen-based fibers used by marine mussels to anchor on hard surfaces. The proximal end of threads is approximately 50-fold less stiff and twice as extensible as the distal end. However, the hierarchical structure of the distal-proximal junction is still not fully elucidated, and it is unclear how it is formed. Using tensile testing coupled with video extensometry, confocal Raman spectroscopy, and transmission electron microscopy on native threads, we identified a continuous graded transition in mechanics, composition, and nanofibrillar morphology, which extends several hundreds of microns and which can vary significantly between individual threads. Furthermore, we performed in vitro fiber assembly experiments using purified secretory vesicles from the proximal and distal regions of the secretory glands (which contain different precursor proteins), revealing spontaneous self-assembly of distinctive distal- and proximal-like fiber morphologies. Aside from providing fundamental insights into the byssus structure, function, and fabrication, our findings reveal key design principles for bioinspired design of functionally graded polymeric materials.

Mussel-Based Biomimetic Strategies in Musculoskeletal Disorder Treatment: From Synthesis Principles to Diverse Applications

Musculoskeletal disorders are the second leading cause of disability worldwide, posing a huge global burden to the public sanitation system. Currently, tissue engineering-based approaches act as effective strategies, which are, however, challenging in limited application scenarios. Mussel-based biomimetic materials, exhibit numerous unique properties such as intense adhesion, biocompatibility, moisture resistance, and injectability, to name only a few, and have attracted extensive research interest. In particular, featuring state-of-the-art properties, mussel-inspired biomaterials have been widely explored in innumerable musculoskeletal disorder treatments including osteochondral defects, osteosarcoma, osteoarthritis, ligament rupture, and osteoporosis. Nevertheless, a comprehensive and timely discussion of their applications in musculoskeletal disorders is insufficient. In this review, we emphasize on (1) the main categories and characteristics of mussel foot proteins and their fundamental mechanisms for the spectacular adhesion in mussels; (2) the diverse synthetic methods and modification of various polymers; and (3) the emerging applications of mussel-biomimetic materials, the future perspectives, and challenges, especially in the area of musculoskeletal disorder. We envision that this review will provide a unique and insightful perspective to improve the development of a new generation of mussel biomimetic strategies.

Divergent Effects of Ocean Warming On Byssal Attachment in Two Congener Mussel Species

Organisms rely on the integrity of the structural materials they produce to maintain a broad range of processes, such as acquiring food, resisting predators or withstanding extreme environmental forces. The production and maintenance of these biomaterials, which are often modulated by environmental conditions, can therefore have important consequences for fitness in changing climates. One well-known example of such a biomaterial is mussel byssus, an array of collagen-like fibers (byssal threads) that tethers a bivalve mollusk securely to benthic marine substrates. Byssus strength directly influences mortality from dislodgement, predation or competition and depends on the quantity and quality of byssal threads produced. We compared the temperature sensitivity of byssal attachment strength of two mussel species common to the west coast of North America, Mytilus trossulus and M. galloprovincialis, when exposed to seawater temperatures ranging from 10 to 24˚C in the laboratory. We found the two species attached equally strong in seawater ≤ 18˚C, but higher temperatures caused byssal thread production rate and quality (break force and extensibility) to be greatly reduced in M. trossulus and increased in M. galloprovincialis, leading to a 2 to 10-fold difference in overall byssus strength between the two species. Using this threshold value (18˚C), we mapped habitat for each species along the west coast of North America based on annual patterns in sea surface temperature. Estimated ranges are consistent with the current distribution of the two species and suggest a potential mechanism by which ocean warming could facilitate the northern expansion of M. galloprovincialis and displacement of native M. trossulus populations.

Viscoelastic analysis of mussel threads reveals energy dissipative mechanisms

Mussels use byssal threads to secure themselves to rocks and as shock absorbers during cyclic loading from wave motion. Byssal threads combine high strength and toughness with extensibility of nearly 200%. Researchers attribute tensile properties of byssal threads to their elaborate multi-domain collagenous protein cores. Because the elastic properties have been previously scrutinized, we instead examined byssal thread viscoelastic behaviour, which is essential for withstanding cyclic loading. By targeting protein domains in the collagenous core via chemical treatments, stress relaxation experiments provided insights on domain contributions and were coupled with in situ small-angle X-ray scattering to investigate relaxation-specific molecular reorganizations. Results show that when silk-like domains in the core were disrupted, the stress relaxation of the threads decreased by nearly 50% and lateral molecular spacing also decreased, suggesting that these domains are essential for energy dissipation and assume a compressed molecular rearrangement when disrupted. A generalized Maxwell model was developed to describe the stress relaxation response. The model predicts that maximal damping (energy dissipation) occurs at around 0.1 Hz which closely resembles the wave frequency along the California coast and implies that these materials may be well adapted to the cyclic loading of the ambient conditions.

The byssal-producing glands and proteins of the silverlip pearl oyster Pinctada maxima (Jameson, 1901)

Pinctada maxima are most well known for their production of high-quality natural pearls. They also generate another natural material, the byssus, an adhesive thread critical for steadfast attachment underwater. Herein, P. maxima byssal threads were analysed via proteotranscriptomics to reveal 49 proteins. Further characterisation was undertaken on five highly expressed genes: glycine-rich thread protein (GRT; also known as PUF3), apfp1/perlucin-like protein (Pmfp1); peroxidase; thrombospondin 1, and Balbiani ring 3 (BR3), which showed localised tissue expression. The spatial distribution of GRT and Pmfp1 via immunodetection combined with histology helped to identify glandular regions of the foot that contribute to byssal thread production: the byssal gland, the duct gland, and two thread-forming glands of basophilic and acidophilic serous-like cells. This work advanced primary knowledge on the glands involved in the creation of byssal threads and the protein composition of the byssus for P. maxima, providing a platform for the design of marine biopolymers.

Counterplotting the Mechanosensing-Based Fouling Mechanism of Mussels against Fouling

Marine organisms react to various factors when building colonies for survival; however, severe accumulation of diverse organisms on artificial structures located close to water causes large industrial losses. Herein, we identify a concept in the development of antifouling surfaces based on understanding the surface stiffness recognition procedure of mussel adhesion at the genetic level. It was found that on a soft surface the combination of decreased adhesive plaque size, adhesion force, and plaque protein downregulation synergistically weakens mussel wet adhesion and sometimes prevents mussels from anchoring, mainly due to transcriptional changes within the mechanosensing pathway and the adhesive proteins in secretory glands. In addition, the use of soft substrates or antagonists of surface mechanosensing behavior suppresses mussel fouling significantly.

Following the thread: Mytilus mussel byssus as an inspired multi-functional biomaterial

Over the last 15 years, the byssus of marine mussels (Mytilus spp.) has emerged as an important model system for the bio-inspired development and synthesis of advanced polymers and adhesives. But how did these seemingly inconsequential fibers that are routinely discarded in mussel hors d’oeuvres become the focus of intense international research. In the present review, we take a historical perspective to understand this phenomenon. Our purpose is not to review the sizeable literature of mussel-inspired materials, as there are numerous excellent reviews that cover this topic in great depth. Instead, we explore how the byssus became a magnet for bio-inspired materials science, with a focus on the specific breakthroughs in the understanding of composition, structure, function, and formation of the byssus achieved through fundamental scientific investigation. Extracted principles have led to bio-inspired design of novel materials with both biomedical and technical applications, including surgical adhesives, self-healing polymers, tunable hydrogels, and even actuated composites. Continued study into the byssus of Mytilid mussels and other species will provide a rich source of inspiration for years to come.

From vesicles to materials: bioinspired strategies for fabricating hierarchically structured soft matter

Certain organisms including species of mollusks, polychaetes, onychophorans and arthropods produce exceptional polymeric materials outside their bodies under ambient conditions using concentrated fluid protein precursors. While much is understood about the structure-function relationships that define the properties of such materials, comparatively less is understood about how such materials are fabricated and specifically, how their defining hierarchical structures are achieved via bottom-up assembly. Yet this information holds great potential for inspiring sustainable manufacture of advanced polymeric materials with controlled multi-scale structure. In the present perspective, we first examine recent work elucidating the formation of the tough adhesive fibres of the mussel byssus via secretion of vesicles filled with condensed liquid protein phases (coacervates and liquid crystals)-highlighting which design principles are relevant for bio-inspiration. In the second part of the perspective, we examine the potential of recent advances in drops and additive manufacturing as a bioinspired platform for mimicking such processes to produce hierarchically structured materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Protein-mediated bioadhesion in marine organisms: A review

Protein-mediated bioadhesion is one of the crucial physiological processes in marine organisms, by which they can firmly adhere to underwater substrates. Most marine adhesive organisms are biofoulers, causing negative effects on marine ecosystems and huge economic losses to aquaculture and maritime industries. Furthermore, adhesive proteins in these organisms are promising bionic candidates for high-performance artificial materials with great application value. In-depth understanding of the bioadhesion in marine ecosystems is of dual significance for resolving biofouling issue and developing marine bionic products. Here, we review the research progress of protein-mediated bioadhesion in marine organisms. The adhesion processes such as protein biosynthesis and secretion are similar among organisms, but the detailed features such as compositions, structures, and molecular functions of adhesive proteins are distinct. Hydroxylation, glycosylation, and phosphorylation are important post-translational modifications during the processes of adhesion. The contents of some amino acids such as glycine, tyrosine and cysteine involved in underwater adhesion are significantly higher, which is a sequence feature of barnacle cement and mussel foot proteins. The amyloid structures and conserved domains/motifs such as EGF and vWFA distributed in adhesive proteins are involved in the underwater adhesion. In addition, the oxidative cross-linking also plays an important role in marine bioadhesion. Overall, the unique and common features identified for the protein-mediated bioadhesion in diverse marine organisms here provide background information and essential reference for characterizing marine adhesive proteins and associated functional domains, formulating antifouling strategies, and developing novel biomimetic adhesives.

Collagen Pentablock Copolymers Form Smectic Liquid Crystals as Precursors for Mussel Byssus Fabrication

Protein-based biological materials are important role models for the design and fabrication of next generation advanced polymers. Marine mussels (Mytilus spp.) fabricate hierarchically structured collagenous fibers known as byssal threads via bottom-up supramolecular assembly of fluid protein precursors. The high degree of structural organization in byssal threads is intimately linked to their exceptional toughness and self-healing capacity. Here, we investigated the hypothesis that multidomain collagen precursor proteins, known as preCols, are stored in secretory vesicles as a colloidal liquid crystal (LC) phase prior to thread self-assembly. Using advanced electron microscopy methods, including scanning TEM and FIB-SEM, we visualized the detailed smectic preCol LC nanostructure in 3D, including various LC defects, confirming this hypothesis and providing quantitative insights into the mesophase structure. In light of these findings, we performed an in-depth comparative analysis of preCol protein sequences from multiple Mytilid species revealing that the smectic organization arises from an evolutionarily conserved ABCBA pentablock copolymer-like primary structure based on demarcations in hydropathy and charge distribution as well as terminal pH-responsive domains that trigger fiber formation. These distilled supramolecular assembly principles provide inspiration and strategies for sustainable assembly of nanostructured polymeric materials for potential applications in engineering and biomedical applications.

Genomics and Transcriptomics of the green mussel explain the durability of its byssus

Mussels, which occupy important positions in marine ecosystems, attach tightly to underwater substrates using a proteinaceous holdfast known as the byssus, which is tough, durable, and resistant to enzymatic degradation. Although various byssal proteins have been identified, the mechanisms by which it achieves such durability are unknown. Here we report comprehensive identification of genes involved in byssus formation through whole-genome and foot-specific transcriptomic analyses of the green mussel, Perna viridis. Interestingly, proteins encoded by highly expressed genes include proteinase inhibitors and defense proteins, including lysozyme and lectins, in addition to structural proteins and protein modification enzymes that probably catalyze polymerization and insolubilization. This assemblage of structural and protective molecules constitutes a multi-pronged strategy to render the byssus highly resistant to environmental insults.

Large-Scale Laboratory Experiments on Mussel Dropper Lines in Ocean Surface Waves

The rapid growth of marine aquaculture around the world accentuates issues of sustainability and environmental impacts of large-scale farming systems. One potential mitigation strategy is to relocate to more energetic offshore locations. However, research regarding the forces which waves and currents impose on aquaculture structures in such conditions is still scarce. The present study aimed at extending the knowledge related to live blue mussels (Mytilus edulis), cultivated on dropper lines, by unique, large-scale laboratory experiments in the Large Wave Flume of the Coastal Research Center in Hannover, Germany. Nine-months-old live dropper lines and a surrogate of 2.0 m length each are exposed to regular waves with wave heights between 0.2 and 1.0 m and periods between 1.5 and 8.0 s. Force time histories are recorded to investigate the inertia and drag characteristics of live mussel and surrogate dropper lines. The surrogate dropper line was developed from 3D scans of blue mussel dropper lines, using the surface descriptor Abbott–Firestone Curve as quality parameter. Pull-off tests of individual mussels are conducted that reveal maximum attachment strength ranges of 0.48 to 10.55 N for mussels that had medium 3.04 cm length, 1.60 cm height and 1.25 cm width. Mean drag coefficients of CD = 3.9 were found for live blue mussel lines and CD = 3.4 for the surrogate model, for conditions of Keulegan–Carpenter number (KC) 10 to 380, using regular wave tests.

Nanoparticles decrease the byssal attachment strength of the thick shell mussel Mytilus coruscus

The extensive application of nanoparticles (NPs) drives their release into the ocean, which may pose a potential threat to marine organisms. Although the byssus is important for the survival of mussels, the effects of NPs on byssal attachment and the underlying molecular byssal responses remain largely unknown. Therefore, the impacts of three metal oxide NPs (nTiO₂, nZnO, and nFe₂O₃) on the production and mechanical properties of byssal thread in the thick shell mussel M. coruscus were investigated in this study. The results showed that both mechanical properties (such as strength and extensibility) and morphology (diameter and volume) of byssal thread newly produced by M. coruscus were significantly affected after NP exposure, which resulted in an approximately 60-66% decrease in mussel byssal attachment strength. Downregulated expression of genes encoding mussel foot proteins, precursor collagen proteins, and proximal thread matrix proteins was also detected in this study, and this alteration may be one of the reasons for the weakened mechanical properties of byssal threads after NP exposure. This study indicated that NP pollution may hamper byssal attachment of M. coruscus and thereby pose a severe threat to the health of mussel individuals and the stability of the intertidal ecosystem.

From Bioinspired Glue to Medicine: Polydopamine as a Biomedical Material

Biological structures have emerged through millennia of evolution, and nature has fine-tuned the material properties in order to optimise the structure-function relationship. Following this paradigm, polydopamine (PDA), which was found to be crucial for the adhesion of mussels to wet surfaces, was hence initially introduced as a coating substance to increase the chemical reactivity and surface adhesion properties. Structurally, polydopamine is very similar to melanin, which is a pigment of human skin responsible for the protection of underlying skin layers by efficiently absorbing light with potentially harmful wavelengths. Recent findings have shown the subsequent release of the energy (in the form of heat) upon light excitation, presenting it as an ideal candidate for photothermal applications. Thus, polydopamine can both be used to (i) coat nanoparticle surfaces and to (ii) form capsules and ultra-small (nano)particles/nanocomposites while retaining bulk characteristics (i.e., biocompatibility, stability under UV irradiation, heat conversion, and activity during photoacoustic imaging). Due to the aforementioned properties, polydopamine-based materials have since been tested in adhesive and in energy-related as well as in a range of medical applications such as for tumour ablation, imaging, and drug delivery. In this review, we focus upon how different forms of the material can be synthesised and the use of polydopamine in biological and biomedical applications.

Hierarchically-structured metalloprotein composite coatings biofabricated from co-existing condensed liquid phases

Complex hierarchical structure governs emergent properties in biopolymeric materials; yet, the material processing involved remains poorly understood. Here, we investigated the multi-scale structure and composition of the mussel byssus cuticle before, during and after formation to gain insight into the processing of this hard, yet extensible metal cross-linked protein composite. Our findings reveal that the granular substructure crucial to the cuticle’s function as a wear-resistant coating of an extensible polymer fiber is pre-organized in condensed liquid phase secretory vesicles. These are phase-separated into DOPA-rich proto-granules enveloped in a sulfur-rich proto-matrix which fuses during secretion, forming the sub-structure of the cuticle. Metal ions are added subsequently in a site-specific way, with iron contained in the sulfur-rich matrix and vanadium coordinated by DOPA-catechol in the granule. We posit that this hierarchical structure self-organizes via phase separation of specific amphiphilic proteins within secretory vesicles, resulting in a meso-scale structuring that governs cuticle function.

The mussel byssus cuticle is a wear-resistant and extensible metalloprotein composite. Here, the authors probed the cuticle nanostructure and composition before, during and after fabrication revealing a crucial role of metal-binding proteins that self-organize via liquid-liquid phase separation.

Cation-π Interactions and Their Contribution to Mussel Underwater Adhesion Studied Using a Surface Forces Apparatus: A Mini-Review

Mussel underwater adhesion is a model phenomenon important for the understanding of broader biological adhesion and the development of biomimetic wet adhesives. The catechol moiety of 3,4-dihydroxyphenyl-l-alanine (DOPA) is known to be actively involved in the mechanism of mussel underwater adhesion; however, other underwater adhesion mechanisms are also crucial. The surface forces apparatus (SFA) has often been used to explore the contributions of other mechanisms to mussel underwater adhesion; e.g., recent SFA-based nanomechanical studies have revealed that cation-π interactions, one of the strongest intermolecular interactions in water, are the pivotal interactions of adhesive proteins involved in underwater mussel adhesion. This mini-review surveys recent research on cation-π interactions and their contributions to strong mussel underwater adhesion, shedding light on some biological processes and facilitating the development of biomedical adhesives.

Fiber Formation from Liquid Crystalline Collagen Vesicles Isolated from Mussels

Marine mussels (Mytilus edulis) fabricate byssal threads, high-performance biopolymeric fibers, which exhibit exceptional toughness and self-healing capacity. These properties are associated with collagenous proteins in the fibrous thread core known as preCols that self-organize into a hierarchical semicrystalline structure. Threads assemble individually in a bottom-up process lasting just minutes via secretion of membrane bound vesicles filled with preCols. However, very little is understood about the details and dynamics of this assembly process. Here, we explore the hypothesis that preCols are stored within the vesicles in a liquid crystalline phase, which contributes to fiber assembly by preordering molecules. To achieve this, a protocol was developed for extracting and isolating intact preCol secretory vesicles in high yield and purity. Vesicles were characterized and were manipulated in vitro, clearly indicating the dynamic liquid crystalline nature of the proteins within. Moreover, mechanical shearing of vesicles led to formation of highly birefringent preCol fibers. These findings have relevance for efforts toward sustainable production of advanced polymeric materials, and possibly for engineering biomedical scaffolds based on collagenous proteins.

The benthic sea-silk-thread displacement of a sessile bivalve, Pinctada imbricata radiata (Leach, 1819) in the Arabian-Persian Gulf

A number of molluscs within the Class Bivalvia are defined by their ability to secrete fine silk like threads known as byssus which are used to anchor themselves to solid substrates. With relatively few exceptions the majority of these species remain in a sedentary state throughout their life attached via their byssal threads. However, observations of adult Pinctada imbricata radiata pearl oysters made during this study revealed this species' ability to implement active movement. Byssal threads were secreted in a sequence of attachment and detachment phases, which resulted in the active displacement of the oyster. The oyster was observed, in the laboratory over a 9 day period, travelling a distance of 28cm in a horizontal path. After horizontal displacement, a vertical climbing phase was observed until the oyster reached the water surface at which point the byssus was discarded and the animal dropped, drifting in accordance with water current intensity. It is possible that these adaptations of byssal use are a result of environmentally induced evolutionary change within P. i. radiata.

Enhanced Adhesion of Carbon Nanotubes by Dopamine Modification

According to the fact that gecko-inspired vertically aligned carbon nanotubes (VA-CNTs) exhibit ultrastrong adhesion, dopamine is utilized to make a modification to this traditional biomimetic material. The composite material is tested for adhesion performance under different environmental conditions by an atomic force microscope. The adhesion force of the modified VA-CNTs does not show obvious fluctuation during the gradual heating process; however, the material gains improved adhesion when increasing the ambient humidity. In addition, the modified CNTs show a stronger adhesion force than the original CNTs in their performance tests. The dopamine polymer has a good combination with CNTs, which is responsible for the aforementioned excellent performance. Overall, this modification method is simple, convenient, efficient, and environmentally friendly, which all indicates a promising future in its application. The modified CNTs are expected to be used for super-adhesion in harsh environments, as well as in the field of microelectronics.

In Silico Analysis of Pacific Oyster (Crassostrea gigas) Transcriptome over Developmental Stages Reveals Candidate Genes for Larval Settlement

Following their planktonic phase, the larvae of benthic marine organisms must locate a suitable habitat to settle and metamorphose. For oysters, larval adhesion occurs at the pediveliger stage with the secretion of a proteinaceous bioadhesive produced by the foot, a specialized and ephemeral organ. Oyster bioadhesive is highly resistant to proteomic extraction and is only produced in very low quantities, which explains why it has been very little examined in larvae to date. In silico analysis of nucleic acid databases could help to identify genes of interest implicated in settlement. In this work, the publicly available transcriptome of Pacific oyster Crassostrea gigas over its developmental stages was mined to select genes highly expressed at the pediveliger stage. Our analysis revealed 59 sequences potentially implicated in adhesion of C. gigas larvae. Some related proteins contain conserved domains already described in other bioadhesives. We propose a hypothetic composition of C. gigas bioadhesive in which the protein constituent is probably composed of collagen and the von Willebrand Factor domain could play a role in adhesive cohesion. Genes coding for enzymes implicated in DOPA chemistry were also detected, indicating that this modification is also potentially present in the adhesive of pediveliger larvae.

The impact of the sea anemone Actinothoe sphyrodeta on Mytilus galloprovincialis mussel cultivation (Galicia, Spain)

Marine mussel aggregations act as a substratum and refuge for many fouling species. Mussel cultivation in Galicia, Spain, is carried out on hanging ropes in subtidal systems. The fauna associated with this cultivation includes a large number of invertebrates that compete for space or food with the mussels, or use their clusters as a refuge from predators or water turbulence. Outbreaks of the epibiont anemone Actinothoe sphyrodeta have been reported in cultivated Galician mussels since 2013, but their impact has not been investigated rigorously. Here, the temporal and spatial variability of Actinothoe sphyrodeta on mussel shells throughout one year is presented. Sampling of mussel size, weight and byssus attachment strength allowed mussel tenacity (attachment strength relative to size) to be calculated. A higher presence of Actinothoe sphyrodeta correlated with lower mussel tenacity and greater biomass losses, suggesting that this species could be an economically important biofouling component.

Hypoxia weakens mussel attachment by interrupting DOPA cross-linking during adhesive plaque curing

Marine mussels (Mytilus spp.) attach to a wide variety of surfaces underwater using a network of byssal threads, each tipped with a protein-based adhesive plaque that uses the surrounding seawater environment as a curing agent. Plaques undergo environmental post-processing, requiring a basic seawater pH be maintained for up to 8 days for the adhesive to strengthen completely. Given the sensitivity of plaques to local pH conditions long after deposition, we investigated the effect of other aspects of the seawater environment that are known to vary in nearshore habitats on plaque curing. The effect of seawater temperature, salinity and dissolved oxygen concentration were investigated using tensile testing, atomic force microscopy and amino acid compositional analysis. High temperature (30°C) and hyposalinity (1 PSU) had no effect on adhesion strength, while incubation in hypoxia (0.9 mg l-1) caused plaques to have a mottled coloration and prematurely peel from substrates, leading to a 51% decrease in adhesion strength. AFM imaging of the plaque cuticle found that plaques cured in hypoxia had regions of lower stiffness throughout, indicative of reductions in DOPA cross-linking between adhesive proteins. A better understanding of the dynamics of plaque curing could aid in the design of better synthetic adhesives, particularly in medicine where adhesion must take place within wet body cavities.

Proteinaceous secretion of bioadhesive produced during crawling and settlement of Crassostrea gigas larvae

Bioadhesion of marine organisms has been intensively studied over the last decade because of their ability to attach in various wet environmental conditions and the potential this offers for biotechnology applications. Many marine mollusc species are characterized by a two-phase life history: pelagic larvae settle prior to metamorphosis to a benthic stage. The oyster Crassostrea gigas has been extensively studied for its economic and ecological importance. However, the bioadhesive produced by ready to settle larvae of this species has been little studied. The pediveliger stage of oysters is characterized by the genesis of a specific organ essential for adhesion, the foot. Our scanning electron microscopy and histology analysis revealed that in C. gigas the adhesive is produced by several foot glands. This adhesive is composed of numerous fibres of differing structure, suggesting differences in chemical composition and function. Fourier transformed infrared spectroscopy indicated a mainly proteinaceous composition. Proteomic analysis of footprints was able to identify 42 proteins, among which, one uncharacterized protein was selected on the basis of its pediveliger transcriptome specificity and then located by mRNA in situ hybridization, revealing its potential role during substrate exploration before oyster larva settlement.

Intertidal exposure favors the soft-studded armor of adaptive mussel coatings

The mussel cuticle, a thin layer that shields byssal threads from environmental exposure, is a model among high-performance coatings for being both hard and hyper-extensible. However, despite avid interest in translating its features into an engineered material, the mechanisms underlying this performance are manifold and incompletely understood. To deepen our understanding of this biomaterial, we explore here the ultrastructural, scratch-resistant, and mechanical features at the submicrometer scale and relate our observations to individual cuticular components. These investigations show that cuticle nanomechanics are governed by granular microinclusions/nanoinclusions, which, contrary to previous interpretations, are three-fold softer than the surrounding matrix. This adaptation, which is found across several related mussel species, is linked to the level of hydration and presumed to maintain bulk performance during tidal exposures. Given the interest in implementing transfer of biological principles to modern materials, these findings may have noteworthy implications for the design of durable synthetic coatings.

There is interest in the development of mussel inspired materials; however, this requires an understanding of the materials. Here, the authors report on an investigation into the properties of mussel cuticle from different species that challenges conventional wisdom about particle filled composites.

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

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

Mussel-Inspired Polyglycerol Coatings with Controlled Wettability: From Superhydrophilic to Superhydrophobic Surface Coatings

Facile approaches to substrate-independent surface coatings with special wettability properties, such as superhydrophobicity, superhydrophilicity, and superamphiphobicity, have been limited. To address this problem, we combined two separate biomimetic concepts of mussel-inspired adhesion and highly hierarchical lotuslike surface structures to develop a universal fabrication method for various superwetting systems on any kind of material. In this feature article, we summarize our work on mussel-inspired polyglycerol (MI-dPG) and its application in the area of superwetting interfacial materials. MI-dPG mimics not only the functional groups of mfp-5 but also their molecular weight and molecular structure, which results in strong and rapid adhesion to the substrate. Furthermore, the MI-dPG coating process provides precise roughness control. The construction of highly hierarchical and superhydrophilic structures was achieved either directly by pH-controlled aggregation or in combination with nanoparticles. Subsequent postmodification of these highly hierarchical structures with different fluorinated or nonfluorinated hydrophobic molecules yielded a surface with superhydrophobic and even superamphiphobic properties.

The purplish bifurcate mussel Mytilisepta virgata gene expression atlas reveals a remarkable tissue functional specialization

BACKGROUND

Mytilisepta virgata is a marine mussel commonly found along the coasts of Japan. Although this species has been the subject of occasional studies concerning its ecological role, growth and reproduction, it has been so far almost completely neglected from a genetic and molecular point of view. In the present study we present a high quality de novo assembled transcriptome of the Japanese purplish mussel, which represents the first publicly available collection of expressed sequences for this species.

RESULTS

The assembled transcriptome comprises almost 50,000 contigs, with a N50 statistics of ~1 kilobase and a high estimated completeness based on the rate of BUSCOs identified, standing as one of the most exhaustive sequence resources available for mytiloid bivalves to date. Overall this data, accompanied by gene expression profiles from gills, digestive gland, mantle rim, foot and posterior adductor muscle, presents an accurate snapshot of the great functional specialization of these five tissues in adult mussels.

CONCLUSIONS

We highlight that one of the most striking features of the M. virgata transcriptome is the high abundance and diversification of lectin-like transcripts, which pertain to different gene families and appear to be expressed in particular in the digestive gland and in the gills. Therefore, these two tissues might be selected as preferential targets for the isolation of molecules with interesting carbohydrate-binding properties. In addition, by molecular phylogenomics, we provide solid evidence in support of the classification of M. virgata within the Brachidontinae subfamily. This result is in agreement with the previously proposed hypothesis that the morphological features traditionally used to group Mytilisepta spp. and Septifer spp. within the same clade are inappropriate due to homoplasy.

Rapid self-assembly of complex biomolecular architectures during mussel byssus biofabrication

Protein-based biogenic materials provide important inspiration for the development of high-performance polymers. The fibrous mussel byssus, for instance, exhibits exceptional wet adhesion, abrasion resistance, toughness and self-healing capacity–properties that arise from an intricate hierarchical organization formed in minutes from a fluid secretion of over 10 different protein precursors. However, a poor understanding of this dynamic biofabrication process has hindered effective translation of byssus design principles into synthetic materials. Here, we explore mussel byssus assembly in Mytilus edulis using a synergistic combination of histological staining and confocal Raman microspectroscopy, enabling in situ tracking of specific proteins during induced thread formation from soluble precursors to solid fibres. Our findings reveal critical insights into this complex biological manufacturing process, showing that protein precursors spontaneously self-assemble into complex architectures, while maturation proceeds in subsequent regulated steps. Beyond their biological importance, these findings may guide development of advanced materials with biomedical and industrial relevance.

Mussels attach to rocks using a byssus, which possesses unique properties of adhesion, toughness and self-healing. Here, the authors explore the fabrication process of mussel byssus demonstrating the self-assembly of specific proteins into multi-scale organized structures using artificially induced byssus threads.

Mussel adhesion - essential footwork

Robust adhesion to wet, salt-encrusted, corroded and slimy surfaces has been an essential adaptation in the life histories of sessile marine organisms for hundreds of millions of years, but it remains a major impasse for technology. Mussel adhesion has served as one of many model systems providing a fundamental understanding of what is required for attachment to wet surfaces. Most polymer engineers have focused on the use of 3,4-dihydroxyphenyl-l-alanine (Dopa), a peculiar but abundant catecholic amino acid in mussel adhesive proteins. The premise of this Review is that although Dopa does have the potential for diverse cohesive and adhesive interactions, these will be difficult to achieve in synthetic homologs without a deeper knowledge of mussel biology; that is, how, at different length and time scales, mussels regulate the reactivity of their adhesive proteins. To deposit adhesive proteins onto target surfaces, the mussel foot creates an insulated reaction chamber with extreme reaction conditions such as low pH, low ionic strength and high reducing poise. These conditions enable adhesive proteins to undergo controlled fluid-fluid phase separation, surface adsorption and spreading, microstructure formation and, finally, solidification.

Does natural selection explain the fine scale genetic structure at the nuclear exon Glu-5' in blue mussels from Kerguelen?

The Kerguelen archipelago, isolated in the Southern Ocean, shelters a blue mussel Mytilus metapopulation far from any influence of continental populations or any known hybrid zone. The finely carved coast leads to a highly heterogeneous habitat. We investigated the impact of the environment on the genetic structure in those Kerguelen blue mussels by relating allele frequencies to habitat descriptors. A total sample comprising up to 2248 individuals from 35 locations was characterized using two nuclear markers, mac-1 and Glu-5', and a mitochondrial marker (COI). The frequency data from 9 allozyme loci in 9 of these locations were also reanalyzed. Two other nuclear markers (EFbis and EFprem's) were monomorphic. Compared to Northern Hemisphere populations, polymorphism in Kerguelen blue mussels was lower for all markers except for the exon Glu-5'. At Glu-5', genetic differences were observed between samples from distinct regions (F CT = 0.077), as well as within two regions, including between samples separated by <500 m. No significant differentiation was observed in the AMOVA analyses at the two other markers (mac-1 and COI). Like mac-1, all allozyme loci genotyped in a previous publication, displayed lower differentiation (Jost's D) and F ST values than Glu-5'. Power simulations and confidence intervals support that Glu-5' displays significantly higher differentiation than the other loci (except a single allozyme for which confidence intervals overlap). AMOVA analyses revealed significant effects of the giant kelp Macrocystis and wave exposure on this marker. We discuss the influence of hydrological conditions on the genetic differentiation among regions. In marine organisms with high fecundity and high dispersal potential, gene flow tends to erase differentiation, but this study showed significant differentiation at very small distance. This may be explained by the particular hydrology and the carved coastline of the Kerguelen archipelago, together with spatially variable selection at Glu-5'.

Interfacial tension of complex coacervated mussel adhesive protein according to the Hofmeister series

Complex coacervation is a liquid-liquid phase separation in a colloidal system of two oppositely charged polyelectrolytes or colloids. The interfacial tension of the coacervate phase is the key parameter for micelle formation and interactions with the encapsulating material. However, the relationship between interfacial tensions and various salt solutions is poorly understood in complex coacervation. In the present work, the complex coacervate dynamics of recombinant mussel adhesive protein (MAP) with hyaluronic acid (HA) were determined in the presence of Hofmeister series salt ions. Using measurements of absorbance, hydrodynamic diameter, capillary force, and receding contact angle in the bulk phase, the interfacial tensions of complex coacervated MAP/HA were determined to be 0.236, 0.256, and 0.287 mN/m in 250 mM NaHCOO, NaCl, and NaNO3 solutions, respectively. The sequences of interfacial tensions and contact angles of the complex coacervates in the presence of three sodium salts with different anions were found to follow the Hofmeister ordering. The tendency of interfacial tension between the coacervate and dilute phases in the presence of different types of Hofmeister salt ions could provide a better understanding of Hofmeister effects on complex coacervated materials based on the protein-polysaccharide system. This information can also be utilized for microencapsulation and adsorption by controlling intramolecular interactions. In addition, the injection molding dynamics of mussel byssus formation was potentially explained based on the measured interfacial tension of coacervated MAP.