Elastomeric gradients: a hedge against stress concentration in marine holdfasts?

Determining hyperelastic properties of the constituents of the mussel byssus system

The mussel byssus system, comprising the adhesive plaque, distal thread, and proximal thread, plays a crucial role in the survival of marine mussels amongst ocean waves. Whilst recent research has explored the stress-strain behaviour of the distal thread and proximal thread through experimental approaches, little attention has been paid to the potential analytical or modelling methods within the current literature. In this work, analytical and finite element (FE) inverse methods were employed for the first time to identify the hyperelastic mechanical properties of both the plaque portion and the proximal thread. The results have demonstrated the feasibility of applied inverse methods in determining the mechanical properties of the constituents of the mussel byssus system, with the residual sum of squares of 0.0004 (N2) and 0.01 (mm2) for the proximal thread and the plaque portion, respectively. By leveraging mechanical and optical tests, this inverse methodology offers a simple and powerful means to anticipate the material properties for different portions of the mussel byssus system, thus providing insights into mimetic applications in engineering and material design.

Three-Dimensional Printed Shape Memory Gels Based on a Structured Disperse System with Hydrophobic Cellulose Nanofibers

Inks for 3D printing were prepared by dispersing bacterial cellulose nanofibers (CNF) functionalized with methacrylate groups in a polymerizable deep eutectic solvent (DES) based on choline chloride and acrylic acid with water as a cosolvent. After 3D printing and UV-curing, the double-network composite gel consisting of chemically and physically crosslinked structures composed from sub-networks of modified CNF and polymerized DES, respectively, was formed. The rheological properties of inks, as well as mechanical and shape memory properties of the 3D-printed gels, were investigated in dynamic and static modes. It was shown that the optimal amount of water allows improvement of the mechanical properties of the composite gel due to the formation of closer contacts between the modified CNF. The addition of 12 wt% water results in an increase in strength and ultimate elongation to 11.9 MPa and 300%, respectively, in comparison with 5.5 MPa and 100% for an anhydrous system. At the same time, the best shape memory properties were found for an anhydrous system: shape fixation and recovery coefficients were 80.0 and 95.8%, respectively.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

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.

High Performance Marine and Terrestrial Bioadhesives and the Biomedical Applications They Have Inspired

This study has reviewed the naturally occurring bioadhesives produced in marine and freshwater aqueous environments and in the mucinous exudates of some terrestrial animals which have remarkable properties providing adhesion under difficult environmental conditions. These bioadhesives have inspired the development of medical bioadhesives with impressive properties that provide an effective alternative to suturing surgical wounds improving closure and healing of wounds in technically demanding tissues such as the heart, lung and soft tissues like the brain and intestinal mucosa. The Gecko has developed a dry-adhesive system of exceptional performance and has inspired the development of new generation re-usable tapes applicable to many medical procedures. The silk of spider webs has been equally inspiring to structural engineers and materials scientists and has revealed innovative properties which have led to new generation technologies in photonics, phononics and micro-electronics in the development of wearable biosensors. Man made products designed to emulate the performance of these natural bioadhesive molecules are improving wound closure and healing of problematic lesions such as diabetic foot ulcers which are notoriously painful and have also found application in many other areas in biomedicine. Armed with information on the mechanistic properties of these impressive biomolecules major advances are expected in biomedicine, micro-electronics, photonics, materials science, artificial intelligence and robotics technology.

Principles of elastic bridging in biological materials

Load-bearing biological materials employ specialized elastic bridging regions to connect material parts with substantially different properties. While such bridging regions emerge in diverse systems of biological systems, their functional-mechanical origins are yet disclosed. Here, we hypothesize that these elastic bridging regions evolved primarily to minimize the near-interface stress effects in the biological material and, supported by experiments and simulations, we develop a simple theoretical model for such stress-minimizing bridging modulus. Our theoretical model describes well extensive experimental data of diverse biomechanical systems, suggesting that despite their compositionally distinct bridging regions, they share a similar mechanical adaptation strategy for stress minimization. The theoretical model developed in this study may directly serve as a design guideline for bio-inspired materials, biomedical applications, and advanced interfacial architectures with high resilience to mechanical failure. STATEMENT OF SIGNIFICANCE: Biological materials exhibit unconventional structural-mechanical strategies allowing them to attain extreme load-bearing capabilities. Here, we identify the strategy of biological materials to connect parts of distinct elastic properties in an optimal manner of stress minimization. Our findings are compatible with broad types of biological materials, including biopolymers, biominerals, and their bio-composite combinations, and may promote novel engineering designs of advanced biomedical and synthetic materials.

Microplastics can aggravate the impact of ocean acidification on the health of mussels: Insights from physiological performance, immunity and byssus properties

Ocean acidification may increase the risk of disease outbreaks that would challenge the future persistence of marine organisms if their immune system and capacity to produce vital structures for survival (e.g., byssus threads produced by bivalves) are compromised by acidified seawater. These potential adverse effects may be exacerbated by microplastic pollution, which is forecast to co-occur with ocean acidification in the future. Thus, we evaluated the impact of ocean acidification and microplastics on the health of a mussel species (Mytilus coruscus) by assessing its physiological performance, immunity and byssus properties. We found that ocean acidification and microplastics not only reduced hemocyte concentration and viability due to elevated oxidative stress, but also undermined phagocytic activity of hemocytes due to lowered energy budget of mussels, which was in turn caused by the reduced feeding performance and energy assimilation. Byssus quality (strength and extensibility) and production were also reduced by ocean acidification and microplastics. To increase the chance of survival with these stressors, the mussels prioritized the synthesis of some byssus proteins (Mfp-4 and Mfp-5) to help maintain adhesion to substrata. Nevertheless, our findings suggest that co-occurrence of ocean acidification and microplastic pollution would increase the susceptibility of bivalves to infectious diseases and dislodgement risk, thereby threatening their survival and undermining their ecological contributions to the community.

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.

The flying insect thoracic cuticle is heterogenous in structure and in thickness-dependent modulus gradation

The thorax is a specialized structure central to insect flight. In the thorax, flight muscles are surrounded by a thin layer of cuticle. The structure, composition, and material properties of this chitinous structure may influence the efficiency of the thorax in flight. However, these properties, as well as their variation throughout the thorax and between insect taxa, are not known. We provide a multi-faceted assessment of thorax cuticle for fliers with asynchronous (honey bee; Apis mellifera) and synchronous (hawkmoth; Manduca sexta) muscles. These muscle types are defined by the relationship between their activation frequency and the insect's wingbeat frequency. We investigated cuticle structure using histology, resilin distribution through confocal laser scanning microscopy, and modulus gradation with nanoindentation. Our results suggest that thorax cuticle properties are highly dependent on anatomical region and species. Modulus gradation, but not mean modulus, differed between the two types of fliers. In some regions, A. mellifera had a positive linear modulus gradient from cuticle interior to exterior of about 2 GPa. In M. sexta, modulus values through cuticle thickness were not well represented by linear fits. We utilized finite element modeling to assess how measured modulus gradients influenced maximum stress in cuticle. Stress was reduced when cuticle with a linear gradient was compressed from the high modulus side. These results support the protective role of the A. mellifera thorax cuticle. Our multi-faceted assessment advances our understanding of thorax cuticle structural and material heterogeneity and the potential benefits of material gradation to flying insects. STATEMENT OF SIGNIFICANCE: The insect thorax is essential for efficient flight but questions remain about the contribution of the exoskeletal cuticle. We investigated the microscale properties of the thorax cuticle, a crucial step to determine its role in flight. Techniques including histology, nanoindentation, and confocal laser scanning microscopy revealed that cuticle properties vary through cuticle thickness, by thorax region, and between species with asynchronous (honey bee; Apis mellifera) and synchronous (hawkmoth; Manduca sexta) muscles. This variation highlights the importance of high resolution cuticle assessment for flying insect lineages and points to factors that may (modulus gradation) and may not (mean modulus) contribute to different flight forms. Understanding material variation in the thorax may inform design of technologies inspired by insects, such as mobile micro robots.

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.

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.

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

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

Recent Advances in Mussel-Inspired Synthetic Polymers as Marine Antifouling Coatings

Synthetic oligomers and polymers inspired by the multifunctional tethering system (byssus) of the common mussel (genus Mytilus) have emerged since the 1980s as a very active research domain within the wider bioinspired and biomimetic materials arena. The unique combination of strong underwater adhesion, robust mechanical properties and self-healing capacity has been linked to a large extent to the presence of the unusual α-amino acid derivative l-DOPA (l-3,4-dihydroxyphenylalanine) as a building block of the mussel byssus proteins. This paper provides a short overview of marine biofouling, discussing the different marine biofouling species and natural defenses against these, as well as biomimicry as a concept investigated in the marine antifouling context. A detailed discussion of the literature on the Mytilus mussel family follows, covering elements of their biology, biochemistry and the specific measures adopted by these mussels to utilise their l-DOPA-rich protein sequences (and specifically the ortho-bisphenol (catechol) moiety) in their benefit. A comprehensive account is then given of the key catechol chemistries (covalent and non-covalent/intermolecular) relevant to adhesion, cohesion and self-healing, as well as of some of the most characteristic mussel protein synthetic mimics reported over the past 30 years and the related polymer functionalisation strategies with l-DOPA/catechol. Lastly, we review some of the most recent advances in such mussel-inspired synthetic oligomers and polymers, claimed as specifically aimed or intended for use in marine antifouling coatings and/or tested against marine biofouling species.

Evading the strength–ductility trade-off dilemma of rigid thermosets by incorporating triple cross-links of varying strengths

A new class of rigid thermosets with simultaneously enhanced strengths and ductilities have been successfully designed and synthesised.

Lipids as integral components in mussel adhesion

Lipids are fundamental components of cells in organisms. Recent studies reveal that lipids are also present in cell-free bioadhesives. Examples include barnacle cement, sea star footprints, hairy and smooth pads of insects and gecko setae. Whether reliance on lipids is universal in bioadhesion is not known. In the present study, we demonstrated, for the first time, the involvement of lipids in mussel adhesion. We extracted, identified and localized lipids in the byssal threads. The lipids were confirmed as fatty acids by gas chromatograpy mass spectrometry. δ13C measurements of the fatty acids in the byssus were also conducted. Results show that byssal fatty acids, with concentrations ranging from 1.10-2.51 mg g-1 by thread dry weight depending on the mussel species, are localized both on the surface of and inside the byssal thread and plaque. Over half of the fatty acids were loosely attached to the surface while a small portion were tightly bound to the byssus. Most of the surface fatty acids disappear within a week of thread deposition. δ13C values of byssal fatty acids show isotope fractionation suggesting that thread fatty acids are derived from the foot. It is possible that fatty acids are key players in expelling water and preparing the substrate surface for adhesion. Using lipids in the adhesion process might be a common strategy for organisms in need of temporary or permanent attachment. The process of lipid participation may be as important as adhesive components for developing more efficient man-made glues.

Composite Reinforcement by Magnetic Control of Fiber Density and Orientation

The flexural rigidity of cylindrical specimens, composed of epoxy reinforced by short, magnetized glass fibers, was enhanced using weak magnetic fields (<100 mT). By spatially controlling the magnitude and direction of the field, and thereby the torques and forces acting locally on the fibers, the orientation and concentration of the fillers in the matrix could be tuned prior to curing. Unidirectional alignment of the fibers, achieved using an air-core solenoid, improved the contribution of the fibers to the flexure modulus by a factor of 3. When a ring-shaped permanent magnet was utilized, the glass fibers were migrated preferentially near the rod boundary, and as a result, the contribution of the fibers to the flexure modulus doubled. The fiber length, density, and orientation distributions were extracted by μCT image analysis, allowing comparison of the experimental flexure modulus to a modified rule of mixtures prediction. The ability to magnetically control the fiber distribution in reinforced composites demonstrated in this study may be applied in the fabrication of complex micro- and macroscale structures with spatially variable anisotropy, allowing features such as crack diversion, strengthening of highly loaded regions, as well as economic management of materials and weight.

3D Printing of Materials with Tunable Failure via Bioinspired Mechanical Gradients

Mechanical gradients are useful to reduce strain mismatches in heterogeneous materials and thus prevent premature failure of devices in a wide range of applications. While complex graded designs are a hallmark of biological materials, gradients in manmade materials are often limited to 1D profiles due to the lack of adequate fabrication tools. Here, a multimaterial 3D-printing platform is developed to fabricate elastomer gradients spanning three orders of magnitude in elastic modulus and used to investigate the role of various bioinspired gradient designs on the local and global mechanical behavior of synthetic materials. The digital image correlation data and finite element modeling indicate that gradients can be effectively used to manipulate the stress state and thus circumvent the weakening effect of defect-rich interfaces or program the failure behavior of heterogeneous materials. Implementing this concept in materials with bioinspired designs can potentially lead to defect-tolerant structures and to materials whose tunable failure facilitates repair of biomedical implants, stretchable electronics, or soft robotics.

Metal-Tunable Self-Assembly of Hierarchical Structure in Mussel-Inspired Peptide Films

Bottom-up control over structural hierarchy from the nanoscale through the macroscale is a critical aspect of biological materials fabrication and function, which can inspire production of advanced materials. Mussel byssal threads are a prime example of protein-based biofibers in which hierarchical organization of protein building blocks coupled via metal complexation leads to notable mechanical behaviors, such as high toughness and self-healing. Using a natural amino acid sequence from byssal thread proteins, which functions as a pH-triggered self-assembly point, we created free-standing peptide films with complex hierarchical organization across multiple length scales that can be controlled by inclusion of metal ions (Zn²⁺ and Cu²⁺) during the assembly process. Additionally, analysis of film mechanical performance indicates that metal coordination bestows up to an order of magnitude increase in material stiffness, providing a paradigm for creating tunable polymeric materials with multiscale organizational structure.

The effect of shape on the fracture of a soft elastic gel subjected to shear load

For brittle solids, the fracture energy is the energy required to create a unit area of new surface through the process of division. For crosslinked materials, it is a function of the intrinsic properties like crosslinking density and bond strength of the crosslinks. Here we show that the energy released due to fracture can depend also on the shape of a joint made of this material. Our experiment involves two gel blocks connected via a thin gel disk. The disk is formed into different regular and exotic shapes, but with identical areas of cross-section. When one of the blocks is sheared with respect to the other, the shear load increases with vertical displacement, eventually causing a fracture at a threshold load. The maximum fracture load is different for different disks and among different regularly shaped disks, it is at a maximum for pentagon and hexagon shapes. The fracture energy release rate of the joint depends also on the aspect ratio (height/width) of the shapes. Our experiments also throw light on possible reasons for such a dependence on the shape of the joints.

Interspecies comparison of the mechanical properties and biochemical composition of byssal threads

Several bivalve species produce byssus threads to provide attachment to substrates, with mechanical properties highly variable among species. Here, we examined the distal section of byssal threads produced by a range of bivalve species (Mytilus edulis, Mytilus trossulus, Mytilus galloprovincialis, Mytilus californianus, Pinna nobilis, Perna perna, Xenostrobus securis, Brachidontes solisianus and Isognomon bicolor) collected from different nearshore environments. Morphological and mechanical properties were measured, and biochemical analyses were performed. Multivariate redundancy analyses on mechanical properties revealed that byssal threads of M. californianus, M. galloprovincialis and P. nobilis have very distinct mechanical behaviors compared to the remaining species. Extensibility, strength and force were the main variables separating these species groups, which were highest for M. californianus and lowest for P. nobilis. Furthermore, the analysis of the amino acid composition revealed that I. bicolor and P. nobilis threads are significantly different from the other species, suggesting a different underlying structural strategy. Determination of metal contents showed that the individual concentration of inorganic elements varies but that the dominant elements are conserved between species. Altogether, this bivalve species comparison suggests some molecular bases for the biomechanical characteristics of byssal fibers that may reflect phylogenetic limitations.

Biotechnological production of the mussel byssus derived collagen preColD

preColD, a mussel byssus derived structural protein with a central collagen, was successfully produced recombinantly in the yeast Pichia pastoris. It shows stable beta-sheet secondary structure (based on its silk-like terminal domains) and undergoes fibrillization as the natural preCols.

Recombinant mussel proximal thread matrix protein promotes osteoblast cell adhesion and proliferation

Background

von Willebrand factor (VWF) is a key load bearing domain for mamalian cell adhesion by binding various macromolecular ligands in extracellular matrix such as, collagens, elastin, and glycosaminoglycans. Interestingly, vWF like domains are also commonly found in load bearing systems of marine organisms such as in underwater adhesive of mussel and sea star, and nacre of marine abalone, and play a critical load bearing function. Recently, Proximal Thread Matrix Protein1 (PTMP1) in mussel composed of two vWF type A like domains has characterized and it is known to bind both mussel collagens and mammalian collagens.

Results

Here, we cloned and mass produced a recombinant PTMP1 from E. coli system after switching all the minor codons to the major codons of E. coli. Recombinant PTMP1 has an ability to enhance mouse osteoblast cell adhesion, spreading, and cell proliferation. In addition, PTMP1 showed vWF-like properties as promoting collagen expression as well as binding to collagen type I, subsequently enhanced cell viability. Consequently, we found that recombinant PTMP1 acts as a vWF domain by mediating cell adhesion, spreading, proliferation, and formation of actin cytoskeleton.

Conclusions

This study suggests that both mammalian cell adhesion and marine underwater adhesion exploits a strong vWF-collagen interaction for successful wet adhesion. In addition, vWF like domains containing proteins including PTMP1 have a great potential for tissue engineering and the development of biomedical adhesives as a component for extra-cellular matrix.

Mussels as a model system for integrative ecomechanics

Mussels form dense aggregations that dominate temperate rocky shores, and they are key aquaculture species worldwide. Coastal environments are dynamic across a broad range of spatial and temporal scales, and their changing abiotic conditions affect mussel populations in a variety of ways, including altering their investments in structures, physiological processes, growth, and reproduction. Here, we describe four categories of ecomechanical models (biochemical, mechanical, energetic, and population) that we have developed to describe specific aspects of mussel biology, ranging from byssal attachment to energetics, population growth, and fitness. This review highlights how recent advances in these mechanistic models now allow us to link them together across molecular, material, organismal, and population scales of organization. This integrated ecomechanical approach provides explicit and sometimes novel predictions about how natural and farmed mussel populations will fare in changing climatic conditions.

Multiscale structural gradients enhance the biomechanical functionality of the spider fang

The spider fang is a natural injection needle, hierarchically built from a complex composite material comprising multiscale architectural gradients. Considering its biomechanical function, the spider fang has to sustain significant mechanical loads. Here we apply experiment-based structural modelling of the fang, followed by analytical mechanical description and Finite-Element simulations, the results of which indicate that the naturally evolved fang architecture results in highly adapted effective structural stiffness and damage resilience. The analysis methods and physical insights of this work are potentially important for investigating and understanding the architecture and structural motifs of sharp-edge biological elements such as stingers, teeth, claws and more.

Crystallization and preliminary X-ray diffraction analysis of proximal thread matrix protein 1 (PTMP1) from Mytilus galloprovincialis

In order to deal with the dynamic ocean environment, blue mussels adhere to various surfaces via their collagenous byssal threads. PTMP1 (proximal thread matrix protein 1) is one identified matrix protein residing in the proximal thread and is capable of collagen binding. Its sequence comprises two von Willebrand factor type A-like repeats. In order to characterize the structure and domain architecture of PTMP1, recombinant protein was crystallized by vapour diffusion. The obtained crystals diffracted to 1.95 Å resolution and belonged to space group P2₁, with unit-cell parameters a=62.0, b=62.3, c=122.6 Å, β=102.2°. The Matthews coefficient suggested the presence of two monomers in the asymmetric unit and 48.3% solvent content.

Peeling off an adhesive layer with spatially varying topography and shear modulus

Inspired by recent experiments on hierarchically structured adhesives, we analyze here the effect of spatial variation in surface topography and shear modulus of an elastomeric adhesive on its ability to adhere strongly to a flexible contactor. The undulation of surface and modulus both were assumed to be periodic with periodicity, which is either identical or different for the two parameters; for identical periodicity, the phase lag between the respective undulations is also systematically varied. Calculations show that during continuous lifting of the flexible contactor from complete initial contact, the interfacial crack between the two adherents does not propagate continuously but intermittently, with crack arrest and initiation at the vicinity of minimum thickness and modulus of the layer; the torque required to initiate an arrested crack increases significantly over that required to propagate it on a smooth adhesive surface. The adhesion strength estimated from the corresponding force vs displacement plot is calculated to be higher than that achieved on a smooth and featureless adhesive surface. For in-phase variation in topography and shear modulus of the layer, the adhesive strength is found to be higher than for nonzero phase lag between the two parameters. The adhesion strength is found to diminish also for nonidentical periodicity between modulus and surface undulation. We have derived a scaling law for relating adhesion strength to several of these parameters.

Structural disorder and protein elasticity

An emerging class of disordered proteins underlies the elasticity of many biological tissues. Elastomeric proteins are essential to the function of biological machinery as diverse as the human arterial wall, the capture spiral of spider webs and the jumping mechanism of fleas. In this chapter, we review what is known about the molecular basis and the functional role of structural disorder in protein elasticity. In general, the elastic recoil of proteins is due to a combination of internal energy and entropy. In rubber-like elastomeric proteins, the dominant driving force is the increased entropy of the relaxed state relative to the stretched state. Aggregates of these proteins are intrinsically disordered or fuzzy, with high polypeptide chain entropy. We focus our discussion on the sequence, structure and function of five rubber-like elastomeric proteins, elastin, resilin, spider silk, abductin and ColP. Although we group these disordered elastomers together into one class of proteins, they exhibit a broad range of sequence motifs, mechanical properties and biological functions. Understanding how sequence modulates both disorder and elasticity will help advance the rational design of elastic biomaterials such as artificial skin and vascular grafts.