Beating the heat--fast scanning melts silk beta sheet crystals

Beta-pleated-sheet crystals are among the most stable of protein secondary structures, and are responsible for the remarkable physical properties of many fibrous proteins, such as silk, or proteins forming plaques as in Alzheimer's disease. Previous thinking, and the accepted paradigm, was that beta-pleated-sheet crystals in the dry solid state were so stable they would not melt upon input of heat energy alone. Here we overturn that assumption and demonstrate that beta-pleated-sheet crystals melt directly from the solid state to become random coils, helices, and turns. We use fast scanning chip calorimetry at 2,000 K/s and report the first reversible thermal melting of protein beta-pleated-sheet crystals, exemplified by silk fibroin. The similarity between thermal melting behavior of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals is confirmed. Significance for controlling beta-pleated-sheet content during thermal processing of biomaterials, as well as towards disease therapies, is envisioned based on these new findings.

Highly stretchable electric circuits from a composite material of silver nanoparticles and elastomeric fibres

Conductive electrodes and electric circuits that can remain active and electrically stable under large mechanical deformations are highly desirable for applications such as flexible displays, field-effect transistors, energy-related devices, smart clothing and actuators. However, high conductivity and stretchability seem to be mutually exclusive parameters. The most promising solution to this problem has been to use one-dimensional nanostructures such as carbon nanotubes and metal nanowires coated on a stretchable fabric, metal stripes with a wavy geometry, composite elastomers embedding conductive fillers and interpenetrating networks of a liquid metal and rubber. At present, the conductivity values at large strains remain too low to satisfy requirements for practical applications. Moreover, the ability to make arbitrary patterns over large areas is also desirable. Here, we introduce a conductive composite mat of silver nanoparticles and rubber fibres that allows the formation of highly stretchable circuits through a fabrication process that is compatible with any substrate and scalable for large-area applications. A silver nanoparticle precursor is absorbed in electrospun poly (styrene-block-butadiene-block-styrene) (SBS) rubber fibres and then converted into silver nanoparticles directly in the fibre mat. Percolation of the silver nanoparticles inside the fibres leads to a high bulk conductivity, which is preserved at large deformations (σ ≈ 2,200 S cm(-1) at 100% strain for a 150-µm-thick mat). We design electric circuits directly on the electrospun fibre mat by nozzle printing, inkjet printing and spray printing of the precursor solution and fabricate a highly stretchable antenna, a strain sensor and a highly stretchable light-emitting diode as examples of applications.

Temperature and exposure dependence of hybrid organic-inorganic layer formation by sequential vapor infiltration into polymer fibers

The characteristic processing behavior for growth of a conformal nanoscale hybrid organic-inorganic modification to polyamide 6 (PA6) by sequential vapor infiltration (SVI) is demonstrated. The SVI process is a materials growth technique by which exposure of organometallic vapors to a polymeric material promotes the formation of a hybrid organic-inorganic modification at the near surface region of the polymer. This work investigates the SVI exposure temperature and cycling times of sequential exposures of trimethylaluminum (TMA) on PA6 fiber mats. The result of TMA exposure is the preferential subsurface organic-inorganic growth by diffusion into the polymer and reaction with the carbonyl in PA6. Mass gain, infrared spectroscopy, and transmission electron microscopy analysis indicate enhanced materials growth and uniformity at lower processing temperatures. The inverse relationship between mass gain and exposure temperature is explained by the formation of a hybrid layer that prevents the diffusion of TMA into the polymer to react with the PA6 upon subsequent exposure cycles. As few as 10 SVI exposure cycles are observed to saturate the growth, yielding a modified thickness of ∼75 nm and mass increase of ∼14 wt %. Removal of the inherent PA6 moisture content reduces the mass gain by ∼4 wt % at low temperature exposures. The ability to understand the characteristic growth process is critical for the development of the hybrid materials fabrication and modification techniques.

Towards high-performance bioinspired composites

Biological composites have evolved elaborate hierarchical structures to achieve outstanding mechanical properties using weak but readily available building blocks. Combining the underlying design principles of such biological materials with the rich chemistry accessible in synthetic systems may enable the creation of artificial composites with unprecedented properties and functionalities. This bioinspired approach requires identification, understanding, and quantification of natural design principles and their replication in synthetic materials, taking into account the intrinsic properties of the stronger artificial building blocks and the boundary conditions of engineering applications. In this progress report, the scientific and technological questions that have to be addressed to achieve this goal are highlighted, and examples of recent research efforts to tackle them are presented. These include the local characterization of the heterogeneous architecture of biological materials, the investigation of structure-function relationships to help unveil natural design principles, and the development of synthetic processing routes that can potentially be used to implement some of these principles in synthetic materials. The importance of replicating the design principles of biological materials rather than their structure per se is highlighted, and possible directions for further progress in this fascinating, interdisciplinary field are discussed.

Healable thermoset polymer composite embedded with stimuli-responsive fibres

Severe wounds in biological systems such as human skin cannot heal themselves, unless they are first stitched together. Healing of macroscopic damage in thermoset polymer composites faces a similar challenge. Stimuli-responsive shape-changing polymeric fibres with outstanding mechanical properties embedded in polymers may be able to close macro-cracks automatically upon stimulation such as heating. Here, a stimuli-responsive fibre (SRF) with outstanding mechanical properties and supercontraction capability was fabricated for the purpose of healing macroscopic damage. The SRFs and thermoplastic particles (TPs) were incorporated into regular thermosetting epoxy for repeatedly healing macroscopic damages. The system works by mimicking self-healing of biological systems such as human skin, close (stitch) then heal, i.e. close the macroscopic crack through the thermal-induced supercontraction of the SRFs, and bond the closed crack through melting and diffusing of TPs at the crack interface. The healing efficiency determined using tapered double-cantilever beam specimens was 94 per cent. The self-healing process was reasonably repeatable.

Mesoporous metal oxides by vapor infiltration and atomic layer deposition on ordered surfactant polymer films

Catalysis, chemical separations, and energy conversion devices often depend on well-defined mesoporous materials as supports or active component elements. Herein, we show that ordered assembled organic surfactant films can directly template porous inorganic solids with surface area exceeding 1000 m(2)/g by infusing the polymers with reactive inorganic vapors, followed by anneal. The specific surface area, pore size, chemical composition, and overall shape of the product material are tuned by choice of the polymer and precursor materials as well as the influsion and postinfusion treatment conditions. X-ray diffraction, infrared spectroscopy, and electron microscopy show that vapor infusion changes both the physical and chemical structure of the starting ordered polymer films, consistent with quantified trends in specific surface area and pore size distribution measured by nitrogen adsorption after film annealing. This method yields porous TiO(2) films, for example, that function as an anode layer in a dye-sensitized solar cell.

Stable anatase TiO₂ coating on quartz fibers by atomic layer deposition for photoactive light-scattering in dye-sensitized solar cells

Quartz fibers provide a unique high surface-area substrate suitable for conformal coating using atomic layer deposition (ALD), and are compatible with high temperature annealing. This paper shows that the quartz fiber composition stabilizes ALD TiO(2) in the anatase phase through TiO(2)-SiO(2) interface formation, even after annealing at 1050 °C. When integrated into a dye-sensitized solar cell, the TiO(2)-coated quartz fiber mat improves light scattering performance. Results also confirm that annealing at high temperature is necessary for better photoactivity of ALD TiO(2), which highlights the significance of quartz fibers as a substrate. The ALD TiO(2) coating on quartz fibers also boosts dye adsorption and photocurrent response, pushing the overall efficiency of the dye-cells from 6.5 to 7.4%. The mechanisms for improved cell performance are confirmed using wavelength-dependent incident photon to current efficiency and diffuse light scattering results. The combination of ALD and thermal processing on quartz fibers may enable other device structures for energy conversion and catalytic reaction applications.

Bioinspired layered composites based on flattened double-walled carbon nanotubes

Inspired by the layered hierarchical nano- and microstructures of natural nacre, flattened double-walled carbon nanotube (FDWCNT) reinforced epoxy composites are fabricated. Impressively, the prepared composites exhibit layered structures analogous to nacre, and the FDWCNT loading can reach 70 wt%, which results in superior mechanical properties that evidently outperform other existing materials.

Rough fibrils provide a toughening mechanism in biological fibers

Spider silk is a fascinating natural composite material. Its combination of strength and toughness is unrivalled in nature, and as a result, it has gained considerable interest from the medical, physics, and materials communities. Most of this attention has focused on the one to tens of nanometer scale: predominantly the primary (peptide sequences) and secondary (β sheets, helices, and amorphous domains) structure, with some insights into tertiary structure (the arrangement of these secondary structures) to describe the origins of the mechanical and biological performance. Starting with spider silk, and relating our findings to collagen fibrils, we describe toughening mechanisms at the hundreds of nanometer scale, namely, the fibril morphology and its consequences for mechanical behavior and the dissipation of energy. Under normal conditions, this morphology creates a nonslip fibril kinematics, restricting shearing between fibrils, yet allowing controlled local slipping under high shear stress, dissipating energy without bulk fracturing. This mechanism provides a relatively simple target for biomimicry and, thus, can potentially be used to increase fracture resistance in synthetic materials.

Solid freeform fabrication of designer scaffolds of hyaluronic acid for nerve tissue engineering

The field of tissue engineering and regenerative medicine will tremendously benefit from the development of three dimensional scaffolds with defined micro- and macro-architecture that replicate the geometry and chemical composition of native tissues. The current report describes a freeform fabrication technique that permits the development of nerve regeneration scaffolds with precisely engineered architecture that mimics that of native nerve, using the native extracellular matrix component hyaluronic acid (HA). To demonstrate the flexibility of the fabrication system, scaffolds exhibiting different geometries with varying pore shapes, sizes and controlled degradability were fabricated in a layer-by-layer fashion. To promote cell adhesion, scaffolds were covalently functionalized with laminin. This approach offers tremendous spatio-temporal flexibility to create architecturally complex structures such as scaffolds with branched tubes to mimic branched nerves at a plexus. We further demonstrate the ability to create bidirectional gradients within the microfabricated nerve conduits. We believe that combining the biological properties of HA with precise three dimensional micro-architecture could offer a useful platform for the development of a wide range of bioartificial organs.

Atomic layer deposition for nanofabrication and interface engineering

Atomic layer deposition (ALD) provides a tool for conformal coating on high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineering of surface properties. This Feature Article highlights the application of ALD in selected fields including photonics, SERS and energy materials. Specifically, the topics include fabrication of plasmonic nanostructures for the SERS applications, fabrication of 3-D nanoarchitectured photoanodes for solar energy conversions (dye-sensitized solar cells and photoelectrochemical cells), and coating of electrodes to enhance the cyclic stability and thus device life span of batteries. Dielectric coating for tailoring optical properties of semiconductor nanostructures is also discussed as exemplified by ZnO nanowires. Future direction of ALD in these applications is discussed at the end.

Nanoscale patterning of organosilane molecular thin films from the gas phase and its applications: fabrication of multifunctional surfaces and large area molecular templates for site-selective material deposition

A simple methodology to fabricate micrometer- and nanometer-scale patterned surfaces with multiple chemical functionalities is presented. Patterns with lateral dimensions down to 110 nm were made. The fabrication process involves multistep gas-phase patterning of amine, thiol, alkyl, and fluorinated alkyl-functional organosilane molecules using PDMS molds as shadow masks. Also, a combination process of channel diffused plasma etching of organosilane molecular thin films in combination with masked gas-phase deposition to fabricate multilength scale, multifunctional surfaces is demonstrated.

The critical role of water in spider silk and its consequence for protein mechanics

Due to its remarkable mechanical and biological properties, there is considerable interest in understanding, and replicating, spider silk's stress-processing mechanisms and structure-function relationships. Here, we investigate the role of water in the nanoscale mechanics of the different regions in the spider silk fibre, and their relative contributions to stress processing. We propose that the inner core region, rich in spidroin II, retains water due to its inherent disorder, thereby providing a mechanism to dissipate energy as it breaks a sacrificial amide-water bond and gains order under strain, forming a stronger amide-amide bond. The spidroin I-rich outer core is more ordered under ambient conditions and is inherently stiffer and stronger, yet does not on its own provide high toughness. The markedly different interactions of the two proteins with water, and their distribution across the fibre, produce a stiffness differential and provide a balance between stiffness, strength and toughness under ambient conditions. Under wet conditions, this balance is destroyed as the stiff outer core material reverts to the behaviour of the inner core.

A route to nanoscopic materials via sequential infiltration synthesis on block copolymer templates

Sequential infiltration synthesis (SIS), combining stepwise molecular assembly reactions with self-assembled block copolymer (BCP) substrates, provides a new strategy to pattern nanoscopic materials in a controllable way. The selective reaction of a metal precursor with one of the pristine BCP domains is the key step in the SIS process. Here we present a straightforward strategy to selectively modify self-assembled polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP thin films to enable the SIS of a variety of materials including SiO(2), ZnO, and W. The selective and controlled interaction of trimethyl aluminum with carbonyl groups in the PMMA polymer domains generates Al-CH(3)/Al-OH sites inside the BCP scaffold which can seed the subsequent growth of a diverse range of materials without requiring complex block copolymer design and synthesis.

Spider silk as a load bearing biomaterial: tailoring mechanical properties via structural modifications

Spider silk shows great potential as a biomaterial: in addition to biocompatibility and biodegradability, its strength and toughness are greater than native biological fibres (e.g. collagen), with toughness exceeding that of synthetic fibres (e.g. nylon). Although the ultimate tensile strength and toughness at failure are unlikely to be limiting factors, its yield strain of 2% is insufficient, particularly for biomedical application because of the inability to mimic the complex ultrastructure of natural tissues with current tissue engineering approaches. To harness the full potential of spider silk as a biomaterial, it is therefore necessary to increase its yield strain. In this paper, we discuss the means by which the mechanical properties of spider silk, particularly the yield strain, can be optimized through structural modifications.

Co-cross-linking silk matrices with silica nanostructures for robust ultrathin nanocomposites

We report on a novel assembly approach to fabricate ultrathin robust freely standing nanocomposite membranes. The materials are composed of a pre-cross-linked silk fibroin matrix with incorporated silica nanoparticles with silsesquioxane cores (POSS) or clay nanoplatelets. These reinforced silk membranes have enhanced mechanical properties as compared to traditional silk-based nanocomposites reported previously. Up to 6-fold and 8-fold increase in elastic modulus and toughness, respectively, were found for these nanocomposites. In contrast, traditional LbL-assembled nanocomposites showed only a 3-fold increase in mechanical strength. The silk nanocomposites obtained also revealed excellent optical transparency in the visible region especially if reinforced with POSS nanoparticles, which suggests their utility as low cost, nontoxic, and easily scalable reinforced biomaterials for mechanically demanding applications.

Cooperative deformation of hydrogen bonds in beta-strands and beta-sheet nanocrystals

Beta-sheet protein domains are stabilized by weak hydrogen bonds, yet materials such as silk--whose ultimate tensile strength is controlled primarily by this secondary structure--can exceed the ultimate tensile strength of steel. Earlier work has suggested that this is because hydrogen bonds deform cooperatively within small protein domains to reach the maximum strength. Here we study the atomistic mechanism of this concerted deformation mechanism by applying an elastic structural model, used to solve the deformation field of the chemical bonds in beta-sheet nanostructures under stretching and thereby identify the number of hydrogen bonds that deform cooperatively. Through this analysis, we predict the optimal beta-strand and beta-sheet nanocrystal size associated with reaching the maximum usage of hydrogen bonds under loading applied per unit material volume. Our results, albeit based on a simple model and analytical equations, quantitatively agree with results based on experimental and molecular-dynamics studies and provide physical insight into the underlying molecular mechanisms of weak bond cooperativity. A comparison with the size of hydrogen bond clusters in biology reveals excellent agreement with the cluster sizes predicted by our analysis, suggesting that perhaps the confinement of hydrogen bonds into nanoscale elements is a universal biological design paradigm that turns weakness to strength. The parameters used in this study could be modified and applied to other protein and polymer structures, which imply potential applications of our model in understanding the physics of deformation and failure in a broader range of biological and polymer materials, as well as in de novo biomaterial design.

Size dictates mechanical properties for protein fibers self-assembled by the Drosophila hox transcription factor ultrabithorax

The development of protein-based materials with diverse mechanical properties will facilitate the realization of a broad range of potential applications. The recombinant Drosophila melanogaster transcription factor Ultrabithorax self-assembles under mild conditions in aqueous buffers into extremely extensible materials. By controlling fiber diameter, both the mechanism of extension and the magnitude of the mechanical properties can be varied. Narrow Ultrabithorax fibers (diameter <10 μm) extend elastically, whereas the predominantly plastic deformation of wide fibers (diameter >15 μm) reflects the increase in breaking strain with increasing diameter, apparently due to a change in structure. The breaking stress/strain of the widest fibers resembles that of natural elastin. Intermediate fibers display mixed properties. Fiber bundles retain the mechanical properties of individual fibers but can withstand much larger forces. Controlling fiber size and generating fiber superstructures is a facile way to manipulate the mechanical characteristics of protein fibers and rationally engineer macroscale protein-based materials with desirable properties.

Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider

Background

Combining high strength and elasticity, spider silks are exceptionally tough, i.e., able to absorb massive kinetic energy before breaking. Spider silk is therefore a model polymer for development of high performance biomimetic fibers. There are over 41.000 described species of spiders, most spinning multiple types of silk. Thus we have available some 200.000+ unique silks that may cover an amazing breadth of material properties. To date, however, silks from only a few tens of species have been characterized, most chosen haphazardly as model organisms (Nephila) or simply from researchers' backyards. Are we limited to ‘blindly fishing’ in efforts to discover extraordinary silks? Or, could scientists use ecology to predict which species are likely to spin silks exhibiting exceptional performance properties?

Methodology

We examined the biomechanical properties of silk produced by the remarkable Malagasy ‘Darwin's bark spider’ (Caerostris darwini), which we predicted would produce exceptional silk based upon its amazing web. The spider constructs its giant orb web (up to 2.8 m²) suspended above streams, rivers, and lakes. It attaches the web to substrates on each riverbank by anchor threads as long as 25 meters. Dragline silk from both Caerostris webs and forcibly pulled silk, exhibits an extraordinary combination of high tensile strength and elasticity previously unknown for spider silk. The toughness of forcibly silked fibers averages 350 MJ/m³, with some samples reaching 520 MJ/m³. Thus, C. darwini silk is more than twice tougher than any previously described silk, and over 10 times better than Kevlar®. Caerostris capture spiral silk is similarly exceptionally tough.

Conclusions

Caerostris darwini produces the toughest known biomaterial. We hypothesize that this extraordinary toughness coevolved with the unusual ecology and web architecture of these spiders, decreasing the likelihood of bridgelines breaking and collapsing the web into the river. This hypothesis predicts that rapid change in material properties of silk co-occurred with ecological shifts within the genus, and can thus be tested by combining material science, behavioral observations, and phylogenetics. Our findings highlight the potential benefits of natural history–informed bioprospecting to discover silks, as well as other materials, with novel and exceptional properties to serve as models in biomimicry.

Limitations of mass spectrometric methods for the characterization of polydisperse materials

This paper is a review of work on the characterization of coal liquids and petroleum residues and asphaltenes over several decades in which various mass spectrometric methods have been investigated. The limitations of mass spectrometric methods require exploration in order to understand what the different analytical methods can reveal about environmental pollution by these kinds of samples and, perhaps more importantly, what they cannot reveal. The application of mass spectrometry to environmental problems generally requires the detection and determination of the concentration of specific pollutants released into the environment by accident or design. The release of crude petroleum or coal liquids into the environment can be detected and tracked during biodegradation processes through specific chemicals such as alkanes or polyaromatic hydrocarbons (PAHs). However, petroleum asphaltenes are polydisperse materials of unknown mass range and chemical structures and, therefore, there are no individual chemicals to detect. It is necessary to determine methods of detection and the ranges of mass of such materials. This can only be achieved through fractionation to reduce the polydispersity of the initial sample. Comparison of mass spectrometric results with results from an independent analytical method such as size-exclusion chromatography with a suitable eluent is advisable to confirm that all the sample has been detected and mass discrimination effects avoided.

New opportunities for an ancient material

Spiders and silkworms generate silk protein fibers that embody strength and beauty. Orb webs are fascinating feats of bioengineering in nature, displaying magnificent architectures while providing essential survival utility for spiders. The unusual combination of high strength and extensibility is a characteristic unavailable to date in synthetic materials yet is attained in nature with a relatively simple protein processed from water. This biological template suggests new directions to emulate in the pursuit of new high-performance, multifunctional materials generated with a green chemistry and processing approach. These bio-inspired and high-technology materials can lead to multifunctional material platforms that integrate with living systems for medical materials and a host of other applications.

Improved mechanical stability of dried collagen membrane after metal infiltration

A few percent of transition metals impregnated inside some biological organisms in nature remarkably improve such organisms' mechanical stability. Although the lure to emulate them for development of new biomimetic structural materials has been great, the practical advances have been rare because of the lack of proper synthetic approaches. Multiple pulsed vapor phase infiltration proved successful for the preparation of such transition metal impregnated materials with highly improved mechanical stability. The artificially infiltrated metals (Al, Ti, or Zn) from gas phase lead to around 3 times increase of toughness (in terms of breaking energy) of natural collagen in a dried state. In addition, the infiltrated metals apparently induce considerable crystallographic changes in the natural collagen structures. This infiltration approach can be used as guide for the synthesis of bioinspired structural materials related to metal infiltration.

Molecular and nanostructural mechanisms of deformation, strength and toughness of spider silk fibrils

Spider dragline silk is one of the strongest, most extensible and toughest biological materials known, exceeding the properties of many engineered materials including steel. Silk features a hierarchical architecture where highly organized, densely H-bonded beta-sheet nanocrystals are arranged within a semiamorphous protein matrix consisting of 3(1)-helices and beta-turn protein structures. By using a bottom-up molecular-based approach, here we develop the first spider silk mesoscale model, bridging the scales from Angstroms to tens to potentially hundreds of nanometers. We demonstrate that the specific nanoscale combination of a crystalline phase and a semiamorphous matrix is crucial to achieve the unique properties of silks. Our results reveal that the superior mechanical properties of spider silk can be explained solely by structural effects, where the geometric confinement of beta-sheet nanocrystals, combined with highly extensible semiamorphous domains, is the key to reach great strength and great toughness, despite the dominance of mechanically inferior chemical interactions such as H-bonding. Our model directly shows that semiamorphous regions govern the silk behavior at small deformation, unraveling first when silk is being stretched and leading to the large extensibility of the material. Conversely, beta-sheet nanocrystals play a significant role in defining the mechanical behavior of silk at large-deformation. In particular, the ultimate tensile strength of silk is controlled by the strength of beta-sheet nanocrystals, which is directly related to their size, where small beta-sheet nanocrystals are crucial to reach outstanding levels of strength and toughness. Our results and mechanistic insight directly explain recent experimental results, where it was shown that a significant change in the strength and toughness of silk can be achieved solely by tuning the size of beta-sheet nanocrystals. Our findings help to unveil the material design strategy that enables silk to achieve superior material performance despite simple and inferior material constituents. This concept could lead to a new materials design paradigm, where enhanced functionality is not achieved using complex building blocks but rather through the utilization of simple repetitive constitutive elements arranged in hierarchical structures from nano to macro.

Strong reversible Fe3+-mediated bridging between dopa-containing protein films in water

Metal-containing polymer networks are widespread in biology, particularly for load-bearing exoskeletal biomaterials. Mytilus byssal cuticle is an especially interesting case containing moderate levels of Fe(3+) and cuticle protein-mussel foot protein-1 (mfp-1), which has a peculiar combination of high hardness and high extensibility. Mfp-1, containing 13 mol % of dopa (3, 4-dihydroxyphenylalanine) side-chains, is highly positively charged polyelectrolyte (pI approximately 10) and didn't show any cohesive tendencies in previous surface forces apparatus (SFA) studies. Here, we show that Fe(3+) ions can mediate unusually strong interactions between the positively charged proteins. Using an SFA, Fe(3+) was observed to impart robust bridging (W(ad) approximately 4.3 mJ/m(2)) between two noninteracting mfp-1 films in aqueous buffer approaching the ionic strength of seawater. The Fe(3+) bridging between the mfp-1-coated surfaces is fully reversible in water, increasing with contact time and iron concentration up to 10 microM; at 100 microM, Fe(3+) bridging adhesion is abolished. Bridging is apparently due to the formation of multivalent dopa-iron complexes. Similar Fe-mediated bridging (W(ad) approximately 5.7 mJ/m(2)) by a smaller recombinant dopa-containing analogue indicates that bridging is largely independent of molecular weight and posttranslational modifications other than dopa. The results suggest that dopa-metal interactions may provide an energetic new paradigm for engineering strong, self-healing interactions between polymers under water.

Mechanical energy transfer and dissipation in fibrous beta-sheet-rich proteins

Mechanical properties of structural protein materials are crucial for our understanding of biological processes and disease states. Through utilization of molecular simulation based on stress wave tracking, we investigate mechanical energy transfer processes in fibrous beta-sheet-rich proteins that consist of highly ordered hydrogen bond (H-bond) networks. By investigating four model proteins including two morphologies of amyloids, beta solenoids, and silk beta-sheet nanocrystals, we find that all beta-sheet-rich protein fibrils provide outstanding elastic moduli, where the silk nanocrystal reaches the highest value of ≈40 GPa. However, their capacities to dissipate mechanical energy differ significantly and are controlled strongly by the underlying molecular structure of H-bond network. Notably, silk beta-sheet nanocrystals feature a ten times higher energy damping coefficient than others, owing to flexible intrastrand motions in the transverse directions. The results demonstrate a unique feature of silk nanocrystals, their capacity to simultaneously provide extreme stiffness and energy dissipation capacity. Our results could help one to explain the remarkable properties of silks from an atomistic and molecular perspective, in particular its great toughness and energy dissipation capacity, and may enable the design of multifunctional nanomaterials with outstanding stiffness, strength, and impact resistance.

Iron-clad fibers: a metal-based biological strategy for hard flexible coatings

The extensible byssal threads of marine mussels are shielded from abrasion in wave-swept habitats by an outer cuticle that is largely proteinaceous and approximately fivefold harder than the thread core. Threads from several species exhibit granular cuticles containing a protein that is rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (dopa) as well as inorganic ions, notably Fe3+. Granular cuticles exhibit a remarkable combination of high hardness and high extensibility. We explored byssus cuticle chemistry by means of in situ resonance Raman spectroscopy and demonstrated that the cuticle is a polymeric scaffold stabilized by catecholato-iron chelate complexes having an unusual clustered distribution. Consistent with byssal cuticle chemistry and mechanics, we present a model in which dense cross-linking in the granules provides hardness, whereas the less cross-linked matrix provides extensibility.

Nanoconfinement controls stiffness, strength and mechanical toughness of beta-sheet crystals in silk

Silk features exceptional mechanical properties such as high tensile strength and great extensibility, making it one of the toughest materials known. The exceptional strength of silkworm and spider silks, exceeding that of steel, arises from beta-sheet nanocrystals that universally consist of highly conserved poly-(Gly-Ala) and poly-Ala domains. This is counterintuitive because the key molecular interactions in beta-sheet nanocrystals are hydrogen bonds, one of the weakest chemical bonds known. Here we report a series of large-scale molecular dynamics simulations, revealing that beta-sheet nanocrystals confined to a few nanometres achieve higher stiffness, strength and mechanical toughness than larger nanocrystals. We illustrate that through nanoconfinement, a combination of uniform shear deformation that makes most efficient use of hydrogen bonds and the emergence of dissipative molecular stick-slip deformation leads to significantly enhanced mechanical properties. Our findings explain how size effects can be exploited to create bioinspired materials with superior mechanical properties in spite of relying on mechanically inferior, weak hydrogen bonds.

Atomic layer deposition and abrupt wetting transitions on nonwoven polypropylene and woven cotton fabrics

Atomic layer deposition (ALD) of aluminum oxide on nonwoven polypropylene and woven cotton fabric materials can be used to transform and control fiber surface wetting properties. Infrared analysis shows that ALD can produce a uniform coating throughout the nonwoven polypropylene fiber matrix, and the amount of coating can be controlled by the number of ALD cycles. Upon coating by ALD aluminum oxide, nonwetting hydrophobic polypropylene fibers transition to either a metastable hydrophobic or a fully wetting hydrophilic state, consistent with well-known Cassie-Baxter and Wenzel models of surface wetting of roughened surfaces. The observed nonwetting/wetting transition depends on ALD process variables such as the number of ALD coating cycles and deposition temperature. Cotton fabrics coated with ALD aluminum oxide at moderate temperatures were also observed to transition from a natural wetting state to a metastable hydrophobic state and back to wetting depending on the number of ALD cycles. The transitions on cotton appear to be less sensitive to deposition temperature. The results provide insight into the effect of ALD film growth mechanisms on hydrophobic and hydrophilic polymers and fibrous structures. The ability to adjust and control surface energy, surface reactivity, and wettability of polymer and natural fiber systems using atomic layer deposition may enable a wide range of new applications for functional fiber-based systems.

Engineering the Salmonella type III secretion system to export spider silk monomers

The type III secretion system (T3SS) exports proteins from the cytoplasm, through both the inner and outer membranes, to the external environment. Here, a system is constructed to harness the T3SS encoded within Salmonella Pathogeneity Island 1 to export proteins of biotechnological interest. The system is composed of an operon containing the target protein fused to an N-terminal secretion tag and its cognate chaperone. Transcription is controlled by a genetic circuit that only turns on when the cell is actively secreting protein. The system is refined using a small human protein (DH domain) and demonstrated by exporting three silk monomers (ADF-1, -2, and -3), representative of different types of spider silk. Synthetic genes encoding silk monomers were designed to enhance genetic stability and codon usage, constructed by automated DNA synthesis, and cloned into the secretion control system. Secretion rates up to 1.8 mg l(-1) h(-1) are demonstrated with up to 14% of expressed protein secreted. This work introduces new parts to control protein secretion in Gram-negative bacteria, which will be broadly applicable to problems in biotechnology.