Biological liquid crystal elastomers

A collagen-rich arch in the urochordate notochord coordinates cell shaping and multi-tissue elongation

Cell and tissue reshaping is crucial for coordinating three-dimensional pattern formation, in which the size and shape of the cells must be accurately regulated via signal transport and communication among tissues. However, the identity of signaling and transportation mechanisms in this process remains elusive. In our study, we identified an extracellular matrix (ECM) structure with a vertebra-like shape surrounding the central notochord tissue in the larval tail of the urochordate Ciona. Additionally, we verified that the ECM structure was formed de novo, mainly from collagens secreted by notochord cells. Fluorescence recovery after photobleaching and simulation results revealed that this structure was formed via diffusional collagen flow from a notochord that was restricted and molded in the spaces among tail tissues. We revealed that the collagen structure was essential for notochord cell arrangement and elongation. Furthermore, we observed that the central notochord connects with the epidermis through this ECM structure. The disruption of this structure by collagen knockdown and loss-of-collagen function caused the failure of notochord elongation. More importantly, the epidermis could not elongate proportionally with notochord, indicating that the collagen-rich structure serves as a scaffold to coordinate the concurrent elongation of the tail tissues. These findings provide insights into how the central tissue forms and molds its surrounding ECM structure, by not only regulating its own morphogenesis but also functioning as a scaffold for signal transmission to orchestrate the coordinated morphologic reshaping of the surrounding tissues.

Natural spider silk nanofibrils produced by assembling molecules or disassembling fibers

Spider silk is biocompatible, biodegradable, and rivals some of the best synthetic materials in terms of strength and toughness. Despite extensive research, comprehensive experimental evidence of the formation and morphology of its internal structure is still limited and controversially discussed. Here, we report the complete mechanical decomposition of natural silk fibers from the golden silk orb-weaver Trichonephila clavipes into ≈10 nm-diameter nanofibrils, the material's apparent fundamental building blocks. Furthermore, we produced nanofibrils of virtually identical morphology by triggering an intrinsic self-assembly mechanism of the silk proteins. Independent physico-chemical fibrillation triggers were revealed, enabling fiber assembly from stored precursors "at-will". This knowledge furthers the understanding of this exceptional material's fundamentals, and ultimately, leads toward the realization of silk-based high-performance materials. STATEMENT OF SIGNIFICANCE: Spider silk is one of the strongest and toughest biomaterials, rivaling the best man-made materials. The origins of these traits are still under debate but are mostly attributed to the material's intriguing hierarchical structure. Here we fully disassembled spider silk into 10 nm-diameter nanofibrils for the first time and showed that nanofibrils of the same appearance can be produced via molecular self-assembly of spider silk proteins under certain conditions. This shows that nanofibrils are the key structural elements in silk and leads toward the production of high-performance future materials inspired by spider silk.

Nanolattice-Forming Hybrid Collagens in Protective Shark Egg Cases

Nanoscopic structural control with long-range ordering remains a profound challenge in nanomaterial fabrication. The nanoarchitectured egg cases of elasmobranchs rely on a hierarchically ordered latticework for their protective function─serving as an exemplary system for nanoscale self-assembly. Although the proteinaceous precursors are known to undergo intermediate liquid crystalline phase transitions before being structurally arrested in the final nanolattice architecture, their sequences have so far remained unknown. By leveraging RNA-seq and proteomic techniques, we identified a cohort of nanolattice-forming proteins comprising a collagenous midblock flanked by domains typically associated with innate immunity and network-forming collagens. Structurally homologous proteins were found in the genomes of other egg-case-producing cartilaginous fishes, suggesting a conserved molecular self-assembly strategy. The identity and stabilizing role of cross-links were subsequently elucidated using mass spectrometry and in situ small-angle X-ray scattering. Our findings provide a new design approach for protein-based liquid crystalline elastomers and the self-assembly of nanolattices.

Liquid Crystal Elastomers for Biological Applications

The term liquid crystal elastomer (LCE) describes a class of materials that combine the elastic entropy behaviour associated with conventional elastomers with the stimuli responsive properties of anisotropic liquid crystals. LCEs consequently exhibit attributes of both elastomers and liquid crystals, but additionally have unique properties not found in either. Recent developments in LCE synthesis, as well as the understanding of the behaviour of liquid crystal elastomers—namely their mechanical, optical and responsive properties—is of significant relevance to biology and biomedicine. LCEs are abundant in nature, highlighting the potential use of LCEs in biomimetics. Their exceptional tensile properties and biocompatibility have led to research exploring their applications in artificial tissue, biological sensors and cell scaffolds by exploiting their actuation and shock absorption properties. There has also been significant recent interest in using LCEs as a model for morphogenesis. This review provides an overview of some aspects of LCEs which are of relevance in different branches of biology and biomedicine, as well as discussing how recent LCE advances could impact future applications.

Structural Diversity of Native Major Ampullate, Minor Ampullate, Cylindriform, and Flagelliform Silk Proteins in Solution

The foundations of silk spinning, the structure, storage, and activation of silk proteins, remain highly debated. By combining solution small-angle neutron and X-ray scattering (SANS and SAXS) alongside circular dichroism (CD), we reveal a shape anisotropy of the four principal native spider silk feedstocks from Nephila edulis. We show that these proteins behave in solution like elongated semiflexible polymers with locally rigid sections. We demonstrated that minor ampullate and cylindriform proteins adopt a monomeric conformation, while major ampullate and flagelliform proteins have a preference for dimerization. From an evolutionary perspective, we propose that such dimerization arose to help the processing of disordered silk proteins. Collectively, our results provide insights into the molecular-scale processing of silk, uncovering a degree of evolutionary convergence in protein structures and chemistry that supports the macroscale micellar/pseudo liquid crystalline spinning mechanisms proposed by the community.

Responsive Liquid-Crystal-Clad Fibers for Advanced Textiles and Wearable Sensors

A simple process to clad conventional monofilament fibers with low-molecular-weight liquid crystals (LCs) stabilized by an outer polymer sheath is demonstrated. The fibers retain the responsive properties of the LCs but in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC-core fibers produced by electrospinning or airbrushing. The microscopic structure and the optical properties of round and flattened fibers are reported. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the LCs. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemical concentrations, and pressure. The fabrics can be fashioned into garments that can sense and report the state of health of the wearer or the status of their environment.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Mineralization and non-ideality: on nature's foundry

Understanding how ions, ion-clusters and particles behave in non-ideal environments is a fundamental question concerning planetary to atomic scales. For biomineralization phenomena wherein diverse inorganic and organic ingredients are present in biological media, attributing biomaterial composition and structure to the chemistry of singular additives may not provide a holistic view of the underlying mechanisms. Therefore, in this review, we specifically address the consequences of physico-chemical non-ideality on mineral formation. Influences of different forms of non-ideality such as macromolecular crowding, confinement and liquid-like organic phases on mineral nucleation and crystallization in biological environments are presented. Novel prospects for the additive-controlled nucleation and crystallization are accessible from this biophysical view. In this manner, we show that non-ideal conditions significantly affect the form, structure and composition of biogenic and biomimetic minerals.

The complexity of silk under the spotlight of synthetic biology

For centuries silkworm filaments have been the focus of R&D innovation centred on textile manufacture with high added value. Most recently, silk research has focused on more fundamental issues concerning bio-polymer structure–property–function relationships. This essay outlines the complexity and fundamentals of silk spinning, and presents arguments for establishing this substance as an interesting and important subject at the interface of systems biology (discovery) and synthetic biology (translation). It is argued that silk is a generic class of materials where each type of silk presents a different embodiment of emergent properties that combine genetically determined (anticipatory) and environmentally responsive components. In spiders’ webs the various silks have evolved to form the interactive components of an intricate fabric that provides an extended phenotype to the spider's body morphology.

Topography from topology: photoinduced surface features generated in liquid crystal polymer networks

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

Silk constructs for delivery of musculoskeletal therapeutics

Silk fibroin (SF) is a biopolymer with distinguishing features from many other bio- as well as synthetic polymers. From a biomechanical and drug delivery perspective, SF combines remarkable versatility for scaffolding (solid implants, hydrogels, threads, solutions), with advanced mechanical properties and good stabilization and controlled delivery of entrapped protein and small molecule drugs, respectively. It is this combination of mechanical and pharmaceutical features which renders SF so exciting for biomedical applications. This pattern along with the versatility of this biopolymer has been translated into progress for musculoskeletal applications. We review the use and potential of silk fibroin for systemic and localized delivery of therapeutics in diseases affecting the musculoskeletal system. We also present future directions for this biopolymer as well as the necessary research and development steps for their achievement.

X-ray diffraction study of nanocrystalline and amorphous structure within major and minor ampullate dragline spider silks

Synchrotron X-ray micro-diffraction experiments were carried out on Nephila clavipes (NC) and Argiope aurantia (AA) major (MA) and minor ampullate (MiA) fibers that make up dragline spider silk. The diffraction patterns show a semi-crystalline structure with β-poly(l-alanine) nanocrystallites embedded in a partially oriented amorphous matrix. A superlattice reflection 'S' diffraction ring is observed, which corresponds to a crystalline component larger in size and is poorly oriented, when compared to the β-poly(l-alanine) nanocrystallites that are commonly observed in dragline spider silks. Crystallite size, crystallinity and orientation about the fiber axis have been determined from the wide-angle X-ray diffraction (WAXD) patterns. In both NC and AA, the MiA silks are found to be more highly crystalline, when compared with the corresponding MA silks. Detailed analysis on the amorphous matrix shows considerable differences in the degree of order of the oriented amorphous component between the different silks studied and may play a crucial role in determining the mechanical properties of the silks.

Spider silk: understanding the structure-function relationship of a natural fiber

Spider silk is of great interest because of its extraordinary physical properties, such as strength and toughness. Here we discuss how these physical properties relate to the way in which spiders have utilized this material in prey capture, forcing its evolution to a high-performance fiber. Female spiders can produce up to seven different types of silk, and all these have different physical properties, which relate to their various functions. The variation in properties are due to underlying differences in the proteins making up these silks. As our understanding of spider silk has increased in the recent years, it has been possible to produce recombinant versions of the respective proteins. Recombinant proteins open up the potential to produce synthetic silk fibers with properties similar to those of the natural spider silk threads.

Role of polyalanine domains in beta-sheet formation in spider silk block copolymers

Genetically engineered spider silk-like block copolymers were studied to determine the influence of polyalanine domain size on secondary structure. The role of polyalanine block distribution on beta-sheet formation was explored using FT-IR and WAXS. The number of polyalanine blocks had a direct effect on the formation of crystalline beta-sheets, reflected in the change in crystallinity index as the blocks of polyalanines increased. WAXS analysis confirmed the crystalline nature of the sample with the largest number of polyalanine blocks. This approach provides a platform for further exploration of the role of specific amino acid chemistries in regulating the assembly of beta-sheet secondary structures, leading to options to regulate material properties through manipulation of this key component in spider silks.

High-resolution NMR characterization of a spider-silk mimetic composed of 15 tandem repeats and a CRGD motif

Multidimensional solution NMR spectroscopic techniques have been used to obtain atomic level information about a recombinant spider silk construct in hexafluoro-isopropanol (HFIP). The synthetic 49 kDa silk-like protein mimics authentic silk from Nephila clavipes, with the inclusion of an extracellular matrix recognition motif. 2D (1)H-(15)N HSQC NMR spectroscopy reveals 33 cross peaks, which were assigned to amino acid residues in the semicrystalline repeat units. Signals from the amorphous segments in the primary sequence were weak and broad, suggesting that this region is highly dynamic and undergoing conformational exchange. An analysis of the deviations of the (13)C(alpha), (13)C(beta), and (13)CO chemical shifts relative to the expected random coil values reveals two highly alpha-helical regions from amino acid 12-19 and 26-32, which comprise the polyalanine track and a GGLGSQ sequence. This finding is further supported by phi-value analysis and sequential and medium-range NOE interactions. Pulsed field gradient NMR measurements indicate that the topology of the silk mimetic in HFIP is nonglobular. Moreover, the 3D (15)N-NOESY HSQC spectrum exhibits few long-range NOEs. Similar spectral features have been observed for repeat modules in other polypeptides and are characteristic of an elongated conformation. The results provide a residue-specific description of a silk sequence in nonaqueous solution and may be insightful for understanding the fold and topology of highly concentrated, stable silk before spinning. Additionally, the insights obtained may find application in future design and large-scale production and storage of synthetic silks in organic solvents.

Silk inverse opals from template-directed β-sheet transformation of regenerated silk fibroin

We describe a novel and facile method for the fabrication of reconstituted silk monoliths with ordered interconnected air spheres based on intercalation and β-sheet transformation of regenerated silk fibroin solutions within the interstitial spaces of a sacrificial colloidal crystal template. The silk inverse opals are elastic and can withstand and recover from compressive loads of up to 112 MPa by reversible pore deformation. They also exhibit super-hydrophobicity with water droplet contact angles of up to 140° due to periodic nanoscale protrusions associated with the surface texture of the inverse opal architecture. These properties indicate that silk inverse opals could have potential applications as biocompatible elastic scaffolds, storage-release, barrier and self-cleaning materials, and in the design of load-responsive microfluidic devices.

Constitutive model for fiber-reinforced materials with deformable matrices

A great number of biological structures are composed of fibers (elastin, collagen, etc.) dispersed on an aqueous matrix in such a complex way that a detailed mechanical analysis based on microconstituents is, for practical purposes, out of reach. Consequently, the preferred approach to the mechanical behavior of these materials is based on setting up of constitutive equations that homogenize the behavior while capturing their main microstructural features. This work presents a simple macroscopic model for fiber-reinforced materials with deformable matrices, especially suited to many biological structural tissues. The constitutive equation is derived by imposing equivalence between the virtual works of both the fiber-reinforced and the equivalent continuum media, showing that it is independent of the control volume used for such equivalence. The model is particularized to incompressible materials, and an extension to orthotropic biological fibers is shown. Numerical simulations of uniaxial tests on silk fibers demonstrate the model's ability to capture the progressive alignment of the microconstituents under large deformations.

Correlation of the β-sheet crystal size in silk fibers with the protein amino acid sequence

Low voltage transmission electron microscopy (LVTEM) and wide angle X-ray scattering (WAXS) are used to independently determine the size of the β-sheet crystalline regions in Bombyx mori silk fibers. The peak in the size distributions of the major and minor axes of the anisotropic crystallites measured from the LVTEM images compare well with the average sizes as determined by Scherrer analysis of the X-ray fiber diagrams. These values are then discussed in the context of the B. mori fibroin heavy chain amino acid sequence, and the underlying mechanism for the organism's control on fiber crystallite size, and therefore mechanical properties, is proposed.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

High-performance elastomeric nanocomposites via solvent-exchange processing

The incorporation of nanoparticles into engineering thermoplastics affords engineers an opportunity to synthesize polymer nanocomposites that potentially rival the most advanced materials in nature. Development of these materials is difficult because thermodynamic and kinetic barriers inhibit the dispersal of inorganic, often hydrophilic nanoparticles in hydrophobic polymer matrices. Using a new solvent-exchange approach, we preferentially reinforce the hard microdomains of thermoplastic elastomers with smectic clay of similar characteristic dimensions. The strong adhesion between the clay and the hard microdomains coupled with the formation of a percolative network not only stiffens and toughens, but increases the heat distortion temperature of the material and induces reversible thermotropic liquid-crystalline transitions. The discotic clay platelets induce morphological ordering over a range of length scales, which results in significant thermomechanical enhancement and expands high-temperature applications. Merging block-copolymer processing techniques with this method for preferential ordering of nanoparticle facilitates the development of new, hierarchically ordered materials.

Silk fibroin as an organic polymer for controlled drug delivery

The pharmaceutical utility of silk fibroin (SF) materials for drug delivery was investigated. SF films were prepared from aqueous solutions of the fibroin protein polymer and crystallinity was induced and controlled by methanol treatment. Dextrans of different molecular weights, as well as proteins, were physically entrapped into the drug delivery device during processing into films. Drug release kinetics were evaluated as a function of dextran molecular weight, and film crystallinity. Treatment with methanol resulted in an increase in beta-sheet structure, an increase in crystallinity and an increase in film surface hydrophobicity determined by FTIR, X-ray and contact angle techniques, respectively. The increase in crystallinity resulted in the sustained release of dextrans of molecular weights ranging from 4 to 40 kDa, whereas for less crystalline films sustained release was confined to the 40 kDa dextran. Protein release from the films was studied with horseradish peroxidase (HRP) and lysozyme (Lys) as model compounds. Enzyme release from the less crystalline films resulted in a biphasic release pattern, characterized by an initial release within the first 36 h, followed by a lag phase and continuous release between days 3 and 11. No initial burst was observed for films with higher crystallinity and subsequent release patterns followed linear kinetics for HRP, or no substantial release for Lys. In conclusion, SF is an interesting polymer for drug delivery of polysaccharides and bioactive proteins due to the controllable level of crystallinity and the ability to process the biomaterial in biocompatible fashion under ambient conditions to avoid damage to labile compounds to be delivered.

β‐Silks: Enhancing and Controlling Aggregation

It appears that fiber-forming proteins are not an exclusive group but that, with appropriate conditions, many proteins can potentially aggregate and form fibrils; though only certain proteins, for example, amyloids and silks, do so under normal physiological conditions. Even so, this suggests a ubiquitous aggregation mechanism in which the protein environment is at least as important as the sequence. An ideal model system in which forced and natural aggregation has been observed is silk. Silks have evolved specifically to readily form insoluble ordered structures with a wide range of structural functionality. The animal, be it silkworm or spider, will produce, store, and transport high molecular weight proteins in a complex environment to eventually allow formation of silk fibers with a variety of mechanical properties. Here we review fiber formation and its prerequisites, and discuss the mechanism by which the animal facilitates and modulates silk assembly to achieve controlled protein aggregation.

Compatibility of lyotropic liquid crystals with viruses and mammalian cells that support the replication of viruses

We report a study that investigates the biocompatibility of materials that form lyotropic liquid crystals (LCs) with viruses and mammalian cells that support the replication of viruses. This study is focused on aqueous solutions of tetradecyldimethyl-amineoxide (C(14)AO) and decanol (D), or disodium cromoglycate (DSCG; C(23)H(14)O(11)Na(2)), which can form optically birefringent, liquid crystalline phases. The influence of these materials on the ability of vesicular stomatitis virus (VSV) to infect human epitheloid cervical carcinoma (HeLa) cells was examined by two approaches. First, VSV was dispersed in aqueous C(14)AO+ D or DSCG, and then HeLa cells were inoculated by contacting the cells with the aqueous C(14)AO + D or DSCG containing VSV. The infectivity of VSV to the HeLa cells was subsequently determined. Second, VSV was incubated in LC phases of either C(14)AO + D or DSCG for 4 h, and the concentration (titer) of infectious virus in the LC was determined by dilution into cell culture medium and subsequent inoculation of HeLa cells. Using these approaches, we found that the LC containing C(14)AO + D caused inactivation of virus as well as cell death. In contrast, we determined that VSV retained its infectivity in the presence of aqueous DSCG, and that greater than 74-82% of the HeLa cells survived contact with aqueous DSCG (depending on concentration of DSCG). Because VSV maintained its function (and we infer structure) in LCs formed from DSCG, we further explored the influence of the virus on the ordering of the LC. Whereas the LC formed from DSCG was uniformly aligned on surfaces prepared from self-assembled monolayers (SAMs) of HS(CH(2))(11)(OCH(2)CH(2))(4)OH on obliquely deposited films of gold in the absence of VSV, the introduction of 10(7)-10(8) infectious virus particles per milliliter caused the LC to assume a non-uniform orientation and a colorful appearance that was readily distinguished from the uniformly aligned LCs. Control experiments using cell lysates with equivalent protein concentrations but no virus did not perturb the uniform alignment of the LC.

Protein fibers as performance proteins: new technologies and applications

Protein fibers are fundamental building blocks of life playing an essential role in motility, elasticity, scaffolding, stabilization and the protection of cells, tissues and organisms. Despite nearly a century of research into the assembly mechanisms and structures of fibrous proteins, only limited information is still available. Within the past decade, however, insights have been provided into how some fibrous proteins assemble and how they function in biology. In addition, efforts are increasingly being made to employ protein fibers as performance molecules in man-made medical and technical applications.

Spider Silk Proteins – Mechanical Property and Gene Sequence

Spiders spin up to seven different types of silk and each type possesses different mechanical properties. The reports on base sequences of spider silk protein genes have gained importance as the mechanical properties of silk fibers have been revealed. This review aims to link recent molecular data, often translated into amino acid sequences and predicted three dimensional structural motifs, to known mechanical properties.