Oriented nucleation of hydroxylapatite crystals on spider dragline silks

The translational paradigm of nanobiomaterials: Biological chemistry to modern applications

Recently nanotechnology has evolved as one of the most revolutionary technologies in the world. It has now become a multi-trillion-dollar business that covers the production of physical, chemical, and biological systems at scales ranging from atomic and molecular levels to a wide range of industrial applications, such as electronics, medicine, and cosmetics. Nanobiomaterials synthesis are promising approaches produced from various biological elements be it plants, bacteria, peptides, nucleic acids, etc. Owing to the better biocompatibility and biological approach of synthesis, they have gained immense attention in the biomedical field. Moreover, due to their scaled-down sized property, nanobiomaterials exhibit remarkable features which make them the potential candidate for different domains of tissue engineering, materials science, pharmacology, biosensors, etc. Miscellaneous characterization techniques have been utilized for the characterization of nanobiomaterials. Currently, the commercial transition of nanotechnology from the research level to the industrial level in the form of nano-scaffolds, implants, and biosensors is stimulating the whole biomedical field starting from bio-mimetic nacres to 3D printing, multiple nanofibers like silk fibers functionalizing as drug delivery systems and in cancer therapy. The contribution of single quantum dot nanoparticles in biological tagging typically in the discipline of genomics and proteomics is noteworthy. This review focuses on the diverse emerging applications of Nanobiomaterials and their mechanistic advancements owing to their physiochemical properties leading to the growth of industries on different biomedical measures. Alongside the implementation of such nanobiomaterials in several drug and gene delivery approaches, optical coding, photodynamic cancer therapy, and vapor sensing have been elaborately discussed in this review. Different parameters based on current challenges and future perspectives are also discussed here.

Biomineralization Directed by Prenucleated Calcium and Phosphorus Nanoclusters Improving Mechanical Properties and Osteogenic Potential of Antheraea pernyi Silk Fibroin-Based Artificial Periosteum

The use of biomacromolecules as templates to control the nucleation and growth of hydroxyapatite crystals to prepare bioactive materials is a valuable approach in bone tissue engineering. Here, an artificial periosteum is prepared by biomineralizing Antheraea pernyi fibroin (AF) membrane with prenucleated nanoclusters, which can promote the osteogenic differentiation of mesenchymal stem cells (MSCs) and induce the formation of bone matrix protein in vivo. To achieve this, a biologically inspired prenucleated calcium and phosphorus nanocluster mineralization system is designed to nucleate and generate hydroxyapatite crystals on the surface of the AF membrane. This biomineralization process provides AF membranes with improved elastic modulus and tensile strength. Subsequently, cell viability assay, hemolysis test, and H&E staining show that the mineralized AF (MAF) membranes has good cytocompatibility, hemocompatibility, and histocompatibility in vitro and in vivo. Additionally, the MAF membranes significantly promote osteogenic differentiation of MSCs in the absence of osteogenic inducer in vitro. Experiments in vivo demonstrate that bone-related matrix proteins are highly expressed in MAF groups with or without MSCs seeded. Therefore, the use of bioinspired prenucleated nanoclusters to prepare artificial periosteum based on biomineralized AF membrane is a promising strategy in the field of bone tissue engineering.

DNA nanostructures as templates for biomineralization

Nature uses extracellular matrix scaffolds to organize biominerals into hierarchical structures over various length scales. This has inspired the design of biomimetic mineralization scaffolds, with DNA nanostructures being among the most promising. DNA nanotechnology makes use of molecular recognition to controllably give 1D, 2D and 3D nanostructures. The control we have over these structures makes them attractive templates for the synthesis of mineralized tissues, such as bones and teeth. In this Review, we first summarize recent work on the crystallization processes and structural features of biominerals on the nanoscale. We then describe self-assembled DNA nanostructures and come to the intersection of these two themes: recent applications of DNA templates in nanoscale biomineralization, a crucial process to regenerate mineralized tissues.

Hybrid Spider Silk with Inorganic Nanomaterials

High-performance functional biomaterials are becoming increasingly requested. Numerous natural and artificial polymers have already demonstrated their ability to serve as a basis for bio-composites. Spider silk offers a unique combination of desirable aspects such as biocompatibility, extraordinary mechanical properties, and tunable biodegradability, which are superior to those of most natural and engineered materials. Modifying spider silk with various inorganic nanomaterials with specific properties has led to the development of the hybrid materials with improved functionality. The purpose of using these inorganic nanomaterials is primarily due to their chemical nature, enhanced by large surface areas and quantum size phenomena. Functional properties of nanoparticles can be implemented to macro-scale components to produce silk-based hybrid materials, while spider silk fibers can serve as a matrix to combine the benefits of the functional components. Therefore, it is not surprising that hybrid materials based on spider silk and inorganic nanomaterials are considered extremely promising for potentially attractive applications in various fields, from optics and photonics to tissue regeneration. This review summarizes and discusses evidence of the use of various kinds of inorganic compounds in spider silk modification intended for a multitude of applications. It also provides an insight into approaches for obtaining hybrid silk-based materials via 3D printing.

Recent Advances in Development of Functional Spider Silk-Based Hybrid Materials

Silkworm silk is mainly known as a luxurious textile. Spider silk is an alternative to silkworm silk fibers and has much more outstanding properties. Silk diversity ensures variation in its application in nature and industry. This review aims to provide a critical summary of up-to-date fabrication methods of spider silk-based organic-inorganic hybrid materials. This paper focuses on the relationship between the molecular structure of spider silk and its mechanical properties. Such knowledge is essential for understanding the innate properties of spider silk as it provides insight into the sophisticated assembly processes of silk proteins into the distinct polymers as a basis for novel products. In this context, we describe the development of spider silk-based hybrids using both natural and bioengineered spider silk proteins blended with inorganic nanoparticles. The following topics are also covered: the diversity of spider silk, its composition and architecture, the differences between silkworm silk and spider silk, and the biosynthesis of natural silk. Referencing biochemical data and processes, this paper outlines the existing challenges and future outcomes.

The Significance and Utilisation of Biomimetic and Bioinspired Strategies in the Field of Biomedical Material Engineering: The Case of Calcium Phosphat-Protein Template Constructs

This review provides a summary of recent research on biomimetic and bioinspired strategies applied in the field of biomedical material engineering and focusing particularly on calcium phosphate-protein template constructs inspired by biomineralisation. A description of and discussion on the biomineralisation process is followed by a general summary of the application of the biomimetic and bioinspired strategies in the fields of biomedical material engineering and regenerative medicine. Particular attention is devoted to the description of individual peptides and proteins that serve as templates for the biomimetic mineralisation of calcium phosphate. Moreover, the review also presents a description of smart devices including delivery systems and constructs with specific functions. The paper concludes with a summary of and discussion on potential future developments in this field.

Genetically Engineered Flagella Form Collagen-like Ordered Structures for Inducing Stem Cell Differentiation

Bacteria use flagella, the protein nanofibers on their surface, as a molecular machine to swim. Flagella are polymerized from monomers, flagellins, which can display a peptide by genetic means. However, flagella as genetically modifiable nanofibers have not been used in building bone extracellular matrix-like structures for inducing stem cell differentiation in non-osteogenic medium. Here we discovered that interactions between Ca2+ ions and flagella (displaying a collagen-like peptide (GPP)8 on every flagellin) resulted in ordered bundle-like structures, which were further mineralized with hydroxyapatite to form ordered fibrous matrix. The resultant matrix significantly induced the osteogenic differentiation of stem cells, much more efficiently than wild-type flagella and type I collagen. This work shows that flagella can be used as protein building blocks in generating biomimetic materials.

Construction of a micro/nano structured surface on a β-TCP/CaSiO3 bioceramic promotes osteogenic differentiation of mBMSCs

We report a simple and practical process to construct surface structures with water as the only reagent system; the additive-free system provides regulated structures with few defects and impurities.

Design, Fabrication, and Function of Silk-Based Nanomaterials

Animal silks are built from pure protein components and their mechanical performance, such as strength and toughness, often exceed most engineered materials. The secret to this success is their unique nanoarchitectures that are formed through the hierarchical self-assembly of silk proteins. This natural material fabrication process in sharp contrast to the production of artificial silk materials, which usually are directly constructed as bulk structures from silk fibroin (SF) molecular. In recent years, with the aim of understanding and building better silk materials, a variety of fabrication strategies have been designed to control nanostructures of silks or to create functional materials from silk nanoscale building blocks. These emerging fabrication strategies offer an opportunity to tailor the structure of SF at the nanoscale and provide a promising route to produce structurally and functionally optimized silk nanomaterials. Here, we review the critical roles of silk nanoarchitectures on property and function of natural silk fibers, outline the strategies of utilization of these silk nanobuilding blocks, and we provide a critical summary of state of the art in the field to create silk nanoarchitectures and to generate silk-based nanocomponents. Further, such insights suggest templates to consider for other materials systems.

Bio-templated silica composites for next-generation biomedical applications

Silica-based materials have extensive biomedical applications owing to their unique physical, chemical, and biological properties. Recently, increasing studies have examined the mechanisms involved in biosilicification to develop novel, fine-tunable, eco-friendly materials and/or technologies. In this review, we focus on recent developments in bio-templated silica synthesis and relevant applications in drug delivery systems, tissue engineering, and biosensing.

Silk scaffolds in bone tissue engineering: An overview

Bone tissue plays multiple roles in our day-to-day functionality. The frequency of accidental bone damage and disorder is increasing worldwide. Moreover, as the world population continues to grow, the percentage of the elderly population continues to grow, which results in an increased number of bone degenerative diseases. This increased elderly population pushes the need for artificial bone implants that specifically employ biocompatible materials. A vast body of literature is available on the use of silk in bone tissue engineering. The current work presents an overview of this literature from materials and fabrication perspective. As silk is an easy-to-process biopolymer; this allows silk-based biomaterials to be molded into diverse forms and architectures, which further affects the degradability. This makes silk-based scaffolds suitable for treating a variety of bone reconstruction and regeneration objectives. Silk surfaces offer active sites that aid the mineralization and/or bonding of bioactive molecules that facilitate bone regeneration. Silk has also been blended with a variety of polymers and minerals to enhance its advantageous properties or introduce new ones. Several successful works, both in vitro and in vivo, have been reported using silk-based scaffolds to regenerate bone tissues or other parts of the skeletal system such as cartilage and ligament. A growing trend is observed toward the use of mineralized and nanofibrous scaffolds along with the development of technology that allows to control scaffold architecture, its biodegradability and the sustained releasing property of scaffolds. Further development of silk-based scaffolds for bone tissue engineering, taking them up to and beyond the stage of human trials, is hoped to be achieved in the near future through a cross-disciplinary coalition of tissue engineers, material scientists and manufacturing engineers.

STATEMENT OF SIGNIFICANCE

The state-of-art of silk biomaterials in bone tissue engineering, covering their wide applications as cell scaffolding matrices to micro-nano carriers for delivering bone growth factors and therapeutic molecules to diseased or damaged sites to facilitate bone regeneration, is emphasized here. The review rationalizes that the choice of silk protein as a biomaterial is not only because of its natural polymeric nature, mechanical robustness, flexibility and wide range of cell compatibility but also because of its ability to template the growth of hydroxyapatite, the chief inorganic component of bone mineral matrix, resulting in improved osteointegration. The discussion extends to the role of inorganic ions such as Si and Ca as matrix components in combination with silk to influence bone regrowth. The effect of ions or growth factor-loaded vehicle incorporation into regenerative matrix, nanotopography is also considered.

Evaluating adhesion and alignment of dental pulp stem cells to a spider silk substrate for tissue engineering applications

A proposed source of stem cells for nerve regeneration are dental pulp stem cells (DPSCs), based on their close embryonic origin to neurons and the ease with which DPSCs can be obtained from a donor. This study evaluated the response of human DPSCs to spider dragline silk fibers, a potential substrate material for tissue regeneration. The DPSCs' morphology and spread pattern were characterized after these cells were plated onto Nephila clavipes dragline fibers in media. In addition, the responses of two other well established cell lines, osteoblasts (7F2s), and fibroblasts (3T3s), were also studied under identical conditions. The inclusion of 3T3s and 7F2s in this study allowed for both direct comparisons to prior published work and a qualitative comparison to the morphology of the DPSCs. After twelve days, the DPSCs exhibited greater relative alignment and adherence to the spider dragline fibers than the 3T3s and 7F2s. The impact of a common sterilization method (ultraviolet light) on the spider dragline fiber surface and subsequent cell response to this modified surface was also characterized. Exposure of the silk to ultraviolet light did not have a measureable effect on cell alignment, but it did eliminate bacterial growth and changed fiber surface roughness. Spiders' exposure to stressful environments did not have an effect on silk to impair cell alignment or adhesion. Synthetic recombinant protein silk did not act as a substrate for cell adhesion or alignment but hydrogels with similar composition supported cell attachment, growth and proliferation. In all cases, natural drawn spider silk acted as an effective substrate for cellular adhesion and alignment of DPSCs and could be used in neural differentiation applications.

"Cleaning" the Surface of Hydroxyapatite Nanorods by a Reaction-Dissolution Approach

Synthetic nanoparticles are always terminated with coating molecules, which are often cytotoxic and not desired in biomedicine. Here we propose a novel reaction-dissolution approach to remove the cytotoxic coating molecules. A two-component solution is added to the nanoparticle solution; one component reacts with the coating molecules to form a salt whereas another is a solvent for dissolving and thus removing the salt. As a proof of concept, this work uses a NaOH-ethanol solution to remove the cytotoxic linoleic acid molecules coated on the hydroxyapatite nanorods (HAP-NRs). The removal of the coating molecules not only significantly improves the biocompatibility of HAP-NRs but also enables their oriented attachment into tightly-bound superstructures, which mimic the organized HAP crystals in bone and enamel and can promote the osteogenic differentiation of mesenchymal stem cells. Our reaction-dissolution approach can be extended to the surface "cleaning" of other nanomaterials.

Biomineralization of Natural Collagenous Nanofibrous Membranes and Their Potential Use in Bone Tissue Engineering

Small intestinal submucosa (SIS) membranes as a decellularized tissue are known to be a natural nanofibrous biomaterial mainly made of type I collagen fibers and containing some growth factors (fibroblast growth factor 2 and transforming growth factor β) desired in tissue engineering. Here we show that the SIS membranes can promote the formation of bone mineral hydroxylapatite (HAP) crystals along the collagen fibers constituting the membranes from a HAP-supersaturated solution. The resultant biomineralized HAP-SIS scaffolds were found to promote the attachment, growth and osteogenic differentiation of mesenchymal stem cells (MSCs) in both basal and osteogenic media by the evaluation of osteogenic marker formation. More importantly, the HAP-SIS scaffolds could induce the osteogenic differentiation in the basal media without osteogenic supplements due to the presence of HAP crystals in the scaffolds. Histological characterization of the MSC-seeded scaffolds showed that HAP-SIS scaffolds are biocompatible and promote the formation of new tissue in vitro. The biomineralized SIS membranes mimic some aspects of natural bone in terms of the composition and nanostructures and can find potential use in bone tissue engineering.

Transformation of amorphous calcium carbonate to rod-like single crystal calcite via "copying" collagen template

Collagen Langmuir films were prepared by spreading the solution of collagen over deionized water, CaCl2 solution and Ca(HCO3)2 solution. Resultant collagen Langmuir monolayers were then compressed to a lateral pressure of 10 mN/m and held there for different duration, allowing the crystallization of CaCO3. The effect of crystallization time on the phase composition and microstructure of CaCO3 was investigated. It was found that amorphous calcium carbonate (ACC) was obtained at a crystallization time of 6 h. The amorphous CaCO3 was transformed to rod-like single crystal calcite crystals at an extended crystallization time of 12 h and 24 h, via "copying" the symmetry and dimensionalities of collagen fibers. Resultant calcite crystallites were well oriented along the longitudinal axis of collagen fibers. The ordered surface structure of collagen fibers and electrostatic interactions played key roles in tuning the oriented nucleation and growth of the calcite crystallites. The mineralized collagen possessing both desired mechanical properties of collagen fiber and good biocompatibility of calcium carbonate may be assembled into an ideal biomaterial for bone implants.

Ca2+-induced self-assembly of Bombyx mori silk sericin into a nanofibrous network-like protein matrix for directing controlled nucleation of hydroxylapatite nano-needles

Bone biomineralization is a well-regulated protein-mediated process where hydroxylapatite (HAP) crystals are nucleated with preferred orientation within self-assembled protein matrix. Mimicking this process is a promising approach to the production of bone-like protein/mineral nanocomposites for bone repair and regeneration. Towards the goal of fabricating such nanocomposites from sericin, a protein spun by Bombyx mori (B.mori) silkworm, and bone mineral HAP, for the first time we investigated the chemical mechanism underpinning the synergistic processes of the conformational change/self-assembly of B.mori sericin ( BS ) as well as the nucleation of HAP on the resultant self-assembled BS matrix. We found that BS , rich in anionic amino acid residues, could bind Ca²⁺ ions from the HAP precursor solution through electrostatic attraction. The Ca²⁺binding drove the conformational change of BS from random coils into β-sheets and its concomitant self-assembly into interconnected nanofibrous network-like protein matrix, which initiated the nucleation and growth of HAP crystals. HAP crystals directed by the resultant self-assembled BS matrix grew preferentially along their crystallographic c-axis, leading to the formation of HAP nano-needles. The HAP nano-needles in the self-assembled BS matrix were subsequently aggregated into globules, probably driven by the hydrogen bonding between C=O groups of BS and O-H groups of HAP nano-needles. The present work sheds light on the chemical mechanisms of BS self-assembly and the controlled mineralization directed by the self-assembled matrix. We also found that the resultant nanocomposites could promote the osteogenic differentiation of human bone marrow-derived mesenchymal stem cells. Thus our work also generates a biomimetic approach to bone-like silk protein/mineral nanocomposite scaffolds that can find potential applications in bone repair and regeneration.

Tuning molecular weights of Bombyx mori (B. mori) silk sericin to modify its assembly structures and materials formation

Bombyx mori (B. mori) silk sericin is a protein with features desirable as a biomaterial, such as increased hydrophilicity and biodegradation, as well as resistance to oxidation, bacteria, and ultraviolet light. In contrast to other widely studied B. mori silk proteins such as fibroin, sericin is still unexplored as a building block for fabricating biomaterial, and thus a facile technique of processing it into a material is needed. Here, electrospinning technology was used to fabricate it into biomaterials from two forms of B. mori silk sericin with different molecular weights, one is a low (12.0 kDa) molecular sericin (LS) form and another is a high (66.0 kDa) molecular weight sericin (HS) form. Circular dichroism (CD) spectra showed that LS in hexafluoroacetone (HFA) solvent adopted a predominantly random coil conformation, whereas HS tended to form a β-sheet structure along with a large content of random coils. In addition, LS and HS in HFA solvent were found to form cylinder-like smaller nanoparticles and larger irregular aggregates before electrospinning, respectively. As a result, biomaterials based on microparticles and nanofibers were successfully fabricated by electrospinning of LS and HS dissolved in HFA, respectively. The cell viability and differentiation assay indicated that nanofibers and microparticles improved cell adhesion, growth, and differentiation, proving that the scaffolds electrospun from sericin are biocompatible regardless of its molecular weight. The microparticles, not common in electrospinning of silk proteins reported previously, were found to promote the osteogenic differentiation of mesenchymal stem cells in comparison to the nanofibers. This study suggested that molecular weight of sericin mediates its secondary structure and assembly structure, which in turn leads to a control of final morphology of the electrospun materials. The microparticles and nanofibers of sericin can be potentially used as building blocks for fabricating the scaffolds for tissue engineering.

Bioinspired amphiphilic phosphate block copolymers as non-fluoride materials to prevent dental erosion

Inspired by the fact that certain natural proteins, e.g. casein phosphopeptide or amelogenin, are able to prevent tooth erosion (mineral loss) and to enhance tooth remineralization, a synthetic amphiphilic diblock copolymer, containing a hydrophilic methacryloyloxyethyl phosphate block (MOEP) and a hydrophobic methyl methacrylate block (MMA), was designed as a novel non-fluoride agent to prevent tooth erosion under acidic conditions. The structure of the polymer, synthesized by reversible addition-fragment transfer (RAFT) polymerization, was confirmed by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). While the hydrophilic PMOEP block within the amphiphilic block copolymer strongly binds to the enamel surface, the PMMA block forms a hydrophobic shell to prevent acid attack on tooth enamel, thus preventing/reducing acid erosion. The polymer treatment not only effectively decreased the mineral loss of hydroxyapatite (HAP) by 36-46% compared to the untreated control, but also protected the surface morphology of the enamel specimen following exposure to acid. Additionally, experimental results confirmed that low pH values and high polymer concentrations facilitate polymer binding. Thus, the preliminary data suggests that this new amphiphilic diblock copolymer has the potential to be used as a non-fluoride ingredient for mouth-rinse or toothpaste to prevent/reduce tooth erosion.

Controlled alignment of filamentous supramolecular assemblies of biomolecules into centimeter-scale highly ordered patterns by using nature-inspired magnetic guidance

We took the advantage of the capability of magnetic nanoparticles (MNPs) being aligned along a magnetic field and reproducibly generated large scale bio-nanofiber assemblies with the orientation of the constituent bio-nanofibers defined by the applied magnetic field. When decorated by MNPs, bio-nanofibers could be guided by the external magnetic field to become oriented either horizontally or vertically, forming single- and multi-orientation layered assemblies.

Nanoscale confinement controls the crystallization of calcium phosphate: relevance to bone formation

A key feature of biomineralization processes is that they take place within confined volumes, in which the local environment can have significant effects on mineral formation. Herein, we investigate the influence of confinement on the formation mechanism and structure of calcium phosphate (CaP). This is of particular relevance to the formation of dentine and bone, structures of which are based on highly mineralized collagen fibrils. CaP was precipitated within 25-300 nm diameter, cylindrical pores of track etched and anodised alumina membranes under physiological conditions, in which this system enables systematic study of the effects of the pore size in the absence of a structural match between the matrix and the growing crystals. Our results show that the main products were polycrystalline hydroxapatite (HAP) rods, together with some single crystal octacalcium phosphate (OCP) rods. Notably, we demonstrate that these were generated though an intermediate amorphous calcium phosphate (ACP) phase, and that ACP is significantly stabilised in confinement. This effect may have significance to the mineralization of bone, which can occur through a transient ACP phase. We also show that orientation of the HAP comparable, or even superior to that seen in bone can be achieved through confinement effects alone. Although this simple experimental system cannot be considered, a direct mimic of the in vivo formation of ultrathin HAP platelets within collagen fibrils, our results show that the effects of physical confinement should not be neglected when considering the mechanisms of formation of structures, such as bones and teeth.

Structure-function-property-design interplay in biopolymers: spider silk

Spider silks have been a focus of research for almost two decades due to their outstanding mechanical and biophysical properties. Recent advances in genetic engineering have led to the synthesis of recombinant spider silks, thus helping to unravel a fundamental understanding of structure-function-property relationships. The relationships between molecular composition, secondary structures and mechanical properties found in different types of spider silks are described, along with a discussion of artificial spinning of these proteins and their bioapplications, including the role of silks in biomineralization and fabrication of biomaterials with controlled properties.

Flagellar display of bone-protein-derived peptides for studying peptide-mediated biomineralization

A bacterial flagellum is self-assembled primarily from thousands of flagellin (FliC), a protein subunit. A foreign peptide can be fully displayed on the surface of the flagellum through inserting it into every constituent protein subunit. To shed light on the role of bone proteins during the nucleation of hydroxyapatite (HAP), representative domains from type I collagen, including part of the N-,C-terminal, N-,C-zone around the hole zone and an eight repeat unit Gly-Pro-Pro (GPP8) sequence similar to the central sequence of type I collagen, were separately displayed on the surface of the flagella. Moreover, eight negatively charged, contiguous glutamic acid residues (E8) and two other characteristic sequences derived from a representative noncollagenous protein called bone sialoprotein (BSP) were also displayed on flagella. After being incubated in an HAP supersaturated precursor solution, flagella displaying E8 or GPP8 sequences were found to be coated with a layer of HAP nanocrystals. Very weak or no nucleation was observed on flagella displaying other peptides being tested. We also found that calcium ions can induce the assembly of the negatively charged E8 flagella into bundles mimicking collagen fibers, followed by the formation of HAP nanocrystals with the crystallographic c axis preferentially aligned with long axis of flagella, which is similar to that along the collagen fibrils in bone. This work demonstrates that because of the ease of the peptide display on flagella and the self-assembly of flagella, flagella can serve as a platform for studying biomineralization and as a building block to generate bonelike biomaterials.

Influence of the charge relay effect on the silanol condensation reaction as a model for silica biomineralization

The catalytic effect of various sequential peptides for silica biomineralization has been studied. In peptide sequence design, lysine (K) and histidine (H) were selected as the standard amino acids and aspartic acid (D) was selected to promote the charge relay effects, such as in the enzyme active site. Therefore, homopolypeptides (K(10) and H(10)), block polypeptides (K(5)D(5) and H(5)D(5)), and alternate polypeptides [(KD)(5) and (HD)(5)] were designed, and the dehydration reaction ability of trimethylethoxysilane was investigated as a quantitative model of silica mineralization. The catalytic activity per basic residue of alternate polypeptide was the highest because of the charge relay effects at the surface of the peptide. In silica mineralization using tetraethoxysilane, spherical silica particles were obtained, and their size is related to the catalytic activities of the peptides in the model systems. From these results, the effect of the functional group combination by the peptide sequence design enables the control of the efficiency of mineralization and preparation of specific inorganic materials.

Transmission electron microscopy as a tool to image bioinorganic nanohybrids: the case of phage-gold nanocomposites

In recent years, bioinorganic nanohybrids composed of biological macromolecules and functional inorganic nanomaterials have revealed many unique properties that show promise for the future. Transmission electron microscopy (TEM) is a popular and relatively simple tool that can offer a direct visualization of the nanomaterials with high resolutions. When TEM is applied to visualize bioinorganic nanohybrids, a treatment of negative staining is necessary due to the presence of biological molecules in the nanohybrids except for those with densely packed inorganic materials. However, the conventional negative-staining procedure for regular biological samples cannot be directly applied to such bioinorganic nanohybrids. To image a specific bioinorganic nanohybrid, negative-staining factors such as negative stain type, working pH, staining time, and drying method, should be identified. Currently, no detailed studies have been done to investigate how to adjust negative-staining factors based on specific bioinorganic nanohybrids. In this study, bacteriophage-gold nanoparticle hybrids were chosen as a model to systematically study the effects of each factor on the negative staining of the nanohybrids. The best staining conditions for gold nanoparticle-phage nanohybrids were obtained and the effects of each factor on the negative staining of general nanohybrids were discussed. This work indicates that with proper staining it is possible to use TEM to visualize directly both biological and inorganic components without introducing any artifact.

Self-assembly and mineralization of genetically modifiable biological nanofibers driven by β-structure formation

Bioinspired mineralization is an innovative approach to the fabrication of bone biomaterials mimicking the natural bone. Bone mineral hydroxylapatite (HAP) is preferentially oriented with c-axis parallel to collagen fibers in natural bone. However, such orientation control is not easy to achieve in artificial bone biomaterials. To overcome the lack of such orientation control, we fabricated a phage-HAP composite by genetically engineering M13 phage, a nontoxic bionanofiber, with two HAP-nucleating peptides derived from one of the noncollagenous proteins, Dentin Matrix Protein-1 (DMP1). The phage is a biological nanofiber that can be mass produced by infecting bacteria and is nontoxic to human beings. The resultant HAP-nucleating phages are able to self-assemble into bundles by forming β-structure between the peptides displayed on their side walls. The β-structure further promotes the oriented nucleation and growth of HAP crystals within the nanofibrous phage bundles with their c-axis preferentially parallel to the bundles. We proposed that the preferred orientation resulted from the stereochemical matching between the negatively charged amino acid residues within the β-structure and the positively charged calcium ions on the (001) plane of HAP crystals. The self-assembly and mineralization driven by the β-structure formation represent a new route for fabricating mineralized fibers that can serve as building blocks in forming bone repair biomaterials and mimic the basic structure of natural bones.

Bacteriophage Bundles with Pre-Aligned Ca Initiate the Oriented Nucleation and Growth of Hydroxylapatite

Inorganic ions may direct the self-assembly of biomacromolecules into nanostructures which can further be used as a reactant and matrix for nanomaterials synthesis and assembly. Here we use bone mineral and filamentous bacteriophage as a model to demonstrate this concept. Divalent calcium ions are found to trigger the electrostatic self-assembly of anionic nanofiber-like bacteriophages into bundle structures where calcium ions are pre-organized between bacteriophage nanofibers. The resultant Ca(2+)-bacteriophage bundles can be separated and purified from the aqueous solution. The nanostructures of the bundles are verified by zeta potential analysis, small angle x-ray scattering and transmission electron microscopy. Because of the transcription of the bacteriophage chiral surface to the periodic alignment of pre-loaded Ca(2+), the Ca(2+)-bacteriphage bundles can serve as both Ca sources and biotemplates to initiate the oriented nucleation and growth of nanocrystalline hydroxyapatite in phosphate solution or in simulated body fluid. This work provides new insights into biomineralization and represents a new approach to the fabrication of biomolecular-inorganic hybrid layered nanostructures.

Engineered disulfides improve mechanical properties of recombinant spider silk

Nature's high-performance polymer, spider silk, is composed of specific proteins, spidroins, which form solid fibers. So far, fibers made from recombinant spidroins have failed in replicating the extraordinary mechanical properties of the native material. A recombinant miniature spidroin consisting of four poly-Ala/Gly-rich tandem repeats and a nonrepetitive C-terminal domain (4RepCT) can be isolated in physiological buffers and undergoes self assembly into macrofibers. Herein, we have made a first attempt to improve the mechanical properties of 4RepCT fibers by selective introduction of AA --> CC mutations and by letting the fibers form under physiologically relevant redox conditions. Introduction of AA --> CC mutations in the first poly-Ala block in the miniature spidroin increases the stiffness and tensile strength without changes in ability to form fibers, or in fiber morphology. These improved mechanical properties correlate with degree of disulfide formation. AA --> CC mutations in the forth poly-Ala block, however, lead to premature aggregation of the protein, possibly due to disulfide bonding with a conserved Cys in the C-terminal domain. Replacement of this Cys with a Ser, lowers thermal stability but does not interfere with dimerization, fiber morphology or tensile strength. These results show that mutagenesis of 4RepCT can reveal spidroin structure-activity relationships and generate recombinant fibers with improved mechanical properties.

Enzyme Directed Templating of Artificial Bone Mineral

Bone is one of Nature's most remarkable materials, not only for its mechanical properties but also for its ability to repair fractures and remodel its microstructure in response to stress. At the nanoscale bone is a supramolecular matrix of collagen fibers reinforced by hydroxyapatite crystals with a high degree of order. Emulating elements of the biological synthesis of this composite could help develop strategies for advanced materials. Previous work has demonstrated the use of functionalized peptide amphiphile nanofibers in a two-dimensional system to emulate hydroxyapatite mineralization in natural bone. We describe here an artificial, in vitro biomineralization process that allows a similar process to occur in three dimensions. The system employs the natural enzyme alkaline phosphatase and a phosphorylated, anionic nanofiber gel matrix to template hydroxyapatite nanocrystals with size, shape, and crystallographic orientation resembling natural bone mineral. The formation of this biomimetic mineral in three dimensions results from the synergy of fiber-induced nucleation and the temporal control of phosphate ion harvesting by the enzyme. Gradual enzymatic harvesting of ions for crystal growth and the strong nucleating ability of the phosphorylated fibers suppresses uncontrolled precipitation of mineral. The strategy could lead to biomimetic materials to promote bone regeneration or the synthesis of hybrid materials with crystallographically defined structures.

Biotemplated syntheses of anisotropic nanoparticles

The technological advances predicted (or, perhaps, demanded) for the twenty-first century are intimately linked to the crystallochemically controlled synthesis of high-performance functional materials. To answer the new hendiatris of ‘smaller, faster, better’, the manufacture of these materials as nanoparticles has become a scientific noblesse oblige . Direct incorporation into the next generation of electronic devices will necessitate anisotropic forms of these materials, be they nanowires, nanotapes or nanotubes. Chemists have recently discovered that, in addition to the classical methods of anisotropic growth, new routes allow more complex materials to be synthesized in these morphologies. This review describes, using a series of examples, how the morphology of functional materials can be controlled using templated growth mediated by a biopolymer. By involving a biopolymer in the synthetic protocol, anisotropic nanoparticles and assemblages of even quite complex materials can be generated in syntheses that are simple, elegant and highly specific.

Templating effect of silk fibers in the oriented deposition of aragonite

Controlled deposition of calcium carbonate crystals can be obtained on degummed Bombyx mori silk fibers through the use of a silk fibroin solution; aragonite crystallites are found on the surface of the fiber with consistent orientation along the longitudinal axis; the results indicate that the combination of the ordered surface structure on the silk fiber and the directing-effect of silk fibroin solution are the key factors in the orientative deposition process of the mineral.