Effect of silicon on the formation of silk fibroin/calcium phosphate composite

The silk fibroin/calcium phosphate composites were prepared by adding the different amount of Na₂SiO₃ to assess the effect of silicon on the HA (hydroxyapatite) formation in the composites. FTIR and XRD results suggested that the inorganic phase was constituted mainly by the amorphous DCPD (dicalcium phosphate dehydrate), a precursor of HA in the bone mineral, when the composites were prepared at the final Na₂SiO₃ concentration lower than 0.008%. Otherwise, HA was formed as the predominant one in the as-prepared composite, accompanied with a conformational transition in the organic phase of silk fibroin protein from silk I (α-helix and/or polyglycine II (3₁–helix) conformations) to silk II (antiparallel β-sheet conformation). SEM images showed the different morphologies with the samples, i.e., sheet-like crystals in the composites prepared at a low Na₂SiO₃ concentration and rod-like bundles in other composites. The rod-like bundles were connected together to form the porous network, due to the fact that the HA crystals grew with the aggregation of silk fibroin, and further accreted onto the silk fibroin fibrils. TG curves indicated that the composites prepared with a certain amount of additional SiO₃²⁻ had the higher thermal stability because of its high molecular orientation and crystallinity, and high water-holding capacity due to the porous microstructure.     

In-vitro study on calcium carbonate crystal growth mediated by organic matrix extracted from fresh water pearls

For the purpose of studying the mediation of organic matrix on the crystallization of calcium carbonate, water soluble matrix (WSM), acid soluble matrix (ASM) and acid insoluble matrix (AIM) were extracted from aragonite pearls and vaterite pearls respectively. Then, in-vitro calcium carbonate crystallization experiments under the control of these six organic matrices were carried out in the present study. Scanning electron microscopy (SEM) was utilized to observe the morphology of CaCO₃ and Raman spectroscopy as a powerful technique was used to distinguish the crystal polymorph. Influences of the six kinds of organic matrices on the calcium carbonate crystal growth are proposed. ASM of vaterite pearls can induce vaterite to crystallize and WSM of aragonite pearls mediates to produce aragonite crystals. The single AIM membranes of the two pearls have no pronounced effect on the CaCO₃ crystallization. Additionally, the crystal size obtained with the additive of WSM of the two kinds of pearls is smaller than that with the additive of ASM. Moreover, self-assembly phenomenon in the biomineralization process and the distorted morphology calcite are observed. Current results demonstrate important aspects of matrix protein-controlled crystallization, which is beneficial to the understanding of nacre biomineralization mechanism. Further study of the precise control of these matrix proteins on CaCO₃ crystal growth is being processed.     

Experimental investigations on the effects of asparagine and serine on the polymorphism of calcium carbonate

The present study investigated the effects of the amino acids asparagine and serine, as additives, on the polymorphic transformation of calcium carbonate (CaCO₃) within the concentration range of 25–100 ppm at 30 °C and pH 12. The structural composition and morphology of the samples prepared with and without additives were evaluated experimentally through X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) analysis. The XRD results showed that the calcite composition of CaCO₃ samples prepared with asparagine and serine reduced with increasing concentrations of both additives. SEM images showed that CaCO₃ prepared with asparagine and serine had three coexisting crystal forms: cubic-shaped calcite, spherical shaped vaterite, and needle-like aragonite crystals. Moreover, the addition of both additives separately was found to increase the average particle size and Brunauer–Emmett–Teller (BET) specific surface area of the crystals. Higher concentrations of serine and asparagine individually in the crystallization media resulted in a more negative zeta potential. Meanwhile, the thermal behavior, kinetics, and thermodynamics of the CaCO₃ crystals were simultaneously evaluated by means of by thermogravimetric analysis (TGA) coupled with Fourier transform infrared (FTIR) and mass spectrometry (MS). The master plots method combined with the Friedman method revealed that the decomposition of CaCO₃ prepared in pure media followed the contracting volume mechanism, R₃. Positive values of ΔH and ΔG were obtained for CaCO₃ decomposition.     

High-concentration dispersions of exfoliated MoS<Subscript>2</Subscript> sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS₂ sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS₂ nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS₂ nanosheets using freeze-dried silk fibroin powders. Compared to MoS₂ dispersion in the absence of silk fibroin powder, sonicated MoS₂ dispersions with silk fibroin powder (MoS₂/Silk dispersion) show noticeably higher exfoliated MoS₂ nanosheet yields, with suspended MoS₂ concentrations in MoS₂/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL^–1, respectively. The MoS₂ concentration in the MoS₂/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS₂ nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS₂ nanosheets spin-cast from MoS₂ /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS₂ crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS₂ nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS₂ nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS₂ nanosheets contents.

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Mechanical properties of talc- and CaCO3-reinforced high-crystallinity polypropylene composites

Toughening mechanisms and mechanical properties of two high-crystallinity polypropylene (hcPP)-based composite systems, hcPP/talc and hcPP/CaCO₃, are investigated. Significant improvement in tensile modulus is observed in the PP/talc composite, but only a moderate improvement is found for hcPP/CaCO₃. The introduction of CaCO₃ nanoparticles to hcPP helps nucleate a measurable amount of β-phase crystals and results in a significant drop in crystallization temperature, suggesting a possible retardation of hcPP crystallization. In addition, the hcPP/CaCO₃ nanocomposite shows more pronounced damping characteristics than that of hcPP/talc, throughout the temperature range studied. A detailed investigation of fracture mechanisms suggests that well-dispersed, highly oriented talc particles cause embrittlement of hcPP. Only when the crack extends toward the edges of the specimen will the crack deflection/bifurcation and microcracking mechanisms initiate. In the case of hcPP/CaCO₃, the CaCO₃ nanoparticles help trigger massive crazing and shear yielding if the testing speed is in quasi-static. The presence of β-phase crystals around the CaCO₃ particles could facilitate the formation of crazes throughout the hcPP matrix. Approaches for toughening hcPP are discussed.     

Formation of Helically Structured Chitin/CaCO3 Hybrids through an Approach Inspired by the Biomineralization Processes of Crustacean Cuticles

Chitin/CaCO₃ hybrids with helical structures are formed through a biomineralization‐inspired crystallization process under ambient conditions. Liquid‐crystalline chitin whiskers are used as helically ordered templates. The liquid‐crystalline structures are stabilized by acidic polymer networks which interact with the chitin templates. The crystallization of CaCO₃ is conducted by soaking the templates in the colloidal suspension of amorphous CaCO₃ (ACC) at room temperature. At the initial stage of crystallization, ACC particles are introduced inside the templates, and they crystallize to CaCO₃ nanocrystals. The acidic polymer networks induce CaCO₃ crystallization. The characterization of the resultant hybrids reveals that they possess helical order and homogeneous hybrid structures of chitin and CaCO₃, which resemble the structure and composition of the exoskeleton of crustaceans.     

Elucidating the role of charge density on the growth of CaCO3 crystals underneath Calix[4]arene monolayers

The amphiphilic 5,11,17,23-tetrakis-(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(2-hydroxyethoxy)calix[4]arene (1) forms stable monolayers at the air–water interface. The growth of CaCO₃ crystals underneath monolayers of 1 is strongly inhibited, in contrast to the corresponding carboxylic acid derivative 5,11,17,23-tetrakis-(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(carboxymethoxy)calix[4]arene (2), the monolayers of which lead to growth of preferentially oriented calcite single crystals. The growth morphology of CaCO₃ crystals is correlated with the phase behaviour and surface potential of the monolayers. The investigations demonstrate that the average charge density is the dominant factor for heterogeneous nucleation of CaCO₃ crystals at the calixarene monolayer/solution interface.     

Preparation and characterization of regenerated fiber from the aqueous solution of Bombyx mori cocoon silk fibroin

The regenerated silk fibers with high strength and high biodegradability were prepared from the aqueous solution of Bombyx mori silk fibroin from cocoons with wet spinning method. Although the tensile strength of the regenerated silk fibroin fiber, 210 MPa is still half of the strength of native silk fiber, the diameter of the fiber is about 100 μm which is suitable for monofilament of suture together with high biodegradability. The high concentration (30%, w/v) of the aqueous solution of the silk fibroin which corresponds to the high concentration in the middle silkgland of silkworm was obtained. This was performed by adjusting the pH of the aqueous solution to 10.4 which corresponds to pK_a value of the OH group of Tyr residues in the silk fibroin. The mixed solvent, methanol/acetic acid (7:3 in volume ratio) was used as coagulant solvent for preparing the regenerated fiber. The structural change of silk fibroin fiber by stretching was monitored with both ¹³C solid state NMR and X-ray diffraction methods, indicating that the high strength of the fiber is related with the long-range orientation of the silk fibroin chain with β-sheet structure.     

Calcium carbonate crystallization in the presence of taurine

The kinetics of calcite (CaCO₃) crystallization on calcite seed crystals in the presence of taurine was investigated by the constant composition method. The presence of taurine (4× 10^{− 5}− 4× 10⁻⁴M) in the supersaturated solutions lead to calcite crystals with a characteristic discontinuous planes of growth and poor habit, as compared to the hombohedral morphology of the seed crystals. The acceleration effect of taurine on the crystal growth rate was 17–96%. The apparent order of the crystal growth was found to be 2.0± 0.2 typical for a surface diffusion-controlled spiral growth processes.     

Biological synthesis of calcite crystals using <Emphasis Type="Italic">Scindapsus aureum</Emphasis> petioles

We report a novel strategy for the biological synthesis of calcite crystals using the petioles of the plant Scindapsus aureum. The resultant calcite crystals were characterized by scanning electron microscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray powder diffractometry, and electron diffraction. The biomolecules of S. aureum petioles were confirmed by UV–Vis and FT-IR analysis. The results showed that the spherical or rhombohedral calcite crystals were obtained in the cells of S. aureum petioles. Biomimetic synthesis of calcium carbonate (CaCO₃) in aqueous solution containing extracts of S. aureum petioles was also performed to investigate the soluble biomolecules’ influence on crystal growth of CaCO₃. It was found that twinborn spherical calcite crystals were formed, suggesting that the soluble biomolecules of S. aureum play a crucial role in directing the formation of calcite spherical particles. The possible mechanism of formation of CaCO₃ crystals using S. aureum is also discussed; the biomolecules of S. aureum may induce and control the nucleation and growth of CaCO₃ crystals.     

The co-effect of organic matrix from carp otolith and microenvironment on calcium carbonate mineralization

In vitro mineralization experiment is an effective way to study the effect of organic matrix on calcium carbonate crystallization, and to reveal the relationship between organic matrix and inorganic crystal in natural biominerals. In natural biominerals, organic matrix plays an important role in crystal formation and stability, together with microenvironment changes, they can affect crystal polymorph, morphology, density, size, orientation etc.In this work, we systematically studied the effects of different organic matrices in fish otoliths, the organic matrix concentration changes, as well as the co-effect of organic matrices with temperature, pH value and Mg ion changes in the in vitro CaCO₃ mineralization experiments.The organic matrix and concentration change experiments prove that water soluble matrix (WSM) plays an important role in crystal form transition. It can induce CaCO₃ crystals with same crystal polymorph as the otolith from which organic matrix was extracted. The temperature change experiment proves that CaCO₃ has a tendency to form calcite, vaterite, and then aragonite in priority as temperature goes up. Under different temperature, WSM from lapillus/asteriscus still has the effect to mediate different CaCO₃ crystals. The pH change experiment shows that, near the neutral environment, as pH value goes up, calcites have a tendency to form crystal aggregates with more faces exposed, the organic matrix still keeps crystal mediation effect. The Mg^2 + experiment shows that, Mg ion can promote aragonite formation, together with lapillus organic matrix, aragonites with different shapes are formed.     

Modification of CaCO3 precipitation rates by water-soluble nacre proteins

Mineral growth in nacre and other CaCO₃-containing biominerals is controlled by biopolymers. Water-soluble proteins were extracted from nacre of the sea snail Haliotis laevigata by dissolving the mineral phase with 6% acetic acid. The influence of this protein mixture on CaCO₃ precipitation rates was investigated at different concentrations. A well-established assay for measuring the pH-value during CaCO₃ precipitation with and without protein additives was extended by calculating maximum precipitation rates from the pH-values. It could be shown that precipitation rates are greatly influenced by the mixtures of water-soluble nacre proteins. At very low protein concentrations (0.02 μg/ml) a rate enhancement in comparison to the pure supersaturated calcium carbonate solution by a factor of 1.4 was observed. At higher protein concentrations, a strong inhibitory effect occurred, with total inhibition at concentrations of 1.0 μg/ml and higher. Two unspecific proteins (bovine serum albumin and lysozyme) showed little or no modification of precipitation rates. In vivo, the function of the strong inhibition of CaCO₃ precipitation by nacre proteins at higher concentrations is presumably to prevent uncontrolled crystallization in the extrapallial fluid. The rate-enhancing capability of proteins at low concentrations may be explained by the presence of acidic and/or hydrophilic moieties.     

Calcium carbonate growth in the presence of water soluble cellulose ethers

Calcium carbonate precipitation was performed in the presence of methyl cellulose (MC) and two kinds of hydroxyethyl cellulose (HEC FD-10000, HEC FD-30000). The results demonstrated that the final product morphology and structure of CaCO₃ crystals are highly sensitive to the concentration of the cellulose ethers aqueous solution. By precisely controlling their concentrations, all these three cellulose ethers solutions have the ability of protecting metastable vaterite from thermodynamically transforming into stable calcite. The intermediate products investigation showed to some extent the phase transformation of calcium carbonate in its growing process from metastable vaterite to calcite and indicated that the calcium carbonate crystal growth in HEC solutions occurs through dissolution and reprecipitation process. Calcium carbonate growth in both presence of HEC and ethanol or Mg²⁺ was also examined. This work demonstrates the potential of water soluble cellulose ethers in controlling biominerals crystallization and growth. The results are revelatory for biomineralization and fabricating new organic–inorganic hybrids based on cellulose derivatives.     

Abnormal polymorph conversion of calcium carbonate from calcite to vaterite

Inspired by mineralization in biological organisms, the production of higher ordered CaCO₃ crystals modified by surfactants has received much attention. Normal micelles formed by anionic surfactant, sodium bis-2-ethylhexyl-sulfosuccinate (aerosol OT, AOT), was used as a template to direct the growth of CaCO₃ crystals in the present work. When the concentration of the AOT increased from 0.5 to 5.0 mM, the polymorph of CaCO₃ crystals changed from almost pure aragonite to almost pure vaterite at 150 °C. In the meantime, various morphologies of vaterite were obtained, such as astral and hexagonal structures. Based on the time-resolved experiments, we suggested that AOT micelles induced the formation of these structures. And the micelles were wrapped in the hexagonal structures, and dissolved in the aqueous solution with increase of the experimental time. Thus, hollow shells were produced. Abnormal polymorph transformation of CaCO₃ crystals from the calcite to the vaterite was found. These effects of AOT on polymorph and morphology of CaCO₃ crystals have not been reported as far as we know.     

Controlled synthesis of hollow calcite microspheres modulated by polyacrylic acid and sodium dodecyl sulfonate

Hollow CaCO₃ microspheres were successfully synthesized through the precipitation reaction of Na₂CO₃ with CaCl₂ in the presence of polyacrylic acid (PAA) and sodium dodecyl sulfonate (SDS) at 80 °C. The concentration of SDS is an important factor to control the synthesis of hollow CaCO₃ microspheres. X-ray diffraction confirmed that the hollow CaCO₃ microsphere consists of calcite crystals. The “pearl-necklace model” of PAA/SDS micellar aggregates serves as the spherical templates to generate hollow microspheres of CaCO₃ crystals in the precipitation system.     

Novel silk fibroin/hydroxyapatite composite films: Structure and properties

Novel composite films of Bombyx mori silk fibroin (SF) and hydroxyapatite (HA) composite films, with glycerin as an additive, were fabricated by means of co-precipitation, where the theoretical HA content was varied from 2 (w/w)% to 31 (w/w)%. The structure and properties of the composite films were investigated by SEM, XRD, AFM, TGA and tensile testing. The results showed that the composite films were smooth and transparent with the uniform distribution of HA into the composites when the final HA content was lower than 21 (w/w)%. XRD and TGA data showed that the silk fibroin in the composites was predominantly in a β-sheet crystalline structure, which was induced not only by the addition of glycerin, also by the HA crystal growth during the composite fabrication, leading to the thermal stable composite films. On the other hand, the HA crystals had the anisotropic growth with high extent of lattice imperfection and the preferential orientation along c-axis, probably promoted by the silk fibroin. The mechanical testing results showed that both break strain and stress were declined with the increase of HA content in the composites, presumably due to the original brittleness of HA compound.     

Effect of CaCO3 on the crystallization behaviour of polypropylene

The effect of CaCO₃ on the crystallization behaviour and the modification of structure in polypropylene (PP) has been investigated using X-ray diffraction and optical microscopy. The crystallization half time deduced from spherulitic growth rate was found to vary sharply and decreased considerably from 5–6 min for pure polymer to less than 2 min in the presence of CaCO₃. The ultimate spherulite size also decreased considerably for PP containing CaCO₃. Its dependance on composition however, showed a plateau region at about 10–15 wt % of additive. The intensities of certain reflections especially the 130 and 040 of the phase were greatly affected by the presence of CaCO₃ and large variations in the crystallinity (Ci) values were observed with composition. The ratio of intensities of 130 and 040 reflections and theCi revealed a maximum at a certain concentration of CaCO₃. The above results can be explained on the basis of nucleation and preferential growth of the phase of PP crystallites.     

Bovine pericardium coated with biopolymeric films as an alternative to prevent calcification: In vitro calcification and cytotoxicity results

Bovine pericardium, for cardiac valve fabrication, was coated with either chitosan or silk fibroin film. In vitro calcification tests of coated and non coated bovine pericardium were performed in simulated body fluid solution in order to investigate potential alternatives to minimize calcification on implanted heart valves. Complementary, morphology was assessed by scanning electron microscopy — SEM; X-ray diffraction (XRD) and infrared spectroscopy (FTIR-ATR) were performed for structural characterization of coatings and biocompatibility of chitosan. Silk fibroin films were assayed by in vitro cytotoxicity and endothelial cell growth tests. Bovine pericardium coated with silk fibroin or chitosan did not present calcification during in vitro calcification tests, indicating that these biopolymeric coatings do not induce bovine pericardium calcification. Chitosan and silk fibroin films were characterized as non cytotoxic and silk fibroin films presented high affinity to endothelial cells. The results indicate that bovine pericardium coated with silk fibroin is a potential candidate for cardiac valve fabrication, since the affinity of silk fibroin to endothelial cells can be explored to induce the tissue endothelization and therefore, increase valve durability by increasing their mechanical resistance and protecting them against calcification.     

Laser patterning and morphology of two-dimensional planar ferroelastic rare-earth molybdate crystals on the glass surface

The laser-induced crystallization method is applied to pattern two-dimensional planar crystals consisting of ferroelastic β′-(Sm,Gd)₂(MoO₄)₃ crystals (designated here as SGMO crystals) on the surface of Sm₂O₃–Gd₂O₃–MoO₃–B₂O₃ glass. By scanning Yb:YVO₄ fiber lasers (wavelength: 1080 nm) continuously with a small pitch (0.7 μm) between laser irradiated parts, planar SGMO crystals with periodic domain structures showing different refractive indices are patterned successfully, and a high orientation of SGMO crystals is confirmed from micro-Raman scattering spectrum and second harmonic intensity measurements. It is found that the crystal growth direction is perpendicular to the laser scanning direction. This relation, i.e., the perpendicular relation, is a different from the behavior in discrete crystal line patterning, where the crystal growth direction is consistent with the laser scanning direction. The present study proposes the possibility of the control of crystal growth direction in laser-induced crystallization in glasses.     

Effects of pH and initial Ca2+?H2PO4? concentration on fibroin mineralization

In the present study, the effects of pH and initial Ca²⁺−H₂PO₄ ⁻ (Ca-P) concentration on fibroin mineralization were studied. The crystal growth of calcium phosphates was regulated by regenerated silk fibroin for 8 h (at pH 4.0, 7.0 and 10.0, respectively). Meanwhile, different concentrations of Ca²⁺ were employed at a certain pH value, keeping the initial Ca-P molar ratio constant at 1.67, i.e., the stoichiometry of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, HAP]. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results demonstrated that, compared to pH 4.0 and 10.0, pH 7.0 promoted the transformation of brushite (CaHPO₄·2H₂O, DCPD) to HAP. In the composites of mineralized fibroin, DCPD is the main inorganic phase at both relatively low and high pH, while HAP is the main inorganic phase at pH 7.0. Additionally, the initial Ca-P concentration does not affect the kind of inorganic phase in the synthesized mineralized fibroin, but induce to different contents of inorganic mineral and different morphology of DCPD at pH 4.0 and pH 10.0.