Nucleation of Hydroxyapatite Crystal through Chemical Interaction with Collagen

The nucleation of hydroxyapatite (HAp) crystal through chemical interaction with collagen was investigated. A collagen membrane was soaked in a supersaturated simulated body fluid (1.5 SBF) solution with ion concentrations at 1.5 times that of normal simulated body fluid (1.0 SBF). A few carbonate-containing HAp crystals were formed mostly on the edge-side of the collagen membrane after 4 weeks. In the Fourier-transform infrared spectometry (FTIR) results, the carboxylate band of the collagen membrane showed red chemical shifts after the formation of HAp crystals, which coincided well with the decrease of the calculated bond orders of the carboxylate group when chelated with a calcium ion, which emulated the first-step nucleation of HAp crystal on the carboxylate group of collagen. The result implies that the binding of a calcium ion to the carboxylate group of collagen is one of the key factors for the nucleation of HAp crystals in a 1.5 SBF solution.     

Investigation of Apatite Deposition onto Charged Surfaces in Aqueous Solutions Using a Quartz-Crystal Microbalance

Hydroxyapatite (HAp) deposition onto positively charged surfaces (i.e., self-assembled monolayers (SAMs) terminated with NH₂ head groups) and negatively charged surfaces (i.e., OH-SAMs (weak) and COOH-SAMs (strong)) soaked at 50°C in aqueous supersaturated solutions (1.5 SBF, pH 7.0–7.6; SBF = simulated body fluid) was investigated using a quartz-crystal microbalance. The results revealed that the solution conditions greatly influenced the formation of HAp on the charged surfaces. In a stable supersaturated solution of simulated body fluid (1.5 SBF, pH <7.4), more strongly negative surfaces had a more powerful induction capability for the heterogeneous nucleation of HAp (COOH > OH), whereas nucleation was obviously prohibited on a positive surface (NH₂-SAM). On the other hand, after the calcium phosphate particles had nucleated homogeneously in an unstable soaking solution (1.5 SBF, pH ≧7.4), adhesion of the HAp microparticles to the NH₂-SAM was observed. A two-step interaction is conceivable to describe the formation of HAp on the positive NH₂-SAM: At the first stage, electrostatic interaction dominates the adhesion of HAp microparticles; at the second stage, hydrogen bonds possibly form between the HAp microparticles and the amino head groups of the NH₂-SAM, for a firm bonding with the substrate, and the microparticles grow progressively into a thin film. The electrophoretic behaviors of the HAp microparticles confirmed this hypothesis.     

ORDERING TRANSITIONS IN LIQUID CRYSTALS PERMIT IMAGING OF SPATIAL AND TEMPORAL PATTERNS FORMED BY PROTEINS PENETRATING INTO LIPID-LADEN INTERFACES

Recent studies have reported that full monolayers of L-α-dilaurylphosphatidylcholine (L-DLPC) and D-α-dipalmitoylphosphatidylcholine (D-DPPC) formed at interfaces between thermotropic liquid crystals (LCs) and aqueous phases lead to homeotropic (perpendicular) orientations of nematic LCs and that specific binding of proteins to these interfaces (such as phospholipase A ₂ binding to D-DPPC) can trigger orientational ordering transitions in the liquid crystals. We report on the nonspecific interactions of proteins with aqueous-LC interfaces decorated with partial monolayer coverage of L-DLPC. Whereas nonspecific interactions of four proteins (cytochrome c, bovine serum albumin, immunoglobulins, and neutravidin) do not perturb the ordering of the LC when a full monolayer of L-DLPC is assembled at the aqueous-LC interface, we observe patterned orientational transitions in the LC that reflect penetration of proteins into the interface of the LC with partial monolayer coverage of L-DLPC. The spatial patterns formed by the proteins and lipids at the interface are surprisingly complex, and in some cases the protein domains are found to compartmentalize lipid within the interfaces. These results suggest that phospholipid-decorated interfaces between thermotropic liquid crystals and aqueous phases offer the basis of a simple and versatile tool to study the spatial organization and dynamics of protein networks formed at mobile, lipid-decorated interfaces.     

Influence of concentration of hydroxyapatite surface modifier agent on bioactive composite characteristics

The characterization of chitosan – hydroxyapatite (CH – HAp) composite sponges prepared via freeze-drying methodology is reported in this study. Stearic acid (SA), added as a surface modifier of the HAp nanoparticles, induced changes in the TG/DTG results, particle size distribution and particle morphology. Composite sponges prepared with SA coated HAp demonstrated enhanced biocompatibility and structural properties, as compared to the composites prepared with uncoated HAp. SA coating modified the morphology of the composite, promoting a better dispersion of HAp particles within the composite sponges, and better homogeneity of the polymeric cover with HAp particles. The viability of the composites for cell culture applications was analyzed, and the results suggest that the sponges are biocompatible. Therefore, SA proved to be a good candidate for surface coating of HAp nanoparticles prevent agglomerations, and could be used effectively in the preparation of biocompatible composite sponges with chitosan.     

Characteristics and iso‐baric relaxation phenomenon of pmma monolayers with different molecular weights at different temperatures

Poly(methyl methacrylate) (PMMA) monolayers with different molecular weights at the air/water interface were investigated at three different temperatures. The monolayer characteristics of PMMA were studied in terms of surface pressure‐area per molecule (π‐A) isotherm and isobaric relaxation experiments. The results show that the π‐A isotherms of PMMA converge at 40°C regardless of molecular weight. The collapse pressure of PMMA monolayers decreases as the temperature is elevated. It was shown the area relaxation process of PMMA could be described by a model considering the nucleation and growth mechanisms. The simulation parameters of area relaxation of PMMA with different molecular weights at 8 mN/m are very similar, indicating similar mechanisms. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A method to fabricate hydroxyapatite/poly(lactic acid) microspheres intended for biomedical application

The interaction between organic and inorganic materials enables one to fabricate inorganic/organic microsphere composites. In a novel microemulsion method, we have prepared surfactant-free poly(lactic acid) (PLA)/hydroxyapatite (HAp) microspheres by using the interfacial interaction between inorganic and organic materials, because surfactants are undesireble materials for biomedical applications due to their non-biodegradability. The studies that varied the ion concentrations in oil/water microemulsions revealed that HAp was nucleated at the end group PLA, COOH, and the array of the PLA end groups determined the crystal phase of the calcium phosphate as HAp under the low ion concentration condition. PLA induced the nucleation and precipitation of HAp at the oil/water interface, simultaneously, the precipitated HAp stabilized the microemulsion without surfactants. This novel process made surfactants unnecessary for organic/inorganic microsphere fabrications.     

Mineralization of hydroxyapatite crystallites on zein microspheres

Bioinspired mineralization process has been extended to the formation of bonelike hydroxyapatite (HAp) coatings on biodegradable polymer substrates. HAp‐coated zein microspheres were prepared through phase separation procedure, followed by incubation in modified simulated body fluids (mSBF). The morphologies of mineralized zein microspheres with different diameters and incubation time were investigated by scanning electron microscopy (SEM). The nature of the deposited mineral phase was characterized by energy‐dispersive spectroscopy (EDS) and wide‐angle X‐ray diffraction (XRD). The results showed that as the average diameter of zein microspheres increased to 1.13 μm, randomly oriented minerals were observed on the surface of microspheres. Two characteristic HAp diffraction reflections 002 and 211 centered at 2θ of 25.6° and 32.0° were ascertained on the XRD patterns of the mineralized microspheres. Incubation of zein microspheres with larger diameters in mSBF led to nucleation and growth of minerals on the surface. Even larger number of minerals were deposited and a porous structure of platelike HAp crystals was obtained after incubation for 21 days. The novel zein/HAp microspheres may have a wide variety of potential applications in bone regeneration and tissue engineering. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Effect of Chondroitin Sulfate on the Crystal Growth of Hydroxyapatite

The effect of chondroitin sulfate (ChS) templates on the crystal growth of hydroxyapatite (HAp) was investigated. HAp particles and 60HAp/40ChS hybrid (in wt%) were synthesized by using a precipitation method with a calcium hydroxide suspension and phosphoric acid solutions without and with ChS, respectively. The sizes of HAp crystals formed without ChS templates were around 75 nm × 10 nm while those with ChS templates were around 12 nm × 6 nm. The inhibitory effect of ChS templates on the crystal growth of HAp is explained in terms of organic matrix regulated HAp formation following steric hindrance by adjacent crystals.     

Effects of humidity and surfaces on the melt crystallization of Ibuprofen

Melt crystallization of ibuprofen was studied to understand the effects of humidity and surfaces. The molecular self-assembly during the amorphous-to-crystal transformation was examined in terms of the nucleation and growth of the crystals. The crystallization was on Al, Au, and self-assembled monolayers with -CH(3), -OH, and -COOH functional groups. Effects of the humidity were studied at room temperature (18-20 °C) with relative humidity 33%, 75%, and 100%. Effects of the surfaces were observed at -20 °C (relative humidity 36%) to enable close monitoring with slower crystal growth. The nucleation time of ibuprofen was faster at high humidity conditions probably due to the local formation of the unfavorable ibuprofen melt/water interface. The crystal morphologies of ibuprofen were governed by the nature of the surfaces, and they could be associated with the growth kinetics by the Avrami equation. The current study demonstrated the effective control of the melt crystallization of ibuprofen through the melt/atmosphere and melt/surface interfaces.     

In situ synthesis and characterization of nano-size hydroxyapatite in poly(vinyl alcohol) matrix

Hydroxyapatite (HAp) crystals were prepared via an in situ biomimetic process in the presence of poly(vinyl alcohol) (PVA). The effect of polymer amount and its molecular weight on the physical properties of the HAp crystals were investigated. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) analysis, confirmed the formation of the crystalline HAp at room temperature. Microstructural features such as size and morphology of the resulting HAp samples were characterized using BET, scanning and transmission electron microscopy. The results indicate that the development (size and shape) of the HAp nanocrystals precipitated in an aqueous solution of PVA was influenced by the molecular weight of the polymer in such a way that smallest crystallite size was observed in the case of PVA with the highest molecular weight. It is believed that the HAp formation was initiated through the interaction of Ca²⁺ ions with the negative side groups on the polymer surface. The larger number of reaction sites in the PVA polymer with higher molecular weight led to a higher number of HAp nuclei and therefore smaller crystallite size.     

Hydrothermal nucleation of hydroxyapatite on titanium surface

Titanium plates were submitted to nucleation and growth of hydroxyapatite (HAp) under hydrothermal conditions. A group of these plates were submitted to nucleation without any previous treatment and another group was treated with NaOH 1 M at 130°C inside an autoclave followed by heat-treatment at 600°C. The nucleation were performed by soaking all these titanium pieces, in a simulated body fluid (SBF) solution, containing calcium, phosphate and other ions, in order to promote the nucleation and growth of hydroxyapatite under hydrothermal conditions on the titanium surface. The results show that hydrothermal nucleation and growth of HAp occurs even on the non-treated titanium surface at 150°C. However, a better uniformity of the hydroxyapatite layer was observed on the pre-treated titanium surface, at 80°C, with the renewal of the SBF solution.     

Effect of side chain structure on aggregation state and mechanical properties of synthetic polypeptide monolayers at the air-water interface

Synopsis Aggregation structure of poly(γ-methyl-L-glutamate)(PMLG) and poly(γ-n-hexyl-L-glutamate)(PnHLG) monolayers at the air-water interface was investigated on the basis of the transmission electron microscopic observation. The bright field image of PMLG and PnHLG monolayers exhibited a homogeneous structure at surface pressure of 18 mN·m⁻¹ and 15 mN·m⁻¹, respectively. The bright field image of PMLG monolayers showed linear humps at high surface pressure, suggesting the collapse of monolayers and the orientation of α-helix axis perpendicular to the compression direction. Surface pressure anisotropy was investigated by means of orthogonal Wilhelmy plate method. The surface pressure anisotropy for PMLG was greater than that for PnHLG. Dynamic viscoelastic properties of polypeptides monolayers at the air-water interface were examined on the basis of transient response of surface pressure. The magnitude of dynamic storage modulus E' of the PnHLG monolayer was lower than that of PMLG monolayer. This is ascribed to the active side chain motion of PnHLG at room temperature. The mechanical properties and aggregation structure of polypeptide monolayers were revealed to depend on their thermal molecular motion of side chain.     

Microwave- versus Conventional-Hydrothermal Synthesis of Hydroxyapatite Crystals from Gypsum

Hydroxyapatite (HAp) crystals were synthesized from gypsum powder with diammonium hydrogen phosphate solution via microwave-hydrothermal treatment at 100°C, and their properties were studied. Gypsum powder could be completely converted to fine HAp crystals 30-300 nm long with 0.5 M diammonium hydrogen phosphate at 100°C in 5 min. Compared to the formation of HAp crystals via conventional-hydrothermal treatment, microwave-hydrothermal treatment led to increased rate of formation (by two orders of magnitude).     

Relaxation phenomena of stereoregular poly(methyl methacrylate) monolayers at the air/water interface

Relaxation behaviors of isotactic, atactic, and syndiotactic poly(methyl methacrylate) (PMMA) monolayers at the air/water interface were investigated at three different temperatures. The monolayer characteristics of the three stereoisomers were studied in terms of surface pressure–area per molecule (π‐A) isotherm, area relaxation, and pressure relaxation. The results show that pressures at inflection points of π‐A isotherms of PMMA decrease with an increase in temperature. The collapse pressure also decreases as the temperature is elevated. It was shown likely for the first time that the relaxation process of PMMA stereoisomers could be described by a model considering the nucleation and growth mechanisms. The simulation parameters of area relaxation of the three stereoisomers at 30 mN/m are very similar, indicating similar mechanisms. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

原位合成制备碳纳米管/羟基磷灰石复合粉体

对多壁碳纳米管进行了浓酸氧化处理,然后采用原位合成法制备了碳纳米管/羟基磷灰石复合粉体,并对氧化处理前后的碳纳米管及所得复合粉体进行了红外光谱、X射线衍射、透射电镜检测.实验表明:经过浓硝酸氧化处理之后,多壁碳纳米管的端口被打开并在缺陷处引入大量的羟基与羧基,这是原位合成复合材料的基础;所得复合粉体中仅有碳管和羟基磷灰石两种物相,其中纳米级的短棒状羟基磷灰石均匀吸附在碳纳米管表面,形成了较强的界面结合.     

Biomimetic apatite coating on P(EMA-co-HEA)/SiO2 hybrid nanocomposites

P(EMA-co-HEA)/SiO₂ nanocomposites with silica contents in the range of 0-30 wt% were prepared by co-polymerization of the organic monomers during the simultaneous sol-gel polymerization of the silica precursor. The ability of the hybrids to form hydroxyapatite (HAp) on their surfaces was tested in vitro by soaking the samples in a simulated body fluid (SBF) solution for different times up to 35 days. On the one hand, the composition and morphology of the HAp layer formed were characterized by SEM, EDS, FTIR and XRD; on the other, the exchange of soluble silicates and calcium and phosphate ions, and the structural changes taking place in the nanohybrids when immersed in SBF were analyzed by SEM/EDS. This is, up to our knowledge, the first time the HAp nucleation mechanism has been proposed for organic-silica nanohybrids and correlated with their respective nanostructures. The results revealed that the formation of a HAp coating was in all cases limited by the nucleation induction time, but the mechanism and rate of HAp nucleation were found to be different depending on the nanostructure of the samples, which differs, in turn, with the silica content as a consequence of the differing connectivity of the silica network. The nanohybrids with silica contents in the range of 10-20 wt% proved to be the most suitable for the development of bioactive synthetic scaffolds for bone or other mineralized tissues.     

Adsorption of amyloid beta (1-40) peptide at phospholipid monolayers

The folding of amyloid beta (1-40) peptide into beta-sheet-containing fibrils is thought to play a causative role in Alzheimer's disease. Because of its amphiphilic character, the peptide can interact with phospholipid membranes. Langmuir monolayers of negatively charged DPPS, DPPG, and DMPG, and also of zwitterionic DPPC and DMPC, have been used to study the influence of the peptide on the lipid packing and, vice versa, the influence of phospholipid monolayers on the peptide secondary structure by infrared reflection absorption spectroscopy and grazing incidence X-ray diffraction. The peptide adsorbs at the air/water (buffer) interface, and also inserts into uncompressed phospholipid monolayers. When adsorbed at the interface, the peptide adopts a beta-sheet conformation, with the long axis of these beta-sheets oriented almost parallel to the surface. If the lipid exhibits a condensed monolayer phase, then compression of the complex monolayer with the inserted peptide leads to the squeezing out of the peptide at higher surface pressures (above 30 mN m(-1)). The peptide desorbs completely from zwitterionic monolayers and negatively charged DPPG and DPPS monolayers on buffer, but remains adsorbed in the beta-sheet conformation at negatively charged monolayers on water. This can be explained in terms of electrostatic interactions with the lipid head groups. It also remains adsorbed at, or penetrating into, disordered anionic monolayers on buffer. Additionally, the peptide does not influence the condensed monolayer structure at physiological pH and modest ionic strength.     

Flunitrazepam effect on distearoylphosphatidylglycerol, cholesterol and distearoylphosphatidylglycerol + cholesterol mixed monolayers structure at a DCE/water interface

The effect of Flunitrazepam (FNTZ) on molecular packing of distearoylphosphatidylglycerol (DSPG), cholesterol (CHOL) monolayers and DSPG + CHOL mixed monolayers adsorbed at the water/1,2-dichloroethane interface was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).The interfacial packing of the monolayer was estimated by the analysis of the voltammetric transfer of tetraethylammonium cation (TEA⁺) from the aqueous to the organic phase in the presence of different cations (Li⁺, Ca²⁺ and K⁺) in water. An important blocking of the transfer process was observed in the case of DSPG monolayer when the aqueous phase contained Li⁺ and Ca²⁺ cations indicating a highly ordered structure. EIS experiments evidenced that these cations adsorb at polar head group leading to a decrease in interfacial dielectric constant. FNTZ produced an enhancement of this blocking effect. When K⁺ was employed as aqueous electrolyte, no blocking effect of DSPG monolayer with or without FNTZ was observed. It was concluded that FNTZ induces lateral interaction with DSPG molecules in highly structured monolayers in which the negative charge of polar head groups is neutralized by cation adsorption.An interesting substitution effect of FNTZ was observed in monolayers containing CHOL.     

Hydroxyapatite formation by solvothermal treatment of α-tricalcium phosphate with water-ethanol solution

We investigated hydroxyapatite (HAp) formation from alpha-tricalcium phosphate (α-TCP) under solvothermal conditions using water-ethanol solution. The rate of HAp formation decreased with increasing ethanol fraction in the solution. Needle-like HAp was formed with a small amount of beta-tricalcium phosphate after solvothermal treatment for 3 h in solutions with water/ethanol volume ratios of 5/15 and 10/10. In the solution with water/ethanol volume ratio of 5/15, needle-like HAp formed with a small amount of dicalcium phosphate anhydrous (DCPA) at 12 h, and the amount of DCPA increased with increasing treatment period. The aspect ratio of the HAp crystals that formed increased with increasing ethanol fraction in the solution. The fraction of ethanol in the solution during the solvothermal processing affects not only the rate of transformation of α-TCP into HAp, but also the morphology of the HAp that is formed.     

The monolayer behavior and transfer characteristics of phospholipids at the air/water interface

The monolayer behavior of phospholipids at the air/water interface and their transfer characteristics on the: solid substrates have been investigated with a constant-perimeter type Langmuir trough. From the surface pressure-area (Π-A) isotherm, evaluated were the cross-sectional area of an oriented molecule, phase transition, and the miscibility of mixed monolayers. The monolayer state depends on the chain length and its mobility, and by adding proper salts, the monolayers were stabilized. The miscibility of mixed monolayers was also discussed with collapse pressure and excess area. The monolayers of all lipids were transferred into only one layer on hydrophobic substrates, and up to two layers on hydrophobic substrates. The multilayer formation of dipalmitoylphosphatidic acid (I)PPA) was significantly affected by the subphase pH and the addition of multivalent salts. As a transfer promoter. DPPA or octadecylamine (ODA) was used as a component in a mixture with lipid materials, and their mixed monolayers resulted in good transfer characteristics. The transferred mass and film thickness of DPPA, determined by using a quartz crystal microbalance (QCM) and by an ellipsometry respectively, were proportional to the number of transfer. The lattice spacing of DPPA was 20.3 Å per layer by ellipsometry, and 22.78 Å per layer by X-ray diffraction. These indicated that the transferred multilayers had well-defined layered structures of Y-type.