Showing papers by "Daryoosh Vashaee published in 2013"

3D conductive nanocomposite scaffold for bone tissue engineering

Abstract: Bone healing can be significantly expedited by applying electrical stimuli in the injured region. Therefore, a three-dimensional (3D) ceramic conductive tissue engineering scaffold for large bone defects that can locally deliver the electrical stimuli is highly desired. In the present study, 3D conductive scaffolds were prepared by employing a biocompatible conductive polymer, ie, poly(3,4-ethylenedioxythiophene) poly(4-styrene sulfonate) (PEDOT:PSS), in the optimized nanocomposite of gelatin and bioactive glass. For in vitro analysis, adult human mesenchymal stem cells were seeded in the scaffolds. Material characterizations using hydrogen-1 nuclear magnetic resonance, in vitro degradation, as well as thermal and mechanical analysis showed that incorporation of PEDOT:PSS increased the physiochemical stability of the composite, resulting in improved mechanical properties and biodegradation resistance. The outcomes indicate that PEDOT:PSS and polypeptide chains have close interaction, most likely by forming salt bridges between arginine side chains and sulfonate groups. The morphology of the scaffolds and cultured human mesenchymal stem cells were observed and analyzed via scanning electron microscope, micro-computed tomography, and confocal fluorescent microscope. Increasing the concentration of the conductive polymer in the scaffold enhanced the cell viability, indicating the improved microstructure of the scaffolds or boosted electrical signaling among cells. These results show that these conductive scaffolds are not only structurally more favorable for bone tissue engineering, but also can be a step forward in combining the tissue engineering techniques with the method of enhancing the bone healing by electrical stimuli.

Surface modification of stainless steel orthopedic implants by sol-gel ZrTiO4 and ZrTiO4-PMMA coatings.

Abstract: In this paper, the biocompatibility of a medical-grade stainless steel coated with sol-gel derived, nanostructured inorganic ZrTiO4 and hybrid ZrTiO4-PMMA thin films is correlated with surface characteristics. The surfaces of the samples are characterized by atomic force microscopy, the sessile drop technique, and electrochemical corrosion experiments. The viability of adult human mesenchymal stem cells on the surfaces after one day of culture is also assessed quantitatively and morphologically. According to the results, both of the coatings improve the hydrophilicity, corrosion resistance, and thereby cytocompatibility of the substrate. Despite the higher corrosion protection by the hybrid coating, the sample coated with the inorganic thin film exhibits a better cell response, suggesting the domination of wettability. In summary, the ZrTiO4-based sol-gel films can be considered to improve the biocompatibility of metallic implants.

Synthesis and Characterization of Encapsulated Nanosilica Particles with an Acrylic Copolymer by in Situ Emulsion Polymerization Using Thermoresponsive Nonionic Surfactant.

Abstract: Nanocomposites of encapsulated silica nanoparticles were prepared by in situ emulsion polymerization of acrylate monomers. The synthesized material showed good uniformity and dispersion of the inorganic components in the base polymer, which enhances the properties of the nanocomposite material. A nonionic surfactant with lower critical solution temperature (LCST) was used to encapsulate the silica nanoparticles in the acrylic copolymer matrix. This in situ method combined the surface modification and the encapsulation in a single pot, which greatly simplified the process compared with other conventional methods requiring separate processing steps. The morphology of the encapsulated nanosilica particles was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM), which confirmed the uniform distribution of the nanoparticles without any agglomerations. A neat copolymer was also prepared as a control sample. Both the neat copolymer and the prepared nanocomposite were characterized by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analyses (TGA), dynamic mechanical thermal analysis (DMTA) and the flame resistance test. Due to the uniform dispersion of the non-agglomerated nanoparticles in the matrix of the polymer, TGA and flame resistance test results showed remarkably improved thermal stability. Furthermore, DMTA results demonstrated an enhanced storage modulus of the nanocomposite samples compared with that of the neat copolymer, indicating its superior mechanical properties.

Multilayer bioactive glass/zirconium titanate thin films in bone tissue engineering and regenerative dentistry

Abstract: Surface modification, particularly coatings deposition, is beneficial to tissue-engineering applications. In this work, bioactive glass/zirconium titanate composite thin films were prepared by a sol-gel spin-coating method. The surface features of the coatings were studied by scanning electron microscopy, atomic force microscopy, and spectroscopic reflection analyses. The results show that uniform and sound multilayer thin films were successfully prepared through the optimization of the process variables and the application of carboxymethyl cellulose as a dispersing agent. Also, it was found that the thickness and roughness of the multilayer coatings increase nonlinearly with increasing the number of the layers. This new class of nanocomposite coatings, comprising the bioactive and inert components, is expected not only to enhance bioactivity and biocompatibility, but also to protect the surface of metallic implants against wear and corrosion.

The effect of hyaluronic acid on biofunctionality of gelatin-collagen intestine tissue engineering scaffolds.

Abstract: The creation of engineered intestinal tissue has recently stimulated new endeavors with the ultimate goal of intestinal replacement for massive resections of bowel. In this context, we investigated the effect of hyaluronic acid (HA) on the physicochemical characteristics of gelatin-collagen scaf- folds and its cytocompatibilty to the human intestinal epithe- lial Caco-2 cell line in vitro. Gelatin/collagen hybrid scaffolds with different concentrations of HA were prepared by solvent casting and freeze-drying techniques and subsequent chemi- cal crosslinking by genipin. The morphologies of the scaffolds were characterized by scanning electron microscopy and Fou- rier transform infrared spectroscopy. In vitro tests were carried out in phosphate-buffered saline (PBS) solution to study the swelling ratio and the biostability of the scaffolds. It was found that the porous structure of the scaffolds could be tailored by further addition of HA. Moreover, both the swelling ratio and the degradation rate of the scaffold increased by addition of HA. A resazurin-based cell viability assay was employed to determine the viability and estimate the number of scaffold- adherent Caco-2 cells. The assay indicated that the scaffolds were all cytocompatible. We concluded that addition of less than 15% HA to scaffolds with a composition of 9:1 gelatin:col- lagen results only in incremental improvement in the struc- tural characteristics and cytocompatibility of the gelatin- collagen scaffolds. However, the scaffolds with 25% HA exhib- ited remarkable enhancement in physicochemical characteris- tics of the scaffolds including cell viability, growth, and attachment as well as their physical structure. V C 2013 Wiley

Prediction of a large number of electron pockets near the band edges in type-VIII clathrate Si46 and its physical properties from first principles.

Abstract: The material design of type-VIII clathrate Si46 is presented based on first principles. The structural, electronic, elastic, vibrational, and thermodynamic properties of this hypothetical material are presented. Our results predict that type-VIII clathrate Si46 is an indirect semiconductor with a bandgap of 1.24 eV. The band structure revealed an interestingly large number of electron pockets near both conduction and valance band edges. Such a large density of states near the band edges, which is higher than that of the best thermoelectric materials discovered so far, can result in a large thermoelectric power factor (>0.004 W m 1 K 2 ) making it a promising candidate for thermoelectric applications. The elastic properties as well as the vibrational modes and the phonon state densities of this material were also calculated. Our calculations predict that the heat capacity at constant volume (isochoric) of this clathrate increases smoothly with temperature and approaches the Dulong‐Petit value near room temperature. The electronic band structure shows a large number of valleys closely packed around the valance band edge, which is rare among the known semiconducting materials. These valleys can contribute to transport at high temperature resulting in a possibly high performance (ZT> 1:5) p-type thermoelectric material. (Some figures may appear in colour only in the online journal)

The effect of phase heterogeneity on thermoelectric properties of nanostructured silicon germanium alloy

Abstract: Detailed examination of the nanostructured bulk Si0.80Ge0.20 alloy synthesized by mechanical alloying and hot-press methods revealed that the alloy composition can unintentionally deviate from its nominal value. The phase deviation is difficult to be detected with x-ray diffraction due to the continuous solid solution characteristics of the Si-Ge alloy. Differential thermal analysis, in particular, showed that the synthesized nanostructured bulk Si0.80Ge0.20 alloy was a composition of two unintentional phases. The dominant phase was Si0.88Ge0.12 with admixture of Si0.58Ge0.42 in a much lower concentration. The two-phase structure is difficult to be detected in X-ray diffraction analysis and is often neglected. Thermoelectric properties of Si1−xGex significantly depend on the Ge content in the synthesized alloy. The thermoelectric properties of the synthesized material were studied experimentally and theoretically. The comparison of the data of the mixed phase nanostructured alloy with those of the single ...

Comparison of boron precipitation in p-type bulk nanostructured and polycrystalline silicon germanium alloy

Abstract: Boron precipitation process and its effect on electronic properties of p-type bulk nanostructured silicon germanium (Si0.8Ge0.2) compared with large grain polycrystalline Si0.8Ge0.2 have been studied. The structures were synthesized and their thermoelectric properties were measured versus temperature during heating and cooling cycles. The experimental data showed stronger temperature variation of Seebeck coefficient, carrier concentration, and conductivity in the nanostructured Si0.8Ge0.2 compared with the polycrystalline form indicating stronger boron precipitation in this structure. The electrical properties of both samples were calculated using a multi-band semi-classical model. The theoretical calculations confirm that the increase of boron precipitation in the nanostructured Si0.8Ge0.2 is responsible for its higher thermal instability. Since the thermoelectric properties of the nanostructured sample degrade as a result of thermal cycling, the material is appropriate only for continuous operation at h...

Digital holographic microscopy of the myelin figure structural dynamics and the effect of thermal gradient

Abstract: Myelin figures (MFs) are cylindrical multilamellar lipid tubes that can be found in various healthy and diseased living cells. Their formation and dynamics involve some of the most mysterious configurations that lipid molecules can adopt under certain conditions. They have been studied with different microscopy methods. Due to the frequent coiling of their structure, the usual methods of microscopy fail to give precise quantitative information about their dynamics. In this paper, we introduced Digital Holographic Microscopy (DHM) as a useful method to calculate the precise dynamical volume, thickness, surface and length of the myelin figures. As an example of DHM imaging of myelin figures, their structure and growth rate in the presence and absence of temperature gradient have been studied in this work. We showed that the thickness of a myelin figure can be changed during the first few seconds. However, after approximately ten seconds, the thickness stabilizes and does not alter significantly. We further studied the effect of the thermal gradient on the length growth. The calculation of the length growth from the measurement of the myelin figure volume shows that the length (L) grows in time (t) as L∝tat the early stage of the myelin protrusion in both the presence and the absence of the thermal gradient. However, thermal gradient facilitates the growth and increases its rate.

Electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 and Ba8Al16Sn30 by first principles

Abstract: We present the results of studying electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 Ba8Al16Sn30 calculated from a first-principles approach. The calculations utilize the generalized gradient approximation to density functional theory. The results indicate that the Ba8Ga16Sn30 and Ba8Al16Sn30 are indirect semiconductors with fundamental band gaps of 160 meV and 315 meV, respectively. It was also found that the stiffness of Al containing type-VIII clathrate does not show any significant change against the uniform pressure, shearing, and linear strains. The phonon spectrum and the phonon state densities of these compounds as well as the Raman and infrared active modes were further calculated and the effects of replacing the Ga with Al atoms on the properties of interest were discussed. The calculated elastic, vibrational, and thermodynamic properties along with Raman and IR spectra are reported for the first time. The identification of the Raman and infrared active modes will be especially useful for the experimental characterizations of these compounds. Our calculations show that the heat capacities of these clathrates increase smoothly with temperature and approach the Dulong-Petit value at about room temperature, which agrees with the existing experimental data.

Interlamellar organization of phase separated domains in multi-component lipid multilayers: energetic considerations.

Abstract: A recent experimental study [1] has demonstrated the alignment of phase separated domains across hundreds of bilayer units in multicomponent stacked lipid bilayers. The origin of this alignment is the interlamellar coupling of laterally phase separated domains. Here, we develop a theoretical model that presents the energetics description of this phenomenon based on the minimization of the free energy of the system. Specifically, we use solution theory to estimate the competition between energy and entropy in different stacking configurations. The model furnishes an elemental phase diagram, which maps the domain distributions in terms of the strength of the intra- and inter-layer interactions and estimates the value of inter-layer coupling for complete alignment of domains in the stacks of five and ten bilayers. The area fraction occupied by co-existing phases was calculated for the system of the minimum free energy, which showed a good agreement with experimental observations.

Preparation of Al-SiC-Al 2 O 3 metal matrix composite powder by mechanochemical reaction between Al, SiO 2 and C

Abstract: In this Letter, Al–SiC–Al2O3 metal matrix nanocomposite powder was successfully synthesised employing a mechanical alloying technique, through a mechanochemical reaction among aluminium (Al), silicon dioxide (SiO2) and carbon (C). For commercial purposes, the materials (Al, SiO2 and C powders) and also the method of synthesis (mechanical alloying) is considered to be cost-effective for production of Al–SiC–Al2O3 nanocomposite. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry, and scanning electron microscopy. The results showed that during ball milling Al, SiO2 and C reacted through a mechanochemical reaction producing Al–SiC–Al2O3 metal matrix nanocomposite powder after 50 h of ball milling and annealing at a temperature of 650°C for 1 h. The crystallite sizes of phases remained in nanometric scale.

Nanofibrous Scaffolds for Tissue Engineering Applications: State-of-the-Art and Future Trends

Abstract: One of the main challenges in tissue engineering is fabrication of nanostructured scaffolds. Conventional techniques are not able to fabricate fi bers below micrometers in diameter. However, the native extracellular matrix (ECM) fi bers are in nanometer range. Hence, there has been an intensive effort to improve the techniques for fi bers fabrication to achieve nano-sized fi bers which adequately simulate the fi brous structure of ECM. Electrospinning can achieve the biomimic non-woven scaffolds that are composed of a large network of interconnected fi bers and pores, resembling the topographic features of the