Medium For Periods Of Time Research Articles

Investigating non-reciprocity in time-periodic media using a perturbative approach.

Lorentz famous theorem leads to clear reciprocity conditions for linear, time-invariant media based on their constitutive parameters. By contrast, reciprocity conditions for linear time-varying media are not fully explored. In this paper, we investigate whether, and how a structure containing a time-periodic medium can be truly identified as reciprocal or not. To that end, a necessary and sufficient condition is derived which requires both the constitutive parameters and the electromagnetic fields inside the dynamic structure. As solving for the fields for such problems is challenging, a perturbative approach is proposed which expresses the aforementioned non-reciprocity condition in terms of the electromagnetic fields and the Green's functions of the unperturbed static problem and is particularly applicable for the case of structures with weak time modulation. Reciprocity of two famous canonical time-varying structures are then studied using the proposed approach and their reciprocity/non-reciprocity is investigated. In the case of one-dimensional propagation in a static medium with two point-wise modulations, our proposed theory clearly explains the often observed maximization of non-reciprocity when the modulation phase difference between the two points is 90 degrees. In order to validate the perturbative approach, analytical and Finite-Difference Time-Domain (FDTD) methods are employed. Then, solutions are compared and considerable agreement between them is observed.

Effects of hydrothermal treatment of cellulose nanocrystal templated TiO2 films on their photodegradation activity of methylene blue, methyl orange, and rhodamine B

Transparent mesoporous TiO2 films with enhanced photocatalytic degradation efficiency were systematically synthesized. The films were prepared using a mixed sol containing a cellulose nanocrystal (CNC) template that was spin-coated onto an ITO substrate, followed by calcination at 400 °C for 1 h. Subsequently, the fabricated films were subjected to hydrothermal treatment in a pure aqueous medium for various periods of time (1, 2, and 4 h) and at different temperatures (150 and 250 °C). The surface area and crystallite size of the prepared TiO2 film were increased and decreased from 76.7 m2/g and 12.78 nm (without the template) to 127.3 m2/g and 9.95 nm (with the template), and then to 233.4 m2/g and 4.43 nm (1 h and 150 °C) with hydrothermal treatment, respectively. Hydrothermal treatment plays an important role in enlarging the surface area, and decreasing the crystallite size. Furthermore, the optimized TiO2 film exhibits exceptional photodegradation activity towards organic dyes with performance nearly equal to that of the standard photocatalyst. Because of the exceptional properties of the prepared optimized TiO2 film, the adsorption of organic dyes on its surface increases, which promotes the photocatalytic degradation efficiency during light irradiation.

Parametric amplification interactions in time-periodic media: coupled waves theory

Waves and their interactions with time-periodic media have recently been of theoretical interest for their unusual properties. It has been revealed that such interactions lead to momentum gaps at ω = N Ω / 2 , where ω is the wave frequency, Ω is the modulation frequency, and N ∈ Z . While waves can propagate at the boundaries of the momentum gaps, it has been shown that waves exhibit parametric amplification in the momentum gaps due to the active nature of such temporally dynamic media. In this paper, the coupled waves theory is used for the modeling of the parametric amplification phenomena in time-periodic optical media. Simple formulas are derived using the 0th and first Fourier components of θ ( t ) = 1 / ε ( t ) , approximating the results of the cumbersome stability analysis of the associated hypergeometric equation of the electric displacement field and providing the conditions to define the wave parameters, which can fall in the momentum gap, resulting in parametric amplification or allow propagating modes. Numerical results are compared with the Hill’s determinant of the Floquet theory and FDTD simulations showing that the coupled waves theory can efficiently model the first and second momentum gaps. Further extensions can be made to higher-order gaps, by considering higher-order harmonics and following the same steps of this analysis.

Nonlocal dispersal equations in time-periodic media: Principal spectral theory, limiting properties and long-time dynamics

The present paper is devoted to the investigation of the following nonlocal dispersal equationut(t,x)=Dσm[∫ΩJσ(x−y)u(t,y)dy−u(t,x)]+f(t,x,u(t,x)),t>0,x∈Ω‾, where Ω⊂RN is a bounded and connected domain with smooth boundary, m∈[0,2) is the cost parameter, D>0 is the dispersal rate, σ>0 characterizes the dispersal range, Jσ=1σNJ(⋅σ) is the scaled dispersal kernel, and f is a time-periodic nonlinear function of generalized KPP type. This paper is a continuation of the works of Berestycki et al. [3,4], where f was assumed to be time-independent. We first study the principal spectral theory of the linear operator associated to the linearization of the equation at u≡0. We establish an easily verifiable, general and sharp sufficient condition for the existence of the principal eigenvalue as well as important sup-inf characterizations of the principal eigenvalue. Next, we study the influences of the principal spectrum point on the global dynamics and confirm that the principal spectrum point being zero is critical. It is followed by the investigation of the effects of the dispersal rate D and the dispersal range characterized by σ on the principal spectrum point and the positive time-periodic solution. In particular, we prove various limiting properties of the principal spectrum point and the positive time-periodic solution as D,σ→0+ or ∞. To achieve these, we develop new techniques to overcome fundamental difficulties caused by the lack of the usual L2 variational formula for the principal eigenvalue, the lack of the regularizing effects of the semigroup generated by the nonlocal dispersal operator, and the presence of the time-dependence of the nonlinearity f. Finally, we establish the maximum principle for time-periodic nonlocal dispersal operators.

General solution to wave propagation in media undergoing arbitrary transient or periodic temporal variations of permittivity

A novel and general formulation for wave propagation in time-varying media is presented. Unlike previous reports, our formalism is able to solve propagation in media with arbitrary time variations of permittivity or permeability, for both transient and steady-state periodic variations. The formulation is approximate yet strikingly accurate in most practical cases. The provided closed-form expressions show that the normalized average power after the transition of the permittivity does not depend on the details of the transition, while the energy density does. Some important discussions are made about time-periodic media, and it is shown that there is an accumulation of energy when the temporal equivalent of the Bragg condition is met. All results are validated through comparison against analytical or numerical solutions.

Flocculation of Microalgae via pH Change in a Turbulent Medium and Subsequent Filtration

Traditional microalgae harvesting techniques consume a lot of energy. Flocculation, or the formation of aggregates, is an energetically favorable process to collect biomass. Flocculation is normally carried in tanks to allow the formation of the aggregates after stirring, however, this consumes time and physical resources. In this work, flocculation of Chlorella vulgaris and Scenedesmus sp. by acidification and alkalization was compared to chemical flocculation in a turbulent medium for short periods of time (30 seconds and 2 minutes). Flocculation with potassium hydroxide at pH=10 showed to be nearly as efficient as traditional flocculation by using ferric sulphatum after two minutes. Acid flocculation with nitric acid was not as effective, even at values of pH=4. Flocculation by pH does not generate toxic wastes and the remaining added flocculants turn into nutrients after harvesting. After flocculation and neutralization, the remaining cells in the medium were viable to recultivate. Since pH-driven flocculation does not allow harvesting the total microalgae culture, remaining cells can be used to keep growing. Based on these results a semi continuous harvesting method incorporated in the microalgae growing phase seems promising.

Mixed Metal Oxides As Electrocatalysts for the Oxygen Evolution Reaction

Introduction The electrolysis of water has been at the forefront of catalysis research for over a decade and has gained tremendous attention since the introduction of metal-air cells and photoelectrochemical (PEC) water splitting devices that bridge the gap between carbon-based fuels and alternative energy resources.1 However, these substantial advancements are still very expensive and inefficient to implement due in part to the non-spontaneous nature of the oxygen evolution reaction (OER). The OER is energy intensive; therefore, developing ways of reducing the overpotential and increasing the current throughput with catalytic materials is required.1,2 The OER is a very complex mechanism that consists of various adsorption interactions at the electrode-electrolyte interface.2 Therefore, efficient catalytic electrodes have a high active surface area for adsorbing intermediates, are stable in the electrolytic medium for long periods of time, and are good at electron transfer processes.2 Mixed metal oxides are supported to be highly catalytic, stable, and can be formed using a variety of different experimental techniques.1 Many of the mixed metal oxides being pursued are abundant and easily accessible. Mixed metal oxides are also often found as by-products of corrosion in structural applications and high temperature processes. These facts have led to our research in developing highly stable and electrochemically catalytic mixed metal oxides using different experimental techniques. Nickel alloy-based oxides are of interest due to their promising electrocatalytic capabilities3,4 when compared to the base metals alone. The ability to produce mixed metal oxides through facile techniques in mild conditions is highly desired. Experimental Catalytic electrodes were prepared using a variety of methods. These electrodes were then compared based on their electrochemical activity for catalyzing the OER. Preliminary experiments show promising and interesting results supporting a shift in overpotential and an increase in current density for the OER when the prepared mixed metal oxides are used as the working electrode in a traditional three-electrode configuration. Surface characterization using XPS, XRD, SEM, and Raman spectroscopy was employed to relate electrochemical response to the surface composition of the different electrodes to aid in determining the overall mechanism of catalytic activity. It is also suggested that the breakdown of the outer surface oxide layer along with re-deposition throughout the electrochemical cycle might affect the overall electrochemical capabilities of these mixed metal oxide electrodes. Results Preliminary investigations suggest that electrode preparation technique and surface oxide film depth play a significant part in the catalytic activity of the mixed metal oxides. These results have led to further understanding of the OER mechanism and to better methods for preparing mixed metal oxide electrodes. Figure I. Linear sweep voltammogram of base metal vs. prepared mixed metal oxide electrocatalyst Acknowledgements: This work was partially funded by Department of Energy under contract DE-NE0008236. References C. Yuan., H. B. Wu, et al. Angew. Chem. Int. Ed., 53(6)1488-1504, (2014).I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102-121, (2014).H.-Y. Wang, Y.-Y. Hsu, R. Chen, T.-S. Chan, H. M. Chen and B. Liu, Adv. Energy. Mater., 5,(2015).H. Zhang, H. Y. Li, H. Y. Wang, K. J. He, S. Y. Wang, Y. G. Tang and J. J. Chen, J. Power Sources, 280, 640-648, (2015). Figure 1

Microscopy of Fission Yeast Sexual Lifecycle.

The fission yeast Schizosaccharomyces pombe has been an invaluable model system in studying the regulation of the mitotic cell cycle progression, the mechanics of cell division and cell polarity. Furthermore, classical experiments on its sexual reproduction have yielded results pivotal to current understanding of DNA recombination and meiosis. More recent analysis of fission yeast mating has raised interesting questions on extrinsic stimuli response mechanisms, polarized cell growth and cell-cell fusion. To study these topics in detail we have developed a simple protocol for microscopy of the entire sexual lifecycle. The method described here is easily adjusted to study specific mating stages. Briefly, after being grown to exponential phase in a nitrogen-rich medium, cell cultures are shifted to a nitrogen-deprived medium for periods of time suited to the stage of the sexual lifecycle that will be explored. Cells are then mounted on custom, easily built agarose pad chambers for imaging. This approach allows cells to be monitored from the onset of mating to the final formation of spores.

Particle-based optical pressure sensors for 3D pressure mapping.

This paper presents particle-based optical pressure sensors for in-flow pressure sensing, especially for microfluidic environments. Three generations of pressure sensitive particles have been developed- flat planar particles, particles with integrated retroreflectors and spherical microballoon particles. The first two versions suffer from pressure measurement dependence on particles orientation in 3D space and angle of interrogation. The third generation of microspherical particles with spherical symmetry solves these problems making particle-based manometry in microfluidic environment a viable and efficient methodology. Static and dynamic pressure measurements have been performed in liquid medium for long periods of time in a pressure range of atmospheric to 40 psi. Spherical particles with radius of 12 μm and balloon-wall thickness of 0.5 μm are effective for more than 5 h in this pressure range with an error of less than 5%.

Cooperative packet-forwarding mechanism for throughput improvement in multi-channel wireless networks

Typical MAC protocols for IEEE 802.11-based ad hoc networks employ a direct transmission strategy whenever the transmitter can directly reach the receiver (one-hop transmission). While such a design enjoys simplicity, it limits the number of admitted transmissions in a given neighborhood. Due to the performance anomaly of 802.11-based wireless networks, transmission with low data rates occupy the shared medium for longer periods of time. Occupying the transmission medium for a longer period of time results in less available transmission time for other nodes, which consequently reduces the number of transmitted packets during the same period of time, leading to a reduction in network throughput. To improve the overall network throughput, we present a cooperative multi-channel MAC protocol for single-hop wireless mobile ad hoc networks that attempts at computing the path with the minimum required transmission time for a given source–destination pair (including the channel assignment along that path). The proposed protocol attempts at improving network performance by means of cooperative communications. According to our approach, if the one-hop (direct) path between communicating nodes supports low data rate (requires longer transmission time), the source selects a multi-hop path to the destination such that the total amount of required transmission time is minimized. Through simulations, we show that our proposed protocol achieves significant throughput and fairness improvement compared to the standard IEEE 802.11-based protocol.

Hydroxyapatite, a biomaterial: Its chemical synthesis, characterization and study of biocompatibility prepared from shell of garden snail, Helix aspersa

The shell of garden snail (Helix aspersa) is basically made of calcium carbonate. An attempt is made to convert calcium carbonate of garden snail shell to hydroxyapatite. The snail shell was found to decompose within 850\(^{\circ}\)C to all the carbonate phases. The calcined snail shells were then treated with acids followed by different chemicals in ammoniacal media maintaining proper stoichiometry to produce fine hydroxyapatite (HAP) as filter cake with a Ca\(/\)P molar ratio of 1\({\cdot }\)67. The dried HAP powder was extremely pure with a specific surface area of 15 m\(^{\bf 2}/\)g. The different characterization techniques were adopted both for calcined snail shell and HAP synthesized by X-ray diffraction (XRD), thermal analysis (DTA\(/\)TGA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The surface area and particle size of HAP powder prepared by chemical precipitation route, were also determined by BET and Malvern particle size analyser, respectively. The synthesized powder was soaked in stimulated body fluid (SBF) medium for various periods of time in order to evaluate its bioactivity. The changes of pH of SBF medium were measured. High bioactivity of prepared HAP powder due to the formation of apatite on its surface was observed.

Uniqueness of monostable pulsating wave fronts for time periodic reaction–diffusion equations

We establish the uniqueness of pulsating wave fronts for reaction–diffusion equations in time periodic media with monostable nonlinearities. For the Kolmogorov–Petrovsky–Piskunov (KPP) type nonlinearity, this result provides a complete classification of all types of KPP pulsating fronts.

The influence of gastric emptying kinetics on the drug release from enteric coated pellets in fasted state: An in vitro/in vivo correlation

The absorption and bioavailability of drugs can be substantially affected by the transit of dosage forms through the gastrointestinal (GI) tract. Gastric emptying is one of the most critical parameters contributing to this inter- and intra-individual GI transit variability. It is especially important for the delayed release dosage forms whose release depends on the local environment and begins when the dosage form passes pylorus and comes into contact with higher pH medium in small intestine. The purpose of our research work was to predict the in vivo dissolution from enteric coated pellets for population and establish a good in vitro/in vivo correlation (IVIVC) with mean in vivo absorption profiles, obtained in a pharmacokinetic study under fasting conditions. The dissolution tests were carried out on a USP 4 – flow-through cell with enteric coated pellets containing an acid-labile drug and formulated as orodispersible tablets. Using several residence times in an acidic medium, we simulated the gastric emptying of the pellets and the exposure of different fractions of the pellets to the gastric medium for different periods of time. The amount of drug released decreased with the increasing time of exposure to the acidic medium due to the drug’s degradation. The mean in vivo dissolution profiles, which were predicted on the basis of experimentally determined dissolution profiles and mathematical model of pellets’ gastric emptying, gave a very good IVIVC with the mean in vivo absorption profiles.

Identification of polyether antibiotics produced by the endophyte Streptomyces platensis RTd22

The endophytic actimomycete S. platensis RTd22 was cultured in rice solid medium and shaken ISP-2 liquid medium for different periods of time. Extracts and sub-fractions obtained after the chromatographic procedures showed remarkable activity against Staphylococcus aureus ATCC 6538, S. saprophyticus ATCC 15305 and S. aureus ATCC 29213. Some samples from both culture media also showed high activity against the antibiotic-resistant bacteria: vancomycin-intermediate S. aureus Mu50 (VISA), methicillin-resistant S. aureus SA16 (MRSA) and vancomycin-resistant Enterococcus faecium VRE16. Analyses by ESI-MS (AcquityTM UPLC/Waters) of the active fractions allowed the rapid identification of the ionophore antibiotics grisorixin (1), nigericin (2), and mutalomycin (3), structurally related polyether polyketides. The structural identification of 1–3 together with the antibiotic activities will be presented.

Amyloid β (Aβ) Peptide Directly Activates Amylin-3 Receptor Subtype by Triggering Multiple Intracellular Signaling Pathways

The two age-prevalent diseases Alzheimer disease and type 2 diabetes mellitus share many common features including the deposition of amyloidogenic proteins, amyloid β protein (Aβ) and amylin (islet amyloid polypeptide), respectively. Recent evidence suggests that both Aβ and amylin may express their effects through the amylin receptor, although the precise mechanisms for this interaction at a cellular level are unknown. Here, we studied this by generating HEK293 cells with stable expression of an isoform of the amylin receptor family, amylin receptor-3 (AMY3). Aβ1-42 and human amylin (hAmylin) increase cytosolic cAMP and Ca(2+), trigger multiple pathways involving the signal transduction mediators protein kinase A, MAPK, Akt, and cFos. Aβ1-42 and hAmylin also induce cell death during exposure for 24-48 h at low micromolar concentrations. In the presence of hAmylin, Aβ1-42 effects on HEK293-AMY3-expressing cells are occluded, suggesting a shared mechanism of action between the two peptides. Amylin receptor antagonist AC253 blocks increases in intracellular Ca(2+), activation of protein kinase A, MAPK, Akt, cFos, and cell death, which occur upon AMY3 activation with hAmylin, Aβ1-42, or their co-application. Our data suggest that AMY3 plays an important role by serving as a receptor target for actions Aβ and thus may represent a novel therapeutic target for development of compounds to treat neurodegenerative conditions such as Alzheimer disease.

Entire solutions of the Fisher-KPP equation in time periodic media

This paper is concerned with the existence of the pulsating type entire solutions of the Fisher-KPP equation with advection term in time peri- odic media. By constructing appropriate subsolutions and supersolutions, we prove that there exists a pulsating type entire solution which behaves as two pulsating traveling fronts coming from two opposite directions and approach- ing each other. The main technique here is to characterize the asymptotic behavior of the pulsating traveling front as t ! 1 .

STIM1 and STIM2 Are Located in the Acidic Ca2+ Stores and Associates with Orai1 upon Depletion of the Acidic Stores in Human Platelets

Mammalian cells accumulate Ca2+ into agonist-sensitive acidic organelles, vesicles that possess a vacuolar proton-ATPase. Acidic Ca2+ stores include secretory granules and lysosome-related organelles. Current evidence clearly indicates that acidic Ca2+ stores participate in cell signaling and function, including the activation of store-operated Ca2+ entry in human platelets upon depletion of the acidic stores, although the mechanism underlying the activation of store-operated Ca2+ entry controlled by the acidic stores remains unclear. STIM1 has been presented as the endoplasmic reticulum Ca2+ sensor, but its role sensing intraluminal Ca2+ concentration in the acidic stores has not been investigated. Here we report that STIM1 and STIM2 are expressed in the lysosome-related organelles and dense granules in human platelets isolated by immunomagnetic sorting. Depletion of the acidic Ca2+ stores using the specific vacuolar proton-ATPase inhibitor, bafilomycin A1, enhanced the association between STIM1 and STIM2 as well as between these proteins and the plasma membrane channel Orai1. Depletion of the acidic Ca2+ stores also induces time-dependent co-immunoprecipitation of STIM1 with the TRPC proteins hTRPC1 and hTRPC6, as well as between Orai1 and both TRPC proteins. In addition, bafilomycin A1 enhanced the association between STIM2 and SERCA3. These findings demonstrate the location of STIM1 and STIM2 in the acidic Ca2+ stores and their association with Ca2+ channels and ATPases upon acidic stores discharge.

Chemical Synthesis, Characterization and Bioactivity Evaluation of Hydroxyapatite Prepared from Garden snail (Helix aspersa)

A novel attempt is made to convert the calcium carbonate skeleton of widely available garden snail shell (Helix aspersa) to hydroxyapatite based bioceramics. The snail shell was found to decompose within 8500C to all the carbonate phases. The calcined snail shells were then treated with acids followed by different chemicals in ammoniacal media maintaining proper stoichemetry to produce fine Hydroxyapatite (HAP) as filter cake with Ca/P molar ratio of 1.67. The dried HAP powder was extremely pure with specific surface area of 15m2/g. The different characterization techniques were adopted both for calcined snail shell and HAP synthesized like X-ray Diffraction (XRD), Thermal Analysis (DTA/TGA), Fourier Transform Infra red Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The surface area and the particle size, of the HAP powder prepared by chemical precipitation route, were also determined by BET and Malvern particle size analyzer respectively. The synthesized powder was soaked in stimulated body fluid (SBF) medium for various periods of time in order to evaluate its bioactivity. The changes of the pH of SBF medium were measured. High bioactivity of prepared HAP powder due to the formation of apatite on its surface was observed.

Investigation on Biological Properties of Dental Implant by Ce-TZP/Al2O3/ HA Bio-nano-composites

An attempt is made to convert the calcium carbonate skeleton of widely available garden snail shell (Helix aspersa) to hydroxyapatite based bioceramics. The snail shell was found to decompose within 850°C to all the carbonate phases. The calcined snail shells were then treated with acids followed by different chemicals in ammoniacal media maintaining proper stoichemetry to produce fine Hydroxyapatite (HAP) as filter cake with Ca/P molar ratio of 1.67. The dried HAP powder was extremely pure with specific surface area of 15m2/g. The different characterization techniques were adopted both for calcined snail shell and HAP synthesized like X-ray Diffraction (XRD), Thermal Analysis (DTA/TGA) Figure 1, Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The surface area and the particle size, of the HAP powder prepared by chemical precipitation route, were also determined by BET and Malvern particle size analyzer respectively. The synthesized powder was soaked in stimulated body fluid (SBF) medium for various periods of time in order to evaluate its bioactivity. The changes of the pH of SBF medium were measured. High bioactivity of prepared HAP powder due to the formation of apatite on its surface was observed.

PLGA doping of PCL affects the plastic potential of human mesenchymal stem cells, both in the presence and absence of biological stimuli

A range of poly epsilon-caprolactone (PCL) films mixed/doped with poly(lactide-co-glycolide) (PLGA) (65:35) in 0, 10, 20, and 30 wt % were produced, sterilized using ethylene oxide, and analyzed using FTIR. Characterized human mesenchymal stem cells (hMSCs) were cultured in contact with the materials in basal, chondrogenic, and osteogenic medium for time periods up to 28 days, to determine if the materials could induce differentiation of MSC both in the presence and absence of biological stimuli. Viable cell adhesion was analyzed under all conditions. Collagen I, collagen II, sox-9, osteocalcin, osteopontin, osteonectin, and CBFA1 were evaluated at both the mRNA (real-time PCR) and protein production levels (fluorescent immunohistochemistry) and used to identify cell differentiation. Pure PCL and PCL mixed with PLGA demonstrated a chondrogenic potential. Only PCL 8 (80 wt % PCL, 20 wt % PLGA) facilitated osteogenic differentiation of MSCs under osteogenic conditions. This was attributed to the increased hydrophilic nature of the surface allowing sufficient homogeneous cell attachment and the formation of filamentous F-actin in the cells, allowing osteogenic differentiation. Of all materials tested, PCL 7 (70 wt % PCL, 30 wt % PLGA) demonstrated the greatest chondrogenic differentiation potential under basal and stimulated conditions at both the mRNA and protein production level.