Enhancing Optical Isolator Performance in Nonreciprocal Waveguide Arrays

We investigate the operation of optical isolators based on magneto-optics waveguide arrays beyond the coupled mode analysis. Semi-vectorial beam propagation simulations demonstrate that evanescent tail coupling and the effects of radiation are responsible for degrading the device’s performance. Our analysis suggests that these effects can be mitigated when the array size is scaled up. In addition, we propose the use of radiation blockers in order to offset some of these effects, and we show that they provide a dramatic improvement in performance. Finally, we also study the robustness of the system with respect to fabrication tolerances using the coupled mode theory. We show that small, random variations in the system’s parameters tend to average out as the number of optical guiding channels increases.

Serine residues 1177/78/82 of the insulin receptor are required for substrate phosphorylation but not autophosphorylation

Serine residues of the human insulin receptor (HIR) may be phosphorylated and negatively regulate the insulin signal. We studied the impact of 16 serine residues in HIR by mutation to alanine and co-overexpression in human embryonic kidney (HEK) 293 cells together with the docking proteins insulin receptor substrate (IRS)-1, IRS-2, or (SHC) Src homologous and collagen-like. As a control, IRS-1 was also cotransfected with an HIR with a juxtamembrane deletion (HIR delta JM) and therefore not containing the domain required for interaction with IRS-1. Coexpression of HIR with IRS-1, IRS-2, and SHC strongly enhanced tyrosine phosphorylation of these proteins. A similar increase in tyrosine phosphorylation was observed in cells overexpressing IRS-1, IRS-2, or SHC together with all HIR mutants except HIR delta JM and a mutant carrying exchanges of serines 1177, 1178, and 1182 to alanine (HIR1177/78/82), although this mutant showed normal autophosphorylation. Analysis of total cell lysates with anti-phosphotyrosine antibodies showed that in addition to the overexpressed substrates, other cellular proteins displayed reduced levels of tyrosine phosphorylation in these cells. To study consequences for phosphatidylinositol 3-kinase (PI 3-kinase) activation, we established stable NIH3T3 fibroblast cell lines overexpressing wild-type HIR, HIR1177/78/82, and other HIR mutants as the control. Again, HIR1177/78/82 showed normal autophosphorylation but showed a clear decrease in tyrosine phosphorylation of endogenous IRS-1 and activation of PI 3-kinase. This decrease in kinase activity also occurred in an in vitro kinase assay towards recombinant IRS-1. Finally, we performed a separation of the phosphopeptides by high-performance liquid chromatography and could not detect any differences in the profiles of HIR and HIR1177/78/82. In conclusion, we have defined a region in HIR that is important for substrate phosphorylation but not autophosphorylation. Therefore, this mutant may provide new insights into the mechanism of kinase activation and substrate phosphorylation.

A Survey of Forage Quality Following a Flood Year: 1994

A team of extension livestock specialists and county extension workers collected 362 forage samples from cooperating producers in 55 Iowa Counties. Summaries of the three forage types showed normal feed analysis for energy and protein. Micro minerals were also analyzed with 11% of samples being below National Research Council 1984 selenium and zinc requirements for beef.

Does tin pre-treatment enhance the bond strength of adhesive systems to enamel?

OBJECTIVES The study investigated the modification of composite-to-enamel bond strength by pre-treatment of enamel with a concentrated, acidic SnCl2-solution. METHODS Six groups of flat human enamel specimens (n=44 per group) were treated as follows: OB-H: H3PO4 etching, Optibond FL application (primer+adhesive; manufacturer's instructions); OB-S: SnCl2 pre-treatment, Optibond FL application (primer+adhesive); OB-HS: H3PO4 etching+SnCl2 pre-treatment, Optibond FL application (primer+adhesive); CF-N: Clearfil SE application (primer+bond; manufacturer's instructions); CF-H: H3PO4 etching, Clearfil SE application (primer+bond); CF-S: SnCl2 pre-treatment, Clearfil SE application (primer+bond). Enamel specimens were then built up with resin composite (Clearfil Majesty Esthetic) and stored (100% humidity, 37 °C, 1 week). μTBS-measurement and failure mode analysis of one-half of the specimens were performed immediately after storage, while the other half was analysed after a thermocycling procedure (8500 cycles; 5 °C and 55 °C; dwell time 30s). Additional specimens were prepared for SEM- and EDX-analysis. RESULTS Highest values were measured for OB-H before and after thermocycling, lowest values for CF-N. Compared to OB-H treatment, OB-S treatment reduced μTBS before/after thermocycling by 23%/28% and OB-HS treatment by 8%/24% (except for OB-SH before (n.s.), all p≤0.001 compared to OB-H). In the Clearfil SE treated groups pre-treatment increased μTBS significantly compared to CF-N (before/after: CF-H: +46%/+70%; CF-S: +51%/42%; all p≤0.001). CONCLUSION Pre-treatment with H3PO4 or SnCl2 markedly increased the μTBS of Clearfil SE to enamel. However, thermocycling partly reduced the gain in μTBS obtained by SnCl2 pre-treatment. CLINICAL SIGNIFICANCE The application of an acidic and highly concentrated SnCl2 solution is a good option to increase the μTBS between enamel and a resin composite mediated by an adhesive system containing the multifunctional monomer MDP.

Development of a rapid bioassessment method and index of biotic integrity for estuaries located along the northwest Gulf of Mexico

Recently it has been proposed that the evaluation of effects of pollutants on aquatic organisms can provide an early warning system of potential environmental and human health risks (NRC 1991). Unfortunately there are few methods available to aquatic biologists to conduct assessments of the effects of pollutants on aquatic animal community health. The primary goal of this research was to develop and evaluate the feasibility of such a method. Specifically, the primary objective of this study was to develop a prototype rapid bioassessment technique similar to the Index of Biotic Integrity (IBI) for the upper Texas and Northwestern Gulf of Mexico coastal tributaries. The IBI consists of a series of "metrics" which describes specific attributes of the aquatic community. Each of these metrics are given a score which is then subtotaled to derive a total assessment of the "health" of the aquatic community. This IBI procedure may provide an additional assessment tool for professionals in water quality management.^ The experimental design consisted primarily of compiling previously collected data from monitoring conducted by the Texas Natural Resource Conservation Commission (TNRCC) at five bayous classified according to potential for anthropogenic impact and salinity regime. Standardized hydrological, chemical, and biological monitoring had been conducted in each of these watersheds. The identification and evaluation of candidate metrics for inclusion in the estuarine IBI was conducted through the use of correlation analysis, cluster analysis, stepwise and normal discriminant analysis, and evaluation of cumulative distribution frequencies. Scores of each included metric were determined based on exceedances of specific percentiles. Individual scores were summed and a total IBI score and rank for the community computed.^ Results of these analyses yielded the proposed metrics and rankings listed in this report. Based on the results of this study, incorporation of an estuarine IBI method as a water quality assessment tool is warranted. Adopted metrics were correlated to seasonal trends and less so to salinity gradients observed during the study (0-25 ppt). Further refinement of this method is needed using a larger more inclusive data set which includes additional habitat types, salinity ranges, and temporal variation. ^

Role of the transcription factor activator protein-2alpha in prostate carcinogenesis

Activator protein 2α (AP-2) is a transcription factor known to play a crucial role in the progression of malignant melanoma, colorectal carcinoma, and breast cancer. Several AP-2 target genes are known to be deregulated in prostate cancer, therefore, we hypothesize that loss AP-2 expression plays a causal role in prostate carcinogenesis. Immunofluorescent staining for AP-2 of 30 radical prostatectomy specimens demonstrated that while AP-2 was highly expressed in normal prostate epithelium, its expression was lost in most cases of high grade prostatic intraepithelial neoplasia (PIN), and all cases of prostate cancer studied. Additional analyses demonstrated that AP-2 was associated with normal luminal differentiation and it was not expressed in the basal cell layer. In cell lines, AP-2 was strongly expressed in immortalized normal prostate epithelial cells, whereas low expression was observed in the LNCaP, LNCaP-LN3, and PC3M-LN4 prostate cancer cell lines. Transfection of the highly tumorigenic and metastatic cell line PC3M-LN4 with the AP-2 gene significantly decreased tumor growth in the prostate of nude mice (p = 0.032) and inhibited metastases to the lymph nodes. Moreover, transfection of the low tumorigenic, low metastatic cell line LNCaP-LN3 with full length AP-2; resulted in complete inhibition of tumor incidence in the AP-2 transfectants (0/19) vs. neo control (10/16). A potential mechanism for this loss of tumorigenicity was the modulation of gene expression in prostate cancer cells that mimicked the normal phenotype. Analysis of differential expression between neo control- and AP-2-transfected cells in vitro and in tumors demonstrated low VEGF expression in AP-2 transfectants. We further demonstrated that AP-2 acted as a transcriptional repressor of the VEGF promoter by binding to a GC-rich region located between −88 and −66. This region contains an AP-2 consensus element overlapping two Sp1 consensus elements. We found that Sp3 and AP-2 bound to this region in a mutually exclusive manner to promote activation or repression. Increased VEGF expression has been observed in high grade PIN and in prostate cancer. Here we provide evidence that this early molecular change could be a result of loss of AP-2 expression in the prostatic epithelium. ^

The heparan sulfate-fibroblast growth factor signaling system in liver growth and function

The heparan sulfate (HS)-fibroblast growth factor (FGF) signaling system is a ubiquitous regulator that senses local environmental changes and mediates cell-to-cell communication. This system consists of three mutually interactive components. These are regulatory polypeptides (FGF), FGF receptor (FGFR) and heparan sulfate proteoglycans (FGFRHS). All four FGFR genes are expressed in the adult liver. Expression of the FGFR1–3 genes is generally associated with non-parenchymal cells while expression of the FGFR4 gene is associated with parenchymal hepatocytes. We showed that livers of mice lacking FGFR4 exhibited normal morphology and regenerated normally in response to partial hepatectomy. However, the FGFR4 (−/−) mice exhibited depleted gallbladders, an elevated bile acid pool and elevated excretion of bile acids. Cholesterol- and bile acid-controlled liver cholesterol 7α-hydroxylase (Cyp7a), the limiting enzyme for bile acid synthesis, was elevated, unresponsive to dietary cholesterol, but repressed normally by dietary cholate. These results indicated that FGFR4 was not directly involved in liver growth but exerted negative control on liver bile acid synthesis. This was confirmed in transgenic mice overexpressing the constitutively active human FGFR4 in livers. The transgenic mice exhibited decreased fecal bile acid excretion, bile acid pool size, and expression of Cyp7a. Introduction of this constitutively active human FGFR4 into FGFR4 (−/−) mice restored the inhibition of bile acid synthesis. Activation of the c-Jun N-terminal Kinase (JNK) pathway by FGFR4 correlated with the repressive effect on bile acid synthesis. ^ To determine whether FGFR4 played a broader role in liver-specific metabolic function, we examined the impact of both acute and chronic exposure to CCl 4 in FGFR4 (−/−) mice. Following acute CCl4 exposure, the FGFR4 (−/−) mice exhibited accelerated liver injury, a significant increase in liver mass and delayed hepatolobular repair, with no apparent effect on liver cell proliferation and restoration of cellularity. Chronic CCl4 exposure resulted in severe fibrosis in livers of FGFR4 (−/−) mice compared to normal mice. Analysis at both mRNA and protein levels indicated an 8 hr delay in FGFR4-deficient mice in the down-regulation of cytochrome P450 2E1 (CYP2E1) protein, the major enzyme whose products underlie CCl 4-induced injury. These results show that hepatocyte FGFR4 protects against acute and chronic insult to the liver and prevents accompanying fibrosis. ^ Of the 23 FGF polypeptides, FGF1 and FGF2 are present at significant levels in the liver. To determine whether FGF1 and FGF2 played a role in CCl 4-induced liver injury and fibrosis, we examined the impact of both acute and chronic exposure to CCl4 in both wild-type and FGF1-FGF2 double-knockout mice. Following acute CCl4 exposure, FGF1(−/−)FGF2(−/−) mice exhibited accelerated liver injury, overall normal liver growth and repair, and decreased liver collagen α1(I) induction. Liver fibrosis resulting from chronic CCl4 exposure was markedly decreased in livers of FGF1(−/−)FGF2(−/−) mice compared to wild-type mice. This study suggests a role for FGF1 and FGF2 in hepatic fibrogenesis. ^ In summary, our three part study shows that specific components of the ubiquitous HS-FGF signaling family in the liver context interfaces with metabolite- and xenobiotic-controlled networks to regulate liver function, but has no apparent direct effect on liver cell growth. ^

The Driven Harmonic Oscillator in Chemistry

The molecular dynamical simulation of the normal vibrational mode of water which involves H-O-H angle deformation, when driven by an external force, can be used to see how a driven harmonic oscillator, classically, is associated with the infra-red spectrum of water (and the absorption for this particular normal mode).

Código híbrido avanzado de motores de plasma de efecto Hall

La propulsión eléctrica constituye hoy una tecnología muy competitiva y de gran proyección de futuro. Dentro de los diversos motores de plasma existentes, el motor de efecto Hall ha adquirido una gran madurez y constituye un medio de propulsión idóneo para un rango amplio de misiones. En la presente Tesis se estudian los motores Hall con geometría convencional y paredes dieléctricas. La compleja interacción entre los múltiples fenómenos físicos presentes hace que sea difícil la simulación del plasma en estos motores. Los modelos híbridos son los que representan un mejor compromiso entre precisión y tiempo de cálculo. Se basan en utilizar un modelo fluido para los electrones y algoritmos de dinámica de partículas PIC (Particle-In- Cell) para los iones y los neutros. Permiten hacer uso de la hipótesis de cuasineutralidad del plasma, a cambio de resolver separadamente las capas límite (o vainas) que se forman en torno a las paredes de la cámara. Partiendo de un código híbrido existente, llamado HPHall-2, el objetivo de la Tesis doctoral ha sido el desarrollo de un código híbrido avanzado que mejorara la simulación de la descarga de plasma en un motor de efecto Hall. Las actualizaciones y mejoras realizadas en las diferentes partes que componen el código comprenden tanto aspectos teóricos como numéricos. Fruto de la extensa revisión de la algoritmia del código HPHall-2 se han conseguido reducir los errores de precisión un orden de magnitud, y se ha incrementado notablemente su consistencia y robustez, permitiendo la simulación del motor en un amplio rango de condiciones. Algunos aspectos relevantes a destacar en el subcódigo de partículas son: la implementación de un nuevo algoritmo de pesado que permite determinar de forma más precisa el flujo de las magnitudes del plasma; la implementación de un nuevo algoritmo de control de población, que permite tener suficiente número de partículas cerca de las paredes de la cámara, donde los gradientes son mayores y las condiciones de cálculo son más críticas; las mejoras en los balances de masa y energía; y un mejor cálculo del campo eléctrico en una malla no uniforme. Merece especial atención el cumplimiento de la condición de Bohm en el borde de vaina, que en los códigos híbridos representa una condición de contorno necesaria para obtener una solución consistente con el modelo de interacción plasma-pared, y que en HPHall-2 aún no se había resuelto satisfactoriamente. En esta Tesis se ha implementado el criterio cinético de Bohm para una población de iones con diferentes cargas eléctricas y una gran dispersión de velocidades. En el código, el cumplimiento de la condición cinética de Bohm se consigue por medio de un algoritmo que introduce una fina capa de aceleración nocolisional adyacente a la vaina y mide adecuadamente el flujo de partículas en el espacio y en el tiempo. Las mejoras realizadas en el subcódigo de electrones incrementan la capacidad de simulación del código, especialmente en la región aguas abajo del motor, donde se simula la neutralización del chorro del plasma por medio de un modelo de cátodo volumétrico. Sin abordar el estudio detallado de la turbulencia del plasma, se implementan modelos sencillos de ajuste de la difusión anómala de Bohm, que permiten reproducir los valores experimentales del potencial y la temperatura del plasma, así como la corriente de descarga del motor. En cuanto a los aspectos teóricos, se hace especial énfasis en la interacción plasma-pared y en la dinámica de los electrones secundarios libres en el interior del plasma, cuestiones que representan hoy en día problemas abiertos en la simulación de los motores Hall. Los nuevos modelos desarrollados buscan una imagen más fiel a la realidad. Así, se implementa el modelo de vaina de termalización parcial, que considera una función de distribución no-Maxwelliana para los electrones primarios y contabiliza unas pérdidas energéticas más cercanas a la realidad. Respecto a los electrones secundarios, se realiza un estudio cinético simplificado para evaluar su grado de confinamiento en el plasma, y mediante un modelo fluido en el límite no-colisional, se determinan las densidades y energías de los electrones secundarios libres, así como su posible efecto en la ionización. El resultado obtenido muestra que los electrones secundarios se pierden en las paredes rápidamente, por lo que su efecto en el plasma es despreciable, no así en las vainas, donde determinan el salto de potencial. Por último, el trabajo teórico y de simulación numérica se complementa con el trabajo experimental realizado en el Pnnceton Plasma Physics Laboratory, en el que se analiza el interesante transitorio inicial que experimenta el motor en el proceso de arranque. Del estudio se extrae que la presencia de gases residuales adheridos a las paredes juegan un papel relevante, y se recomienda, en general, la purga completa del motor antes del modo normal de operación. El resultado final de la investigación muestra que el código híbrido desarrollado representa una buena herramienta de simulación de un motor Hall. Reproduce adecuadamente la física del motor, proporcionando resultados similares a los experimentales, y demuestra ser un buen laboratorio numérico para estudiar el plasma en el interior del motor. Abstract Electric propulsion is today a very competitive technology and has a great projection into the future. Among the various existing plasma thrusters, the Hall effect thruster has acquired a considerable maturity and constitutes an ideal means of propulsion for a wide range of missions. In the present Thesis only Hall thrusters with conventional geometry and dielectric walls are studied. The complex interaction between multiple physical phenomena makes difficult the plasma simulation in these engines. Hybrid models are those representing a better compromise between precision and computational cost. They use a fluid model for electrons and Particle-In-Cell (PIC) algorithms for ions and neutrals. The hypothesis of plasma quasineutrality is invoked, which requires to solve separately the sheaths formed around the chamber walls. On the basis of an existing hybrid code, called HPHall-2, the aim of this doctoral Thesis is to develop an advanced hybrid code that better simulates the plasma discharge in a Hall effect thruster. Updates and improvements of the code include both theoretical and numerical issues. The extensive revision of the algorithms has succeeded in reducing the accuracy errors in one order of magnitude, and the consistency and robustness of the code have been notably increased, allowing the simulation of the thruster in a wide range of conditions. The most relevant achievements related to the particle subcode are: the implementation of a new weighing algorithm that determines more accurately the plasma flux magnitudes; the implementation of a new algorithm to control the particle population, assuring enough number of particles near the chamber walls, where there are strong gradients and the conditions to perform good computations are more critical; improvements in the mass and energy balances; and a new algorithm to compute the electric field in a non-uniform mesh. It deserves special attention the fulfilment of the Bohm condition at the edge of the sheath, which represents a boundary condition necessary to match consistently the hybrid code solution with the plasma-wall interaction, and remained as a question unsatisfactory solved in the HPHall-2 code. In this Thesis, the kinetic Bohm criterion has been implemented for an ion particle population with different electric charges and a large dispersion in their velocities. In the code, the fulfilment of the kinetic Bohm condition is accomplished by an algorithm that introduces a thin non-collisional layer next to the sheaths, producing the ion acceleration, and measures properly the flux of particles in time and space. The improvements made in the electron subcode increase the code simulation capabilities, specially in the region downstream of the thruster, where the neutralization of the plasma jet is simulated using a volumetric cathode model. Without addressing the detailed study of the plasma turbulence, simple models for a parametric adjustment of the anomalous Bohm difussion are implemented in the code. They allow to reproduce the experimental values of the plasma potential and the electron temperature, as well as the discharge current of the thruster. Regarding the theoretical issues, special emphasis has been made in the plasma-wall interaction of the thruster and in the dynamics of free secondary electrons within the plasma, questions that still remain unsolved in the simulation of Hall thrusters. The new developed models look for results closer to reality, such as the partial thermalization sheath model, that assumes a non-Maxwellian distribution functions for primary electrons, and better computes the energy losses at the walls. The evaluation of secondary electrons confinement within the chamber is addressed by a simplified kinetic study; and using a collisionless fluid model, the densities and energies of free secondary electrons are computed, as well as their effect on the plasma ionization. Simulations show that secondary electrons are quickly lost at walls, with a negligible effect in the bulk of the plasma, but they determine the potential fall at sheaths. Finally, numerical simulation and theoretical work is complemented by the experimental work carried out at the Princeton Plasma Physics Laboratory, devoted to analyze the interesting transitional regime experienced by the thruster in the startup process. It is concluded that the gas impurities adhered to the thruster walls play a relevant role in the transitional regime and, as a general recomendation, a complete purge of the thruster before starting its normal mode of operation it is suggested. The final result of the research conducted in this Thesis shows that the developed code represents a good tool for the simulation of Hall thrusters. The code reproduces properly the physics of the thruster, with results similar to the experimental ones, and represents a good numerical laboratory to study the plasma inside the thruster.

Platelet signal transduction defect with Gα subunit dysfunction and diminished Gαq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5′-[γ-thio]triphosphate (GTP[γS]) was diminished in the patient’s platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of α-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[γS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Gαs) and its inhibition (mediated by Gαi) by thrombin in the patient’s platelet membranes were normal. Immunoblot analysis of Gα subunits in the patient’s platelet membranes showed a decrease in Gαq (<50%) but not Gαi, Gαz, Gα12, and Gα13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein α-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Gαq in thrombin-induced responses.

Symmetry in chemistry from the hydrogen atom to proteins

The last 2 decades have seen discoveries in highly excited states of atoms and molecules of phenomena that are qualitatively different from the “planetary” model of the atom, and the near-rigid model of molecules, characteristic of these systems in their low-energy states. A unified view is emerging in terms of approximate dynamical symmetry principles. Highly excited states of two-electron atoms display “molecular” behavior of a nonrigid linear structure undergoing collective rotation and vibration. Highly excited states of molecules described in the “standard molecular model” display normal mode couplings, which induce bifurcations on the route to molecular chaos. New approaches such as rigid–nonrigid correlation, vibrons, and quantum groups suggest a unified view of collective electronic motion in atoms and nuclear motion in molecules.

Soins a prendre des animaux domestiques : ouvrage conforme au programme officiel du 5 septembre 1896 /

At head of title: Enseignement normal.

On the dimensionality of the Attachment Style Questionnaire in Italian clinical and nonclinical participants

This study examined the utility of the Attachment Style Questionnaire (ASQ) in an Italian sample of 487 consecutively admitted psychiatric participants and an independent sample of 605 nonclinical participants. Minimum average partial analysis of data from the psychiatric sample supported the hypothesized five-factor structure of the items; furthermore, multiple-group component analysis showed that this five-factor structure was not an artifact of differences in item distributions. The five-factor structure of the ASQ was largely replicated in the nonclinical sample. Furthermore, in both psychiatric and nonclinical samples, a two-factor higher order structure of the ASQ scales was observed. The higher order factors of Avoidance and Anxious Attachment showed meaningful relations with scales assessing parental bonding, but were not redundant with these scales. Multivariate normal mixture analysis supported the hypothesis that adult attachment patterns, as measured by the ASQ, are best considered as dimensional constructs.

Determination of crystal orientation from micrographs using a MATLAB program

Crys.m is a MATLAB routine that combines a micrograph of a crystal with a scaleable, rotatable three-dimensional cage structure to determine the orientation of the crystal axes. The example presented here uses the morphology of tetragonal lysozyme. Rotation of the cage until it aligns with the crystal in the image yields the orientation of the c axis of the crystal relative to the image normal. This analysis can be used for quantitative determination of crystal orientation effects induced by electric, magnetic and/or gravitational fields.

Evidence-based nanoscopic and molecular framework for excipient functionality in compressed orally disintegrating tablets

The work investigates the adhesive/cohesive molecular and physical interactions together with nanoscopic features of commonly used orally disintegrating tablet (ODT) excipients microcrystalline cellulose (MCC) and D-mannitol. This helps to elucidate the underlying physico-chemical and mechanical mechanisms responsible for powder densification and optimum product functionality. Atomic force microscopy (AFM) contact mode analysis was performed to measure nano-adhesion forces and surface energies between excipient-drug particles (6-10 different particles per each pair). Moreover, surface topography images (100 nm2-10 μm2) and roughness data were acquired from AFM tapping mode. AFM data were related to ODT macro/microscopic properties obtained from SEM, FTIR, XRD, thermal analysis using DSC and TGA, disintegration testing, Heckel and tabletability profiles. The study results showed a good association between the adhesive molecular and physical forces of paired particles and the resultant densification mechanisms responsible for mechanical strength of tablets. MCC micro roughness was 3 times that of D-mannitol which explains the high hardness of MCC ODTs due to mechanical interlocking. Hydrogen bonding between MCC particles could not be established from both AFM and FTIR solid state investigation. On the contrary, D-mannitol produced fragile ODTs due to fragmentation of surface crystallites during compression attained from its weak crystal structure. Furthermore, AFM analysis has shown the presence of extensive micro fibril structures inhabiting nano pores which further supports the use of MCC as a disintegrant. Overall, excipients (and model drugs) showed mechanistic behaviour on the nano/micro scale that could be related to the functionality of materials on the macro scale. © 2014 Al-khattawi et al.