The reconstruction of stratified dielectric profiles using successive approximations

The problem of reconstructing the one-dimensional refractive index profiles from electromagnetic reflection data is considered for lossless, dispersionless dielectrics. The Balanis integral equation is solved by the method of successive kernel approximation. Both continuous and discontinuous profile examples demonstrate the accuracy of this method by comparing and contrasting the results with those of a leapfrogging numerical solution and associated analytical results. The convergence criterion for this method is specified.     

Increasing the Efficiency of Rake Receivers for Ultra-Wideband Applications

In diversity rich environments, such as in Ultra-Wideband (UWB) applications, the a priori determination of the number of strong diversity branches is difficult, because of the considerably large number of diversity paths, which are characterized by a variety of power delay profiles (PDPs). Several Rake implementations have been proposed in the past, in order to reduce the number of the estimated and combined paths. To this aim, we introduce two adaptive Rake receivers, which combine a subset of the resolvable paths considering simultaneously the quality of both the total combining output signal-to-noise ratio (SNR) and the individual SNR of each path, reducing the number of combined paths, while keeping the desirable performance. These schemes achieve better adaptation to channel conditions compared to other known receivers, without further increasing the complexity. Their performance is evaluated in different practical UWB channels, whose models are based on extensive propagation measurements. The proposed receivers compromise between the power consumption, complexity and performance gain for the additional paths, resulting in important savings in power and computational resources.     

Design of corrugated optical waveguide filters through a direct numerical solution of the coupled Gel'fand-Levitan-Marchenko integral equations

Regarding the design problem of corrugated planar optical waveguide filters, a new numerical method is presented consisting of a direct numerical solution of the coupled Gel'fand-Levitan-Marchenko integral equations. This method, which uses leapfrogging in space and time, is exact in principle and avoids some difficulties encountered in previously derived analytical methods of solution. Straightforward numerical calculations permit the design of several classes of filters such as Butterworth, Chebyshev, Cauer (elliptic) and others, as presented in the paper. The accuracy of our proposed method of design is checked in several ways, mainly through the numerical solution of the corresponding direct-scattering problem (Riccati differential equation).     

Osteoblast hydraulic conductivity is regulated by calcitonin and parathyroid hormone

Many aspects of cell social behavior, including aspects of tumor invasiveness, embryonic development, and wound healing, can be explained by the principle of contact inhibition (CI) of cell movement. CI refers to the tendency of fibroblasts cultured on a plane substratum to cease movement on contacting other fibroblasts. A problem in studying collisions between cells on a flat substratum is that it is difficult to control the specific regions of the cell that come in contact. In this study we used grooved micromachined titanium substrata to produce collisions between the following cell combinations: fibroblast/fibroblast, fibroblast/epithelium, and epithelium/epithelium. The cells were oriented by the substratum so that the leading lamellae of the cells confronted each other. Cell behaviors before and after contact were observed and recorded using time-lapse cinemicrography employing Nomarski reflected light differential interference microscopy. Electron-microscopy sections were prepared from areas where cell interactions occurred. Fibroblasts (F) moved significantly faster and more persistently on grooved than on smooth surfaces, but the speed of epithelial (E)-cell locomotion was not significantly altered. The grooves, however, guided the direction of locomotion for both cell types. When cultured on grooved surfaces in such a manner that the F and E cells collided head-on, the F, but not the E cells, frequently demonstrated contact inhibition of movement. However, after such collisions, significantly more F continued to invade the E sheet than were observed after F-E collisions on smooth surfaces. After F-F collisions on grooved surfaces, most cells moved to the sides of the grooves and continued in their original directions, while on smooth surfaces they moved off in various different directions. A possible explanation of these observations is that a grooved surface produces and maintains F polarity so that the direction of locomotion is less readily altered by cell-cell interactions.     

Liver sinusoidal endothelial cells are responsible for nitric oxide modulation of resistance in the hepatic sinusoids

The mechanisms that regulate vascular resistance in the liver are an area of active investigation. Previously, we have shown that nitric oxide (NO) modulates hepatic vascular tone in the normal rat liver. In this study, the production of NO is examined in further detail by isolating sinusoidal endothelial cells (SEC) from the rat liver. Endothelial NO synthase (eNOS) was present in SEC based on Western blotting and confocal immunofluorescence microscopy. Exposure of SEC to flow increased the release of NO. To investigate the relevance of these in vitro findings to the intact liver, we modified an in situ perfusion system to allow for direct measurement of NO release from the hepatic vasculature. NO was released from the hepatic vasculature in a time-dependent manner, and administration of N-monomethyl-L-arginine reduced NO release and increased portal pressure. Immunostaining of intact liver demonstrated eNOS localization to endothelial cells lining the hepatic sinusoids. These findings demonstrate that SEC in vitro and in vivo express eNOS and produce NO basally, and increase their production in response to flow. Additionally, an increase in portal pressure concomitant with the blockade of NO release directly demonstrates that endogenous endothelial-derived NO modulates portal pressure.     

Modulation of GTPase activity of G proteins by fluid shear stress and phospholipid composition

Mechanical forces arising from strain, pressure, and fluid shear stress are sensed by cells through an unidentified mechanoreceptor(s) coupled to intracellular signaling pathways. In vascular endothelial cells, fluid shear stress is transduced via pathway(s) involving heterotrimeric guanine nucleotide-binding proteins (G proteins) by molecular mechanisms that are unknown. In the present study, we investigated the activation of purified G proteins reconstituted into phospholipid vesicles. Vesicles containing G proteins were loaded with [gamma-32P]GTP and subjected to physiological levels of fluid shear stress in a cone-and-plate viscometer. Steady-state GTP hydrolysis was measured as an index of G protein function. Shear stress (0-30 dynes/cm2) activated G proteins in dose-dependent manner (0.48-4.6 pmol/min per microg of protein). Liposomes containing lysophosphatidylcholine (30 mol %) or treated with benzyl alcohol (40 mM), conditions that increase bilayer fluidity, exhibited 3- to 5-fold enhancement of basal GTPase activity. Conversely, incorporation of cholesterol (24 mol %) into liposomes reduced the activation of G proteins by shear. These results demonstrate the ability of the phospholipid bilayer to mediate the shear stress-induced activation of membrane-bound G proteins in the absence of protein receptors and that bilayer physical properties modulate this response.     

Maneuvering control of a rolling four-links robot

This work deals with the maneuvering control of the planar motion of a rolling four-links robot as described in Figure 1. The system is composed of four links, two identical wheels, and a mass m_O attached to the joint O. The problem that is addressed is to develop control laws for the rolling four-links robot such that the mass m_O performs prescribed maneuvers in the vertical (X, Z)-plane.     

Rapid magnetic cell delivery for large tubular bioengineered constructs

Delivery of cells into tubular tissue constructs with large diameters poses significant spatial and temporal challenges. This study describes preliminary findings for a novel process for rapid and uniform seeding of cells onto the luminal surface of large tubular constructs. Fibroblasts, tagged with superparamagnetic iron oxide nanoparticles (SPION), were directed onto the luminal surface of tubular constructs by a magnetic field generated by a k4-type Halbach cylinder device. The spatial distribution of attached cells, as measured by the mean number of cells, was compared with a conventional, dynamic, rotational cell-delivery technique. Cell loading onto the constructs was measured by microscopy and magnetic resonance imaging. The different seeding techniques employed had a significant effect on the spatial distribution of the cells (p < 0.0001). The number of attached cells at defined positions within the same construct was significantly different for the dynamic rotation technique (p < 0.05). In contrast, no significant differences in the number of cells attached to the luminal surface were found between the defined positions on the construct loaded with the Halbach cylinder. The technique described overcomes limitations associated with existing cell-delivery techniques and is amenable to a variety of tubular organs where rapid loading and uniform distribution of cells for therapeutic applications are required.     

Shear stress-induced release of PGE2 and PGI2 by vascular smooth muscle cells

This study addresses the direct effect of fluid flow shear stress on production of the vascular mediators, PGE2 and PGI2 by vascular smooth muscle cells (SMC). Results indicate that shear stress increases PGE2 and PGI2 release in SMC. The production patterns, however, differ between PGE2 and PGI2. For PGE2, the rate of production is moderate for the first three hours after the onset of shear stress, then dramatically increases between the fourth and fifth hours, returning to basal levels in the sixth hour. On the other hand, the rate for PGI2 production is maximal right after the onset of shear and remains elevated for the first three hours. The rate then plateaus and remains at a moderate level during the next three hours. The results also indicate that SMC production of PGI2 is more sensitive to shear stress than PGE2 production since a level of 0.5 dynes/cm2 produces a maximal PGI2 release whereas 1 dyne/cm2 produces only 1/4 the response seen at 20 dynes/cm2 for PGE2. The physiological implications of fluid shear stress regulation of SMC are discussed.     

Computerized methods for X-ray-based small bone densitometry

Animal models have been widely used to correlate in vivo changes in bone mineral density (BMD) with changes in disease state of bone. In small animal models, e.g. the hindlimb suspension model of bone loss, a non-invasive assessment of BMD is required. X-ray radiography has been surpassed in some cases by quantitative computed tomography (QCT) and dual X-ray absorptiometry (DEXA) quantitation. However, there are drawbacks in using the computerized methods, especially for small animals. In this paper, we present image-processing algorithms to quantitatively determine bone area and mineral density in digitized radiographs. Image calibration is based on a calibration step wedge, and the algorithm automatically detects the steps and computes the calibration data. In addition, we demonstrate how the algorithm can accurately determine the cortical outline of the bone and provide reliable data and statistics for small animal studies. A downloadable implementation example for the popular NIH Image package is provided.