Desired Gradient Research Articles

Polymer-stabilized liquid crystal microlens array with large dynamic range and fast response time

We report a polymer-stabilized liquid crystal (LC) microlens array with a large dynamic range and fast response time. The top substrate has a planar indium-tin oxide (ITO) electrode, while the bottom substrate has two patterned ITO electrodes for generating a fringing field and uniform longitudinal field. The fringing field is utilized to create the desired gradient refractive index profile in the LC/monomer layer, which is later stabilized by UV curing to form polymer networks. To tune the focal length, we apply a longitudinal field to change the lens shape. This microlens array offers several attractive features, such as large dynamic range, fast response time, and good mechanical stability.

Co-Spray Forming of Gradient Deposits from Two Sprays of Different Tool Steels Using Scanning Gas Atomizers

A co-spray forming process was developed to produce gradient deposits from two sprays of different tool steels using scanning gas atomizers. The tool steel melts were atomized and co-sprayed simultaneously onto a horizontally moving flat substrate, resulting in a flat deposit with a gradient zone between two layers of the different steels when the two sprays were overlapped. The gradient deposits were investigated with respect to the porosity, element distributions, microstructure, and hardness. It showed that a sound deposit with a desired gradient zone could be achieved under proper processing conditions.

Characterization of divinyl benzene aerogels with density gradient using X-ray tomography technique

Production of density gradient aerogels with predetermined density steps or gradient is challenging, particularly when the experiments would demand a prescribed gradient in density along the axis of cylindrical shaped aerogel. In order to achieve this, it is vital to characterize and accurately measure the density gradient in order to “design” synthetic routes to achieve the desired density gradient that can be used in plasma physics experiments using high-energy lasers. X-ray tomography was used for the characterization of these aerogels and it is demonstrated that it is the most reliable and quick method for characterization of gradient density aerogels. Divinyl benzene aerogels samples were synthesized by Lewis acid catalysis and samples were supercritically dried, characterized and their parameters measured to realize that the necessary properties were achieved. The change in density from solid density to 100 mg/cm3 is registered and the accuracy is evaluated.

Development of functionally graded vapor-grown carbon-fiber/polymer materials

We successfully produced vapor-grown carbon-fiber (VGCF)-incorporated polymer-based functionally graded materials (FGMs) using a centrifugal method. Gradual VGCF incorporation within an epoxy resin effectively produced depth gradients in the fiber distribution, microstructure, mechanical, and electrical conductivities and microwave absorbing properties. This VGCF-grading capability indicated that it is possible to tailor desired gradient filler content distributions by careful selection of the processing parameters to control variations in the property and microstructure precisely. The results confirmed that the volume content of VGCF in the epoxy substrate increased as a function of the normalized thickness along the centrifugal force direction, which caused a gradient. A uniform VGCF gradient in the composite can also be observed using field-emission scanning electron microscopy. In the case of the electrical properties, for example, the volume resistance exhibited a depth-graded distribution in the matrix as the electrical conductivity of the FGM nicely followed the grading direction; this is considered to be ideal for applications demanding an electrically conductive surface and an insulating core for FGMs. The results of microwave absorption behavior of FGMs indicate that the grading structure can lead to a graded absorption ability, which could be a better design for microwave absorption materials. The concept of FGMs bridges conventional materials and nanocomposites and will be effective for wider material applications. POLYM. COMPOS., 34:1774–1781, 2013. © 2013 Society of Plastics Engineers

Functionally heterogeneous porous scaffold design for tissue engineering

Porous scaffolds with interconnected and continuous pores have recently been considered as one of the most successful tissue engineering strategies. In the literature, it has been concluded that properly interconnected and continuous pores with their spatial distribution could contribute to perform diverse mechanical, biological and chemical functions of a scaffold. Thus, there has been a need for reproducible and fabricatable scaffold design with controllable and functional gradient porosity. Improvements in Additive Manufacturing (AM) processes for tissue engineering and their design methodologies have enabled the development of controlled and interconnected scaffold structures. However homogeneous scaffolds with uniform porosity do not capture the intricate spatial internal micro architecture of the replaced tissue and thus are not capable of capturing the design. In this work, a novel heterogeneous scaffold modeling is proposed for layered-based additive manufacturing processes. First, layers are generated along the optimum build direction considering the heterogeneous micro structure of tissue. Each layer is divided into functional regions based on the spatial homogeneity factor. An area weight based method is developed to generate the spatial porosity function that determines the deposition pattern for the desired gradient porosity. To design a multi-functional scaffold, an optimum deposition angle is determined at each layer by minimizing the heterogeneity along the deposition path. The proposed methodology is implemented and illustrative examples are also provided. The effective porosity is compared between the proposed design and the conventional uniform porous scaffold design. Sample designed structures have also been fabricated with a novel micro-nozzle biomaterial deposition system. The result has shown that the proposed methodology generates scaffolds with functionally gradient porosity.

HPLC Analysis of Egg Yolk Phosphatidylcholine by Evaporative Light Scattering Detector

Egg yolk phosphatidylcholine (EYPC) is being widely used in food and pharmaceutical industries nowadays owing to its surface activity, pharmaceutical usefulness, and so on. Common determination methods of phospholipids were based on the American Oil Chemists' Society (AOCS) Official Method Ja7b-91, in which n-hexane/2-propanol/acetate buffer was used as the mobile phase. In order to achieve desired results, gradient elution or buffer solution was used, which made the detection process more complicated. Moreover, water or buffer solution could affect the silica gel column both on its lifespan and the separation efficiency significantly. In this study, different mobile phase and detector were used to simplify EYPC analyzing process instead of using water within the mobile phase. The optimized HPLC operating conditions are as follows: pure methanol as a mobile phase, flow rate of 1.0 ml·min−1, silica gel column (250 mm×4.6 mm, 5 μm, Inertsil GLTM), column temperature 30°C and low temperature evaporative light scattering detector (40°C, 0.35 MPa) as used. Under this optimal condition, the linear relative coefficient of the standard curve is 0.998 and the recovery was in the range of 96.83%-101.58% with a relative standard deviation of 1.79% (n=6).

Modeling and Numerical Optimization of Withdrawal Rate in Directional Solidification Process

Ni-based superalloys are currently used as materials for single-crystal (SC) or directionally solidified (DS) turbine blades produced in Bridgman or Liquid Metal Cooling (LMC) furnaces. Valid selection of the process parameters providing proper quality of the casting depends on casting geometry and furnace design. Process parameters should vary with the solidified cross section of the casting. Withdrawal rate is the most important parameter which affects solidification process more severely than the heater temperature. The technique providing for a valid withdrawal rate selection is based on the critical solidification rate calculation from experimental or numerical modeling data. A range of withdrawal rates providing for the required type and quality of the macrostructure is an output of the technique application. Full-scale and computing experiments were carried out to validate the optimization technique. The performance capability of the optimization technique is demonstrated in terms of the in situ composite solidification process. As a result of the solidification process, a real turbine blade was obtained at the desired temperature gradient and solidification rate supporting conditions of the planar solidification front.

Segmentation and tracking of live cells in phase‐contrast images using directional gradient vector flow for snakes

Cell shape is an important characteristic of the physiological state of a cell and is used as a primary read-out of cell behaviour in various assays. Automated accurate segmentation of cells in microscopy images is hence of large practical importance in cell biology. We report a simple algorithm for automated cell segmentation in high-magnification phase-contrast images, which takes advantage of the characteristic directionality of the local image intensity gradient at cellular boundaries due to the 'halo-effect'. We employ a two-step algorithm in which a gradient vector flow (GVF) field is first used to direct active contours to an approximate cell boundary. A directional GVF (DGVF) field is then calculated by considering only edges for which the image intensity gradient is directed outwards with respect to the approximate cell contour. Subsequently, the DGVF field is used to refine the cell contour, by directing active contours to edges with the desired gradient directionality. This method allows us to accurately segment cells in an image series, as well as follow the dynamics of cell shape over time in an automated fashion.

Coupled Dictionary Training for Image Super-Resolution

In this paper, we propose a novel coupled dictionary training method for single image super-resolution based on patchwise sparse recovery, where the learned couple dictionaries relate the low- and high-resolution image patch spaces via sparse representation. The learning process enforces that the sparse representation of a low-resolution image patch in terms of the low-resolution dictionary can well reconstruct its underlying high-resolution image patch with the dictionary in the highresolution image patch space. We model the learning problem as a bilevel optimization problem, where the optimization includes an 1-norm minimization problem in its constraints. Implicit differentiation is employed to calculate the desired gradient for stochastic gradient descent. We demonstrate that our coupled dictionary learning method can outperform the existing joint dictionary training method both quantitatively and qualitatively. Furthermore, for real applications, we speed up the algorithm approximately 10 times by learning a neural network model for fast sparse inference and selectively processing only those visually salient regions. Extensive experimental comparisons with stateof- the-art super-resolution algorithms validate the effectiveness of our proposed approach.

Nanoparticle manipulation by thermal gradient

A method was proposed to manipulate nanoparticles through a thermal gradient. The motion of a fullerene molecule enclosed inside a (10, 10) carbon nanotube with a thermal gradient was studied by molecular dynamics simulations. We created a one-dimensional potential valley by imposing a symmetrical thermal gradient inside the nanotube. When the temperature gradient was large enough, the fullerene sank into the valley and became trapped. The escaping velocities of the fullerene were evaluated based on the relationship between thermal gradient and thermophoretic force. We then introduced a new way to manipulate the position of nanoparticles by translating the position of thermostats with desirable thermal gradients. Compared to nanomanipulation using a scanning tunneling microscope or an atomic force microscope, our method for nanomanipulation has a great advantage by not requiring a direct contact between the probe and the object.

Gradient TiO2 Nanotube Arrays via Asymmetric Anodization

Gradient self-organized TiO2 nanotube arrays with the tube diameters and length gradually changing along the sample plane are conveniently fabricated. The fabrication method is based on asymmetric anodization with the point cathode placed close to the edge of the titanium foil anode. Mechanism study reveals that the drastic variance in the voltage drop over the tube bottom at different locations along the film is responsible for the formation of the gradient morphology. Experimentally, moderate electrolyte conductivities and sufficiently high anodization voltages are necessary to enable the desired gradient TiO2 nanotube arrays. Moreover, the structural features of the fabricated gradient TiO2 nanotube arrays can be further adjusted by controlling the imperfection level of the titanium substrate.

Evaluating passively shielded gradient coil configurations for optimal eddy current compensation

In magnetic resonance imaging, rapidly switching magnetic fields are used to spatially encode the signal. The temporal change of these fields induces eddy currents in nearby conducting structures of the scanner. These eddy currents, in turn, generate a secondary magnetic field that opposes and distorts the desired gradient field. Eddy current compensation methods are generally applied assuming that the primary and secondary magnetic field gradients possess similar spatial characteristics in the imaging volume (field matching). In this work an optimization method is used to deform the shape of the coil support and/or a highly conductive passive shield in order to improve the field matching and reduce the inductive coupling between the gradient coil and the passive shield. Using the residual field after eddy current compensation as the objective function, the coil support and/or conducting surfaces were deformed to obtain passively shielded x- and z-gradient coils with improved field matching and eddy current compensation. Assuming a single frequency, quasi-static simulation, it was demonstrated that the residual field was reduced up to 24 times by reshaping the coil and passive shield surfaces due to the improved field matching. However, using transient analyses we showed that in the case of the passively shielded x-gradient coil the residual field may only be reduced by five times from a cylindrical coil configuration. A bulge shape is created in the conducting surface as a mechanism of matching the field and at the same time reducing the mutual inductive coupling between the coil and the passive shield. An actively shielded coil with control over the magnetic field produced by the induced current was used as a reference coil that produces the minimal residual field. The actively shielded gradient coil produces minimal residual field for short and long pulses in the transient analyses.

Preparation of Si<sub>3</sub>N<sub>4</sub>-Al<sub>2</sub>O<sub>3</sub> Functional Gradient Joining Material Using Transient β-Sialon Interlayer

The joining of high temperature structural ceramics to other materials is an important process commercially and technologically. Due to the existence of physical and economic limitations for the manufacture of large parts, joining is essential. Functional gradient materials (FGMs) are designed to exhibit a desirable gradient in a property, due ether to a gradient in composition, or microstructure, or both. In this paper, Si3N4-Al2O3 functional gradient joining material was prepared by hot press sintering of multilayered FGMs with 20 layers of thickness 400um each using β-Sialon as transient interlayer. The structure and properties of the materials were analyzed by the means of scan electron microscopy(SEM), X-ray diffraction(XRD) and micro-sclerometer. The components of the materials have gradient distribution and successional. The microhardness is gradient changing with the change of micro-structure. The interface between layers is tightly coupling and has no stress convergence. The test of thermal shock and fatigue cracks of FGM between 1300 °C and 900 °C show good properties of thermal stress relaxing and resistance for high temperature.

Bilinear electric field gradient focusing

Electric field gradient focusing (EFGF) uses an electric field gradient and a hydrodynamic counter flow to simultaneously separate and focus charged analytes in a channel. Previously, most EFGF devices were designed to form a linear field gradient in the channel. However, the peak capacity obtained using a linear gradient is not much better than what can be obtained using conventional CE. Dynamic improvement of peak capacity in EFGF can be achieved by using a nonlinear gradient. Numerical simulation results indicate that the peak capacity in a 4-cm long channel can be increased from 20 to 150 when changing from a linear to convex bilinear gradient. To demonstrate the increased capacity experimentally, an EFGF device with convex bilinear gradient was fabricated from poly(ethylene glycol) (PEG)-functionalized acrylic copolymers. The desired gradient profile was confirmed by measuring the focusing positions of a standard protein for different counter flow rates at constant voltage. Dynamically controlled elution of analytes was demonstrated using a monolith-filled bilinear EFGF channel. By increasing the flow rate, stacked proteins that were ordered but not resolved after focusing in the steep gradient segment were moved into the shallow gradient segment, where the analyte peak resolution increased significantly. In this way, the nonlinear field gradient was used to realize a dynamic increase in the peak capacity of the EFGF method.

Effects of Liquid-Phase Composition on Its Migration during Liquid-Phase Sintering of Cemented Carbide

Functionally graded composite materials (FGM composites) with a gradient of matrix phase can offer improved properties. Liquid-phase sintering is one of the approaches for making such materials with a desired gradient of the matrix phase by controlling the redistribution of the liquid phase during sintering. The present study on cemented carbide, WC-Co, demonstrates that the composition of the liquid phase (cobalt phase) is one of the key factors controlling the liquid redistribution. The dependence of the final gradient of the cobalt phase after sintering on its own chemical composition profile is quantitatively established, enabling the design and manufacture of WC-Co with a cobalt-phase-volume gradient via predesigned gradients of carbon content in the system.

Immobilized pH gradients

In this short review, we give an account of the steps through which the protocols for operation with IPGs were set up in the early '80s. One of the main achievements by our group was the development of a computer program, pH GRADIENT, for the calculation of pH, buffering power and ionic strength of a mixture of monoprotic buffers titrated within any pH span and for the linearization of the desired pH gradient. Using this program, in 1984 we could devise formulations for IPGs covering up to six pH units, which was the subject of a publication in Electrophoresis (volume 5, pages 88-97). This was the starting point for the use of IPGs for the resolution of protein samples of any composition and for their application as first dimension of 2-DE separations. Currently IPGs are in common use in proteomics investigations, not only along classical protocols but also for sample prefractionation and in shotgun approaches. Much less frequently are they used for 1-DE analytical applications, a field for which in recent years much attention has instead more often focused on capillary electrophoresis/IEF procedures.

MSE Bounds With Affine Bias Dominating the CramÉr–Rao Bound

In continuation to an earlier work, we further develop bounds on the mean-squared error (MSE) when estimating a deterministic parameter vector thetas <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> in a given estimation problem, as well as estimators that achieve the optimal performance. The traditional Cramer-Rao (CR) type bounds provide benchmarks on the variance of any estimator of thetas <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> under suitable regularity conditions, while requiring <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">priori</i> specification of a desired bias gradient. To circumvent the need to choose the bias, which is impractical in many applications, it was suggested in our earlier work to directly treat the MSE, which is the sum of the variance and the squared-norm of the bias. While previously we developed MSE bounds assuming a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">linear</i> bias vector, here we study, in the same spirit, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">affine</i> bias vectors. We demonstrate through several examples that allowing for an affine transformation can often improve the performance significantly over a linear approach. Using convex optimization tools we show that in many cases we can choose an affine bias that results in an MSE bound that is smaller than the unbiased CR bound for all values of thetas <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> . Furthermore, we explicitly construct estimators that achieve these bounds in cases where an efficient estimator exists, by performing an affine transformation of the standard maximum likelihood (ML) estimator. This leads to estimators that result in a smaller MSE than ML for all possible values of thetas <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> .

Microstructural design of connective base cells for functionally graded materials

Control of compositions and microstructures plays a significant role on developing functionally graded material (FGM). The existing fabrication technologies allow placing multiple compositions in such a way that a desirable gradient of effective physical properties can be achieved. It is interesting to explore how to design microstructure for the same purpose. This paper introduces an inverse homogenization procedure to FGM design. A quasi-periodic base cell (PBC) is optimized for attaining a specific gradient of Young's modulus. To ensure proper connectivity between adjacent PBCs, heat sinks originally used in thermal conduction problem are employed as connection constraints. The capability of this technique is demonstrated by the examples of FGM microstructural design.

Variability of pressure provided by sustained compression.

Compression therapy is the cornerstone of treatment for patients with venous leg ulcers (VLUs). Although it is generally accepted that the therapeutic outcomes are directly related to the quality of compression therapy, delivering precise and sustained compression therapy is an ongoing challenge for health care professionals. Several factors influence quality of compression therapy: physical structure and elastomeric properties of the compression system, size and shape of the leg, skill and technique of the bandager and physical activity undertaken by the patient. Graduated compression is achieved by applying a bandage at the same tension from ankle to knee, providing the shape of the leg is normal. Many patients with VLUs have distorted legs, challenging the delivery of a desired pressure gradient. Poor bandaging technique can result in little or no benefit or may deliver too high a pressure causing a detrimental effect to the wearer. If the wearer is unable to tolerate the compression, patient concordance and effectiveness are affected. Training has been shown to reduce variability of sub-bandage pressure. Sub-bandage pressure increases during standing and walking. These pressure changes are related to the elastomeric properties of the compression systems. Health care professionals need to understand the properties of the available compression systems and how their application technique must be adjusted.

On-Chip Isoelectric Focusing Using Photopolymerized Immobilized pH Gradients

We present the first successful adaptation of immobilized pH gradients (IPGs) to the microscale (muIPGs) using a new method for generating precisely defined polymer gradients on-chip. Gradients of monomer were established via diffusion along 6 mm flow-restricted channel segments. Precise control over boundary conditions and the resulting gradient is achieved by continuous flow of stock solutions through side channels flanking the gradient segment. Once the desired gradient is established, it is immobilized via photopolymerization. Precise gradient formation was verified with spatial and temporal detection of a fluorescent dye added to one of the flanking streams. Rapid (<20 min) isoelectric focusing of several fluorescent pI markers and proteins is demonstrated across pH 3.8-7.0 muIPGs using both denaturing and nondenaturing conditions, without the addition of carrier ampholytes. The muIPG format yields improved stability and comparable resolution to prominent on-chip IEF techniques. In addition to rapid, high-resolution separations, the reported muIPG format is amenable to multiplexed and multidimensional analysis via custom gradients as well as integration with other on-chip separation methods.