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Several Difficulties (several + difficulty)

Distribution by Scientific Domains
Engineering50%

Selected Abstracts

Vertical double-gate MOSFET device technology

ELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 1 2008
Meishoku Masahara
Abstract Silicon device technology is facing several difficulties. Especially, explosion of power consumption due to short-channel effects (SCEs) becomes the biggest issue in further device scaling down. Fortunately, double-gate (DG) MOSFETs have promising potential to overcome this obstacle. The DG-MOSFET is recognized to be the most scalable MOSFET for its high SCE immunity. In addition, independent DG-MOSFET (4T-DG-MOSFET) has great advantage to enable the threshold voltage control for the flexible power management. Through this work, we have realized ideal DG-MOSFETs using newly developed vertical DG-MOSFET device technology. This article examines the effectiveness of the vertical DG-MOSFETs in future high-performance and ultralow-power CMOS circuits. © 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(1): 46, 51, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10021 [source]

Recent studies of dental development in Neandertals: Implications for Neandertal life histories

EVOLUTIONARY ANTHROPOLOGY, Issue 1 2009
Debbie Guatelli-Steinberg
Abstract Did Neandertals share with modern humans their prolonged periods of growth and delayed ages of maturation? During the past five years, renewed interest in this question has produced dental studies with seemingly contradictory results. Some suggest fast dental growth,1, 2 while others appear to suggest a slower, modern-human dental growth pattern.3, 4 Although some apparent contradictions can be reconciled, there remain questions that can be resolved only with additional data and cross-validation of methods. Moreover, several difficulties are inherent in using dental development to gauge Neandertal life histories. Even with complete data on Neandertal dental development, questions are likely to remain about the meaning of those data with regard to understanding Neandertal life histories. [source]

Three-dimensional Mohr,Coulomb limit analysis using semidefinite programming

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2008
K. Krabbenhøft
Abstract Recently, Krabbenhøft et al. (Int. J. Solids Struct. 2007; 44:1533,1549) have presented a formulation of the three-dimensional Mohr,Coulomb criterion in terms of positive-definite cones. The capabilities of this formulation when applied to large-scale three-dimensional problems of limit analysis are investigated. Following a brief discussion on a number of theoretical and algorithmic issues, three common, but traditionally difficult, geomechanics problems are solved and the performance of a common primal,dual interior-point algorithm (SeDuMi (Appl. Numer. Math. 1999; 29:301,315)) is documented in detail. Although generally encouraging, the results also reveal several difficulties which support the idea of constructing a conic programming algorithm specifically dedicated to plasticity problems. Copyright © 2007 John Wiley & Sons, Ltd. [source]

A simple method for compressible multiphase mixtures and interfaces

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2003
Nikolai Andrianov
Abstract We develop a Godunov-type scheme for a non-conservative, unconditional hyperbolic multiphase model. It involves a set of seven partial differential equations and has the ability to solve interface problems between pure materials as well as compressible multiphase mixtures with two velocities and non-equilibrium thermodynamics (two pressures, two temperatures, two densities, etc.).Its numerical resolution poses several difficulties. The model possesses a large number of acoustic and convective waves (seven waves) and it is not easy to upwind all these waves accurately and simply. Also, the system is non-conservative, and the numerical approximations of the corresponding terms need to be provided. In this paper, we focus on a method, based on a characteristic decomposition which solves these problems in a simple way and with good accuracy. The robustness, accuracy and versatility of the method is clearly demonstrated on several test problems with exact solutions. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Materials in particulate form for tissue engineering.

JOURNAL OF TISSUE ENGINEERING AND REGENERATIVE MEDICINE, Issue 2 2007

Abstract Materials in particulate form have been the subjects of intensive research in view of their use as drug delivery systems. While within this application there are still issues to be addressed, these systems are now being regarded as having a great potential for tissue engineering applications. Bone repair is a very demanding task, due to the specific characteristics of skeletal tissues, and the design of scaffolds for bone tissue engineering presents several difficulties. Materials in particulate form are now seen as a means of achieving higher control over parameters such as porosity, pore size, surface area and the mechanical properties of the scaffold. These materials also have the potential to incorporate biologically active molecules for release and to serve as carriers for cells. It is believed that the combination of these features would create a more efficient approach towards regeneration. This review focuses on the application of materials in particulate form for bone tissue engineering. A brief overview of bone biology and the healing process is also provided in order to place the application in its broader context. An original compilation of molecules with a documented role in bone tissue biology is listed, as they have the potential to be used in bone tissue engineering strategies. To sum up this review, examples of works addressing the above aspects are presented. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Maximum Likelihood Methods for Nonignorable Missing Responses and Covariates in Random Effects Models

BIOMETRICS, Issue 4 2003
Amy L. Stubbendick
Summary. This article analyzes quality of life (QOL) data from an Eastern Cooperative Oncology Group (ECOG) melanoma trial that compared treatment with ganglioside vaccination to treatment with high-dose interferon. The analysis of this data set is challenging due to several difficulties, namely, nonignorable missing longitudinal responses and baseline covariates. Hence, we propose a selection model for estimating parameters in the normal random effects model with nonignorable missing responses and covariates. Parameters are estimated via maximum likelihood using the Gibbs sampler and a Monte Carlo expectation maximization (EM) algorithm. Standard errors are calculated using the bootstrap. The method allows for nonmonotone patterns of missing data in both the response variable and the covariates. We model the missing data mechanism and the missing covariate distribution via a sequence of one-dimensional conditional distributions, allowing the missing covariates to be either categorical or continuous, as well as time-varying. We apply the proposed approach to the ECOG quality-of-life data and conduct a small simulation study evaluating the performance of the maximum likelihood estimates. Our results indicate that a patient treated with the vaccine has a higher QOL score on average at a given time point than a patient treated with high-dose interferon. [source]