3,197 research outputs found
The role of structural viscoelasticity in deformable porous media with incompressible constituents: applications in biomechanics
The main goal of this work is to clarify and quantify, by means of
mathematical analysis, the role of structural viscoelasticity in the
biomechanical response of deformable porous media with incompressible
constituents to sudden changes in external applied loads. Models of deformable
porous media with incompressible constituents are often utilized to describe
the behavior of biological tissues, such as cartilages, bones and engineered
tissue scaffolds, where viscoelastic properties may change with age, disease or
by design. Here, for the first time, we show that the fluid velocity within the
medium could increase tremendously, even up to infinity, should the external
applied load experience sudden changes in time and the structural
viscoelasticity be too small. In particular, we consider a one-dimensional
poro-visco-elastic model for which we derive explicit solutions in the cases
where the external applied load is characterized by a step pulse or a
trapezoidal pulse in time. By means of dimensional analysis, we identify some
dimensionless parameters that can aid the design of structural properties
and/or experimental conditions as to ensure that the fluid velocity within the
medium remains bounded below a certain given threshold, thereby preventing
potential tissue damage. The application to confined compression tests for
biological tissues is discussed in detail. Interestingly, the loss of
viscoelastic tissue properties has been associated with various disease
conditions, such as atherosclerosis, Alzheimer's disease and glaucoma. Thus,
the findings of this work may be relevant to many applications in biology and
medicine
PADDLE: Proximal Algorithm for Dual Dictionaries LEarning
Recently, considerable research efforts have been devoted to the design of
methods to learn from data overcomplete dictionaries for sparse coding.
However, learned dictionaries require the solution of an optimization problem
for coding new data. In order to overcome this drawback, we propose an
algorithm aimed at learning both a dictionary and its dual: a linear mapping
directly performing the coding. By leveraging on proximal methods, our
algorithm jointly minimizes the reconstruction error of the dictionary and the
coding error of its dual; the sparsity of the representation is induced by an
-based penalty on its coefficients. The results obtained on synthetic
data and real images show that the algorithm is capable of recovering the
expected dictionaries. Furthermore, on a benchmark dataset, we show that the
image features obtained from the dual matrix yield state-of-the-art
classification performance while being much less computational intensive
Tribally-Driven Participatory Research: State of the practice and potential strategies for the future
This paper discusses current practice of research with and by American Indian tribal governments in the United States. It begins with a brief overview of Community-Based Participatory Research and compares and contrasts its principles and methods with what this paper terms Tribally-Driven Participatory Research. The paper analyzes current challenges and offers concepts for continuing to improve the effectiveness of Tribally-Driven Participatory Research
Solution Map Analysis of a Multiscale Drift-Diffusion Model for Organic Solar Cells
In this article we address the theoretical study of a multiscale
drift-diffusion (DD) model for the description of photoconversion mechanisms in
organic solar cells. The multiscale nature of the formulation is based on the
co-presence of light absorption, conversion and diffusion phenomena that occur
in the three-dimensional material bulk, of charge photoconversion phenomena
that occur at the two-dimensional material interface separating acceptor and
donor material phases, and of charge separation and subsequent charge transport
in each three-dimensional material phase to device terminals that are driven by
drift and diffusion electrical forces. The model accounts for the nonlinear
interaction among four species: excitons, polarons, electrons and holes, and
allows to quantitatively predict the electrical current collected at the device
contacts of the cell. Existence and uniqueness of weak solutions of the DD
system, as well as nonnegativity of all species concentrations, are proved in
the stationary regime via a solution map that is a variant of the Gummel
iteration commonly used in the treatment of the DD model for inorganic
semiconductors. The results are established upon assuming suitable restrictions
on the data and some regularity property on the mixed boundary value problem
for the Poisson equation. The theoretical conclusions are numerically validated
on the simulation of three-dimensional problems characterized by realistic
values of the physical parameters
Feature learning in feature-sample networks using multi-objective optimization
Data and knowledge representation are fundamental concepts in machine
learning. The quality of the representation impacts the performance of the
learning model directly. Feature learning transforms or enhances raw data to
structures that are effectively exploited by those models. In recent years,
several works have been using complex networks for data representation and
analysis. However, no feature learning method has been proposed for such
category of techniques. Here, we present an unsupervised feature learning
mechanism that works on datasets with binary features. First, the dataset is
mapped into a feature--sample network. Then, a multi-objective optimization
process selects a set of new vertices to produce an enhanced version of the
network. The new features depend on a nonlinear function of a combination of
preexisting features. Effectively, the process projects the input data into a
higher-dimensional space. To solve the optimization problem, we design two
metaheuristics based on the lexicographic genetic algorithm and the improved
strength Pareto evolutionary algorithm (SPEA2). We show that the enhanced
network contains more information and can be exploited to improve the
performance of machine learning methods. The advantages and disadvantages of
each optimization strategy are discussed.Comment: 7 pages, 4 figure
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