10,368 research outputs found
Low-Dimensional Stochastic Modeling of the Electrical Properties of Biological Tissues
Uncertainty quantification plays an important role in biomedical engineering
as measurement data is often unavailable and literature data shows a wide
variability. Using state-of-the-art methods one encounters difficulties when
the number of random inputs is large. This is the case, e.g., when using
composite Cole-Cole equations to model random electrical properties. It is
shown how the number of parameters can be significantly reduced by the
Karhunen-Loeve expansion. The low-dimensional random model is used to quantify
uncertainties in the axon activation during deep brain stimulation. Numerical
results for a Medtronic 3387 electrode design are given.Comment: 4 pages, 5 figure
A modeling framework for contact, adhesion and mechano-transduction between excitable deformable cells
Cardiac myocytes are the fundamental cells composing the heart muscle. The
propagation of electric signals and chemical quantities through them is
responsible for their nonlinear contraction and dilatation. In this study, a
theoretical model and a finite element formulation are proposed for the
simulation of adhesive contact interactions between myocytes across the
so-called gap junctions. A multi-field interface constitutive law is proposed
for their description, integrating the adhesive and contact mechanical response
with their electrophysiological behavior. From the computational point of view,
the initial and boundary value problem is formulated as a structure-structure
interaction problem, which leads to a straightforward implementation amenable
for parallel computations. Numerical tests are conducted on different couples
of myocytes, characterized by different shapes related to their stages of
growth, capturing the experimental response. The proposed framework is expected
to have impact on the understanding how imperfect mechano-transduction could
lead to emergent pathological responses.Comment: 31 pages, 17 figure
Nonlinear diffusion & thermo-electric coupling in a two-variable model of cardiac action potential
This work reports the results of the theoretical investigation of nonlinear
dynamics and spiral wave breakup in a generalized two-variable model of cardiac
action potential accounting for thermo-electric coupling and diffusion
nonlinearities. As customary in excitable media, the common Q10 and Moore
factors are used to describe thermo-electric feedback in a 10-degrees range.
Motivated by the porous nature of the cardiac tissue, in this study we also
propose a nonlinear Fickian flux formulated by Taylor expanding the voltage
dependent diffusion coefficient up to quadratic terms. A fine tuning of the
diffusive parameters is performed a priori to match the conduction velocity of
the equivalent cable model. The resulting combined effects are then studied by
numerically simulating different stimulation protocols on a one-dimensional
cable. Model features are compared in terms of action potential morphology,
restitution curves, frequency spectra and spatio-temporal phase differences.
Two-dimensional long-run simulations are finally performed to characterize
spiral breakup during sustained fibrillation at different thermal states.
Temperature and nonlinear diffusion effects are found to impact the
repolarization phase of the action potential wave with non-monotone patterns
and to increase the propensity of arrhythmogenesis
Dynamics of Current, Charge and Mass
Electricity plays a special role in our lives and life. Equations of electron
dynamics are nearly exact and apply from nuclear particles to stars. These
Maxwell equations include a special term the displacement current (of vacuum).
Displacement current allows electrical signals to propagate through space.
Displacement current guarantees that current is exactly conserved from inside
atoms to between stars, as long as current is defined as Maxwell did, as the
entire source of the curl of the magnetic field. We show how the Bohm
formulation of quantum mechanics allows easy definition of current. We show how
conservation of current can be derived without mention of the polarization or
dielectric properties of matter. Matter does not behave the way physicists of
the 1800's thought it does with a single dielectric constant, a real positive
number independent of everything. Charge moves in enormously complicated ways
that cannot be described in that way, when studied on time scales important
today for electronic technology and molecular biology. Life occurs in ionic
solutions in which charge moves in response to forces not mentioned or
described in the Maxwell equations, like convection and diffusion. Classical
derivations of conservation of current involve classical treatments of
dielectrics and polarization in nearly every textbook. Because real dielectrics
do not behave in a classical way, classical derivations of conservation of
current are often distrusted or even ignored. We show that current is conserved
exactly in any material no matter how complex the dielectric, polarization or
conduction currents are. We believe models, simulations, and computations
should conserve current on all scales, as accurately as possible, because
physics conserves current that way. We believe models will be much more
successful if they conserve current at every level of resolution, the way
physics does.Comment: Version 4 slight reformattin
Competing mechanisms of stress-assisted diffusivity and stretch-activated currents in cardiac electromechanics
We numerically investigate the role of mechanical stress in modifying the
conductivity properties of the cardiac tissue and its impact in computational
models for cardiac electromechanics. We follow a theoretical framework recently
proposed in [Cherubini, Filippi, Gizzi, Ruiz-Baier, JTB 2017], in the context
of general reaction-diffusion-mechanics systems using multiphysics continuum
mechanics and finite elasticity. In the present study, the adapted models are
compared against preliminary experimental data of pig right ventricle
fluorescence optical mapping. These data contribute to the characterization of
the observed inhomogeneity and anisotropy properties that result from
mechanical deformation. Our novel approach simultaneously incorporates two
mechanisms for mechano-electric feedback (MEF): stretch-activated currents
(SAC) and stress-assisted diffusion (SAD); and we also identify their influence
into the nonlinear spatiotemporal dynamics. It is found that i) only specific
combinations of the two MEF effects allow proper conduction velocity
measurement; ii) expected heterogeneities and anisotropies are obtained via the
novel stress-assisted diffusion mechanisms; iii) spiral wave meandering and
drifting is highly mediated by the applied mechanical loading. We provide an
analysis of the intrinsic structure of the nonlinear coupling using
computational tests, conducted using a finite element method. In particular, we
compare static and dynamic deformation regimes in the onset of cardiac
arrhythmias and address other potential biomedical applications
Virtual cardiac monolayers for electrical wave propagation
The complex structure of cardiac tissue is considered to be one of the main determinants of an arrhythmogenic substrate. This study is aimed at developing the first mathematical model to describe the formation of cardiac tissue, using a joint in silico-in vitro approach. First, we performed experiments under various conditions to carefully characterise the morphology of cardiac tissue in a culture of neonatal rat ventricular cells. We considered two cell types, namely, cardiomyocytes and fibroblasts. Next, we proposed a mathematical model, based on the Glazier-Graner-Hogeweg model, which is widely used in tissue growth studies. The resultant tissue morphology was coupled to the detailed electrophysiological Korhonen-Majumder model for neonatal rat ventricular cardiomyocytes, in order to study wave propagation. The simulated waves had the same anisotropy ratio and wavefront complexity as those in the experiment. Thus, we conclude that our approach allows us to reproduce the morphological and physiological properties of cardiac tissue
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