1,721,000 research outputs found
Modeling the deterioration of the stiffness and of the collagen fibril distribution in a discrete model of the cornea microstructure
No full textWe present a stochastic approach to model the mechanical deterioration of the reinforcing microstructure of the human cornea. The fundamental structural micro-components of the stroma, collagen and crosslinks, are assembled deterministically into an elementary trusswork cell, multiply repeated and distorted to form a three-dimensional shell with the shape of a cornea. The spatial orientation of the collagen -like elements of each cell is thus characterized stochastically with a non correlated random angle field, obeying an assigned probability density function, leading to a non-deterministic structural stiffness. It follows that the mechanical response of the model to the action of deterministic forces equivalent to the intraocular pressure is stochastic due to the random spatial orientation of collagen fibers. The deterioration of the mechanical stiffness of the collagen components is described through a scalar variable field, evolving in space and in time, representative of a progressive damage which causes heterogeneity and asymmetric behavior. The damage variable acts in two ways on the global stiffness: (i) by reducing the stiffness of the collagen components; (ii) by modifying the dispersion coefficient of the probability density function. The equilibrium equations of the damaging model are solved at discrete time steps, with a fully explicit solution scheme, by means of the stochastic finite element improved perturbation method. The results show that when the collagen fibril stiffness reduces to 10% of the healthy value, as expected in the case of the vision-impairing condition known as keratoconus, the displacement field due to intraocular pressure is significantly affected in terms of both average and variance distributions. This effect confers a typical conical shape to the cornea. In particular, the analysis shows that high values of the response variances are confined in the keratoconus area, which agrees with a high level of uncertainties due to loss of fibril organization and thickness reduction under pathologic conditions
Coupled electro-mechanical models of fiber-distributed active tissues
No full textWe discuss a constitutive model for stochastically distributed fiber reinforced tissues, where the active behavior of the fibers depends on the relative orientation of the electric field. Unlike other popular approaches, based on numerical integration over the unit sphere, or on the use of second order structure tensors, for the passive behavior we adopt a second order approximation of the strain energy density of the distribution. The purely mechanical quantities result to be dependent on two (second and fourth order, respectively) averaged structure tensors. In line with the approximation used for the passive behavior, we model the active behavior accounting for the statistical fiber distribution. We extend the Helmholtz free energy density by introducing a directional active potential, dependent on a stochastic permittivity tensor associated to a particular direction, and approximate the total active potential through a second order Taylor expansion of the permittivity tensor. The approximation allows us to derive explicitly the active stress and the active constitutive tensors, which result to be dependent on the same two averaged structure tensors that characterize the passive response. Active anisotropy follows from the distribution of the fibers and inherits its stochastic parameters. Examples of passive and active behaviors predicted by the model in terms of response to biaxial testing are presented, and comparisons with passive experimental data are provided. © 2016 Elsevier Lt
Modeling the biomechanics of the human cornea accounting for local variations of the collagen fibril architecture
No full textWe present a finite element model of the human cornea describing the in-plane organization of the stromal collagen, modified variously to include features of the collagen architecture. We investigate numerically the implication of the local organization of collagen in the stroma on the response of the human cornea to mechanical tests. We compare four different models by simulating three ideal mechanical tests, i. e., the ex-vivo inflation test, the in-vivo probe indentation, and the in-vivo air puff tests. Numerical results show slight differences between the models in terms of global response and stress distribution. Differences in the overall mechanical response are observed in dynamic tests, while quasi-static tests are not able to differentiate between the models. Stress distributions differ markedly when a variation of the shear stiffness across the thickness is considered. We conclude that the actual architecture of the collagen across the thickness of the cornea or at the limbus has a minor relevance from the mechanical point of view with respect to the main anisotropic orthogonal collagen structure that has been considered and acknowledged in the literature
GIMC-GMA 2016 - XXI Convegno Italiano di Meccanica Computazionale e VIII Riunione del Gruppo Materiali AIMETA - Atti del Convegno
No full textViscoelectromechanics modeling of intestine wall hyperelasticity
Full text linkABSTRACT: Elastic-electroactive biological media are sensitive to both mechanical and electric forces. Their active behavior is often associated with the presence of reinforcing fibers and their excitation-contraction coupling is due to the interplay between the passive elastic tissue and the active muscular network. In this paper we focus on the theoretical framework of constitutive equations for viscous electroactive media. The approach is based on the additive decomposition of the Helmholtz free energy accompanied to the multiplicative decomposition of the deformation gradient in elastic, viscous and active parts. We describe a thermodynamically sound scenario that accounts for geometric and material nonlinearities. © 2016 Taylor & Francis Group, LLC
The role of coherence in emergent behavior of biological systems
No full textIn his research activity, Emilio Del Giudice explored the possibility to move towards a unified view of some long-range dynamics in nature, ranging from quantum field theory in physics up to biology. Such a view is adopted in this contribution by discussing a mathematical model for synchronized electrical behavior of pancreatic beta cells. The stochasticity is a fundamental component of the physiological synchronized behavior of this system. On the contrary, in a pathological type I diabetes scenario, the cells are destroyed by the autoimmune system and their coherent behavior is lost. This phenomenology conceptually links to ideas of coherent dynamics in quantum physics. Possible implications both for physical sciences and for the epistemology of life sciences are outlined
A spectral decomposition approach for the mechanical statistical characterization of distributed fiber-reinforced tissues
Full text linkWe discuss a spectral decomposition formulation for the mechanical statistical characterization of the anisotropic strain energy density of soft hyperelastic materials embedded with distributed fibers. We consider a generalized angular probability density function (PDF) of the reinforcement built upon the local eigenvalue and eigenvector system of the Cauchy–Green deformation tensor. We focus our analysis on material models dependent on the fourth pseudo-invariant of the deformation, I4, and on exponential forms of the fiber strain energy function. Within such a spectral reference system, we derive the closed-form expression of the PDF for I4 by generalizing the multi-value random variable transformation procedure recently developed in Gizzi et al. 2016. Our formulation bypasses the cumbersome extension–contraction switch, commonly adopted for shutting down the contribution of contracted fibers in models based on generalized structure tensors. Accordingly, we identify analytically the support of the fibers in pure extension for significant loading conditions. We can readily compute any statistics of the fourth pseudo-invariant and we can derive the direct definition of the average second Piola–Kirchhoff stress tensor according to the second order approximation
Statistical characterization of the anisotropic strain energy in soft materials with distributed fibers
Full text linkWe discuss analytical and numerical tools for the statistical characterization of the anisotropic strain energy density of soft hyperelastic materials embedded with fibers. We consider spatially distributed orientations of fibers following a tridimensional or a planar architecture. We restrict our analysis to material models dependent on the fourth pseudo-invariant I4 of the Cauchy-Green tensor, and to exponential forms of the fiber strain energy function Ψaniso. Under different loading conditions, we derive the closed-form expression of the probability density function for I4 and Ψaniso. In view of bypassing the cumbersome extension-contraction switch, commonly adopted for shutting down the contribution of contracted fibers in models based on generalized structure tensors, for significant loading conditions we identify analytically the support of the fibers in pure extension. For uniaxial loadings, the availability of the probability distribution function and the knowledge of the support of the fibers in extension yield to the analytical expression of average and variance of I4 and Ψaniso, and to the direct definition of the average second Piola-Kirchhoff stress tensor. For generalized loadings, the dependence of I4 on the spatial orientation of the fibers can be analyzed through angle plane diagrams. Angle plane diagrams facilitate the assessment of the influence of the pure extension condition on the definition of the stable support of fibers for the statistics related to the anisotropic strain energy density. © 2015 Elsevier Ltd. All rights reserved
Mechanical behavior of metastatic femurs through patient-specific computational models accounting for bone-metastasis interaction
No full textThis paper proposes a computational model based on a finite-element formulation for describing the mechanical behavior of femurs affected by metastatic lesions. A novel geometric/constitutive description is introduced by modelling healthy bone and metastases via a linearly poroelastic constitutive approach. A Gaussian-shaped graded transition of material properties between healthy and metastatic tissues is prescribed, in order to account for the bone-metastasis interaction. Loading-induced failure processes are simulated by implementing a progressive damage procedure, formulated via a quasi-static displacement-driven incremental approach, and considering both a stress- and a strain-based failure criterion. By addressing a real clinical case, left and right patient-specific femur models are geometrically reconstructed via an ad-hoc imaging procedure and embedding multiple distributions of metastatic lesions along femurs. Significant differences in fracture loads, fracture mechanisms, and damage patterns, are highlighted by comparing the proposed constitutive description with a purely elastic formulation, where the metastasis is treated as a pseudo-healthy tissue or as a void region. Proposed constitutive description allows to capture stress/strain localization mechanisms within the metastatic tissue, revealing the model capability in describing possible strain-induced mechano-biological stimuli driving onset and evolution of the lesion. The proposed approach opens towards the definition of effective computational strategies for supporting clinical decision and treatments regarding metastatic femurs, contributing also to overcome some limitations of actual standards and procedures
Gambling, games of skill and human ecology. A pilot study by a multidimensional analysis approach
No full textThe present pilot study aims at analyzing the human activity of playing in the light of
an indicator of human ecology (HE). We highlighted the four essential anthropological
dimensions (FEAD), starting from the analysis of questionnaires administered to actual
gamers. The coherence between theoretical construct and observational data is a remarkable
proof-of-concept of the possibility of establishing an experimentally motivated link
between a philosophical construct (coming from Huizinga’s Homo ludens definition) and
actual gamers’ motivation pattern. The starting hypothesis is that the activity of playing
becomes ecological (and thus not harmful) when it achieves the harmony between
the FEAD, thus realizing HE; conversely, it becomes at risk of creating some form of
addiction, when destroying FEAD balance. We analyzed the data by means of variable
clustering (oblique principal components) so to experimentally verify the existence of the
hypothesized dimensions. The subsequent projection of statistical units (gamers) on the
orthogonal space spanned by principal components allowed us to generate a meaningful,
albeit preliminary, clusterization of gamer profiles.The present pilot study aims at analyzing the human activity of playing in the light of
an indicator of human ecology (HE). We highlighted the four essential anthropological
dimensions (FEAD), starting from the analysis of questionnaires administered to actual
gamers. The coherence between theoretical construct and observational data is a remarkable
proof-of-concept of the possibility of establishing an experimentally motivated link
between a philosophical construct (coming from Huizinga’s Homo ludens definition) and
actual gamers’ motivation pattern. The starting hypothesis is that the activity of playing
becomes ecological (and thus not harmful) when it achieves the harmony between
the FEAD, thus realizing HE; conversely, it becomes at risk of creating some form of
addiction, when destroying FEAD balance. We analyzed the data by means of variable
clustering (oblique principal components) so to experimentally verify the existence of the
hypothesized dimensions. The subsequent projection of statistical units (gamers) on the
orthogonal space spanned by principal components allowed us to generate a meaningful,
albeit preliminary, clusterization of gamer profiles
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