1,721,115 research outputs found
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
Improved Design of Low-Pressure Fluidic Microvalves
Full text linkMultilayer soft lithography (MSL) is used to fabricate monolithic elastomeric on-off microvalves by adopting a two-layer cross-channel architecture. The performance of microvalves is strongly dependent on the two-channel geometry (width, height and shape) and on the thickness of the interlayer membrane. Using a finite element model previously validated against experiments, we propose a new fluidic microvalve design, based on the concept of chemically swelling the thin interlayer membrane so as to induce two stable equilibrium configurations. The complete closure of the new valve may then be achieved by applying a much reduced actuation pressure, down to 1/4 of the pressure needed by the standard monostable design. The maximum stress in the interlayer membrane of the bistable valve also drops to 1/3 of the value corresponding to the standard design
A Numerical Model of Light Adjustable Lens
No full textWe model numerically the mechanical effects of UV induced photo-polymerization in elastomeric artificial lens. The elastomer is originated upon cross-linking of a silicone matrix. UV irradiation of one side of the lens polymerizes selectively a photosensitive macromer, causing local variations of its concentration. The subsequent diffusion of macromers from high concentration to low concentration zones modifies the shape of the lens and thus its dioptric power. In vitro experiments on artificial lens showed that the power change is dependent on UV exposure time, irradiation intensity and light pattern. With the aim to define a numerical tool able to predict the dioptric power adjustment as a function of the UV irradiation parameters, we setup a purely mechanic finite element model of the lens, adopting a hyperelastic material model embedded with eigen-deformations. Numerical simulations of axis-symmetric irradiation closely reproduced the experimental results, in terms of both lens geometry and dioptric power, for positive, negative and lock-in corrections
Numerical Analysis of Elastomeric Fluidic Microvalves
No full textWe present a finite element model of polydimethylsiloxane (PDMS) fluidic microvalves. A valve is fabricated by assembling two patterned layers in a two-channel crossed architecture. The valve closes as a consequence of the motion of the interlayer membrane. The membrane is deformed by the pressure of the actuation fluid, flowing in one of the two channels. By using a soft rubber material model, we setup a numerical model of the microvalve and validate it against experiments. The numerical model allows to evaluate the mechanical engagement of commonly used microvalve architectures and to analyze the performance of alternative geometries
Biomechanical and optical behavior of human corneas before and after photorefractive keratectomy
No full textPurpose To evaluate numerically the biomechanical and optical behavior of human corneas and quantitatively estimate the changes in refractive power and stress caused by photorefractive keratectomy (PRK). Setting Athineum Refractive Center, Athens, Greece, and Politecnico di Milano, Milan, Italy. Design Retrospective comparative interventional cohort study. Methods Corneal topographies of 10 human eyes were taken with a scanning-slit corneal topographer (Orbscan II) before and after PRK. Ten patient-specific finite element models were created to estimate the strain and stress fields in the cornea in preoperative and postoperative configurations. The biomechanical response in postoperative eyes was computed by directly modeling the postoperative geometry from the topographer and by reproducing the corneal ablation planned for the PRK with a numerical reprofiling procedure. Results Postoperative corneas were more compliant than preoperative corneas. In the optical zone, corneal thinning decreased the mechanical stiffness, causing local resteepening and making the central refractive power more sensitive to variations in intraocular pressure (IOP). At physiologic IOP, the postoperative corneas had a mean 7% forward increase in apical displacement and a mean 20% increase in the stress components at the center of the anterior surface over the preoperative condition. Conclusion Patient-specific numerical models of the cornea can provide quantitative information on the changes in refractive power and in the stress field caused by refractive surgery. Financial Disclosures No author has a financial or proprietary interest in any material or method mentioned. © 2014 ASCRS and ESCRS
The influence of intraocular pressure and air jet pressure on corneal contactless tonometry tests
No full textThe air puff is a dynamic contactless tonometer test used in
ophthalmology clinical practice to assess the biomechanical properties
of the human cornea and the intraocular pressure due to the filling
fluids of the eye. The test is controversial, since the dynamic response
of the cornea is governed by the interaction of several factors which
cannot be discerned within a single measurement. In this study we
describe a numerical model of the air puff tests, and perform a
parametric analysis on the major action parameters (jet pressure and
intraocular pressure) to assess their relevance on the mechanical
response of a patient-specific cornea. The particular cornea considered
here has been treated with laser reprofiling to correct myopia, and the
parametric study has been conducted on both the preoperative and
postoperative geometries. The material properties of the cornea have
been obtained by means of an identification procedure that compares the
static biomechanical response of preoperative and postoperative corneas
under the physiological IOP. The parametric study on the intraocular
pressure suggests that the displacement of the cornea's apex can be a
reliable indicator for tonometry, and the one on the air jet pressure
predicts the outcomes of two or more distinct measurements on the same
cornea, which can be used in inverse procedures to estimate the material
properties of the tissue. (C) 2015 Elsevier Ltd. All rights reserved
Material and geometric instabilities in the rate elsatic-plastic finite deformation problem
No full text
- …
