1,720,977 research outputs found
Acoustic waves in hydrogels: A bi-phasic model for ultrasound tissue-mimicking phantom
No full textIn the present paper a continuum poroelastic model for high frequency acoustic waves in hydrogels has been developed. The model has been used to obtain the acoustic longitudinal wave equation for ultrasound. In order to obtain a satisfactory model for hydrogels, a viscoelastic force describing the interaction between the polymer network of the matrix and the bounded water is introduced. The model is validated by means of ultrasound (US) wave speed and attenuation measurements in polyvinylalcohol (PVA) hydrogel samples as a function of their water volume fraction "β" and polymer matrix cross-linking. The model predicts that the law ∝ ν(1 + δ) for ultrasound attenuation can be applied as a function of the frequency ν, where δ is the frequency exponent of the polymer-bounded water viscosity. This outcome can well explain the attenuation of the US frequency in natural gels where δ is typically about 0.25÷0.50 while the value for pure water is 1. The theory and experiments show that US attenuation in hydrogels decreases steadily with the increase of its water volume fraction β in a linear. The new proposed dissipative mechanism leads to a US wave speed c that follows the law: c = cw(β - φsymbol)- 3/2, where cw is the US wave speed in water and φsymbol is the volume fraction of the bounded water. Since 0 0, the hydrogel US velocity is always higher than that of pure water. If β tends to 1 (100% water), then the US speed in hydrogels converges to a higher value than that of pure water. The US speed gap at β = 1, between hydrogels and water, is the direct consequence of the introduction of the polymer network-bounded water interaction. This is in line with the experimental results that show that the US speed gap at β = 1 decreases in the gel samples with a more cross-linked polymer matrix that has a lower bounded water volume fraction. On the contrary, if the water content is very low (i.e., β < 0.4), the measured US speed converges to that of the dry hydrogel matrix which increases in the samples with a higher degree of network cross-linking with greater elastic moduli. © 2008 Elsevier B.V. All rights reserved
Determination of mechanical parameters related to the kinetics of swelling in an electrically activated contractile gel
No full textModelling and mechanical characterization of thin fibres of contractile polymer hydrogels
No full textThe purpose of this work is to propose a methodology for characterizing the dynamic mechanical properties of thin fibers of hydrogel. Analytical solutions of mo tion equations of charged gels, based on a continuum model, whose internal consistency is demonstrated, are compared with experimental results in two paradigms: free-swelling and stress-relaxation. In addition we present a new method to estimate the gel shear and bulk moduli and its hydraulic permeability
Ultrasound Poroelastic Tissue Typing
Full text linkEmploying the poroelastic theory of acoustic waves in gels, the ultrasound (US) propagation in a gel medium filled by poroelastic spherical cells is studied. The equation of fast compressional wave, the phase velocity and the attenuation as a function of the elasticity, porosity and concentration of the cells into the gel matrix are investigated. The outcomes of the theory agree with the preliminary measurements done on PVA gel scaffolds inseminated by porcine liver cells at various concentrations. The feasibility of a non-invasive tech-nique for the health assessment of soft biological tissues steaming by the model is analyzed
The dynamics of a hydrogel strip
No full textA hydrogel strip relaxes when it is stretched. The decay in tensile stress can be ascribed primarily to strain-induced swelling of the polymer network--a result that follows from a continuum model of the gel-solvent system. An equation of motion and a linear constitutive law of the polymer network, Darcy's law, and the conservation of mass of the network and interstitial fluid are solved with boundary and initial conditions appropriate for a stress-relaxation experiment. This model predicts that the time constant of decay depends inversely upon the square of the thickness of the sample. This result is confirmed by experiments. In addition, the network shear modulus, mu, bulk modulus, k, and hydraulic permeability, 1/f, which are estimated by non-linear regression, all agree with measurements obtained using other methods
Electromechanical characterization of dielectric elastomer planar actuators: comparative evaluation of different electrode materials and different counterelectrodes
No full textHydrogel stress relaxation
No full textHydrogels exhibit stress-relaxation when stretched. The decay in tensile stress can be ascribed primarily to strain-induced swelling of the polymer network. This assertion is formalized in a biphasic continuum model of the gel-solvent system. An equation of motion and a linear consti tutive law of the polymer network are solved with boundary and initial conditions appropriate for a stress-relaxation experiment The model predicts that the time constant of decay depends upon the square of the significant length of the sample (i.e., thickness of thin films, diameter of sphere or thin cylindrical fibers), a result which is confirmed by experiments. In addition, the values of the material constants (μ, the shear modulus; k, the bulk modulus; and 1/f, the hydraulic permeability) which are estimated using a non-linear regression algorithm, agree well with measurements of the material constants made by other method
Poroelastic longitudinal wave equation for soft living tissues
No full textMaking use of the poroelastic theory for hydrated polymeric matrices, the ultrasound (US) propagation in a gel medium filled by spherical cells is studied. The model describes the connection between the poroelastic structure of living tissues and the propagation behavior of the acoustic waves. The equation of fast compressional wave, its phase velocity and its attenuation as a function of the elasticity, porosity and concentration of the cells into the gel external matrix are investigated. The outcomes of the theory agree with the measurements done on Alginic acid gel scaffolds inseminated by porcine liver cells at various concentrations. The model is promising in the quantitative non-invasive estimation of parameters that could assess the change in the tissue structure, composition and architecture
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