PUMA
Istituto di Fisiologia Clinica     
Chiarelli P. Acoustic waves in hydrogels: A bi-phasic model for ultrasound tissue-mimicking phantom. In: Journal of Intelligent Material Systems and Structures, vol. - (-) p. -. Elsevier, 2008.
 
 
Abstract
(English)
In 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. Themodel 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-boundedwater viscosity. This outcome canwell explain the attenuation of the US frequency in natural gelswhere δ is typically about 0.25¸0.50while the value for purewater 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 newproposed dissipativemechanismleads to a USwave speed c that follows the law: c=cw(β−ϕ)−3/2, where cw is the US wave speed in water and ϕ is the volume fraction of the bounded water. Since 0bβb1 and ϕN0, 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 linewith the experimental results that show that the US speed gap atβ=1 decreases in the gel samples with amore cross-linked polymermatrix that has a lower bounded water volume fraction. On the contrary, if thewater content is very low(i.e., βb0.4), the measured US speed converges to that of the dry hydrogelmatrixwhich increases in the samples with a higher degree of network cross-linkingwith greater elastic moduli.
Abstract
(Italiano)
Nel presente lavoro viene sviluppato un modello poroealastico per ultrasuoni longitudinali negli idrogeli. La teoria Ŕ sviluppata introducendo un appropriato termine di interazione tra l'acqua libera e l'acqua legata presente in questi materiali. Per la ubiqua presenza degli idrogeli nei tessuti biologici come matrici extracellulari,il modello Ŕ un utile mezzo per lo sviluppo di fantom di tessuti molli. I risultati sperimentali confermano le previsioni della teoria, mostrando un aumento della attenuazione ultrasonora al diminuire del contenuto d'acqua dell'idrogele; ed un aumento della velocitÓ di propagazione rispetto a quella dell'acqua.
URL: http://www.sciencedirect.com
DOI: http://dx.doi.org/10.1016/j.msec.2008.07.036
Subject Ultrasound
Hydrogels
tissue phantom
Therapeutic ultrasound
Diagnostic ultrasound


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