Exploring the Viscoelastic Properties of PAA Phantoms

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A gel that has similar thermodynamic properties to human tissue is necessary for determining the safety of implanted medical devices during magnetic resonance imaging (MRI). One particular gel recommended by the ASTM standard (F218209) is the polyacrylic acid (PAA) phantom. In this work, PAA mixtures were characterized by measuring viscosity (as a function of shear rate), electrical conductivity, thermal conductivity, and elastic and viscous moduli (as a function of frequency). Experiments compared samples with blend times between 30 seconds and 9 minutes, and measurements were taken over a period of weeks to document the aging process in the phantoms. Results suggest that 3 minutes or more of blending 500 mL quantities causes the sample to transform from a gel (which has a well-defined yield stress) into a viscous liquid. The same transformation was observed in a single sample over a  period of two weeks. These results are important because the current ASTM standard does not specify blending time in detail. It is therefore possible that variability in the gel preparation methods could affect the results of experiments to determine the safety of implanted medical devices. These results will help to strengthen the ASTM standard procedure in future revisions.