Quantification of the Modulus of Elasticity and Dynamic Properties of Sylgard for Various Mixing Ratios, 2003-2006
Sylgard Elastomer is used to construct phantom models of human tissue. The mechanical properties of Sylgard 184 are tested over a range of mixing ratios of Sylgard 184. Young’s modulus is determined from the tests and compared to the moduli of human tissue. Although more data is needed to validate the preliminary results, Sylgard’s mechanical properties at the range of ratios tested correlate with properties measured on human tissue. However, Sylgard 184 is a linearly elastic material while human tissue is not. Simulation of human tissue using Sylgard should only be in ranges where a linear relationship can be approximated.
The following mixing ratios were created: 10:1, 15:1, 20:1, 25:1, 30:1, and 35:1. The ratio was of base to curing agent by mass. A convenient amount of Sylgard 184 base was poured into a container and weighed on a mass balance. While the container remained on the mass balance, the curing agent was slowly added until the desired mixture mass ratio was achieved. The mixture was stirred for five minutes with a stirring rod and then degassed several times to remove air bubbles. Once the mixture was free of air bubbles, the mixture was cast into a cylindrical shaped mold. Five molds of each mixing ratio were formed. The molds were oversized drinking straws (~23 cm lenght by 0.635 cm diameter). A pump sucked the mixture up through the straws causing the mixture to slowly move up the straw. Sufficient time was available to stop the pumping when the straw was filled.
The bottom of each straw was was folded and taped closed and the molded mixture was allowed to cure for at least two days. The curing period want not consistent. Once the material was solid the straws, were cut lengthwise with an exacto knife while being careful to not slice the sample. Some samples were exposed to water before testing.
The samples were loaded in uniaxial tension. In order to reduce stress at the extreme ends of the samples, and to concentrate strains in the middle test section of the sample, tape is wrapped around the ends. Without the tape to distribute the load of the clamp and protect from cutting into the sample, the high stress caused the first sample to break at the clamped end. The improved design of the apparatus included the fixed ruler because, originally, measurements would make contact with the samples. This introduced unwanted loading. Two lines were marked on the sample at a distance of a few diameters from the ends to ensure no localized stresses. This gauge length was used for analysis of strain.
The diameter was measured at five locations along each sampel and averaged. Known masses were added into the attached tray one at a time, and the corresponding change in length between the tick marks wa measured with the ruler.
Measurements were made by adding and then removing weights to allow testing for hysteresis. Measurements were confined to the linear elastic region. No samples were stressed to yielding or breaking. Some tests were performed to test for creep over a period of one day. Insufficient data was accumulated for this test.
The average strain curve of each mixing ratio was plotted against stress. Young’s modulus for each sample was obtained by fitting a line to the data and calculating its slope (based on linear best fit of data). A plot of Young’s modulus versus mixing ratio is indicated.