Frequency dependence of viscoelastic properties of medical grade silicones

Abstract

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37 degrees C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 +/- 5 N was applied over a frequency range, f, of 0.02-100 Hz. Values of the storage, E', and loss, E'', moduli for the cylinders were found to depend on f; the dependence of E' or E'' on the logarithm (base 10) of f was represented by a third-order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23 degrees C; pretreatment of specimens in physiological saline at 37 degrees C for 24 h and 29 days had no appreciable effect on the results.

Swelling of medical grade silicones in liquids and calculation of their cross-link densities

“…The resulting values were of the order of 10 −4 mol cm −3 , which is consistent with results from silicones used for breast implants [9,14]. Table 5 compares the values of (measured in n-heptane) with Durometer hardness (Shore A), the latter obtained from the suppliers' data sheets, and the Young's modulus [56], for the four grades of silicone investigated here. The harder grades (C6-180 and MED-4080) had higher values than the softer grades (C6-165 and MED-82-5010-80), which swelled the most and had the lowest .…”

“…Grade Table 5 Values of the cross-link density, , (measured in n-heptane), Durometer hardness (Shore A), which was obtained from the suppliers' data sheets, and Young's modulus, E* (obtained from previous literature [56]; values measured at 1 Hz), for each grade of silicone; the mean value is followed by the standard deviation (in brackets). a higher modulus are more cross-linked than silicones with a lower modulus [9,[11][12][13][14].…”

Effect of accelerated aging on the viscoelastic properties of a medical grade silicone

“…The specimens were tested, one at a time, in a cell containing physiological saline solution (9.5 g.L -1 of sodium chloride in deionised water) surrounded by a circulating water jacket at 37°C, to simulate in vivo conditions, as in previous work on silicone elastomers [15,16]. Testing did not begin until the temperature of the physiological saline solution and silicone was stable at 37°C, which took approximately 30 minutes.…”

“…Mean values and standard deviations for E' and E'' were calculated at each value of f, from the six measurements taken for each specimen. Results were plotted against the logarithm (base 10) of f, measured in Hz and second order polynomials fitted to give a smooth representation of a line through the data points [15,16]. The results before and after accelerated aging were compared by calculating the upper and lower 95%…”

“…confidence intervals [15,16]. These intervals represent the regions in which there is a 95% probability of finding the true mean value [15,16].…”