Viscoelastic properties of Silastic medical grade silicones

Cylinders (29 mm in diameter and 13 mm thick) of medical grade silicone elastomers intended for medium-term implantation (up to 90 days), were immersed in physiological saline solution at 37 degrees C, and investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 N +/- 5 N was applied over a range of frequency, f, values of either 0.02-50 Hz or 0.02-100 Hz. Values of the storage, E', and loss, E", moduli for the cylinders were found to depend on f. There was no significant difference (at the 95% level), of the moduli obtained from silicones intended for short (29 day) and medium-term (90 day) implantation; the assumption, in previously published work, that silicones intended for short-term implantation can be used as models for silicones used for long-term implantation appears reasonable. Although E' increases with f, the ratio E'/E" suggests that this increase does not lead to excessive storage of deformation energy, that could lead to fracture, at the higher f values likely to be encountered in vivo.