Kent R. Larson

Effect of polymer composition on steroid permeation: Membrane permeation kinetics of androgens and progestins

Abstract The permeation rates of 10 androgenic and 7 progestagenic steroids through synthetic membranes fabricated from poly(trifluoropropylmethylsiloxane) (PFMS), poly (dimethylsiloxane) (PDMS) and the copolymer of PDMS with poly(ethylene oxide) (PEO) and poly (methyl methacrylate) (PMMA) were investigated and found to be profoundly affected by the chemical structure of penetrants and/or the copolymer composition. The permeation rate for both groups of steroids was observed to decrease in the order of PDMS > PFMS > PDMS/PEO/PMMA copolymer. This can be attributed to the increase in membrane polarity, which causes a decrease in the polymer solubility of steroids, and the incorporation of the high-Tg glassy polymer segments, which leads to a reduction in the diffusivity in the polymer. The increase in the PDMS block size tends to promote the permeation of steroids, which can be explained in light of the increase in the size of more permeable PDMS domain. Incorporation of PMMA blocks into PDMS drastically decreases the rate of steroid permeation due to the presence of less permeable “hard” PMMA domains, while addition of PEO produces an enhancing effect on the permeation of less lipophilic steroids such as testosterone.

Kinetics and Thermodynamics of Drug Permeation Through Silicone Elastomers iv. Effect of Polymer Backbone Structure

AbstractThe effect of silicone polymer backbone structure on the permeability of progesterone and testosterone through silicone membranes was investigated in the capsule-type drug delivery system at 25, 37 and 50°C, respectively. Three types of polymer were studied: (I) (Me2SiCH2)x, (II) (Me2Sl-CH4-SiMe2O)x, and (III) [Me2Si(CH2)n SiMe2O] x, where n=2, 6 and 8.Permeabilities of progesterone in these polymers were found to decrease in the following order: Polymer (III) < Polymer (I) < Polymer (II). The same order was also observed for the permeabilities of testosterone, but their values were one order of magnitude lower than those of progesterone. Incorporation of the phenylene group in the polymer backbone in Polymer (II) caused a significant decrease in the diffusion coefficient but an increase in the solubility coefficient. The net effect is a 50% decrease in the permeability with respect to (Me2 SiO)x. This effect was found to be less pronounced with alkylene group in Polymer (III).The activation energ...

Kinetics and Thermodynamics of Drug Permeation Through Silicone Elastomers III. Effect of Alkyl Substituent (R) in (MeRSiO) Polymer

AbstractThe effect of silicone polymer structure on the permeability of drug through silicone membranes was investigated in the capsule-type drug delivery system. The permeability, diffusivity and solubility of progesterone and testosterone through polymethylalkylsiloxane membranes, (MeCn H2n+1 SiO)x, where n = 1, 2, 3, 6 and 8, were determined at 25, 37 and 50°C respectively.As the size of the alkyl substituent was increased from methyl to octyl, the diffusivity decreased, whereas the solubility increased. As a result, the permeability of progesterone decreased initially but increased gradually thereafter, going from (Me2SiO)x to (MeOctSiO)x, in this series of polymer. The same results were obtained with testosterone; however, permeabilities of testosterone were found to be one order of magnitude lower than those of progesterone. This was attributed to the lower solubilities of testosterone in (MeRSiO)x polymers when compared to those for progesterone. The activation energies of permeation for progestero...

Elastomer modulus and dielectric strength scaling with sample thickness

Material characteristics such as adhesion and dielectric strength have well recognized dependencies on material thickness. There is disagreement, however, on the scale: the long held dictum that dielectric strength is inversely proportional to the square root of sample thickness has been shown to not always hold true for all materials, nor for all possible thickness regions. In D-EAP applications some studies have postulated a “critical thickness” below which properties show significantly less thickness dependency. While a great deal of data is available for dielectric strength, other properties are not nearly as well documented as samples get thinner. In particular, elastic modulus has been found to increase and elongation to decrease as sample thickness is lowered. This trend can be observed experimentally, but has been rarely reported and certainly does not appear in typical suppliers’ product data sheets. Both published and newly generated data were used to study properties such as elastic modulus and dielectric strength vs sample thickness in silicone elastomers. Several theories are examined to explain such behavior, such as the impact of defect size and of common (but not well reported) concentration gradients that occur during elastomer curing that create micron-sized layers at the upper and lower interfaces with divergent properties to the bulk material. As Dielectric Electro-Active Polymer applications strive to lower and lower material thickness, changing mechanical properties must be recognized and taken into consideration for accurate electro-mechanical predictions of performance.