Compared to some organic thermoplastics, the transparency of silicone is described as “translucent”. This result is due to the fact that the silicone elastomer used to make the hose consists of a silicone polymer and amorphous silica. Because the two materials have different refractive indices and there is no specific mixing method to match them, all silicone tubing is translucent.
Chemical Resistance
Although they are unlikely to be present in pharmaceutical processes such as fermentation or infusion operations, two factors limit the chemical resistance of silicones: swelling caused by certain organic solvents, and chemical degradation caused by strong bases or acids.
The swelling of silicones occurs in hydrocarbon non-polar organic solvents such as toluene. During swelling, the weight ratio (w/w) increases by 200%, resulting in a weakening of the mechanical properties of the elastomer, but the bonds are not actually broken, but the elastomer is “diluted”. The swelling depends on time and molecular weight, as it is controlled by diffusion. Silicone tubes with low molecular weight silicones swell quickly, while high molecular weight silicones swell more slowly.
Purity and Dissolution
Drug testers now divide the issue of migration of substances from tubes and containers into “precipitates” and “leachates”. The former refers to substances that migrate under normal conditions of use, while the latter requires excessive temperatures or strong solvents (“bad cases”). Dissolved material should include precipitates, and this term will be discussed further here.
In both cases, hoses made with plasticizers may be expected to produce more dissolved material than hoses without additives. Silicone itself does not require plasticizers, stabilizers, UV absorbers or antioxidants. Because of the way they are manufactured, silicones contain very little heavy metals, usually less than 10 ppm. platinum complexes are used as catalysts for cross-linking reactions, but in very small amounts (10 ppm platinum); once cured, no quantifiable levels of platinum are detectable in the leachate, even when strong solvents are used. For silicones, the dissolved product consists mostly of the short-chain oligomer 6-(SiMe2O)n-, so acceptable residue levels can be determined by risk assessment.
The recommended conditions allow separation of the solvate from the product and minimize swelling, which may affect the interpretation of the data due to the small solvent recovery and the fact that the solvate is trapped in a swollen elastomeric web.
Among the solvents used, acetone obtained a high concentration of solubles (about 2% by weight), while ethanol, water or other aqueous media obtained a lower concentration of extracts. For the purposes of this study, acetone may be the ideal solvent for “over” studies. The structure of the silicone tube sample is critical, as the thicker the tube sample, the lower the dissolution rate. As expected, the lysate decreases upon storage or after sterilization.
Cleaning and sterilization
Tubes are packaged in an “extruded” form. An article comparing silicone with other hoses in relation to phytoplankton incubation mentions the importance of cleaning before use: silicone did not exert a significant effect, while other hoses reduced the growth rate of phytoplankton, suggesting that in some cases the effect was eliminated after cleaning. Some were cleaned with water for injection (WFI) prior to use and subsequently dried with compressed air in an air-conditioned room, but few details were provided. Because of their stability, silicones are easily sterilized.
General sterilization procedures include.
● Using an autoclave (steam) up to one standard gravity steam sterilization cycle (30 min, 15 psi, 121°C) or high speed steam sterilization cycle (15 min, 30 psi, 132°C). Note that silicone materials are more difficult to heat than materials such as thermoplastics because they are adiabatic in nature and therefore may require more time to heat.
● Gamma radiation, doses up to 2.5 Mrad (25 kGy) do not negatively affect mechanical properties (higher doses may cause some changes).
● Ethylene oxide (ETO), given sufficient time to completely remove residual hexane oxide gas. Residual levels of ETO have been studied in different hoses after sterilization and silicones absorb less ETO and can release it more quickly than PVC or polyester-polyurethane hoses.
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