Polydimethylsiloxanes and Bio-compatibility Property of Silicone:
In medical devices and pharmaceutical applications, silicones are used because of their bio-compatibility in a wide variety of physical forms. These forms range from volatile and low oligomers to high molecular weight polymers with viscosities from 0.65 cSt to 20x106 cSt to viscoelastic compounds and cross-linked elastomers.
What is Bio-compatibility?
It is the ability of a material to perform with an appropriate host response in a specific situation. The impact of the bio-material on its host environment is assessed according to approved standards aligned with the performance requirements for the intended applications.
Overall, FDA grade silicone tubing for medical use, and in particular PDMS fluids or PDMS-based elastomers, satisfy the criteria of the above standards, including nonirritating and non-sensitizing behaviors, which explain their wide use in personal care and skin topical applications. A long history of use in medical devices, including long term implants, has made silicones widely recognized as bio-compatible. These standards are yet addressing the impact from the host on the foreign material to a lesser extent, as data on bio-durability are difficult to acquire.
Silicones with side-chain groups other than methyl (Me) are less used; Polydimethylsiloxanes (PDMS) polymers are the 'preferred material,' even if some unlisted non Compendia/non Pharmacopoeia materials are now also well established. Potential improvements with new silicones are hindered by rigid regulatory requirements, and innovation is sometimes limited to the use of current materials in new applications.
Linking physicochemical properties to bio-compatibility is not yet fully understood for many materials. Various factors are involved to explain the successful use of PDMS-based materials in medical devices or pharmaceutical applications:
Because of their backbone flexibility, PDMS materials can preferably expose their low interacting Me group substituents at many interfaces, leading to low surface tension, low surface energy and low inter-molecular interactions, resulting in a low overall level of interactions at their surfaces. Therefore PDMS materials are among the most favored polymers when considering bio-compatibility.
Their composition is well established. PDMS polymers do not require stabilizers because of their intrinsic stability. PDMS elastomers do not require plasticizers because of their low Tg. Hemocompatibility studies have suggested that silicone tubing may be superior to PVC tubing. Impurities are well characterized siloxane oligomers and the toxicology profile of these oligomers has been investigated recently in detail. Other impurities are catalyst traces, such as acids or bases used in polymerization, but these are easy to eliminate and usually not an issue. Similarly, traces of platinum catalyst used at very low levels in cross-linking reactions may be present, and again this is usually not an issue. Only some tin catalysts used in room temperature curing materials or byproducts of peroxides used as initiators in some high consistency rubbers (HCR) have raised concerns.
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