Silver-Based Antimicrobials in Medical Silicone Elastomers
Because of the inherently low toxicity, medical silicones present a low risk of unfavorable biological reactions and have thus gained recognition and acceptance for food and medical applications.
Bacteria and other microorganisms can affect silicone rubber properties. Biocides are used to fight with these micro-organisms and prevent the buildup of harmful bacteria that increase the risk of contamination or transmission of infections in humans.
Improving the Microbial Resistance of Medical Silicone Rubbers
Medical grade silicones are silicones tested for bio-compatibility and are appropriate to be used for medical applications. In the United States, the Food and Drug Administration (FDA) regulates materials implanted in the body. Medical grade silicones are generally grouped into three categories: non-implantable, short-term implantable, and long-term implantable.
The United States Pharmacopeia (USP) is a private and non-governmental organization that “promotes the public health by establishing state-of-the-art standards to ensure the quality of medicines and other health care technologies”. The USP mono-graph (88) describes the classification of plastics into six classes based on responses to a series of in vivo tests for which extracts, materials, and routes of administration are specified.
Materials approved as USP Class V and VI are considered medical grade. Class VI requires the most stringent testing of the six classes. Extracts of the test material are prepared in saline, alcohol, polyethylene glycol (PEG 400), and vegetable oil. The extracts (diluted in the case of the PEG extract) and blanks are injected into mice and rabbits, which are observed several times over a 72 hour period. The animals’ responses to the sample extracts and the blank are compared to determine test passage.
The tests of the extracts are called Systemic Injection Tests and Intra-cutaneous Tests. Materials that pass these two tests are compliant with Class V regulations. Despite all of the controls for medical silicone, the incidence of hospital-acquired infections (HAIs) is increasing around the world, driving the need for better preventative techniques and technologies to reduce the infection rates. Developing effective solutions will involve efforts by both healthcare providers and medical device manufacturers.
In the United States, due to changing health insurance policies, hospitals increasingly have to bear the costs of treating HAIs. Many healthcare institutions are, therefore, attempting to reduce infection rates by modifying their infection control protocols to improve aseptic techniques and compliance. In addition, manufacturers of medical devices are an important part of the solution as they seek new and better technologies to help reduce the growth of microorganisms on their devices. For that reason, additives known as antimicrobials or biocides are used.
The most common antimicrobials or biocides used in silicone rubber are N-butyl-1, 2-benziso-thiazolin-3-one, OBPA in diisodecyl phthalate plasticizer and, as of late, silver-based antimicrobials have also been used. The latter has the best performance. Typical silver antimicrobial technologies available include: Pure silver, elementary silver compounds, and silver cation ion exchange systems. The key to antimicrobial action in silver-based systems is dependent on its ability to generate silver cations. As such, a number of silver-based systems are available, and all derive their activity on the ability to supply, under the right circumstances, a critical concentration of silver cations necessary for the antimicrobial effect. Some silver-based antimicrobials have very efficient silver cation release mechanisms, good control over release rates, and can be incorporated into silicone without affecting the mechanical properties of the base material.
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