Medical grade silicones have many advantageous physical and chemical properties that have been utilized for over 40 years. The main difference between conventional and medical grade silicones is the curing system used.
Selecting an acceptable silicone for medical applications can be a challenge. It is important to determine the biological response of the materials and/or formulations to direct or indirect human patient contact.
Understanding Medical Grade Silicone
Silicones have gained widespread medical acceptance because of their low toxicity and low risk of unfavorable biological reactions. The first question asked when any material for a medical application is considered is: Is the material compatible with the human body?
The main difference between conventional silicone and medical grade silicone is the curing system used. The conventional silicone uses the peroxide cure system, which is often referred to as a free radical cure system or a high temperature vulcanizing (HTV) system. This chemistry utilizes free radicals generated by organic peroxides that decompose at elevated temperatures, initiating a cross linking reaction.
Peroxides and their derivative radicals are consumed during the reaction, but by-products are generated, normally acid residue. Therefore, they are not suitable for medical applications because they do not pass biocompatibility testing. As a result, the best cure system for medical silicone is the platinum cure system, also known as the addition cure. This system makes use of vinyl-functional polymers, oligomers with Si-H groups, and a metal complex catalyst, such as Platinum (Pt).
Addition cure has no by-products. Platinum, in the form of an organometallic complex of salts, is used. Catalyst costs are higher than for peroxide systems, which can be reflected in product prices. The post baking of medical silicone parts is done to assure the parts are fully crossed linked. If the medical silicone parts are not fully cross linked, there could be an effect on compression set as well as out gassing.
Silicone formulations for medical applications are subjected to specific evaluations using the standard ISO10993, which evaluates the bio-safety of materials in contact with the body. According to this standard, the silicone can be grouped into three categories: limited exposure, prolonged exposure, and permanent contact. Limited exposure products have a service life of up to 24 hours in contact with skin, mucosal membranes, or breached surfaces. Prolonged exposure products are those which have contact with surfaces or are implanted more than 24 hours and up to 30 days; they are called heath care grade. Silicones used to be implanted for more than 30 days are classified as permanent or long-time contact. The test methods and results must be accessible for the customer and validated by US Food and Drug Administration (FDA). The test made for silicones are based on cytotoxicity, sensitization, intra-cutaneous reactivity, toxicity, haemolysis, histopathology, cancernogenicity, among others.
When a medical silicone provider is evaluated, a number of factors should be considered: time of residence in the body, the regulations that apply the qualifications of the provider and if the grade available serves the application.
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