All materials used in such medical devices including the plastics used in them must be capable of being sterilized without loss of performance.
Sterilization can be defined as the removal or destruction of all living organisms including resistant forms such as bacterial and fungal spores.
Pyrogens are substances that can cause a fever. A product may be sterile but it still may contain pyrogens.
Cleanliness does not mean sterile.
The main sterilization methods used in medical devices are:
Autoclaving (steam, dry heat)
Ethylene oxide (EtO)
Radiation (gamma radiation, electron beam radiation)
Gamma radiation can negatively affect plastics in the following ways:
Formation of radicals leading to chain scission and degradation.
Formation of radicals leading to cross linking.
Color change
Chain scission leads to degradation and reduces toughness, elongation, and impact strength. The high energy gamma radiation forms radicals along the polymer chain. These radicals subsequently degrade the polymer to lower molecular weight chains leading to reduce physical properties. However many stabilizers like phenols, HALS (hindered amine light stabilizers),
phosphates etc have been used to absorb the energy or quench and capture the free radicals formed, thus preventing degradation. Color correction tints like ultramarine blue are used to compensate for the color change and maintain a clear, transparent plastic after radiation. Those polymers that require stabilization are given bellow:
Transparent polymers like polyvinyl chloride, acrylics, polycarbonates, and polyurethanes have a tinting agent and some polymers also require free radical scavengers or quenchers to prevent degradation.
Plastics used in medical devices can come into contact with various solvents and chemicals either during the manufacturing process or during end use.
The medical devices must maintain their integrity, performance, and aesthetics when exposed to such solvents and chemicals. Chemical can react with the additives causing them to leachout of the part or form unwanted byproducts. Chemicals used in a manufacturing environment include the following:
Acids
Bases
Solvents (methylethyl ketone, tetrahydrofuran, methylene chloride, ethylene oxide, alcohols, ethyl acetate)
Processing aids-greases, oils, mold release like silicones etc.Solvents like MEK and THF are used in the joining of plastics. Ethylene oxide and ethylene glycols are used in the sterilization of plastics. Many plastics are exposed to mold release agents like silicon sprays during production.
The following are some of the sources of possible toxic substances in medical device materials. Each source is then discussed separately.
Residual monomers
Residual solvents
Degradation products
By-products from irradiation
Sterilization residuals
Formulation additives
Inadvertent contaminants
Bacterial endotoxins
