Cell culture assays are used to assess the biocompatibility of a material or extract through an in-vitro reaction of mammalian cells following exposure. These may be plastics or elastomers used as containers (or to seal containers) which will hold drugs or other solutions for parenteral administration (e.g. Intravenous (IV) bags, intravenous (IV) tubing), plastics that will directly contact the patient (e.g. knee brace material, dentures, contacts, catheters), materials that may be implanted (e.g. medical devices: joint replacements or cochlear implants). These techniques are useful in evaluating the potential of materials and chemicals to have toxic properties. They provide a way to screen materials prior to, or in lieu of, in-vivo tests. There are three methods outlined in USP for cytotoxicity tests:
- The Elution assay uses different extracting media and extraction conditions to test devices according to actual use conditions or to exaggerate those conditions. Extracts can be titrated to yield a semi-quantitative measurement of cytotoxicity. After preparation, the extracts are transferred onto a layer of cells and incubated. Following incubation, the cells are examined microscopically for malformation, degeneration and/or lysis of the cells. At least one type of cytotoxicity test should be performed on each component of any device to ensure each material, or any process which produces each component, does not imbue cytotoxic effects.
- The Direct Contact procedure is recommended for low-density materials, such as contact lens polymers. In this method, a piece of test material is placed directly onto cells growing on culture medium. During incubation of the cells in contact with test material, leachable chemicals in the material may diffuse into the culture medium to reach the cell layer. Reactivity of the test sample is indicated by malformation, degeneration and/or lysis of cells around the test material.
- The Agar Diffusion assay is appropriate for high-density materials, such as elastomeric closures. In this method, a thin layer of nutrient-supplemented agar is placed over the cultured cells. The test material (or an extract of the test material dried on filter paper) is placed on top of the agar layer and incubated. A zone of malformed, degenerated and/or lysed cells under and around the test material indicates cytotoxicity.
ISO or custom methods may also be used for cytotoxicity tests, which include the tests outlined in USP, but also include materials other than polymers in scope, as well as quantitative assays that employ techniques to measure of cell viability after exposure.
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