The sensitization test assesses the potential of a material or product to cause a sensitizing effect or allergic reaction in a patient over a long period of exposure. Sensitivity or hypersensitivity is tested by exposing the skin of an animal to the material or by extracting and injecting it from the device / material and / or applying it topically and looking for adverse reactions.

Sensitivity or hypersensitivity reactions are usually caused by repeated or prolonged contact with a chemical substance that interacts with the body's immune system. Since most such reactions to biomaterials are of the dermal cell-mediated type rather than the humoral or antigen-antibody type, the skin of laboratory animals is used in the susceptibility test. Dermal sensitization reactions in laboratory animals are marked with redness and swelling.

Biomaterials and devices that cause sensitizing reactions do so through their extractable chemicals. In some cases, an individual may develop a reaction only after repeated exposure to a material or after continuous, prolonged contact, such as an implant. Or, after wearing daily natural latex gloves for several weeks or months, a previously unaffected person may develop a permanent rash on hands and wrists. This sensitization may result from some chemical components of the gloves that act as an allergen.

In other cases, when a person becomes susceptible to a chemical such as that already in the environment, a reaction will occur when first exposed to a device containing the chemical. Therefore, an individual who has previously been sensitized to nickel will produce a rash through the temples a few days after he or she begins wearing nickel-plated rimmed glasses.

Test Methods

Biomaterials and other device materials are tested for the presence of sensitive chemicals using guinea pigs, a species almost responsive to dermal sensitizers as humans. Guinea pig sensitization tests require six to eight weeks and therefore take the longest time to complete all acute biocompatibility tests described in the 10993 standards.

The repeated or Buehler test method involves directly exposing the shaved backs of the guinea pigs to the test material under occlusive dressings for at least six hours. This procedure is repeated three times a week for three weeks. This part of the test is often referred to as the induction phase. After a rest or recovery period of two weeks to allow the development of a delayed response, animals are challenged with a final exposure to a patch of biomaterial. The repeated patch model is primarily used for dermal electrodes and topical devices such as surgical gowns and drapes, since the method of applying test materials to animals simulates clinical use.

In the maximization or Magnuson-Kligman test method, liquid extracts of the test material are prepared in saline and vegetable oil, and separate groups of guinea pigs are repeatedly placed in two groups of extracts. Guinea pigs are first injected with an extract together with an excipient to enhance an immune response, followed by a topical application. After two weeks of rest or recovery, the animals are covered with a topical patch containing the extract. The maximum valuation test, which is generally considered more sensitive than the repeated patch pattern, is used for device materials that contact areas outside the skin. The use of both a saline extract and an oil extract simulates extraction from body fluids and first through the device and then through intravenous fluids and other pharmaceutical products that come in contact with the patient.

In both techniques, the area of ​​combat patches is examined for reactions (redness and swelling) that are not found in negative control animals. In addition, known sensitizing chemicals are used periodically to verify the model and technician.