USP Class VI Silicone Rubber

USP Class VI: The Highest Biological Test Standard for Medical Elastomers

USP Class VI is a classification from the United States Pharmacopeia (USP) General Chapter <88> "Biological Reactivity Tests, In Vivo." It is the most stringent of the six USP plastic/elastomer biological test classes (Classes I through VI), requiring three separate in vivo animal tests to confirm that the test material does not produce acute biological reactions in living systems.

For silicone rubber used in pharmaceutical and medical applications — drug delivery tubing, catheter bodies, implant-adjacent components, and device seals — USP Class VI compliance is the minimum biological safety baseline expected by regulatory agencies and medical device OEMs worldwide.

The Three USP Class VI Tests

All three tests must be passed to achieve USP Class VI classification. A material that passes two tests but fails one does not qualify as Class VI.

Test 1 — Systemic Injection Test: A chemical extract of the test material is prepared in sesame oil and physiological saline. The extracts are injected into mice (five mice per extract). Animals are observed for 72 hours for systemic toxic signs including: unusual behavior, changes in body weight, and mortality. Pass criterion: no significant difference in response compared to the vehicle control group.

Test 2 — Intracutaneous Reactivity Test: Extracts in sesame oil, polyethylene glycol, and physiological saline are injected intradermally into rabbits (five injection sites per extract per rabbit). Skin reactions (erythema, edema) are scored at 24, 48, and 72 hours. Pass criterion: reaction score does not differ significantly from the vehicle control injection sites.

Test 3 — Implantation Test: Test material implants (strips of the actual silicone rubber compound, typically 1 mm × 1 mm × 10 mm) are implanted into the paraspinal muscles of rabbits. At the 7-day observation point (short-term implantation test) and the 90-day point (long-term), the implant sites are histologically examined for tissue reaction (necrosis, fibrosis, inflammation). Pass criterion: tissue reaction does not differ significantly from the negative control implant (USP-specified reference material).

USP Class VI vs USP Class II: What's the Difference?

USP <88> defines six classes of tests of increasing stringency. The key distinction:

• Class II: Only the systemic injection test (sesame oil extract) and the intracutaneous test are required. No implantation test.
• Class VI: All three tests (systemic injection in two solvents, intracutaneous in three solvents, and implantation at 7 and 90 days) are required. Class VI is significantly more demanding than Class II.

Class VI is required for materials in contact with body fluids, for pharmaceutical processing equipment seals, and for any implant-adjacent application. Class II is acceptable for some external contact applications.

USP Class VI and ISO 10993: Critical Distinction

USP Class VI passing does NOT satisfy ISO 10993 requirements for medical devices. They are different frameworks:

USP Class VI tests a specific lot of material extracted under defined conditions for acute biological reaction. It is a material-level test, not a device-level evaluation.

ISO 10993 (Biological Evaluation of Medical Devices) is a risk-based framework that considers the device's nature, duration, and degree of body contact and requires a customized testing program that may include cytotoxicity (ISO 10993-5), sensitization (ISO 10993-10), systemic toxicity (ISO 10993-11), genotoxicity (ISO 10993-3), hemocompatibility (ISO 10993-4), and chronic toxicity for long-term implants.

For regulatory submissions to FDA (510(k), PMA), EU MDR Notified Bodies, and NMPA (China), ISO 10993 biocompatibility evaluation is required. USP Class VI data is valuable supporting evidence but does not replace ISO 10993.

How to Request USP Class VI Documentation

When sourcing silicone rubber for medical applications, request from your supplier:

1. USP Class VI test report (per compound designation and per lot if lot-to-lot testing is performed)
2. Identify the testing laboratory (should be ISO/IEC 17025 accredited, preferably GLP-compliant)
3. Confirm which extract concentrations and solvents were tested
4. Confirm the compound designation and lot number tested matches the product you are purchasing

Certificates that state "manufactured to be compliant with USP Class VI" without an actual test report are insufficient for regulatory submissions. Request the actual test data.

Contact us to verify USP Class VI certification documents and request tested medical-grade silicone samples.