How to Select USP Class VI Medical Silicones

USP Class VI silicones are biocompatible elastomers that are safe for contact with human skin. They have low levels of extractables and chemicals that can migrate from a material at elevated temperatures or with surface exposure. Applications for USP Class VI silicones include seals, gaskets, tubing, o-rings, cable jacketing, and other components for medical devices and equipment. This article provides an overview of USP Class VI silicones and explains what medical device manufacturers need to consider when selecting these materials.

Understanding USP Class VI

The U.S. Pharmacopeia (USP) defines the testing requirements that plastics, a category of materials that include silicones, need to meet in order to belong to one of six individual plastics classes numbered I through VI. Each USP Class and its testing requirements are defined in Chapter <88>: Biological Tests, In Vivo. USP Class I has the least rigorous testing requirements. USP Class VI has the most rigorous testing requirements and includes an acute systemic toxicity test, an intracutaneous toxicity test, and an implantation test. Specifics of the end use and contact duration will determine which class of material is required for a given application. For example, an application where contact of under 24 hours with intact skin is expected will likely require a material with less rigorous testing than an application where contact is long-term, over 30 days, or the skin is breached or compromised.

USP Class VI silicones may meet requirements that are more rigorous than what you need, but it’s sometimes better to be safer by overspecifying materials. ISO-10993, an international standard for the biological evaluation of medical devices, has testing requirements that are much more extensive than USP Chapter <88>. However, testing medical silicones to ISO-10993 is more time consuming and costly than is necessary for most non-implantable applications.

Key Considerations

The USP is not a regulatory agency, but its standards inform decision-making at the FDA. In Europe, medical devices are regulated under the Medical Device Regulation (EU 2017/745), which requires that materials used in certain types of devices are free of chemicals considered to be carcinogenic, mutagenic, or toxic to reproduction (defined in Annex VI of (EC) 1272/2008), or are considered to be endocrine-disruptors (defined either in the REACH SVHC list — Article 59 of (EC) 1907/2006 — or Annex VI of (EC) 1272/2008).

Medical device manufacturers need to meet regulatory requirements, but it’s also important to find the right balance between material prices and performance. When selecting USP Class VI silicones, consider the ingredients and certifications, colors and curing systems, material tolerances, sizes, and lead times. The durometer, or hardness, of a USP Class VI silicone is also important because it determines the elastomer’s compressibility, which supports sealing. Finally, consider vendor value.

Ingredients and Certifications

Material manufacturers may substitute ingredients because of shortages, discontinued inputs, cost pressures, or supplier consolidation. If the formula for the end-product changes, the elastomer may no longer meet all USP Class VI test requirements. There are also instances of materials that were never validated by a third party and vendors who stopped using certified compounds without giving notice.

To mitigate these risks, medical device manufacturers need to ask suppliers for certificates, or certs. It’s also important to understand the difference between a Certificate of Analysis (COA) and a Certificate of Conformance (COC). COAs report the test results for a specific batch of materials. By contrast, COCs simply attest to a material’s compliance. If the formula for a USP Class VI silicone changes, a COC from a supplier’s quality department may contain out-of-date or inaccurate information.

Colors, Durometers, and Curing Systems

USP Class VI silicones are naturally translucent, but a compounder can add pigments to support distinctive product designs. What medical manufacturers need to consider, however, is that pigmented grades need separate testing per color by the end user and advance communication with the pigment suppliers to prevent the use of pigments that may cause failures in biocompatibility testing. In addition, a supplier that makes silicones in different durometers, such as from 10 to 80 (Shore A), may have all of the required USP Class VI testing for a single durometer rather than for all durometers. If that is the case, USP Class VI equivalency in these other durometers may still be sufficient for the application.

The curing system for USP Class VI silicones is especially important. Additional curing through the use of platinum catalysts produces no by-products. By contrast, peroxide-cured silicones contain by-products in the form of organic acids. For this reason, platinum-cured silicones have inherently higher purity than their peroxide-cured counterparts. Platinum curing can also offer greater control over cure rates and cross-linking for silicones that are more consistent in terms of appearance. In addition, platinum-cured silicones show less degradation of properties under sterilization by autoclaving, irradiation, and ethylene oxide. This combination of high purity, which translates to lower toxicity, and superior property retention under sterilization makes platinum-cured silicones preferable for medical applications.

Material Tolerances, Sizes, and Lead Times

Finally, it’s important to consider the tolerances, sizes, and lead times of USP Class VI silicones. When sheets are compression molded, they come in standard sizes based on available tooling. Continuous roll technology can produce 10 times as many sheets per hour, and with tight tolerances. Importantly, rolls can also be converted to custom sheet sizes. For applications where a medical device manufacturer needs a ready-to-mold compound instead, compression molding can be used for parts production.

Vendor Value

USP Class VI silicones can provide the right balance between cost savings and patient protection, but medical manufacturers need to protect themselves against supplier cost-cutting that can invalidate compliance. It’s also important to evaluate all of a material’s attributes, such as color and durometer, and to choose USP Class VI silicones that have passed third-party testing.

Whether it’s for sheets, rolls, or ready-to-mold compounds, it pays to partner with a supplier whose products are truly USP Class VI compliant. It’s also worth picking a partner that is willing to answer questions about chemical compliance and that will facilitate additional testing if necessary. By sourcing USP Class VI silicones from the right vendor, medical device manufacturers can mitigate risk and get greater value.

This article was written by Dominic Testo, Business Development Manager, Specialty Silicone Products, Bollston Spa, NY. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it. or visit here  .

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