The sterilization of medical devices – particularly reprocessed medical devices – has been in the news in recent years, as healthcare associated infections have soared as the result of improper sterilization of medical instruments, such as scopes, by the end user. Whether a device will be reused or not, sterilization is a significant concern for the manufacturer, who is ultimately responsible for shipping a safe, sterile device.  But is sterilization enough?

With so many different parts and materials incorporated in a single device, manufacturers often express concern about the effect of sterilization on medical silicone molded components.

According to silicone manufacturer Nusil, “Silicone elastomers in general are temperature and moisture resistant and therefore are not adversely affected by most sterilization methods.” However, NuSil offers guidance on the possible effects that different techniques may have on silicone’s properties and urges manufacturers to test those effects on silicone components. “Prior to determining the right method of sterilization, each user must identify and perform tests and analysis on several samples to ensure that its finished parts will be safe and suitable for end-use conditions,” NuSil states.[

But sterilization alone is not enough. The medical silicone molded component first needs to be as clean as possible in order for sterilization techniques to be the most effective.

The importance of cleaning for medical silicone molded components

Medical silicone molded components are often found within the device, rather than on a surface that would come in contact with the patient. Other medical silicone molded parts are intended for use inside the body, such as joint implants, and come in contact with tissues and fluids. The application of the medical device determines the type of medical grade silicone used and the level of cleanroom manufacturing required.

Typically, the molder does not sterilize the components it produces. Instead, standard practice is for the manufacturer to sterilize the entire device after production and assembly, prior to shipping to the end user.

To ensure the efficacy of the eventual sterilization, the medical silicone molded components should be produced in a cleanroom environment and delivered free of contaminants. The cleaner the part, the easier sterilization will be for the manufacturer.

Ensuring cleanliness in silicone molding starts with following the FDA’s Current Good Manufacturing Practices (CGMP), which covers such steps as clearing the line after production to prepare for the next production run and to avoid cross-contamination.

Cleanroom standards are maintained by the management of particulates in the air through filters, pressurization to push contaminants out of the room, and proper humidity levels to control bacteria growth, metal corrosion, and general human comfort in the molding cleanroom, as well as an overall cleaning regimen. The right cleaning procedures ensure that contaminants are not brought into the cleanroom as staff members enter or by the introduction of products or materials that contain contaminants.

After production, the silicone molder may need to perform cleaning operations and test for contaminants. Special packaging for shipping also can help to ensure cleanliness. The silicone molder can ensure that the packaging meets the manufacturer’s standards for shipping, including double bagging, safety seals, shelf life data, and integrity testing.

Sterilization methods and potential effects on silicone

Medical device manufacturers are responsible for the sterilization of finished devices before shipping. In addition, the device must be designed in a way that ensures that it can be sterilized by the end user if it will be reused.

Common methods of sterilization for medical devices include heat (dry heat or steam autoclave), chemical (ethylene oxide or EtO) and radiation (gamma radiation or electronic beam).

In an article in Medical Design Technology, NuSil authors state, “Dry heat or steam autoclaving will most likely have little effect on silicone’s physical properties because of its moisture and heat resistance.” They note that steam autoclave, which is often used to sterilize reusable devices, “is typically not recommended for use on medical grade adhesives since many substrates and adhesives do not survive well under the high-pressure steam environment. The drawback to heat sterilization of silicone materials is that the high heat may cause the silicone to expand which must be taken into consideration in how the device is configured and packaged.” EtO will diffuse throughout the silicone polymer network, so the authors advise, “The only precaution is to ensure that all of the EtO has been removed from the silicone device before it is used, which usually takes 24 hours.”

Radiation methods present more of a concern for sterilization, as the techniques may damage the silicone. “The result can be net loss of flexibility, tensile strength, elongation, and/or an increase in durometer… Since these losses may be within the application’s specification limits, the minor loss in physical properties may not adversely affect the device’s ultimate function.”

The authors recommend that manufacturers consult with sterilization service providers for recommendations on the best sterilization method for the particular device.

Design for manufacturability and sterilization

The design of the medical silicone molded component also must be considered, as more complex designs run the risk of harboring microbes or bacteria, due to the difficulty of cleaning the part and ultimately sterilizing the entire device. Silicone molding engineers can advise on new part design and its ability to be molded and cleaned effectively.

“When it comes to cleanliness, there is no current consensus on how to measure cleanliness or what level of cleanliness is acceptable. Sterility is typically defined as acceptable if the process results in a sterility assurance level (SAL) of 10-6. This SAL can only be reached if the device is adequately cleaned first. This point cannot be stressed enough” writes Donna Swenson, president of Sterile Processing Quality Standards. She also points out, “Both disinfection and sterilization rely on adequate cleaning. Sterilization does not override inadequate cleaning.”

When working with a medical silicone molder, the device manufacturer will need to specify the level of cleanroom production required for the component. Medical silicone molded components that will be used in implantable devices receive the highest level of cleanroom production, while parts for disposable devices may not need the same level of cleanroom handling during molding.

Details on cleanroom molding can be found in a previous article, How Silicone Molding Cleanrooms Factor into Patient Safety.

(i) NuSil’s publication, “Methods Used to Sterilize Silicone Based Medical Devices,” covers dry heat, steam autoclave, ethylene oxide, gamma radiation and electron beam methods of sterilization