By Sean Fenske, Editor-in-Chief

Materials are critical to the success of a medical device project. Selecting the wrong one can add delays and costs or even kill the project outright. When the right material is aligned to a design, however, additional benefits may be realized that bring added value to the device. Getting this early step correct is crucial to moving forward into the next stages of development.

One material that’s not always well understood is silicone rubber. Since it offers specific properties and advantages, it’s ideal for several medical device applications while not being suited for others. Unfortunately, many device designers aren’t always clear on what these are, if it’s suitable for a specific application, or what considerations must be taken into account before selecting it.

As such, working with an expert who supplies not only the material but also all the knowledge necessary to get the most from silicone rubber is paramount. Fortunately, one such individual—Ronnie Fierro, Project Engineer at Jefferson Rubber Works—shared insights on silicone rubber, the best applications for it, and considerations to keep in mind, in the following Q&A.

Sean Fenske: What is silicone rubber? What are its most important/primary characteristics for medical devices?

Ronnie Fierro: Silicone rubber is built on a silicon-oxygen backbone, often mixed with silica to adjust physical properties, and a curing agent to crosslink the material from a paste to a solid rubber during curing. Additional additives can also be mixed to adjust stability and appearance. Uncured silicone is often prepared as a two-part LSR (liquid silicone rubber) or single-component HCR (high consistency rubber), where manufacturers mold or extrude it into the final product.

For medical devices, silicone’s key advantages are from its silicon oxygen backbone, which is naturally resistant to microbial colonization, followed by its inherent chemical, heat, and UV resistance. Medical-grade silicone components are suitable for both extended single-use applications and multi-use devices that undergo repeated autoclave and sterilization cycles.

Fenske: Where is silicone rubber most commonly used in medical devices? What applications?

Fierro: Silicone rubber is most commonly used for IV and infusion tubing, urinary and vascular catheters, and respiratory masks used for gases like oxygen and anesthesia. Other common applications include diaphragms in pumping systems or pre-pressurized devices, as well as stoppers for vials or syringes.

Fenske: What makes silicone rubber a better choice for a particular application over other flexible polymers?

Fierro: By chemical nature, silicone outperforms many thermoset rubbers and flexible thermoplastics in many applications due to its biocompatibility and chemical resistance that other materials lack. In the case of a catheter, some materials like polyurethane and PEBA are also used, but are not as biocompatible and, as a result, are often used for shorter-term implants.

Fenske: Can you add substances to silicone rubber to gain additional benefits, like strength, lubricity, or another favorable attribute?

Fierro: While there are many readily available silicone products from various manufacturers that are medical-grade or UL 94-rated, silicone can be customized for specific applications. Both LSR and HCR can be modified at the raw material manufacturer with additives like silica fragments for strength, lubricity enhancers, radiopaque fillers for X-ray visibility, pigments for color, and ceramic fillers for increased flame resistance. In the case of LSR, these additives can also be added in the molding process with precision dosing equipment. This flexibility allows manufacturers to utilize standard materials and customize internally, simplifying the supply chain and reducing lead times, while still meeting specific customer requirements.

Fenske: Can silicone rubber be used as a carrier of a pharmaceutical element for a drug delivery application?

Fierro: Silicone products are commonly used as carriers in drug delivery systems—most commonly in long-term applications. Many contraceptive implants consist of silicone components and are designed to be inserted into the body to slowly release the drug, often up to a few years. Another common application is for glaucoma inserts, where medication is automatically released and reduces the need for eye drops. A growing application is chemotherapy ports, where the port is installed under the skin and a silicone tube connects to a vein, reducing the number of needle sticks.

Fenske: What often overlooked aspect of silicone rubber should device developers be aware of?

Fierro: An overlooked aspect of silicone is its high permeability compared to other rubber products. Being permeable is not a standard measured value like typical physical properties; it is not commonly understood. However, some products utilize its high permeability for controlled and automatic drug delivery. While good for drug delivery applications, this needs to be considered when used in pressurized sealing applications. Should lower permeability be required, this is commonly accomplished by increasing the silica content, using a platinum curing agent, or using fluorinated silicone.

Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell medical device manufacturers?

Fierro: Many medical device manufacturers are generally familiar with silicone—more specifically LSR. Though LSR has its advantages of low cycle time and low material waste, it requires a higher initial investment for new product introduction and prototyping compared to medical-grade HCR. For prototyping and lower-volume production, HCR is often a feasible and cost-efficient option and should be considered by manufacturers when developing new products.

Click here to learn more about Jefferson Rubber Works >>>>>