#3 - Shaping medical device manufacturing with silicone elastomers

Silicone high-consistency rubber (HCR) has a long history of use in medical devices and other healthcare applications due to its biocompatibility, versatility, and excellent physical properties. This type of silicone elastomer is composed of reactive high-molecular weight polymer reinforced with silica filler. In their uncured form, HCRs have a clay-like consistency, making them well suited for fabrication processes like extrusion, calendering, and compression molding. HCRs are also typically stronger, with more robust physical properties compared with other silicones, such as liquid silicone rubbers (LSRs), making HCR a choice for components such as tubing, balloons, sheeting, and complex parts.

Why manufacturers choose

HCR There are advantages in choosing an HCR when manufacturing a medical device, and a primary factor is its biocompatibility. There are HCRs available with varying levels of regulatory support; some are qualified for long-term (>29 days) implantable medical devices, while others are qualified for short-term implantable devices as well as external applications. Additionally, HCRs can reduce the cost of tooling required to make molded parts, ideal for low-volume manufacturing.

HCRs possess very high mechanical properties when compared to LSRs due to the high molecular weight polymers and fillers used in HCR formulations. The strength of HCRs in their uncured form, known as green strength, also serves as an advantage when manufacturing medical devices, as the uncured parts can hold their shape until cured.

HCR curing systems: peroide- catalyzed and platinum-catalyzed

Generally, HCRs are supplied in a one- or two-part kit using peroxide-catalyzed or platinum-catalyzed cure systems. The cure system selected depends on the application and processing parameters. Peroxide- and platinum-catalyzed HCRs can be supplied catalyzed (the catalyst is already incorporated into the system) or uncatalyzed (the catalyst is packaged separately). In peroxide-catalyzed systems, the curing mechanism is not initiated until the HCR is exposed to heat.

This translates into extended work time for extrusion or molding. Peroxide-catalyzed systems require a post-curing process to remove residual byproducts – an important consideration for medical device applications. This curing mechanism provides unique elastomeric properties, which can be useful for manufacturing balloons or similar components where tension set and hysteresis is important.

Platinum-catalyzed HCRs typically consist of two components: one with the platinum catalyst and the other with hydride functional cross-linkers. For medical device manufacturers, one significant advantage to using platinum-catalyzed HCRs over peroxide-catalyzed HCRs is curing without any byproducts. Platinum-catalyzed cure systems don’t require a post-cure and improve manufacturing efficiency.

Processing HCRs

There are three methods to process HCRs into finished components for medical devices: extrusion, compression molding, and calendering. Extrusion forces the HCR through a die to form an intended shape, after which it’s heat-cured. This process is ideal for manufacturing tubing, ribbon, or rods that can be assembled into catheters or other devices. In addition to producing large volumes of products at low cost, the extrusion process can be optimized to control specific factors, such as wall thickness.

Compression molding forms the HCR into a desired shape that’s then heat-cured. Compression molding is ideal when manufacturing a solid or hollow part, such as balloons, O-rings, valves, and gaskets. During compression molding, the HCR is placed into the cavity of the mold and compressed between two heated platens under high pressure. Compression molding is often used when producing low-volume parts or parts requiring high physical properties that can’t be met with LSRs.

Before extruding or molding, HCRs must be processed using a two-roll mill to soften the material. If the HCR is a two-part material, softening each part on a cooled mill before combining the two parts is recommended to extend the working time of the material. Fillers or pigments can also be added during this step.

Calendering produces flat, uniform sheets of HCR with a specific thickness that can be used for further processing. Often the calendered sheets are used for die cutting or applied to a substrate to create a rubberized composite. The calender is made up of rollers applying pressure to the material, working together to create the desired thickness, typically between 0.005" to 0.25", then heat-cured and rolled or cut and processed.

Four key factors for using HCRs

HCRs are well suited for medical device applications because of their versatile material properties, processing features, and cure options. When choosing an HCR, consider these four factors:

1. Cross-contamination prevention: Some chemicals can negatively impact platinum-cured HCRs if they come into contact prior to curing. The contaminants can partially or completely inhibit the platinum-catalyzed cure system. Adherence to clean manufacturing practices, such as cleaning all surfaces between uses and relying on dedicated instruments like spatulas for subdividing HCRs, helps prevent this contamination.
2. Integration/interaction with other materials: For medical device manufacturers, one key advantage to using an HCR is the ability to incorporate additives into the pre-cured formulation. This allows the use of important components such as colorants, radiopaque fillers, antimicrobial agents, or active pharmaceutical ingredients (APIs). Additives or other materials within the device can interact with the silicone and the molding process. For example, use of a temperature-sensitive additive or one adversely interacting with formulary components can cause an incomplete cure or altered physical properties.
3. Formulation flexibility: Silicone materials can be processed to manufacture numerous medical devices. Silicone materials are formulated to assist process requirements by matching desired cure profile and work time. Device engineers can use the flexibility of silicone formulations to optimize for unique device requirements.
4. Expertise in a silicone supplier: For medical device manufacturers, there are advantages to collaborating with a silicone supplier who can provide extensive experience in HCR formulation and use for medical devices. When evaluating suppliers, consider:
   • Do they have a robust quality system? A materials partner should have ISO 9001 certification, deep knowledge of ISO 13485 quality management requirements for medical systems, and experience with the U.S. Food and Drug Administration (FDA) Master File submissions (MAFs).
   • Will they provide support throughout the entire design and regulatory submission process? A silicone supplier with established regulatory body relationships can provide significant value by saving time and money while navigating the device’s regulatory path to market.

With a long history of use in medical devices, HCRs provide manufacturers with a robust, high-performance material option for a wide variety of medical applications. Whether you’re manufacturing the next generation of an established diabetes care device or pioneering a novel neurological device, these silicone elastomers may be the right choice to help meet your manufacturing and performance needs.

NuSil
https://www.avantorsciences. com/pages/en/nusil

About the author: Dela Morgan is an associate applications engineer for NuSil – part of Avantor.