How to ensure optimal use of high-consistency rubber silicones

Brian Reilly, business development director, biomaterials, NuSil – part of Avantor, discusses how to ensure the optimal use of high-consistency rubber silicones.

High-consistency rubber silicone (HCR) is a versatile material with a long history of use in medical devices and other industries. Commonly found in a wide variety of applications, such as tubing, balloons, sheeting and some molded parts, HCRs consist of a high molecular weight polymer combined with silica to produce a silicone that has a clay-like consistency in its uncured form.

HCRs in medical devices

Medical device manufacturers often choose silicone because of its established biocompatibility pedigree, broad processing parameter ranges and excellent physical properties.

One of HCRs key appealing characteristics is versatility. It can be processed using different fabrication methods, including extrusion, calendering and compression or transfer molding. Compared to other silicones such as liquid silicone rubber (LSR), HCRs are generally stronger with more robust physical properties. Another key aspect is their proven history of use in countless approved implant and non-implant applications.

Versatility in HCRs

The physical properties and process considerations of HCRs are important to understand along with the advantages that both peroxide- and platinum-catalyzed solutions provide.

A key advantage of peroxide-catalyzed systems is that their curing mechanism is not initiated until the HCR is exposed to heat. This translates into a very long work time which is beneficial for molding or extrusion. Consider that peroxide-catalyzed systems call for a post-curing process to remove residual byproducts.

This curing mechanism provides unique elastomeric properties that can be useful in balloons, or similar applications where 'tension set' is important. If a balloon needs the ability to be inflated and then return to its original shape when deflated, a peroxide-cure HCR may provide the best solution.

Platinum-catalyzed systems typically consist of two components; one contains the platinum catalyst, and the other contains hydride functional crosslinkers and cure inhibitors. When combined, the HCR retains its pre-cure consistency for one to two hours. A key advantage of platinum-catalyzed HCRs is the ability to heat-accelerate the cure for faster cure times and increased throughput without corrosive byproducts. Platinum-catalyzed HCRs typically yield much higher physical properties than traditional peroxide-catalyzed HCRs, which may be valuable for specific applications that use molded or extruded components.

HCR processing

When using HCRs, certain steps must be taken depending on the fabrication methods. For example, prior to molding or extruding, they need to be processed with a two-roll mill. The mill imparts shear to the material and modifies its consistency, making it softer and able to flow more easily through the die or mold.  

If the HCR is a two-part material, each part will need to be separately softened on a cooled mill before being combined to prevent premature curing. If the HCR is a one-part material, it will also need to be mill-softened prior to use.

Different end-use applications call for different fabrication methods. For example, in a medical device that incorporates silicone tubing, the most efficient production method would be extrusion. Once the processing method is established, consider additional requirements:

• What properties should the tube have?
• Does it need to be soft and flexible, or stiff and rigid?

These and other factors can help decide whether a device requires a peroxide-cure or platinum-cure system.

The broad processing parameters of silicone elastomers make them ideal for molding. For molded products, like hydrocephalic shunts, consideration needs to be given to the shunt’s valve mechanism. With this product type, the device manufacturer should choose a HCR that reliably provides the proper modulus, tension set and other elastomeric properties to ensure the valve functions properly.

HCRs are often calendered into flat sheets that may be die cut. A major application for this processing method is a gasket used as a device seal. With a peroxide-cure HCR, it’s possible to create gaskets with self-adhering properties. Stored properly, an uncured peroxide HCR sheet has almost infinite shelf life. A supplier could fabricate the sheet, package it and ship it to another manufacturer who die-cuts the gaskets which are pressed in place and then cured to seal completely.

Key factors for using HCRs

The versatile material properties, processing features and cure options make HCRs well-suited for a broad range of medical device applications. It is important to consider the following factors when choosing HCRs.

• Integration/interaction with other materials: One of HCRs advantages is the ability to incorporate additives into the pre-cured formulation.
• How additives or other materials within the device interact with the silicone and the molding process: If any of the additives are temperature-sensitive or adversely interact with formulary components resulting in an incomplete cure, the silicone supplier may be able to customize solutions to address the issue.
• Avoiding cross-contamination: Platinum-cured HCRs can be negatively impacted by chemicals that may come into contact with the silicone prior to curing. These contaminants can partially or completely inhibit the platinum-catalyzed cure system. To prevent contamination, clean manufacturing practices should be followed. This includes having dedicated instruments like spatulas for subdividing HCR and cleaning all surfaces between uses.
• Flexibility in HCRs: Since no two medical devices or fabrication processes are exactly the same, leading-edge silicone providers will have solutions that provide greater flexibility for the manufacturing process. An optimization system can help device engineers optimize process requirements, such as work time and cure profile.

Final considerations for HCR selection

There is a great advantage to working with a silicone supplier with extensive experience in providing HCRs for medical devices. Suppliers that can demonstrate expertise in working with regulatory authorities, along with a focus on supplying materials specifically for medical device applications, can help clear regulatory hurdles efficiently.

When evaluating suppliers, consider whether they have a robust quality system with ISO 9001 certification, deep knowledge of ISO 13485 quality management requirements for medical systems and experience with the U.S. Food and Drug Administration Master File submissions. A medical device manufacturer should also work with a partner that can provide support throughout the entire design and regulatory submission process. A materials partner with established regulatory body relationships can prove invaluable, saving time and money, while navigating the path of getting a device to market.