From tubing and sheeting to valves and balloons, manufacturers have long relied on high-purity silicones to develop medical-grade products for a range of therapeutic applications. High consistency rubbers (HCRs), in particular, have a combination of mechanical properties and highly specific characteristics that give medical device manufacturers the versatility they need for these demanding applications.

Typically used to fabricate various components, such as tubing, balloons, sheeting, and molded parts, HCRs consist of a high molecular weight siloxane polymer combined with silica to produce a silicone that has a claylike consistency in its uncured form.

Why Manufacturers Choose HCRs for Fabrication

Silicones offer elastomeric properties coupled with the biocompatibility that medical manufacturers seek when specifying materials. However, these applications may also demand more complex capabilities than can be attained with thermoset or thermoplastic hydrocarbon-based elastomers. Medical device designers sometimes need materials able to achieve more specific — and often more rigorous — performance characteristics while maintaining design flexibility.

HCRs offer versatility for several reasons, including the following.

Established history of use in medical applications. HCRs were used in the hydrocephalus shunt, one of the first implantable devices. Since then, their track record of reliability gives established operations and start-ups alike the confidence that their HCR components can contribute to regulatory approval. Their long track record can be especially valuable for manufacturers creating a novel device, where the use of a well-established material can help the regulatory process run smoothly.

Fig. 1 - Cure profile for platinum-catalyzed HCR at various temperatures.

Biocompatibility. Because HCRs are available with varying degrees of regulatory support, depending on the specific material application, qualified products can be used in long-term (>29 days) implantable medical devices as well as for short-term implantable devices or external medical applications.

Very high mechanical properties. When compared with other elastomers, like liquid silicone rubbers (LSRs), the high molecular weight of HCRs’ polymers combined with the reinforcing fillers allow for desirable mechanical properties, like greater elongation, and a lower stress-strain modulus, both ideal for balloons.

Green strength. One of HCR’s unique properties is its claylike consistency in the uncured state. It can be formed into a dimension and will hold that shape until it is cured. This green strength is a key characteristic for medical device manufacturers because it’s ideal for a variety of fabrication methods, such as extrusion and calendering.

Cost efficiency. In general, the tooling used to mold HCR components is less complex and relatively inexpensive compared with other tooling. This makes HCRs ideal materials for low production volumes, as well as prototyping molded parts.

Customization. Experienced silicone providers can synthesize monomers and polymers to enable complete customization. For example, an HCR can be customized to have minimal hysteresis compared to standard HCRs. In other cases, a device manufacturer may want to pigment the material with color masterbatches supported with the same regulatory filings and biological testing support.

Understanding HCR Curing Systems

Adding to the material’s overall versatility, HCRs can be supplied with curing options that suit specific applications or processing methods. HCRs can be supplied uncatalyzed, allowing medical device manufacturers to incorporate their own preferred catalysts. Uncatalyzed HCRs are supplied in a one-, two-or three-part system, which can be ideal for a manufacturer that would like maximum flexibility to adjust work time, table life, or cure profile directly on the production floor.

Table 1. Platinum vs. peroxide cure HCR systems.

More commonly, HCRs are supplied precatalyzed in either a one- or two-part system, using either a peroxide catalyst or a platinum catalyst already incorporated in the system. Table 1 provides technical characteristics to assess when choosing between peroxide-cured and platinum-cured HCRs:

Peroxide catalyzed. Peroxide-cured HCRs have a long history in implant applications. It’s an excellent material for low-volume device or parts production. This one-part system contains either a vinyl, or a nonvinyl specific peroxide catalyst ready for use. In peroxide-catalyzed systems, curing is not initiated until the HCR is exposed to heat. In some peroxide cures, the process produces a mild acid as a byproduct, which must be removed with a high-temperature post-cure.

Platinum catalyzed, addition cure. Medical devices made with silicone material commonly use platinum-cure systems due to the fact that, unlike peroxide-catalyzed systems, there are no by-products of the cure mechanism, post cure is not required, and the end product experiences less shrinkage. During the curing process, platinum-catalyzed HCRs can also be heat-accelerated for increased throughput. Platinum-catalyzed systems commonly consist of two components: one with a platinum catalyst, and the other with cross-linkers and inhibitors.

Since many factors can influence conditions required to successfully process HCRs, optimizing these variables can ensure the fastest and most repeatable process possible. Temperature is critical when optimizing cure times based on processing needs. Formulation adjustments can customize the time needed for a platinum-catalyzed HCR to cure at a particular temperature. However, increasing or decreasing the processing temperature can also change the time required to achieve the desired state of cure for any given formulation.

Figure 2 depicts how changes to temperature affect the time needed to cure the silicone. There are practical limits to increasing processing temperatures to reduce cure time, so care must be taken to avoid issues such as scorch. Determining the optimal processing temperature is commonly done through experimental evaluation.

Understanding the Three Main HCR Processing Methods

Based on the desired end component, there are a variety of ways an HCR can be processed. The most common HCR processing methods include extrusion, transfer or compression molding, and calendaring.

Extrusion. If manufacturing continuous profiles, such as tubing, rod, or ribbon, extruding is often the ideal manufacturing process. Extrusion is a very efficient and relatively low-cost way to manufacture high-volume parts. Parts produced through extrusion can have typical wall thicknesses ranging from less than 0.01 inches up to 3.00 inches and have a wide range of elastomeric properties.

Transfer or compression molding. Best suited for creating solid or hollow parts, such as cuffs, O-rings, valves, or balloons, these processing methods provide an economic molding process for low- to medium-volume parts compared to LSR injection molding. Transfer and compression molding require the use of less expensive equipment, offering simpler mold/part design and less set up time. Although transfer and compression molding are both manual processes, they can still be productive, cost-effective ways to fabricate unique components.

Calendering. This processing method creates a flat continuous sheet with a uniform thickness that can be used for further processing (e.g., die-cutting). Calendering using HCRs offers a variety of processing options, depending on the final desired sheet, whether it is a single sheet of material being produced or whether the HCR is being applied to a substrate like fabric or between substrates, multi-layered.

The sheets can be provided in either cured or uncured forms. Uncured sheeting can be used as an adhesive between two devices or components. Calendered HCRs are typically in sheets with thicknesses ranging between 0.005 and 0.250 in. The sheeting is cured by applying heat after the calendering process is complete.

Choosing a Specialty Silicone Manufacturer

Material selection is about more than choosing a silicone with the right elastomeric properties and performance characteristics. Specialty silicone manufacturers have the capability to formulate a wide variety of silicones, from adhesives to elastomers and offer customization to help device manufacturers gain more control of their final component or device. Material selection depends on three key factors:

  • The intended use of the implant (external device or disposable device).

  • The cure profile (the need to control the HCR’s cure rate with an adjustable cure profile).

  • The table life (the need for a standard or extended work time).

It is essential for medical device manufacturers to collaborate closely with suppliers. A supplier should be brought into the process as early as possible, preferably during the product idea or design stage. An expert supplier can narrow down the material selection based on performance requirements for the application. The supplier can also help achieve performance of the end product, which is especially critical in a highly regulated industry like medical devices. During downstream processes, like assembly, a supplier can also help ensure that materials from different parts of the device are compatible.

Just as importantly, they can provide guidance and regulatory support that can lead to faster and smoother commercialization and more successful final end-use. A materials partner with established relationships with regulatory bodies across the globe can prove invaluable and save the device manufacturer time and money. From established medical use to high mechanical properties to cost-effective fabrication, HCRs offer manufacturers the versatility they need to produce safe devices that provide essential therapeutic support for people.

This article was written by John Freedman, Director of Biomaterials, and Alex Santayana, Applications Engineer, NuSil from Avantor, Radnor, PA. For more information, visit here .