Streamlining Medical Device Development and Manufacturing Using Silicone Elastomers

Matthew Kihara, NuSil Technology LLC — part of Avantor

Silicone elastomers have become a vital material in the medical device industry due to their unique properties, including biocompatibility, durability and chemical inertness. Silicone materials are categorized based on their unvulcanized consistency, which significantly affects their processability and their physical properties. This article compares high consistency silicone rubbers (HCRs), liquid silicone rubbers (LSRs), and low consistency elastomers (LCEs), analyzing their characteristics and the implications in selecting each during different phases in the development of silicone medical devices.

High Consistency Rubber (HCR). HCR is a silicone elastomer manufactured with a blend of fillers and high molecular weight silicone polymers, which result in a solid, gum-like consistency in its unvulcanized state. It is primarily used in extrusion, calendaring, and compression molding processes due to its very high viscosity.

The advantages of HCRs include their high tear strength, resistance to deformation under stress, and their ability to retain shape during processing prior to curing due to their high green-strength. However, the thick gum-like consistency of HCRs makes them less suitable for intricate designs, including complex shapes with thinwalled sections, and they require the use of a two-roll mill for mixing the Part A and Part B components together.

Liquid Silicone Rubber (LSR). LSR is a silicone elastomer manufactured with a blend of fillers and medium molecular weight silicone polymers, which result in a thick paste consistency in its unvulcanized state. It is primarily used in liquid injection molding processes but may also be used in transfer molding and knife-coating applications.

The advantages of LSRs include their ability to mold strong parts with intricate designs with thin-walled sections while using high-volume injection molding processes. However, the thick paste-like consistency of LSRs make them less suitable for lab-scale mixing and molding during the initial stages of feasibility and development.

Low Consistency Elastomers (LCE). LCE is a silicone elastomer manufactured with a blend of fillers and low molecular weight silicone polymers, resulting in a flowable liquid-like consistency in its unvulcanized state. It is primarily used in transfer molding processes but may also be used in knife-coating and potting applications.

The advantages of LCEs are that their low viscosity allows them to be easily used for lab-scale mixing and molding, as well as for potting and filling applications. However, the notably lower physical properties of low consistency elastomers and the difficulty to use in liquid injection molding equipment often make them less suitable for large-scale manufacturing.

Comparison of Long-Term Implantable Silicone Elastomers

When comparing the types of platinum curable silicone elastomer options, it is also critical to understand the differences in physical properties that each type exhibits. Table 1 shows typical properties of three ~30 shore A durometer long-term implantable silicone elastomers. As shown in the table, HCRs will typically exhibit the highest cured physical properties, followed by LSRs, when comparing elastomers with similar softness (durometer).

Material and Process Selection

During the feasibility and research and development phases of new device development, manufacturers must evaluate the suitability of these silicone elastomers for their specific medical device applications. Factors such as the desired product properties, processing methods, production scale and regulatory requirements all play a crucial role in material selection.

For example, HCRs may be preferred for applications requiring high tear strength and elongation which have designs that can be extruded or compression-molded, while LSRs may be more suitable for intricate high-volume molded designs with tight tolerances. LCEs will often be restricted to the feasibility and prototyping phases for molding applications due to difficulty scaling to high-volume production; however, they are often preferable for potting and filling applications.

The selection of the molding process is critical in the development of silicone medical devices. HCRs are typically processed using extrusion or compression molding, while LSRs are commonly processed using injection molding. The choice of molding process is influenced by factors such as the desired product properties, production volume, and cost considerations.

During the commercial production phase, manufacturers must ensure that the selected silicone elastomer and molding processes are scalable and cost-effective. HCRs, LSRs, and LCEs each have their advantages and disadvantages in terms of scalability and cost-effectiveness. For example, LSRs may be more suitable for high-volume production due to their ease of processing, but require investment in silicone liquid injection molding equipment, while LCEs may be more cost-effective for low-volume production as parts may be molded by using simple transfer molding processes.

Silicone Expertise

For medical device manufacturers, there are advantages to collaborating with a silicone supplier, such as NuSil^®, with extensive experience in formulating silicone elastomers for medical devices. For these applications, the supplier 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 and Drug Master File submissions (MAFs and DMFs).

A silicone supplier with established regulatory body relationships can provide significant value to medical device manufacturers by offering cost and resource efficiencies while aiding in the device’s regulatory path to market.

Conclusion

Understanding the similarities and dissimilarities between HCRs, LSRs and LCEs is essential for streamlining each phase of silicone medical device development. By carefully evaluating the specific properties and applications of each material, manufacturers may be able to optimize the development process, ensuring efficiency, cost-effectiveness and product quality. Furthermore, considering factors such as the desired physical properties, molding processes and regulatory compliance will enable manufacturers to make informed decisions, ultimately leading to the successful development of silicone medical devices.

This article was written by Matthew Kihara, Senior Applications Engineer at NuSil Technology LLC — part of Avantor, Carpinteria, CA. For more information, e-mail This email address is being protected from spambots. You need JavaScript enabled to view it. or visit here  .

---