Silicone elastomers have long been a popular material for medical devices and medical device components due to many factors, including durability, ease of molding by many methods, wide temperature range, chemical inertness, high tensile strength, vast range of available durometers, low toxicity, and compatibility with many sterilization methods. Furthermore, silicone is compatible with human tissue and body fluids, has a very low tissue response when implanted, and does not support bacteria growth — making it a perfect option for implants due to its excellent biocompatibility.
Silicone elastomers are available in two commercial forms: liquid silicone rubber (LSR) and high consistency rubber (HCR). HCR is known for its gummy consistency and mostly comes in partially vulcanized sheets. LSR is a newer technology and starts out as a two-part liquid that cures into a solid form when mixed. LSR generally comes in buckets and has a longer shelf life than HCR.
Medical device OEMs often face a tough decision: should we use HCR or LSR for our medical device component manufacturing? LSR and HCR are both used to manufacture medical device products; however, there are some key differences. The following compares LSR and HCR to shed some light on their differences and when each should be utilized.
Viscosity Difference Leads to Different Manufacturing Techniques
The performance characteristics of HCR and LSR are relatively similar; however, viscosity is a key differentiator between LSR and HCR, and has a significant impact on the equipment and processes used to manufacture each of these elastomers.
Simply put, viscosity is a measure of a material’s ability to flow. A low viscosity indicates a material is less viscous and more readily flows where a high viscosity indicates a material is more viscous and less apt to flow well. For reference, water has a relatively low viscosity and easily flows whereas molasses has a higher viscosity and is more resistant to flow.
LSR has a lower viscosity than HCR. Due to the lower viscosity, LSR is most often processed via injection molding. LSR’s desirable handling properties and lower shrink rate make it an excellent choice for manufacturing highly complex geometries and intricate products. Additionally, due to the automated nature of injection molding, LSR can produce high volumes of components in a short period of time. For this reason, deciding whether HCR or LSR injection molding is the better choice for a project largely depends on the production volume required.
A lower viscosity makes it easier for manufacturers to mix additives into LSR. Additives that can readily be incorporated into a batch of LSR include colorants, desiccants, barium, and pharmaceuticals such as hormones or steroids. For these reasons, LSR is a great option for medical devices such as combination products. The low viscosity of LSR and the temperatures needed to vulcanize LSR are usually low enough that significant degradation of compounded substances, like active pharmaceutical ingredients (APIs) that are used in combination products, can be avoided.
Due to its higher viscosity and more challenging handling properties, HCR is typically processed using compression and transfer molding methods, which are more labor intensive. In some cases, HCR is used in injection molding projects.
For companies already using HCR to manufacture medical device components, it may make sense to continue using this elastomer especially since the initial capital equipment costs have already been made. For new product development, however, LSR is often the best choice given the lower capital costs and labor associated with processing this elastomer. Because of its lower manufacturing cost and versatility with formulations, companies often prefer LSR over HCR — but the decision is on a case-by-case basis.
ProMed, for example, combines medical-grade LSR and HCR expertise with the latest developments in silicone materials and technology. The company’s expertise in implantable silicone components and assemblies enables it to help OEMs incorporate the latest medical-grade silicone formulations in - to their designs to deliver rapid silicone prototypes.
Materials include LSR from 5 to 80 durometer, HCR from 20 to 80 durometer, and room temperature vulcanizing silicone (RTV). An expert assists OEMs in material selection to help ensure that all design requirements are met. The company’s manufacturing facilities and equipment are designed for specifically for molding medical and implantable silicone, combination components, and biomaterial grade plastics.
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