Silicones are widely used in medical devices because they are biocompatible, biodurable (do not degrade over time), do not produce extractables (a concern with leaching plasticizers from flexible PVC), and are unlikely to cause allergic reactions (a concern with latex). Medical applications for silicones are far ranging from wound care gels to flexible drainage tubes to semi-rigid implants.
The diversity of silicone forms is attributed to the unique polymer chemistry. These synthetic polymers have a silicon-oxygen backbone, onto which organic side groups, such as methyl, are bonded. When discovered in the early 1900’s they were compared in structure to ketones, which resulted in the similar sounding name commonly used today (silicone). However, siloxanes or polymerized siloxanes are the true technical descriptions of these polymers, of which the most common is the silicone oil polydimethylsiloxane (PDMS).
Silicones can vary in consistency from liquid to gel to elastomeric to rigid by varying the silicon-oxygen chain lengths, using alternative side groups, and cross-linking of the polymer chains to each other. Cross-linking converts the inherently liquid or gel form of silicone into dimensional structures. These are thermoset structures that cannot be re-melted and processed into alternative forms like thermoplastics (e.g., PVC, nylon).
There are essentially three different methods of cross-linking silicones, depending on the desired process or application. ‘Cross-linking with radicals’ uses vinyl groups on the silicone chain and organic peroxide for cross-linking. This reaction has been used for high-consistency silicone rubbers (HCR). ‘Cross-linking by condensation’ uses hydroxyl end-blocked PDMS, excess saline, and typically tin catalyst, that results in cross-linking when exposed to moisture in ambient air. This is useful for sealants, such as those used for pacemaker leads (or household caulks). ‘Cross-linking by addition-curing’ uses vinyl groups on the polymer chain, a saline oligomer and a platinum catalyst for cross-linking. This two-part system, with the polymer and catalyst in one part and the oligomer in the other, is used in liquid silicon rubber (LSR) molding of medical devices.
The majority of silicone elastomers incorporate fillers to improve mechanical properties, such as strength or hardness, and reduce tackiness. The most common filler is fumed silica, an extremely small and amorphous particle (10nm diameter). Typically this is added to the silicone before cross-linking.
There are several different types of commercially available silicones; however, the most commonly used for medical devices are HCR and LSR. HCR involves the manufacture of solid silicone rubber into rolled sheets that are partially cured using peroxide catalyst additives. Extrusion of silicone shapes, such as tubing, involves feeding strips from these sheets into the extruder. Hot-air vulcanizing ovens are used to increase mechanical properties of the finished extrusion. HCR produces considerable scrap and involves high labor cost, but the cost of equipment and tooling is low.
LSR uses raw, uncured material in two parts that are mixed using metering pumps prior to processing. A platinum catalyst in one part begins the cross-linking and curing process, which is accelerated with the application of heat. Since the injecting and molding process are often highly automated, labor costs are relatively low. Mold design and fabrication is often the greatest capital expense for LSR processing. This technique is frequently used for precision medical components as well as over-molding applications.
Albright Technologies. “ Silicone: Expanding the horizon for modern medical devices.” Medical Device Outsourcing. June 2015
Colas, Andre and Curtis, Jim. “Silicone Biomaterials: History and Chemistry & Medical Applications of Silicone”. Dow Corning Corporation. 2005