An MD&DI November 1999 Column
A review of material properties and processing characteristics highlights silicone's enduring popularity for fabricating a range of medical products.
For those involved in medical product development, selecting a high-quality elastomer for critical applications can be a challenge. Designers, engineers, and managers must carefully evaluate a wide array of material properties and processing possibilities in order to meet demanding performance specifications and budget requirements. With so many materials and fabrication methods available today, it is often difficult to recognize the optimum solution, and the consequences of selecting an inferior material can range from lost time and money to total project failure.
To make the most informed decision, product developers should gather as many facts as they can about each material. This article is intended to provide a general overview of silicone rubber elastomers, including physical properties, fabrication methods, and potential advantages for device manufacturing.
CHEMICAL STRUCTURE AND CURE MECHANISMS
Since the 1960s, silicone rubber has found widespread use in medical, aerospace, electrical, construction, and industrial applications. Flexibility over wide temperature ranges, good resistance to compression set, a wide range of durometers, and inert and stable compounds are among the reasons for its popularity. Common silicone medical components and assemblies include airways; balloon catheters; tubing for feeding, drainage, and use with peristaltic pumps; compression bars; electrosurgical handpieces; infusion sleeves and test chambers; introducer tips and flexible sheaths; wire/fluid-path coextrusions; ear plugs and hearing aids; shunts and septums; and a variety of seals, stoppers, valves, and clips.
The unique properties of silicones enable them to be used for a variety of devices and components. Photo courtesy of Vesta Inc.
Silicone rubbers are synthetic polymers with an unusual molecular structurea giant backbone of alternating silicon and oxygen atoms. This structural linkage is similar to that found, for example, in a mineral such as quartz, and silicones have superior heat resistance compared with other elastomers. There are two popular catalyst systems used to cross-link silicone polymers: peroxide (free-radical) systems and platinum (addition-cure) systems.
Early-generation silicones used peroxide as the catalyst to initiate curing of the silicone. However, the peroxide reaction leaves an acid residue in the rubber that can deposit a powder or "bloom" on the part surface if not removed through a postcure oven baking process. Though peroxide is still used, addition-cured, platinum-catalyzed silicones have gained wide acceptance among fabricators because of their faster cure rates, lack of peroxide bloom, and availability in an injectable, liquid form.
Platinum-catalyzed (addition-cured) silicone is supplied to fabricators in a kit containing two components, which are mixed in a fixed ratio such as 10:1 or 1:1. The kit contains a catalyst, a filler, and polydimethyl siloxane polymer. Blending of these components forms a compound ready for the vulcanization process. Besides high-consistency (gumstock) silicone rubberwhich is also the form taken by peroxide-cured materialfabricators can purchase addition-cured silicone as liquid silicone rubber (LSR).
The strong silicon-oxygen chemical structure of silicone gives the elastomer its unique performance properties, including biocompatibility, superior temperature and chemical resistance, good mechanical and electrical properties, and natural clarity or translucence.
Biocompatibility. In extensive testing, silicone rubbers have exhibited superior compatibility with human tissue and body fluids and an extremely low tissue response when implanted, compared with other elastomers. Odorless and tasteless, silicones do not support bacteria growth and will not stain or corrode other materials. They are often formulated to comply with FDA, ISO, and Tripartite biocompatibility guidelines for medical products.
Temperature Resistance. Silicones can withstand a wider range of temperature extremes than nearly all other elastomers, remaining stable through temperature variations from 75° to 500°F. They can be sterilized via EtO gas, gamma or E-beam irradiation, steam autoclaving, and various other methods.
Chemical Resistance. Silicones resist water and many chemicals, including some acids, oxidizing chemicals, ammonia, and isopropyl alcohol. Concentrated acids, alkalines, and solvents should not be used with silicones.
Mechanical Properties. Silicone rubbers have high tear (to 250 ppi) and tensile (to 1500 psi) strength, good elongation (to 1250%) and flexibility, low compression set, and a durometer range of 5 to 80 Shore A. The softer forms of silicone have the ability to retain their softness indefinitely, with the softest durometers available in the form of reinforced gels.
Electrical Properties. Silicones exceed all comparable materials in their insulating properties as well as in their versatility for electrical applications. They are nonconductive and can maintain dielectric strength in temperature extremes far higher or lower than those in which conventional insulating materials are able to perform.
Molding. Silicone elastomers are typically molded by three main methods: liquid injection molding (LIM), transfer molding, or compression molding. The LIM process, often chosen for high-volume applications, employs lower pressures and higher temperatures than the other molding methods250 to 2000 psi injection pressure and temperatures of 245° to 485°F. By contrast, transfer and compression molding operate at pressures of 2000 to 8000 psi and temperatures of 200° to 370°F. In designing for the molding process, designers should take into account the material shrinkage rate, which can range from 2 to 4% depending on the type of silicone.
During molding, the three variables that must be controlled are temperature, pressure, and time. The temperature must be high enough to minimize cure times, yet low enough to prevent scorching of the elastomer. The pressure selected must allow for complete mold filling while permitting the escape of all the air, and must be optimized to prevent voids and flash. As in most molding, precise timing of all functions is critical for the production of consistently high-quality, fully cured parts.
Figure 1. The LIM process comprises meter mixing followed by mold forming/vulcanization.
Liquid Injection Molding. LIM offers many benefits in the fabrication of silicone rubber, including cleanliness and speed. In the LIM process, pumping systems deliver the two-part liquid silicone (catalyst and crosslinker) directly into a mixer for homogenization and then directly into the mold cavity, in a completely closed process (Figure 1). Molding and vulcanization (curing) occur rapidly within the mold cavity at a high temperature.
Overall, injection can take as little as 3 to 10 seconds, whereas molding and vulcanization take from 10 to 90 seconds or more, depending upon shot weight and ultimate section thickness of the part.
LIM minimizes contamination due to its closed process. Additionally, because it employs a single automated step, it provides consistent part quality with less chance for variation or human error. Other potential benefits of LIM include little material preparation labor, lower injection pressures, faster cycle rates, and the availability of fully automated systems.
Transfer and Compression Molding. Both transfer and compression molding of silicones are widely accepted and often used for medical products. Unlike the LIM process, transfer and compression molding are more labor intensive and require separate premixing of the rubber on a two-roll mill. Also, because these processes must be operated at lower temperatures, operating cycles are longer. Given the slower curing times, it is not uncommon to see large molds with 100 cavities or more for diaphragms, bottle closures, O-ring seals, and other applications.
In transfer molding, a hydraulic ram displaces rubber through the gates and sprues into the cavities. Compression molding differs in that the rubber is physically placed into the cavities and it is the closing action of the mold that completes the fill.
Figure 2. Extrusion steps include (left to right) mill blending, extrusion feed, pin and die profile forming, and vulcanization.
Extrusion. High-consistency silicone rubbers can be extruded to yield a broad range of tubing and profiles (extrusion is not generally feasible with liquid silicones). The extrusion process begins with the two-part gumstock (catalyst and crosslinker) being blended on a two-roll mill (Figure 2). The blending yields a homogeneous compound, which is formed into strips and fed continuously into the extruder. A variable-speed screw feed is used to maintain proper pressure at the pin and die. Once extruded, the tubing passes through hot-air vulcanization ovens, in which heated air or radiant heat cures the product. During the extrusion process, laser micrometer checks are often performed to help ensure proper dimensional control.
The extrusion process is able to produce single-lumen, multilumen, and coextruded tubing in a variety of diameters. Profiles or nonround cross sections can also be made from silicone for such applications as instrument stands, clips, gaskets, seals, ties, and markers. Specialized properties include x-ray opacity in stripe or opaque form, to enhance product visualization, and reinforcing to provide added strength, electrical conductivity, and kink or stretch resistance.
Examples of extruded silicone products include catheters, drain and fluid-path tubes, gaskets, ribbon, sheathing, balloon cuffs, and coextruded electrical conductors with integrated fluid-path lumens.
Figure 3. An example of a multicomponent silicone assembly, the catheter pictured above was produced using molding, wire encapsulation, bonding, tip beveling and coating, and hole drilling.
Assembly. There is almost no limit to the configurations in which two or more silicone rubber components can be joined to create assemblies for particular functions (Figure 3). The most common assembly methods for joining multiple silicone subcomponents include insert molding and bonding. The insert molding process involves injection molding around an existing part or parts. Bonding normally entails joining one or more silicone components together with silicone adhesives. Other assembly methods include tipping, reinforcing, dipping, and cuffing.
Silicone tubing can accommodate a range of lumen configurations to conform to application requirements.
Some silicone fabricators can provide subcomponent assembly in special environmentsfor example, cleanrooms and HEPA-filtered facilitiesto meet OEM cleanliness requirements.
Secondary Operations. Full-service silicone fabricators can offer a range of secondary operations to satisfy specialized product requirements. Among such processes are silk-screening, slitting, punching, beveling, bundling, and functional testing.
Silicone rubber compares favorably with a number of other materials that might be considered for similar device applicationsespecially in the critical area of biocompatibility, but also regarding a variety of physical characteristics. For example, silicone offers greater clarity, better electrical-insulation properties, and superior lot-to-lot consistency than latexnot to mention the concerns regarding allergic reactions in latex-sensitive individuals. Silicone is more inert than PVC and contains no additives (such as plasticizers) that can leach out of the material. In general, silicone is clearer, more stable over a broad temperature range, and has a lower compression set than polyurethane, besides being provided in softer grades. And compared with most thermoplastic elastomers, silicone provides enhanced chemical resistance as well as more options for sterilization.
When evaluating silicone rubber as a potential material for a medical component or product, manufacturers should examine their possible need for design assistance, prototyping, material and part testing, and determination of cost-effectiveness. Whereas silicone rubber may have a higher cost per pound than other common elastomers, the potential for tooling, prototyping, and manufacturing efficiencies with silicone can often help manufacturers realize substantial savings.
Precision inspection capabilities are among the qualifications that manufacturers should expect in a silicone fabricator. Photo courtesy of Vesta Inc. (Franklin, WI)
Once a device company has determined that silicone rubber is the right material for a part and has selected a fabrication process, the firm should consider diverse qualifications when choosing a silicone fabricator.
Engineering and Design. A potential fabricator should be well staffed with experienced engineers who can help a company refine its concepts and design the custom tools to produce them. The fabricator should offer a choice of prototyping methods and be able to provide the required secondary operations.
Materials Expertise. Does the fabricator have the expertise and experience to aid in selecting the proper silicone compound for a particular application, and can it supply that grade? Engineers should also be available to help evaluate the physical specifications of the product and determine the optimum process parameters.
Manufacturing Capabilities. An important qualification is whether the fabricator maintains manufacturing facilities compliant with the quality system regulation. Depending on the project, it may be required for the vendor to be capable of both short and long runs and both low and high volumes. Does the vendor have advanced molding and extrusion equipment, including LIM equipment? Does it offer assembly, wash, and packaging operations? If cleanliness is an important specification for a part, does the vendor have a controlled environment in its manufacturing area?
Quality. With quality demands becoming more stringent throughout the device industry, a preferred fabricator should practice audited quality control production in its facility. Other important capabilities include the availability of advanced inspection equipment such as video microscopes and laser micrometers and the experience in raw-material testing, in-process inspection, statistical process control, and end-product testing. The vendor should also be able to perform 100% inspection or testing, when specified.
Silicone offers a range of well-established benefits for the production of medical products, including superior biocompatibility, good mechanical characteristics, chemical and temperature resistance, and processing flexibility. With its long history of successful use in the industry and unique combination of properties, silicone is well positioned to meet the ever more demanding material requirements of device manufacturers.
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Winn, Alastair. "Factors in Selecting Medical Silicones." Medical Plastics and Biomaterials 3, no. 2 (1996): 1619.
Wolf, Byron E. "Comparing Liquid and High Consistency Silicone Rubber Elastomers: Which Is Right for You?" Medical Plastics and Biomaterials 4, no. 4 (1997): 3440.
Charles Heide is market development manager at Vesta Inc. (Franklin, WI). The company manufactures silicone products through extrusion, liquid injection molding, and transfer molding.