Silicone tubing has made significant strides in the medical device market over the last five years. From surgical instrumentation to implantable drug-delivery systems, silicone tubing is methodically replacing tubing made from more-traditional plastics and thermoplastics such as polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), and nonplastics such as urethanes. Medical device designers are drawn to silicone tubing for a variety of reasons, including biocompatibility and ease of manufacture relative to plastic. In addition, silicone often offers flexibility in design that does not sacrifice quality or durability.

The migration from thermoplastic tubing to silicone tubing is occurring for several reasons. From a biocompatibility standpoint, silicone scores very well. This is primarily due to the following characteristics of the material:

•Low toxicity.

•Physiological inertness.

•Low occurrence of leachables and extractables.

•Hypoallergenicity.

Silicone has been widely used in both implantable applications such as defibrillator and pacemaker lead covers and stents and nonimplantable applications such as wound drainage. The material has also found various applications in biological sensors, prosthetics, and drug-delivery systems. It is possible that the long-term implantable product market has played a more significant role in the drive for silicone tubing than previously thought. Medical device manufacturers may look to differentiate their product from those of competitors by opting for silicone tubing instead of more traditionally used materials.

Silicone tubing offers medical OEMs a growing selection of custom options. It can be provided in a variety of shapes and sizes, single or multiple lumens, ribbons, and combination materials. It is also available with balloon properties and variable cross sections that are not available in products in the plastic tubing market. Designers can also take advantage of its gas permeability properties. Another unique quality in the processing of silicone tubing is that unlike thermoplastic extrusion, water does not come into contact with silicone during any step of the extrusion process. (In the extrusion of thermoplastics, water is required to size and cool the product.) Water can potentially add pyrogen contaminants to the material, which can negatively affect the biocompatibility properties of the tubing.

Silicone Extrusion

Silicone tubing is available in a wide durometer range (20–90 Shore A hardness), and its color characteristics can include clear, translucent, or opaque versions. In addition, radiopaque tubing in either a striped or solid form can be produced from silicone. The material can be continuously extruded to encompass a broad spectrum of sizes. For example, silicone can be extruded for very small diameters and tolerances (e.g., 0.01-in. outer diameter, ±0.0005 in.). Conversely, the material can be extruded to sizes exceeding 3 in.

The tubing can also be tapered (bumped), a process in which the inner or outer diameter is stretched down to smaller diameters and then back up to original size. This is useful in applications in which the tubing may need to be more rigid or withstand higher pressures on one end of the tubing, or where a larger profile would facilitate insert molding operations. Some of these applications include peripherally inserted central (PIC) catheters and 7 FR double-D insert-molded tubing. Both inner and outer diameters can be adjusted by changing the shape or size of the tube on the fly, prior to curing. Extruders can accomplish this by moving the extrusion mandrel or changing the die shape.

Designers sometimes desire a specific shape along the tube length. For example, peritoneal dialysis tubing may require a spiral shape to aid in device functionality. Because silicone is a heat-cured material, partially cured or uncured extruded profiles can be placed into forming trays or other specialized molds to help set a specified shape without altering physical properties or crucial dimensions. Expertise gained from working experience with silicone becomes pivotal in achieving desired results, through a combination of extruding, shaping, and curing techniques.

Braiding and Reinforcement

Silicone tubing carries a reputation for not being able to withstand extreme pressures as well as comparably sized thermoplastic tubing. However, a designer requiring a softer or more flexible tube that still has resistance to pressure can specify a reinforced version. Silicone extrusions can be embedded or reinforced by braiding or spiral winding with a variety of materials. Polyester and stainless steel are common supplemental materials.

Braiding occurs as a secondary operation and provides kink resistance as well as an increased working pressure range. The reinforced silicone tube's inner layer is extruded (either hollow or over a core), braided or spiral wrapped, then reextruded with an additional layer of silicone, encapsulating all the components or layers. The process can be repeated for multiple layers.

Due to advances in precision braiding equipment, very small stranded materials and steel wire or ribbon can add significant pressure resistance, kink resistance, and rigidity to small extruded shapes. Braiding requires a minimum inner tubing wall thickness and a minimum encapsulation thickness. Dimensional tolerances are typically larger due to stack-ups, which is a term used to describe the addition of different tolerances that must be considered in the production and functionality of each layer of the part. Core removal may also need to be considered to limit piece length.

A Sampling of Applications for Silicone Tubing

Silicone is versatile (see the sidebar, “A Beginner's Guide to Silicone”) and has high dielectric strength. It can be electrically insulating or conductive, and manufacturers can harness these potential properties by controlling the additives used during compounding. Silicone is also semipermeable to certain materials, which is helpful in drug-eluting applications. It is gas permeable, a useful trait for devices used to grow cell cultures.

Silicone multilumen tubing is used extensively in long-term applications such as cardiac rhythm management. Multilumen silicone tubing is the conduit for electrical leads and provides an insulated path for signals from the pacemaker or defibrillator. It has also found use in short-term applications such as urinary catheters and PIC catheters.

Processing the Material

Typically, extrusion outer diameters are round, but other shapes are possible through wire EDM technology. Interior lumens do not need to be round or balanced, although great care must be exercised to calculate the total stack-up of dimensioned tolerances. Most applications require strict attention to the wall thickness of each lumen, but the webbing and space between the lumens is often neglected. Frequently the lack of land area and eventual loss of total tolerance is not discovered until the tubing manufacturer reviews design specifications for design of the die and mandrel tooling. The Rubber Manufacturers Association's tolerance guidelines are a good starting reference, although precision silicone extrusion manufacturers typically hold significantly tighter specifications. Even though lumen location variations of a few thousandths of an inch may be inconsequential to the functioning of the part, such details are crucial to both tooling design and the process for the extruder and, by extension, for the designer.

Radiopaque stripes are required for many tubing applications that require radiographic (x-ray) confirmation of placement within the body. These stripes can be coextruded into the tube wall (either embedded or surface location) with precision, allowing lumen geometry and placement to take precedence. Lumens used for single-lumen or multilumen tubing may be lined with wire, PTFE, PEEK, or other tubing.

Another characteristic of silicone tubing is its ability to separate multilumen tubing for multifunctional applications. This strippable tubing is achieved by including a thin web (e.g., 0.007 × 0.010 in.) between discrete tubes. Multilumen tubing, with certain design restrictions, can transition to fewer lumens or change lumen size and location within the same tube. This has multiple uses in the wound-drainage market, for example.

Silicone extrusions are ideal for prototype development—not only because they allow for easy feasibility testing but also because start-up costs are manageable. Silicone is a thermoset material that is extruded cold and heat cured. The process allows the extrusion operator to make quick setup and die and mandrel adjustments without the high-temperature hazards inherent with thermoplastics. Silicone extrusion also avoids problems with moisture, pellet mixing and melting, and scorch that challenge plastic operations—resulting in potentially fewer process variables.

Identifying and controlling process variables is critical to precise extrusion operations. Silicone processes, whether molding, extrusion, or sheeting, require accurate, stable, and repeatable processes and equipment that minimize variation. Examples of variables are screw speed, infrared oven temperature, die pressure, pot life of catalyzed material, etc. The goal is to minimize such variations because they affect the final dimensions, yield, and the designer's satisfaction with both product performance and cost.

Limitations

Although characteristics such as inertness and ease of manufacture make this product attractive, silicone does have some limitations, including lower tear strength when compared with its urethane counterparts. It also has a lower burst pressure in an unreinforced state. Silicone tubing also has limitations with respect to the type of fluid that is passing through it. It is not recommended for use with concentrated acids or bases, organic solvents, or oils. However, silicone compounders are working on improved silicone chemistry to enhance its appeal to OEMs.

Establishing Partnerships

When choosing an extrusion provider, medical OEMs should seek out the following capabilities:

•Extrusion tooling design expertise.

•Engineering support.

•Engineers and operators with an extensive silicone extrusion background.

Extrusion R&D capabilities that provide short turnaround times should be a top priority, especially when determining the feasibility of several designs. Selecting fabricators that have good relationships with raw silicone material suppliers, as well as those possessing a thorough understanding of available silicone compounds, is key. Partnerships should be formed with companies that are willing to invest in the latest technology and test and develop new processes.

Conclusion

The market for silicone tubing seems ripe for growth due to its use for wound-drainage devices, catheters, and other easily sterilized disposable products. Silicone enables the creation of products that can be provided in a multitude of designs, shapes, and hardnesses. The ability of silicone tubing to meet current and emerging demands and help forge new ideas depends heavily on the medical OEM's ability to receive consistent, high-quality product. When that happens, OEMs can focus on the more difficult functional aspects of their devices and trust the product designer to understand and apply silicone extrusion processes. Both the superior product and the designer's thorough understanding can be achieved through a partnership, with good communication, between the silicone raw material supplier, extrusion specialist, and the device designer.

Pradnya Parulekar is custom manufacturing services brand market manager for Helix Medical LLC (Carpinteria, CA). Reach her at [email protected].

Copyright ©2009 Medical Device & Diagnostic Industry