With the variety of tubing options available to medical device manufacturers, it is often difficult to determine which type would be best suited to a particular application. This article highlights the benefits and physical properties of silicone elastomer tubing and provides a comprehensive look at its most prominent indication.

On the cover: Fluid delivery tubing made from GE Advanced Material's tufel III high consistency elastomers.

By Mel Toub and Sharon Shatto

AT A GLANCE

• Physical characteristics
• Curing options
• Peristaltic pump tubing
• Testing results

The constant evolution of new medical devices designed to meet the growing needs of our aging population has increased the demand for reliable elastomeric components. Many devices are now designed not only for utilization in hospital and long term care facilities, but also for ambulatory and in-home use. The appropriate use of devices in contact with the human environment has thus become increasingly important. The distinctive properties of silicone elastomers—purity, clarity, strength—coupled with ease of processability, have resulted in its usage in a broad range of healthcare tubing applications such as fluid and drug delivery tubing, enteral feeding pumps/devices, wound drains, anesthesia/respiratory tubing and circuits, and catheters.

GE Silicones' LIM2 6050-D2 LSR for liquid injection molding provides design precision for ZEVEX’s EnteraLite® Infinity® feeding pump and tubing.

Numerous silicone formulations exist, such as platinum and peroxide cure high consistency elastomers and liquid silicone rubbers, which have been tested to USP Class VI, ISO10993, and/or tripartite biocompatibility1 requirements. Silicone tubing can be supplied in a variety of configurations including single or multi-lumen designs, reinforced constructions for elevated pressure applications, and radio-opaque striping for x-ray detection. The high thermal stability of silicone has resulted in its use in applications requiring repeated sterilization performance where other elastomers have limitations. In addition, its low compression set properties and high resilience coupled with low extractables has resulted in silicone gaining in popularity in highly dynamic, peristaltic pump tubing applications. The absence of allergic ingredients coupled with its freedom form migratory plasticizers has helped the usage of silicone elastomer to grow in areas that were traditionally served by lower cost materials such as PVC and latex.

This article will review current technology related to the manufacture and production of silicone tubing used in the healthcare and pharmaceutical industries, highlighting recent advancements in peristaltic pump tubing that extend pump life and improve delivery accuracy. In addition, the basic cure chemistry of silicone elastomers will be presented, with emphasis on platinum cured systems that offer the optimum combination of physical attributes and biocompatibility.

Peroxide vs. Addition Cure

Organic peroxide curing agents were used in conjunction with hot air vulcanizing (HAV) ovens to vulcanize early generation silicone tubing. These ovens may be multi-stage horizontal units utilizing conveyor belts to support the extrudate as it passes through the heated tunnel, or vertical units that require no supporting conveyor, and therefore, have the advantage of not marking the uncured profile. The curing/crosslinking mechanism is straight forward and consists of thermal decomposition of the peroxide, forming free radicals which then abstract hydrogen atoms from pendant methyl and/or vinyl groups. The methyl and vinyl radicals then combine to form a crosslink between the participating siloxane polymer chains (Figure 1). Although several organic peroxides will vulcanize silicone rubber, only 2,4 dichlorobenzoyl peroxide offers a tight surface cure with no tackiness or porosity and reasonable odor when cured. This particular peroxide has a one hour half-life at 73°C and is therefore, extremely efficient at inducing crosslinking at typical HAV temperatures of 1,000 to 1,400°F.

Figure 1: Peroxide cure mechanism (click to enlarge)

Figure 2: Platinum cure mechanism (click to enlarge)

The use of 2,4 dichlorobenzoyl peroxide has two potential drawbacks. The first is environmental in that a small amount of polychlorinated biphenyls are generated as a by-product of the crosslinking reaction. The second drawback is that the primary decomposition product, 2-4 dichlorobenzoic acid, manifests itself as a white crystalline powder that tends to diffuse to the surface of the tubing, causing a white film known as bloom. To avoid future reversion of the rubber and to meet general healthcare requirements, it is required that this material be eliminated by postbaking the tubing in forced hot air convection ovens for several hours at elevated temperatures. The precise postbake cycle is a function of tubing dimension, number of coils, air flow, etc., but typical cycles are often eight to 12 hours at 350 to 450°F.

Addition or platinum cured silicone rubber offers an increasingly popular alternative to peroxide cure, especially for healthcare applications that require the utmost purity and cleanliness of materials. The name “addition cure” derives from the vulcanization reaction that consists of a multi-functional silicon hydride crosslinker that, in the presence of a precious metal catalyst such as platinum, adds across the double bond of the vinyl group on the siloxane polymer chain. Since the crosslinker contains several hydride groups, it can react with consecutive chains, thereby forming the crosslink (Figure 2).

This crosslinking reaction proceeds quite vigorously at room temperature, so it is essential to incorporate an inhibitor into the system that inactivates the platinum catalyst at room temperature thereby preventing premature cure. As the temperature is increased, the inhibitor is driven off, allowing the catalyst to function as a crosslinking promoter. Typical inhibitors are materials that have higher affinity for platinum than the siloxane polymer vinyl group and include such materials as peroxides, hydroperoxides, and acetylenic derivatives. These types of materials along with sulfur compounds and amines can also act as permanent catalyst deactivators if not controlled, so it is important that addition cure silicones be mixed in clean environments, preferably separated from areas using organic materials.

Traditionally, platinum cured silicone elastomers are supplied as two component systems which are mill blended prior to fabrication. The shelf life of these materials, once blended, is relatively short, so these materials are usually extruded and cured within several hours of blending to avoid premature structuring. Recent advancements in platinum catalyst technology, however, have resulted in new one-component, addition cured silicone elastomers. These materials are supplied to the fabricator in a “ready to use” formulation in which the catalyst and inhibitor are premixed into the base rubber thus eliminating the need to mill blend. The typical shelf life of these materials is three months and offers the fabricator “out of the box” convenience and cost savings in a one-component material.

Peristaltic Pumps

Peristaltic pumps, also referred to as dispensing pumps, dosing pumps, and flexible tube pumps, consist of a continuous length of flexible tubing that resides in a circular channel in a rigid housing. A motor-driven rotor containing multiple rollers alternatively compresses the tubing and releases it, resulting in transfer of a given volume of fluid determined by the tubing inside diameter, roller spacing, and rotor speed. The primary advantage of this approach is that the fluid only contacts the inside surface of the tubing so that the fluid is never contaminated by the pump mechanism, and, conversely, the pump mechanism is not contaminated by the fluid. These types of pumps are popular in medical applications where transfer of product can be achieved without damage or contamination, such as delivering nutrients and medications to patients, movement of blood during open-heart surgery, and in pharmaceutical production processes.

Table 1	Traditional Platinum	Peristaltic Pump Platinum
Shore A, points	50	50
Tensile Strength, psi	1300	1100
Elongation, %	900	450
Tear B, ppi	250	100
100% Modulus, psi	190	200
Compression Set, % (22 hrs @ 177C)	50	30
Bayshore Resilience, %	48	61
Hysteresis, %	53	41
Specific Gravity	1.15	1.12

In order to function properly, the tubing in a peristaltic pump must be totally compressed to provide sufficient vacuum on the inlet side. This results in differential flexural stress in the tubing walls, with the inside tubing surface in compression and the outside surface in tension. These stresses across the tubing wall can eventually lead to rupture which ultimately depends upon the tube geometry, degree of compression, or occlusion of the rubber, rotor speed, and material characteristics. Short of catastrophic failure (i.e., cracking), tubing can also experience permanent deformation resulting in transformation of the original circular cross-section to an elliptical configuration, which, in turn, impacts flow rates.

Silicone Rubber for Peristaltic Pump Tubing

Traditionally, high tear strength, peroxide cured silicone rubber has been the benchmark for peristaltic pump applications, offering long pump life and tight dimensional tolerance control. Peroxide cured silicone, however, requires postbake to remove acidic by-products and is also generally associated with higher levels of extractables compared to platinum cured silicone rubber. Most commercially available platinum cured silicone elastomers, however, were designed to maximize extension and tear strength with little attention to properties such as elastic memory, compression set, resilience, and hysteresis. Since peristaltic pump tubing can experience millions of compressive cycles, these latter characteristics impact the elastomer’s resistance to dynamic fatigue failure and thus, most strongly influence tubing longevity. Most recently, a new class of platinum cured silicone elastomers has been introduced which optimizes these critical properties (Table 1).

Pump Testing and Performance Validation

Figure 3: Pump life at 600 RPM using a Cole Parmer peristaltic pump with a dual LS pump head and 3/8 in. OD x ¼ in. ID tubing to achieve a 1.7 L/min flow rate (Click to enlarge).

Two commercial grades of standard platinum cured silicone tubing and a benchmark peroxide grade were compared to the platinum cured peristaltic pump grade using a Cole Parmer Masterflex L/S variable speed peristaltic pump with 0.38 in. OD, 0.25 in. ID nominal tubing size. The pump head contains three rollers that compress the tubing to 20% occlusion. The pump was run with water at room temperature at maximum speed of 600 RPM corresponding to a nominal flow rate of 1.7 L/minute, so that the tubing is compressed 1800 times per minute and the test is run until tubing rupture (Figure 3). The two commercial Pt cured tubing samples displayed a range in pump life of 55 to 78 hours. The peroxide cured tubing lasted 218 hours, and the platinum cured pump specific grade lasted 209 hours, essentially equivalent to the peroxide cured material. Post baking extended the tubing life to 290 hours, bettering the benchmark peroxide cured grade (also postbaked) by 33%.

Injection Molded Tubing Segments

Historically, medical device applications have incorporated silicone rubber tubing that has been extruded into a continuous cylindrical profile cut to an appropriate length to accommodate specific device requirements. While dimensional tolerances on ID and wall thickness can be held to within several thousandths of an inch, some cyclical variation is to be expected with a continuous process. Injection molded silicone rubber (LIM 2) offers an alternative process for making high quality, cost effective elastomeric parts and has recently been used to make a silicone rubber tube for disposable peristaltic pump cassettes used for enteral feeding (ZEVEX EnteraLite® Infinity® Feeding Pump; GE silicones LIM6050-D2). LIM is a two-component, platinum curable, and pumpable silicone elastomer that can be molded and cured at elevated temperatures using injection molding machines specifically designed to process thermoset materials. In contrast to extrusion, the molding process allows for complex part geometry and exacting dimensions. In the enteral pump example, a tubing segment with molded-in flanges and posts was designed that tightly integrate the tube with the pump and sensing system. This ability to more closely control the tube geometry enables creative new pump designs with improved portability and excellent fluid delivery performance. As a result, LIM will experience an expanded role in many future innovative healthcare tubing applications.

Conclusion

Platinum cured silicone rubber is an excellent candidate for many healthcare and pharmaceutical applications and the recent introduction of one-component materials facilitates its use in both extrusion and molding applications. Platinum cured silicone elastomers can be designed to provide peristaltic pump performance on par with commercial peroxide cured materials. These new silicone elastomers possess the traditional advantages of low extractables, freedom from peroxide by-products, and smooth surfaces. In addition, they display good elastic memory, low hysteresis, and low compression set for improved pump tube longevity and flow rate accuracy. Injection molded grades of silicone rubber offer a fresh alternative that incorporates new functionality and improved dependability into medical devices.

Notes

1 USP Class VI equivalent test were tested according to USP Class VI methods: Intramuscular implantation, intracutaneous injection, and systemic injection. ISO 10993 for <29 days contact duration only—products tested for: Systemic Injection, Intracutaneous Injection, Intramuscular Implantation, Hemolysis (Rabbit Blood), Kligman Maximization, Ames Test, L929 MEM Elution, and Agar Diffusion.
2 LIM is a registered trademark of General Electric Company.

ONLINE

For additional information on the technologies and products discussed in this article, see the following websites:

www.gesilicones.com

www.zevex.com

Mel Toub is the Elastomers Group leader for GE Advanced Materials, Silicones, 260 Hudson River Road, Waterford, NY 12188, where he directs application development activities for silicone heat cured elastomers. He joined the company in 1978 and has developed silicone elastomers for a variety of industries. Toub can be reached at mel.toub@ge.com.

Sharon Shatto is Americas Marketing Manager, Health Care for GE Advanced Materials, Silicones. She joined the company in 1995, and has served in a variety of marketing and Six Sigma roles. Shatto can be reached at sharon.shatto@ge.com.