Feature Article

MATERIALS

Keeping pace with change

A number of factors, including an ageing population, are putting greater demand on manufacturers for single-use medical equipment and innovative devices. In response, the range and complexity of devices that incorporate some form of tubing continues to expand. A variety of materials including silicone, poly(vinyl chloride) and polyurethane are called upon to meet evolving needs. Silicone is notable because of its physical properties, demonstrated biocompatibility and other attributes, which means it is able to meet specific requirements in medical device new product development.

The availability of high performance materials can help developers and fabricators continually push the limits of technology. Silicone is among the most extensively tested materials used in medical device applications.¹ With its documented biocompatibility, long history of use and established base of knowledge about its performance, silicone often stands out as the material of choice in device applications that incorporate tubing. Because silicones do not support microbial growth and have a low extractables content relative to many other tubing materials, they may be adapted to a wide range of applications that involve contact with body tissues.

Device developers increasingly require smaller silicone tubing with more stringent dimensional tolerances. Where a tolerance of ±0.005 in. (0.127 mm) may have historically been sufficient, a tolerance of ±0.001 in. (±0.025 mm) or less is desired today. Similarly, as designers focus on techniques for minimally invasive surgery, the demand for smaller tubing diameters, thinner walls and in some cases more complex design, is increasing. Smaller tubing with stringent dimensional tolerances is particularly important in implant applications such as insulation on pacemaker leads. The dimensions of these leads continue to decrease to less than 0.030 in. (0.76 mm); therefore, silicone tubing dimensions must also decrease while maintaining its insulating properties.

With the general trend towards smaller electronic devices and components, tubing flexibility becomes a critical physical characteristic for supporting designs that require tubing in tight spaces. For example, in infusion or drug delivery pumps, tubing may need to adapt to tight bends or crevices. Silicone tubing can be twisted without showing stress or breakage, and it can be bent or folded in half in use, then return to its original shape without retaining a permanent crease.

In implant applications, and specifically with pacemaker leads, the developments in silicone tubing are also towards a higher degree of resilience and abrasion resistance. In the case of pacemaker leads, the human body moves and flexes and tubing may rub against the internal body structure. If a lead can move even slightly within that tubing, abrasion becomes a possibility; therefore developers in that market sector require a greater degree of abrasion resistance.

Resilience is also vital in applications such as peristaltic pumps, where the tubing is required to return to its original shape after compression with a roller. Because roller pumps typically run at several hundred rotations per minute, the tubing experiences repeated flexing and releasing. As indicated in Figure 1, performance of silicone tubing in peristaltic pump applications can be formulation dependent. Although six tubing products are prepared from similar polydimethylsiloxane formulations (A to F), their composition or physical profiles may differ in important aspects such as filler type and concentration,

cross-linking density and cure mechanism. This formulation flexibility imparts significant performance differences. This illustrates the importance of identifying critical process needs to ensure the most effective tubing formulation is selected for the intended application. A reputable silicone tubing supplier can work with device developers and fabricators to help identify the most appropriate silicone formulation for their needs.

Tubing for specialised applications

The biocompatibility, flexibility and resilience of silicone tubing facilitate its use in many medical devices, yet some applications require further tubing enhancement. One example is the delivery or transfer of high pressure gases or liquids, which can benefit from reinforced tubing. By completely embedding a polyester braid into the tubing (with no exposure of polyester to the fluid contact surface) to provide enhanced strength, the silicone biocompatibility properties of the tubing are maintained.

With evolving higher resolution, diagnostic technologies and minimally invasive therapies, the need for radio- paque tubing for X-ray visibility is likely to increase. For applications where a physician must be certain that a feeding tube or catheter is properly inserted or needs to observe implant applications, striped or solid tubing with a barium sulphate additive can offer radiopacity. The type and concentration of radiopaque additives can affect X-ray image contrast and sharpness. The radiopaque properties of the tubing can be adjusted by varying the additive concentration or by using an alternative X-ray absorbing material such as tungsten.

Global regulatory requirements have shown an increased concern for biocompatibility of materials that have body contact or that may leach into drug products being administered. As a result, suppliers of silicone tubing are receiving more questions related to whether silicone materials contain genetically modified organisms (GMOs), animal-derived ingredients, natural rubber, latex, organic impurities, preservative impurities or phthalates. Because GMO-based materials are widely available in the market place, there is general consumer concern related to how thoroughly these materials have been studied and whether they are present in materials that could come in contact with a drug delivered to the body. Therefore, it is critical that device developers and fabricators work with suppliers that fully understand and manage their supply chains to ensure they produce products in compliance with current regulatory standards and guidelines.

Sterilisation compatibility

Figure 2: Inside diameter, pre- and post-irradiation. (click image to enlarge)

Silicones remain stable when exposed to a range of sterilisation techniques. They are readily sterilised by ethylene oxide, autoclave and irradiation with a cobalt-60 gamma source, a method that can be performed once a device is packaged. A potential downside of this method for some materials is the possibility of degradation or decomposition as a result of radiation exposure. In general, silicone tubing

withstands gamma irradiation up to 50 kGy without significant degradation or change in dimensions. Figure 2 illustrates the ability of the tubing to retain its inside diameter before and after exposure to a gamma radiation dose of 50 kGy. The four products were formulated with polydimethylsiloxane polymers; however, varying levels of filler, cross- link density and other compositional components were employed. The resulting tubing properties ranged in hardness from 50 to 80 Shore A durometer.

A versatile material

The greater use of disposable devices means that silicone tubing is increasingly being specified for single use applications. The broad temperature of the tubing allows it to be used in a variety of applications involving high or low temperatures.

Requirements associated with trends towards smaller dimensions with tighter tolerances, greater flexibility and increased resilience whether for shorter term pump applications or longer term implant use can be met by the versatility of silicone tubing. Coupled with the relatively low extractables profile of the material, its stability when subjected to sterilisation and its long history of use in health care applications, silicone tubing continues to meet the expanding needs of the medical device market.

Donald Jahn is Pharma Processing Programme Leader at Dow Corning Corp., Midland, Michigan, 48686-0994, USA, tel. +1 989 642 5224, e-mail: don.jahn@dowcorning.com, www.dowcorning.com