Medical devices must maintain a high level of sanitation at all costs. Unfortunately, these same products come into contact with dangerous bacteria and other harmful organisms, and when not sterilized, they can serve as vectors of disease. Barrier films and membranes serve as the first line of defense for a variety of medical applications such as drug containment, medical liners, fluid bags and more.

Why fluoropolymer films?

Figure 1: Films are a crucial part of medical sterilization. Source: Saint-Gobain Specialty Films

No matter the final medical setting use, a variety of fluoropolymer films usually serve as the barrier material of choice. In general, fluoropolymer films offer many desirable properties that make them particularly attractive candidates in medical applications:

• Resistance to both high and low temperatures
• Resistance to cracks
• Flexibility
• Chemical inertness
• Anti-stick surface
• Biocompatibility
• Low coefficient of friction
• Conformance to FDA and USP (United States Pharmacopeia) requirements

Figure 2: Film in the medical sterilization process. Source: Saint-Gobain Specialty FilmsThe specific kind of fluoropolymer film used can vary depending on the final application, but manufacturers typically choose among PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy polymer), FEP (fluorinated ethylene-propylene), ETFE (polyethylenetetrafluoroethylene) and ECTFE (polyethylenechlorotrifluoroethylene). All these fluoropolymers offer the advantages listed above, so which one medical manufacturers choose depends on final application, strength, manufacturing process and cost.

PTFE films, for example, cover any application that needs a high-temperature and a chemically resistant material. PFA offers the same properties but in a clear, transparent form. In addition, it can be heat-sealed, thermoformed, welded, metalized or laminated to a wide variety of materials.

FEP films offer excellent chemical resistance and release properties, and can also be easily fabricated. They are also suitable for use in cryogenic and high-temperature applications.

ETFE film, which has excellent weatherability, is often used in release applications and is also suitable for cryogenic and high-temperature applications.

ECTFE is called for when manufacturers need a fluoropolymer film for lightweight and high-strength composite fabrication. It is also highly resistant to weathering and high-energy radiation.

Medical sterilization

Manufacturers of medical devices must consider the properties of individual barrier film materials against the application challenges, but they also have to consider another factor: sterilization.

When medical devices coated with these films have to be sterilized repeatedly, manufacturers have to evaluate the long-term performance of the film. This varies depending on the kind of fluoropolymer and the sterilization method used, and adds an additional point of complexity in the design and manufacturing equation. Under repeated harsh sterilization fluoropolymer films can be damaged. Some might discolor and others might warp. Knowing how the fluoropolymers perform under different sterilization methods helps determine the most suitable film material.

Most sterilization methods can be classified under physical, chemical and radiation. Professionals choose the sterilization method depending on a variety of conditions, including the volume of products to be sterilized and the economics of the process.

Physical sterilization is usually done in an autoclave, where heat, humidity and pressure combine to clean objects thoroughly. Most autoclave sterilizations can take between 15 minutes to an hour or more depending on the pressure used and the deep cleaning needed.

Chemical sterilization uses either ethylene oxide (ETO) or chlorine dioxide (CD) gas to get the job done. Both processes involve cycling the devices through a range of steps, including introduction of the chemical and air washes, all of which take time. While efficient at killing a wide variety of bacteria, the use of toxic chemicals combined with the hours it may take to thoroughly sterilize equipment might place these methods out of reach for most operating conditions. In addition, while most fluoropolymer films perform well under chemical sterilization, the growing call for the decreased use of toxic materials is leading to less demand for this approach.

Over the years, radiation sterilization has increasing favorability among users of medical devices and equipment. Those users elect gamma ray sterilization, in which items are exposed to a strong gamma radiation source to kill contaminants, or electron beam sterilization, which uses cathode rays.

Gamma ray radiation offers many advantages over other kinds of sterilization methods, especially in terms of time efficiency. For this reason, it might soon be the method of choice in the industry. The problem is that in addition to the harmful organisms it is designed to kill, radiation sterilization can also have an adverse effect on the polymer film coating. The exact nature of the problem depends on the coating used, the radiation levels and a whole host of additional factors. Nevertheless, given the growing popularity of radiation sterilization, especially gamma ray, manufacturers need to evaluate commonly used fluoropolymer films and gauge how each endures these sterilization techniques.

The effects of radiation sterilization

High-energy radiation from X-rays, gamma rays and electron beams have a few basic effects on fluoroplastics: they can affect tensile, impact and shear strength and elongation. Exactly how radiation affects these characteristics, as well as parameters such as odor and color, depends on how each polymer reacts to the radiation. Under radiation, polymers can undergo one of two changes: scission or cross-linking. Polymer scissions can cause reduced toughness and elongation, while cross-links increase strength and stiffness. Both reactions occur simultaneously, but one is usually dominant depending on the particular polymer. These effects are also especially more pronounced with higher temperatures and in air, rather than a vacuum.

The radiation dosage at which the negative effects occur varies depending on the polymer, but can also change based on residual or functional stress from design and production, the product cross-section thickness, dose rate or radiation dose absorbed. The chemical composition of the polymer, its morphology (percent crystallinity) and design of the device (such as physical dimensions) and post-radiation storage environment conditions, such as the temperature and oxygen atmosphere, also play a role in degradation of the films. Discoloration, usually yellowing, might occur before loss of properties, signaling potential trouble.

One of the difficulties in ascertaining which film to move forward with lies in the challenges of figuring out when the material has degraded to the point where it is unusable. The industry standard is usually set at a 25% reduction in tensile strength or other parameters. At this cutoff, the material generally needs to be discarded. Fluoroplastic films usually are classified as good, better or best, depending on performance characteristics.

A general guide is that partially fluorinated fluoropolymers such as ETFE and ECTFE perform better under radiation sterilization than fully fluorinated ones such as PTFE, PFA and FEP. A comparative ranking, from most to least favorable:

1. ECTFE
2. ETFE
3. FEP
4. PFA
5. PTFE

The right choice

The type of fluoropolymer films manufacturers can work with for medical applications depends on many variables, including the kinds of sterilization conducted. The right choice can extend the life of the film and the devices, even under harsh sterilization procedures.

Saint-Gobain Specialty Films is there to help medical device manufacturers select the optimal polymer film for any application. Engineers have designed and tested each material, so manufacturers know Saint-Gobain materials are high quality and suitable for their applications. Find out more at the Saint-Gobain Specialty Films homepage.