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Efficacy and sterility in flexible packaging
Karen Berger and Dhuanne Dodrill | Thursday, December 2, 2004, 08:00 Hrs  [IST]

Packaging plays an important role in many industries, but few applications are as demanding as those found in the medical device and diagnostic industry. Packaging for medical devices may lack the glamour and glitz of that for consumer goods; however, nowhere else is package integrity as important. The job of the package is to maintain the sterility of the product through its intended shelf life, as well as ensure its efficacy at the time of use. The impressive range of flexible packaging materials available today helps to achieve these goals. This article will discuss tried-and-true methods of packaging, as well as introduce several new technologies for meeting packaging requirements.

Flexible Packaging

With the tremendous variety of packaging materials available today, there are many possible combinations. The simplest structures may consist of just one or two layers-for example, a monolayer forming web or the OPET/polyethylene web commonly found in a basic pouch. The more complicated structures can easily exceed eight layers, including all of the components such as primers, inks, and tie layers. Flexible packaging materials for medical packaging applications incorporate one or several elements.

Heat-Stable Material. This material serves as the outer layer of a flexible packaging composite. This side of the web is typically exposed to a heat-sealing die, so good thermal stability is important. OPET is most commonly used. A suitable print surface may be needed for in-line printing applications. Oriented polypropylene (OPP) film can be used where less thermal stability can be tolerated, although not all OPP films can be used for radiation sterilization. Biaxially oriented nylon (BON) is more expensive than OPET but has greater flexibility for better stresscrack and pinhole resistance. Cast nylon film is used as the heatstable layer in many forming applications. Paper can be used for its excellent heat resistance and print surface, but it is prone to tearing and fiber generation.

The sterilization method is a vital factor to consider in specifying packaging for medical devices, because once the sterilization method is known, the range of appropriate materials narrows. For each of the major sterilization processes, there are 2-D and 3-D packaging systems in place.

EtO Sterilization. For effective EtO sterilization the packaging material must be breathable to allow the high-humidity EtO gas mixture to infiltrate the package. A partial vacuum is drawn before and after the cycle to facilitate the movement (in and out) of the EtO and moisture vapor. If the package does not have sufficient permeability, the process will be ineffective. As stresses in the seal area are introduced due to the pressure difference between the inside and outside of the package, a seal failure phenomenon known as sterilizer creep may occur. The package must also withstand the moderately elevated process temperatures, although this is typically not a problem for most materials.

A typical 2-D breathable pouch consists of a PET/PE bottom web combined with a Tyvek or paper top web. The Tyvek or paper top web is the breathable portion, and it may be coated or uncoated. Uncoated Tyvek has a Gurley Hill porosity of about 20-25 sec/100 cm3 air per square inch; typical coated Tyvek has a porosity of 80-100 sec/100 cm3 air per square inch. A less-breathable coating can be applied as a patterned coating, such as a grid or dot pattern. Tyvek or paper also provides a suitable print surface and good aesthetics. Because one disadvantage to Tyvek is its expense, a great many 2-D pouch applications use a Tyvek header strip or vent, to save on material cost.

For 3-D radiation, a formable bottom web is used. For nonbarrier applications, the same webs used as bottom forming webs for 3-D EtO packages are employed. For barrier applications, one of the formable barrier webs, such as a coextruded film containing EVOH, an Aclar lamination, or a formable foil, can be used.

Retort Sterilization (Nonbreathable Autoclave). In some nonbreathable autoclave applications, a liquid-containing package is sterilized in a retort process. Seal integrity is extremely important in these applications because, unless the engineer has precise overpressure control, the pressure difference between the inside and outside of the package may easily result in seal failures. This is an area where very strong peelable seals are often specified. The same high heat resistance needed for breathable steam applications is also required. In general, a barrier package for this type of sterilization can use either clear barrier or foil barrier composites. Not all of the barrier materials are autoclaveable, and not all of the autoclaveable materials are thermoformable. Table I indicates which barrier materials apply for retort and autoclave sterilization.

A 2-D barrier pouch for retort or autoclave sterilization can be made from aluminum foil or from one of the nonfoil autoclaveable barrier materials. Autoclaveable clear barrier materials are often more expensive than foil; therefore, a pouch can be made of foil on one side and the clear barrier material on the other. This will suffice provided the end-user does not need clarity on both sides of the package and the packaging equipment can accommodate two different webs.

For this sterilization method, 3-D packaging usually consists of a rigid thermoformed tray and a flexible film lidstock. The lidstock can be foil based or use a nonfoil barrier material.

Other Sterilization Methods. Although the previously mentioned sterilization methods represent the major applications, other methods are well worth mentioning. Dry-heat sterilization is used for products that do not lend themselves well to other sterilization techniques, do not contain moisture, may be damaged by contact with moisture, and are stable at elevated temperatures. This technique requires a long exposure time (as long as several hours) at elevated temperatures (275°F and above). Heat resistance is the main packaging requirement. Some examples of dry-heat-sterilized products include orthopedic implants, collagen products, and surgical instruments.

Hydrogen peroxide (H2O2) gas plasma sterilization is drawing interest as a substitute for EtO sterilization. Developed by Advanced Sterilization Products (Irvine, CA), the Sterrad system is gaining acceptance in both institutional and industrial use. A breathable package is required for the gas to permeate the package. Although this is done in a drier and slightly cooler environment than EtO sterilization, the same types of package materials are appropriate. The only known exception is paper, which cannot be used; the cellulose fiber in paper can absorb enough H2O2 to make the process ineffective. Since this method is relatively new, not all materials have been thoroughly tested. One of the main benefits of this process is greatly improved cycle time; since the primary by-products are water and oxygen, there is no aeration required. This is very important to help achieve quick turnaround in clinical settings. Compared to EtO, the environmental and regulatory concerns are minimal.

- Canon Communications LLC

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