Sterilization Process Compatibility with Tyvek
Choosing the proper packaging material for a medical device's primary package requires the careful consideration of three major factors: the nature of the device, the sterilization process that will be used, and the characteristics of the packaging materials. Packaging engineers must also consider the effect each one has on the others, because these are not independent, but interdependent, variables. For example, a device manufactured from non-radiation-stable polymers can only be sterilized using EtO, or a low-temperature oxidative method, or steam. But for any of these methods to be successful, the package must be designed with a porous component. One of the most widely used porous materials is Tyvek brand spunbonded olefin.
The porosity and moisture-vapor transmission rate of the porous packaging component (as well as its porous surface area) are key to reducing cycle times in EtO sterilization. Tyvek—with its porous nature (22 Gurley seconds for uncoated Tyvek) and high moisture-vapor transmission rate—is suitable for use as a top-web material for EtO-sterilized packages. Recent technological advances have further enhanced the performance of this nonwoven material. For example, several new films have recently become commercially available for use with uncoated Tyvek, enabling users to take advantage of the greater porosity of the uncoated styles of Tyvek while still maintaining a relatively wide sealing window. Combined, these two factors have the potential to reduce sterilization cycle times significantly. Traditionally, adhesive-coated Tyvek has been the major form used in medical device packaging. While these adhesives open the heat-sealing window, they also make the material less porous.
Tyvek also has a low propensity for absorbing EtO. Therefore, it does not significantly inhibit EtO aeration during the outgassing cycle. The radiation stability of HDPE provides excellent physical strength retention to the nonwoven, even after radiation exposure in excess of 60 kGy. This is especially important in gamma or E-beam sterilization where seal creep or a loss of seal integrity can occur in completely nonporous packages because of pressure changes during the distribution cycle. However, with a porous component in the package design, the primary package is allowed to breathe, eliminating the likelihood of seal creep or a loss of seal integrity.
Low-temperature oxidative sterilization methods, such as Sterrad from Advanced Sterilization Products (Irvine, CA), also require that package designs include a porous component. At first, paper may seem a logical choice, but the reaction of paper's cellulose with the sterilant can result in incomplete sterilization cycles. On the other hand, HDPE has relative chemical inertness that does not interfere with the sterilization cycle. In fact, it actually allows the sterilant to pass through the package and reach the device.
High-temperature steam sterilization can also be used with packages containing Tyvek—if certain safeguards are in place. The nonwoven retains its dimensional stability and integrity at 250°–260°F (121°–127°C) at 15 psi for 30 minutes with no discoloration. Therefore, sterilization temperatures must be accurately controlled.
Sterilizer hot spots can also present potential problems by exceeding the melting point of the polymer. But many medical device manufacturers have found that they can successfully use Tyvek in steam sterilization by simply using sterilizers with adequate temperature control and appropriate sterilization cycles.
Tyvek is compatible with all of the major sterilization methods used in the manufacture of presterilized medical devices. This compatibility range allows medical device packaging engineers to focus their attention on more critical issues without worrying that the chosen materials will not perform in a given application.