Accelerated Aging Protocols Established

ASTM's new standard shows how to set up and conduct an accelerated aging test.

Medical device manufacturers now have a standard to follow when conducting accelerated aging tests for their packages. The American Society for Testing and Materials (ASTM) recently published ASTM F1980-99, "Standard Guide for Accelerated Aging of Sterile Medical Device Packages."

The need for such tests is underscored in ISO 11607, "Packaging for Terminally Sterilized Medical Devices," which states that manufacturers "shall demonstrate that, under the rigors of distribution, storage, handling, and aging, the integrity of the final package is maintained at least for the claimed shelf life of the medical device under storage conditions specified by the manufacturer, as long as the package is undamaged or unopened."

Testing must be done before a product is made available for sale. But determining the effects of aging on a package in real time is a lengthy process that could slow the introduction of new devices to the market. Hence, there is the need for accelerated aging programs, which store samples at elevated temperatures to simulate the effects of real-time aging, and protocols for them. A product can be released after accelerated aging tests are done, provided that real-time studies that can eventually evaluate the accelerated aging results are being conducted concurrently.

The key to setting up a protocol is determining the appropriate accelerated aging factor (AAF), which is a ratio of the time needed to achieve the same level of physical property change to a package stored at real-time conditions.

The standard states that the fewer the available data on a particular package, the more conservative the AAF must be. If a lot of data about the package are available, then the AAF can be more aggressive. A more aggressive AAF means a shorter test, because accelerated aging time (AAT) = desired real-time aging / AAF.

Among the needed data are a package's processing history, morphology, chemical structure, molecular weight and molecular weight distribution, additives, processing agents, catalysts, lubricants, solvents, fillers; and glass, moisture, and heat-distortion transition temperature data.

The standard shows how to determine the real-time temperature, the accelerated aging temperature, and the AAF, and the standard plots out the steps by which to follow the protocol. The process starts with determining the rate of the chemical reactions that cause package deterioration and includes defining the test conditions and the desired shelf life of the package.

The process ends with the comparison of package performance, such as seal strength and integrity, after accelerated aging (and, eventually, after real-time aging) to the initial package requirements. The tests for these evaluations should challenge the most critical parts of the package or the parts most likely to fail.

Strength properties to consider are flexure, puncture, tensile and elongation, tear, impact resistance, abrasion resistance, yellowness index, microbial barrier, seal strength, and burst strength. Integrity tests, whose methods must be validated, include trace gas, dye leak, bubble leak, and microbial challenge. Acceptance criteria must be selected prior to the entire process.

Among the things that need to be documented are a written test protocol specifying the accelerated aging conditions, time frame, sample sizes, and other factors; the test standard methods and references; the temperature of the oven used and the resources used for measuring and monitoring the aging conditions; and the postaging test results, including statistical methods used to arrive at particular conclusions.

In some cases, the guide states, subjecting the package to elevated temperatures will not be feasible. Examples include packaging materials that have a very low heat-distortion temperature or that experience significant morphological changes at slightly increased temperatures. In these instances, only real-time aging can be done.

The guide does not cover real-time aging protocols, product/package compatibility, process validation methods, modeling the kinetics of material deterioration, and safety issues.

Copies of the standard may be purchased from ASTM for $30 at http://www.astm.org or by calling 610/832-9585.

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