Appendix A I Appendix B I Appendix C

Accelerated aging is widely used for sterile package integrity testing to determine shelf life. (EN ISO 10993). It is also widely used for polymer and material testing; even the library of congress uses it to determine optimum storage conditions. So why not use accelerated aging for devices? The problem here is that no standards exist or other widely accepted models are available. The key factor with accelerated aging is to know and understand the use environment first, before any test can be specified. The purpose is not to reach the destruct limits but to find the extended operating or storage range in direct relation to the life of the product. To be valid, accelerated aging must have a 1:1 correlation relationship to the products normal life. Accelerate aging when properly applied will allow the normal product life of 80,000 hours to be reduced to less that 168 hours of testing with a 95% reliability confidence level. In other words, a logarithmic aging process is applied. This method has been successfully developed and applied by Dr Joseph Capitano, Ph.D. ³

The theory is based on the fact that any component or device will age depending on its environment. A component can be stressed in three different ways, thermally, mechanically or chemically. By knowing first the temperature and mechanical limits of the components of the device, an accurate product model can be developed. (For simplification for this paper, the chemical factor is left out.) This is an exercise of reviewing all specification sheets of components and materials to determine the maximum allowed temperatures and mechanical stresses.

With this information on hand, an accurate model can be developed to apply an accelerated aging test. The object is to subject the non-operating product to rapid thermal cycling to achieve a mass rate of change of approximately 20ºC - 24 ºC per minute. (This is approximate value for surgical devices.) The most thermally sensitive components are instrumented with thermal couples to observe the mass rate of change. The temperature is alternated to increasing levels of hot and cold temperatures to achieve the desired 24 ºC mass rate of change. The object is not to dwell on the hot and cold temperatures but to achieve the rapid temperature change. In fact, during the initial investigation, the operator has to be careful not to exceed the maximum component temperature limits. In fact it is advisable to leave a 5 to 10 degree safety margin, because we do not want to go into the destruct region. The maximum hot and cold temperatures are determined by the achieved mass rate of change. If the desired mass rate of change of 24 ºC cannot be achieved because of component or test chamber limitation, more cycles are required.

After several temperature cycles, the product is allowed to reach ambient and is tested. If a defect is found, it has to be mitigated before the testing resumes. Then more thermal cycles are applied, until the maximum calculated cycles have been applied that is representative of the expected product's life.

Similar test are conducted mechanically by subjecting the product to a vibration table. Again, the object is not to reach the destruct limits of the device, but to simulate an extended operating range during operating conditions. By operating the device during this test, the software or monitoring system typically will cause error messages if out of range conditions occur. On all of the above testing, variations can be implemented depending on the use environment and physical limitations of the device.

From the previous series of test, a P/F screen can be developed for product qualification testing and more samples can be tested to verify the derived model.

Then from this model a manufacturing screening test can be derived to age the product before it is shipped to the customer. This not only will catch the typical manufacturing defects but also catch the latent defects. One might asked "why age a product before you ship it?" This can be explained that if the product robust design would last 15 years, six months of aging could not hurt it.

More information on Accelerated Aging can be found in Appendix B.