Page 235 - Biomedical Engineering and Design Handbook Volume 2, Applications

214 MEDICAL DEVICE DESIGN

The Q 10 method assumes that the ratio of the times to equivalent damage at low temperatures
(usually 10°C apart) has a constant value. In fact, the value of Q will decrease with increasing tem-
10
perature. Donohue and Apostolou suggest the use of a modified or variable Q method in which the
10
ratio of the time to equivalent damage between two temperatures is used as a variable. In this method
the TED ratio is equal to the Arrhenius equation, and the Q is determined with the TED ratio as a
10
variable as follows:
Q (T – T )/10 = TED T /TED T
H
L
10 L H
So Q = (TED T /TED T ) 1/[(T H – T L )/10]
10 L H
Again, it is necessary to acquire performance data for ambient storage conditions as well as for
elevated conditions in order to determine the TED ratio, and before this method can be employed for
predicting a variable Q .
10
Lambert and Tang describe a method of aging using an iterative process that provides an oppor-
tunity to refine and validate the initial, conservative aging factor (Q ). The basic concept is to col-
10
lect a number of parallel real-time aged and accelerated aged data points at early time points such that
a correlation between the two can be developed, thereby defining the actual aging factor of the system
under investigation. One limitation of this method is that real-time aged package performance data are
required in which to compare accelerated aged data and make iterations on the conservative Q . A
10
basic eight-step concept was flowcharted by Lambert and Tang as shown in Fig. 7.5.
7.8.3 Guidance Documents
The American Society for Testing and Materials (ASTM) Committee F2 on Flexible Barrier Materials
published ASTM F-1980, “Standard Guide for Accelerated Aging of Sterile Medical Device Packages.”
The scope of the guide is to provide information for developing accelerated aging protocols to rapidly
determine the effects due to the passage of time and environmental effects on the sterile integrity of
packages and the physical properties of their component packaging materials. Additional guidance on
accelerated aging protocols is provided in the AAMI Technical Information Report 17 (TIR 17). The
information obtained from utilizing these guides may be used to support expiration date claims for
medical device packages. It is hoped that it will provide the necessary rationale for accelerated aging
protocols which satisfies both the FDA’s Quality System Regulations (QSR) and the essential require-
ments for packaging in the MDD.
The ASTM Guide provides referenced documents (many of which are cited in this chapter) which
render credibility to the current suggested methodology for aging medical device packages. The guide
condones the simplified Q method as rationale for using accelerated aging for medical device pack-
10
ages. The guide states, “Conservative accelerated aging factors must be used if little information is
known about the package under investigation.” The ASTM Guide was revised in 2007 to clarify the
use of humidity conditions in the accelerated aging protocol. As the Sterilization Packaging
Manufacturers Council (SPMC) stated in their whitepaper on “The Role of Humidity on the
Accelerated Aging of Sterilizable Medical Packaging,” “While the role of temperature is well docu-
mented and understood in the aging process, the impact of humidity is not.”
The guide simply stated that “The effects of humidity may need to be considered. . . .” For this
reason the guide was revised to provide clarity on when to use humidity in the protocol, and how
much. So the concept of absolute humidity (water concentration) was included for guidance in
selecting a realistic humidity condition for accelerated aging protocols. The chart of “Concentration
of Water in Air as Function of Temperature and Relative Humidity,” as shown in Fig. 7.6, was added.
So if a fixed absolute humidity is established for accelerated aging protocols and the test tempera-
ture is varied, the corresponding relative humidity can be determined so that the test equipment can
be set up for the study. This conversion is necessary since test chambers are controlled based on the
relative humidity in the interior of the chamber.
Although the method provides conservative estimates of product/package shelf life resulting in
longer test durations than would be necessary using more complex aging methods, it does not require