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Supplementary notes for expected shelf life calculation

Since the sterility of a product cannot be confirmed with a non-destructive test before its application in patients, special techniques of quality assurance are required. In addition to the visual inspection of failures such as punctures, tears etc. it is necessary to consider other possible factors that impair sterility during transport and storage. One factor is the microbial challenge as a result of environmental air flow through the porous component of packaging. Whereas the relevant physical parameters for sterilization are used to control sterilization processes, the relationship between flow of environmental air into the sterile barrier systems and the probability of loss of sterility is not introduced as a data-based approach for shelf life calculation. Such an approach seems to be obvious because porous packaging material can only maintain sterility if its filtration efficiency is sufficient and not overburdened by the airborne microbial challenge so that sterility is not compromised.

The compatibility of the capacity to retain the airborne micro-organisms and the airborne microbial challenge is calculated according to following formula in order to meet the maintenance of sterility at the sterility assurance level (SAL) of 1:1,000,000:

The airborne microbial challenge (No) is obtained by addition of the inflow of air volumes that occur in a period of time and by multiplication with the expected airborne microbial concentration (CFU/m³, particle size < 10 µm). The atmospheric air pressure changes day by day. Consequently, these changes should be continuously measured on site for a reliable data base to calculate the cumulative airflow into the packaging. A software is used for the desktop monitoring application in order to plot the measured air pressure values and the calculated cumulative air inflow into the packaging on a graph. The increase of air volume which has been entered the packaging after the start of the recording is used to calculate a linear trend line. After inputting the packaging volume, the filtration efficiency of the porous component of the packaging and the estimated airborne microbial concentration, the program is able to calculate the estimated expiration date, i. e. the maximum shelf life.

Expected Shelf Life Calculator - instruction for use. The expected shelf life is calculated by using the Menu “INPUT MASK”. Currently, alone the atmospheric air pressure changes are included in the calculation program. Other effects such as temperature changes are not considered in this program and need to be estimated by the operator. Using a real time data logger, the expected shelf life calculator can be used for calculating the maximum shelf life according to the SAL of 1:1,000,000 during stoarge,  transport by airplane, car and train or in combination of different exposure scenarios.  
The input of the following parameters is required: volume of the packaging (button “Packaging Volume [ml]”, estimated or measured mean airborne microbial concentration (button “Colony Forming Units [m³]”) and filtration efficiency of the packaging material for airborne micro-organisms (button “Filtration Efficiency [%]”). A real time data logger equipped with an SD card should be used to record on-site the atmospheric air pressure and to save the values into the SD memory card (capacity about 4 GB). When using the data logger PCE-THB 40 (PCE Americas Inc.) following steps are recommended. The card has to be inserted in a computer PC card slot. The saving data are shown with EXCEL software. The data file can be put into a folder. The data file is uploaded using the button “Upload Air pressure Profile” provided on the menu page “INPUT MASK”. Thereafter the graph of atmospheric air pressure slope, the cumulative increase of air volume inflow [ml] and the date of maximum shelf life is shown on the screen.