Tech Corner: Studying Oxygen Barrier
By Rick Thompson, Manager, Applications Development & Support
Sealed Air Corp.
Many complex medical products and pharmaceutical preparations require an oxygen barrier in order to maintain safety and efficacy over shelf life. Included within this segment are complex large-volume parenterals (LVPs) such as parenteral nutrition and specialty drugs. To achieve a high level of oxygen barrier, producers continue to use glass bottles or flexible bags in combination with foil or clear barrier overwraps, sometimes also containing oxygen-scavenging sachets. A transparent, flexible, steam-autoclaveable barrier material would be advantageous for several reasons, including improved quality inspectability, label verification at point of use, and potentially extended shelf life and reduced cost.
An effective package for oxygen-sensitive products must maintain its barrier properties over a 12–24 month (minimum) shelf life. Terminal sterilization by steam autoclave presents a challenging environment for flexible packaging materials with temperatures often exceeding 121°C, wet conditions, and pressure and overpressure competing to maintain system equilibrium. A temporary loss or permanent reduction in oxygen-barrier properties within the packaging system would be detrimental to highly sensitive products. Oxygen-scavenging sachets may be used inside flexible overwraps to consume permeating oxygen, but scavenging capacity, added cost, and the placement and use of a foreign object can be detractors.
Data on the subject of required barrier for medical products and pharmaceutical preparations is somewhat limited. Therefore the purpose of this study was to understand oxygen ingress during and after steam autoclave sterilization and to measure the effectiveness of available active and passive oxygen barrier technologies.
MATERIALS and PROCEDURES
Six experimental oxygen-barrier flexible film structures were compared with two controls (a foil lamination and a nonbarrier film) in a real-time study that measured oxygen ingress for 24 months after steam autoclave sterilization (see Table I). For the purposes of this study, the test films were converted into and used as IV bag overwrap pouches.
Primary IV bags were fashioned from a multilayer coextruded non-PVC film. The bags were filled with 100 ml water, purged of air, and heat sealed. Overwrap pouches were produced from the experimental and control films using a straight bar heat sealer.
The IV bags were inserted into overwrap pouches, which were gas flushed with nitrogen and heat sealed in order to create a modified atmosphere package (MAP).
Destructive testing was performed at regular intervals throughout sample preparation using a CheckMate 9900 headspace analyzer to confirm a modified atmosphere of less than 1.0% O₂ within the overwrap headspace.¹ The mean value of 0.23% O₂ resulting from this in-process control testing was used as the Time Zero-Before Autoclave (T0BA) data point for all samples. Enough samples were produced such that hermetically sealed samples were available at each subsequent time point.
The samples were sterilized by steam water spray autoclave for 25 minutes at 121°C. Overpressure, or counter pressure, was controlled to 35 PSI (2.4 bar) to compensate for the internal pressures within the pouches to prevent them from bursting. A schematic of the autoclave cycle is presented in Figure 1.
Immediately following sterilization, the Time Zero-After Autoclave (T0AA) data points were established by sampling headspace gas from six samples of each variable by syringe and measuring the gas with the headspace analyzer. The remaining samples were then stored at room conditions (approximately 21°C/50% RH) and monitored for oxygen ingress using the same method and headspace analyzer at select time points throughout the duration of the study.
RESULTS and DISCUSSION
The study confirmed that traditional steam-autoclave sterilization is a challenging processing step for those that wish to minimize product exposure to oxygen. A temporary loss and/or permanent reduction in barrier properties, sometimes referred to as autoclave shock, is to be expected for many materials due to the elevated temperature, relative humidity, and pressure gradients present during the process. Oxygen ingress was evident in all films immediately after sterilization, including the laminated structures with aluminum foil and AlOx coating (see Figure 2).
The films that incorporated active scavenging technologies showed the lowest initial percent oxygen after sterilization, and were able to reach and maintain a modified atmosphere at or below 1% oxygen for a number of months. The sustained drop in oxygen concentration confirms the scavenging behavior of these materials.
The data suggests the scavenging EVOH film reached its capacity for scavenging permeating oxygen at about 90 days, maintaining lower total oxygen concentrations than the foil and AlOx materials for at least the first 6 months. The films that incorporated the scavenging iron and ethylenically unsaturated hydrocarbon oxygen scavenging polymer (OSP) showed the greatest separation in oxygen barrier performance by providing a virtually oxygen-free headspace for at least a full 12 months.
The iron-containing film was able to preserve the MAP headspace the longest (throughout the duration of the entire study) but the limitations of incorporating this technology into a flexible film for such an application were quite evident, such as the inherent high haze and visible iron oxide that appeared after exposure to wet conditions. In any case, the capacities of these active scavengers can be tailored to the needs of a particular application.
The passive barrier films based on blended EVOH experienced the highest initial ingress among the experimental group, which was similar to that of the nonbarrier film control. For the duration of the study, this grouping demonstrated the gradual but steady increase in headspace oxygen that one might expect based on their oxygen transmission rates.
The laminated structures with aluminum foil and AlOx coating experienced about the same initial increase in headspace oxygen to about 3.5%. The foil structure was able to maintain this T0AA condition throughout the duration of the test, while the AlOx coated structure experienced some slight additional ingress.
The samples were handled carefully and did not experience a distribution environment (real or simulated) that may have led to
additional ingress due to flex cracking.
The following conclusions can be made after this study:
• Terminal sterilization by steam autoclave presents a challenging environment for oxygen-sensitive products.
• Traditional flexible barrier materials, such as EVOH, aluminum foil, and AlOx-coated PET, are susceptible to measurable oxygen ingress as a result of the steam autoclave process, and optics can be negatively affected. ²
• An active scavenging technology can be discretely incorporated into a medical or pharmaceutical packaging film to provide and preserve a modified atmosphere over a typical industry shelf life. Several of these technologies demonstrated their ability to scavenge the initial surge of oxygen ingress resulting from the autoclave process, and by maintaining a headspace at or below 1% oxygen for a number of months afterward.
1. Employed during analysis was the Checkmate 9900, which is a product of PBI-Dansensor America Inc.
2. The active films used in this experiment were not necessarily formulated for autoclave resistance, but were included because of their availability and interest.