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The Science of Particle Counting

Medical Device & Diagnostic Industry Magazine MDDI Article Index Originally Published August 2000 INSIDE MDDI Robert Drummond

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published August 2000

Robert Drummond

According to Earl Hackett, technical marketing associate at DuPont Medical Packaging (Wilmington, DE), it has long been understood that both pinholes and seal defects can provide unobstructed paths for microbes to enter a package (see "How Small Is Small? The Effects of Pinholes on Medical Packaging"). As a result, he says, medical device manufacturers have one recurring question when it comes to packaging their devices: "How big does a defect have to be before it significantly increases the risk of contamination?"

In studies performed as a part of the ASTM F2.6 working group, Hackett has made it his business to find the answer. "We have been actively developing and evaluating techniques for detecting pinholes in flexible medical packaging for many years," he says. The primary method of evaluation, he explains, is to take sterile devices in packages with known seal defects and place them in bacterial exposure chambers, then measure the degree of contamination. "These experiments show that seal defects on the order of a few thousandths of an inch across do not increase the probability of device contamination," he says.

Hackett explains that most pinholes in packaging materials are generated mechanically. They occur while the material is being manufactured, when it is being assembled, or during shipping. "The common mechanisms are abrasion, impact, or contact with a sharp object," he says.

Particle-counting equipment is used to test the effects of pinholes on their packaging. This equipment can detect particles with diameters as small as 0.005 mm. "We have shown in previous work that porous sterile medical packaging materials act as depth filters," Hackett says. "Particulate aerosols behave exactly like suspended microbial spores of the same aerodynamic diameter. If we can present a particulate aerosol to a test material at very low pressure and then collect and count the particles that come through, we can quantify the barrier characteristics of that material."

To count these invading particles, Hackett and his staff use condensation particle counters, which draw the aerosol through an alcohol-saturated atmosphere. "The particles act as nuclei and rapidly grow to a detectable size," he says.

Hackett and the working group have concluded that a greater material permeability imparts a greater tolerance of defects. This finding does not apply in situations where there is absolutely still air, but that is a relatively rare environment. "Even when a package is sitting on a shelf," Hackett explains, "ventilation systems are constantly creating pressure variations."

The finding is especially significant, Hackett says, "in the transportation and handling environments, where a significant exchange of air occurs." With any air flow, all of the particles entering a pinhole emerge from the other side. The porous material then acts as a vent, and any accompanying air flow from the outside environment is divided between the vent material and the pinhole. "The less resistance the vent material poses to the passage of air," he explains, "the greater the percentage of air that will pass through the vent compared to the percentage that passes through the pinhole." Thus, if the pinhole creates no measurable difference in the number of particles entering the package, "the pinhole can be said to have no effect on the ability of the system to maintain sterility."

Looking toward the future of medical packaging, Hackett sees a movement away from coated porous materials, as a means of cost savings. The development of improved film adhesives, he adds, will only further this trend. "Without a coating, the porous material should have greater permeability," he says. "As long as the base material has similar barrier properties to its coated counterpart," Hackett says, "the reliability of the sterile package could well improve at the same time that costs are substantially reduced."

Robert Drummond is managing editor of MD&DI.



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