Medical Device Link.

Originally Published EMDM October 2004

Technology News

Developed as a sensible alternative to traditional testing methods, virtual leak diameter flow standards can be applied on a contract basis by Ion Medicare Ltd. (Click to enlarge).

In leak testing, as in other endeavours, size matters. Specifically, it’s important to determine a package’s defect size limit to prevent the rejection of packs that are, in fact, perfectly suitable. Virtual leak diameter (VLD) flow standards were developed as an alternative to traditional testing standards. Ion Medicare Ltd. (Cambridge, UK) is able to apply VLD principles on a contract basis and will advise companies on how best to use the test method to achieve compliance.

Under ISO 11607, manufacturers must perform both package-integrity and seal-strength tests on terminally sterilized medical packaging. Integrity testing measures a package’s sterile barrier. Pressure- decay testing has been shown to be an effective method for determining barrier integrity. It is well suited for identifying leaks in flexible packaging with nonporous surfaces and seals. Seal-strength testing includes burst, creep, and creep-to-failure techniques. Inflation tests are suitable for most materials and package forms.

Conventional instruments combine these tests with statistical and documentation packages. Because the tests are pneumatic, however, the resulting data are expressed in terms of pressure decay over time. “This bears no resemblance to defect size, which is the object of the test,” says Mel Vinton, design engineer at Ion Medicare. “In addition, standards tend to be comparative and usually based on ‘known-OK’ parts with little or no consideration given to the package contents or conditions of storage.” VLD provides a means to perform adequate testing while not rejecting packages that are fit for their purpose, adds Vinton.

Sealing integrity can be related to defect size by applying the Poiseuilles-Hagan equation, notes Vinton. The operative standard can then be based on gas laws and physics. This is achieved by calculating the maximum allowable hole for a given package and using that value as your standard.

“For example, take a product that will be stored and sold in a location where the humidity is high and where freight conditions produce rapid changes in barometric pressure and temperature,” explains Vinton. “Under those circumstances, a blister pack containing a hydroscopic material would have a maximum 5-µm VLD, a sterile-sealed device would be 10 µm, and hand lotion would be about 20 µm. It is not correct or cost-effective to test these products to the same standard,” says Vinton. “The specification is relevant only to the requirement. Leave the decision to the logic of gas laws, physics, the product, and the conditions it must endure.”

Everything leaks, notes Vinton. “Setting a no-leaks standard is like asking an engineer to design a zero-tolerance part. It can’t be done.” Basing a standard on the maximum allowable hole for a given package represents a realistic and cost-effective alternative to current techniques, Vinton adds.

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