The present invention relates to a method for measuring the amount of gas that leaks through sealed packages. More specifically, the invention relates to gas leakage through packages that have been sealed by a cover of porous material fabricated as a mat of polyethylene fibers. This material acts as a permeable membrane to gases, but an impermeable membrane to bacteria. The membrane comprises a layer having pores which provide a tortuous path to the passage of bacteria; the material is commonly sold under the trademark designation xe2x80x9cTYVEC.xe2x80x9d The packages which use this material are typically semi-rigid plastic cases which protect medical devices and appliances after manufacture and before actual use.
The invention relates to U.S. Pat. No. 5,939,619, issued Aug. 17, 1999, entitled xe2x80x9cMethod and Apparatus for Detecting Leaks in Packaging,xe2x80x9d and U.S. Pat. No. 6,050,133, issued Apr. 18, 2000, entitled xe2x80x9cMethod and Apparatus for Detecting Leaks in Packaging.xe2x80x9d Both of these patents are owned by the assignee of the present invention. The present invention also relates to co-pending application entitled xe2x80x9cMethod for Measuring Gas Leakage From Sealed Packages,xe2x80x9d Ser. No. 09/676,621, filed Oct. 2, 2000, and owned by the assignee of the present invention. The present invention permits a measurement of leakage that is wholly non-destructive to the package.
Gas sterilization is widely used for medical devices that must be sterile at the time of use, but cannot be subjected to sterilization by the application of high temperatures. Examples of such medical devices include cardiac pacemakers and catheter-based monitoring devices such as blood pressure probes. Typically, the medical device is sealed within a package that is permeable to gases but impermeable to bacteria. The package is then placed in a gas sterilization chamber, and a sterilizing gas such as ethylene oxide is introduced into the gas-permeable package to achieve sterilization. The sterilizing gas is then removed from the package, leaving the interior of the package sterile and non-toxic.
In a typical design, the medical device is placed within a thermoformed rigid plastic tray equipped with a flat sealing flange. A sheet of gas-permeable membrane, such as DuPont TYVEK(copyright) 1073-B (medical grade) brand membrane, which is available from E. I. duPont de Nemours and Co., is then sealed to the sealing flange, typically by using an adhesive. The integrity of the seal is critically important to maintaining sterility. Leaks can result from incorrect setting of parameters in the automated sealing process, or from physical defects such as burrs on the face of the sealing equipment.
According to the known practice described in the prior art patents listed herein, a temporary barrier is formed over the gas-permeable layer, wherein the temporary barrier has an aperture with the gas-permeable layer to temporarily seal the gas-permeable layer except where the aperture is located. A tracer gas is applied under low pressure through the aperture so that it can enter into the interior chamber of the package. The entire package is placed into a larger sealed second chamber, and the concentration of tracer gas in the second chamber is measured, outside the package, to thereby measure the amount of tracer gas which has leaked through the package, presumably via leaks in the sealing flange, although leakage can also occur through pinhole defects in the plastic tray itself.
The methods described in the foregoing patents provide very accurate measurements and evaluations for sealed packages, under controlled conditions. However, in many applications, it is not necessary to achieve a high degree of accuracy in the leakage measurement, but is desirable to provide a quick evaluation of leakage as a production line test, to determine whether packages are leaking excessively. Excessive leakage is defined in terms of leakage beyond a predetermined range of acceptability, as a pass/fail parameter, and the precise degree of leakage does not need to by quantified. Methods for making this type of determination should produce results more quickly and at a lesser cost.
The present invention provides a pass/fail test which can be quickly performed at considerably less cost in terms of test equipment and testing time, because it relies on measurement of internal package gas pressure, and specifically pressure drop, caused by leakage of gas from within the package. The method of the present invention can be performed in a short time, perhaps 30-60 seconds, using very much less expensive equipment than prior art methods.
The present invention comprises a method for testing sealed packages of the type described herein for leakage, according to a more efficient and faster process than is known in the prior art. The method requires a barrier layer to be overlaid on the gas-permeable membrane of the package, the barrier layer having a single aperture to the gas-permeable membrane. A pressure-sensitive monitor is affixed over the aperture, to monitor the pressure inside the package, and more particularly, to monitor the relative drop in pressure caused by leakage from the package. The internal package gas pressure is initially incrementally increased as a step of the method, and the incremental drop in pressure is measured as a function of time, usually by determining whether pressure drops incrementally between two desired set points within a predetermined time increment.
It is a principal object and advantage of the present invention to provide a method for measuring leakage through packages under faster test conditions than have heretofore been known.
It is another object and advantage of the present invention to provide a leakage test method which does not destroy the package being tested.
Other and further objects and advantages of the invention will become apparent from the following specification and claims and with reference to the appended drawings.