MARKET PLACE
The finer points of leak testing
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Image: Uson LP
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With the possible exception of putting a man in space, no other industry demands as high a degree of fail-safe operation as the medical device industry. Yet, unlike the single-digit iterations of space travel products, the output of medical devices is often in the millions, which can complicate the challenge of maintaining quality. Leak testing equipment is often at the forefront in the effort to maintain standards. It is responsible for non-destructive, production line testing of leaks, flow and operation in an array of medical devices that handle fluids, gases and even solids.
Thanks to improvements in design and manufacturing, many currently available makes and models of industrial leak testing equipment are capable of detecting breaches in containment, or the exclusion, of elements. However, medical device manufacturers must be more discerning than most when selecting test equipment. With advanced knowledge of some of the finer points and lesser known considerations of leak testing, quality engineers can ensure that their devices meet the unique demands of medical device manufacturing, without having quality issues limit production schedules.
Complex requirements
Often a medical device will require more than one type of test, which increases cycle time and reduces throughput. For example, a check valve requires testing from both directions to test opening and closed seating, as well as flow measurement at design specifications. This requires three discrete tests, each one capable of being programmed to abort the test sequence on failure.
“Our CozMore insulin pump must endure showers and swimming environments, therefore in addition to several operational and testing requirements, it is important to ensure that these devices do not permit water ingress,” explains Bill Parish, Senior Industrial Engineer at Smiths Medical MD Inc. (St Paul, Minnesota, USA, www.smiths-medical.com) a provider of medical devices for the hospital, emergency, home and specialist environments. “We must also accommodate altitude differences and pressure test accordingly,” continues Parish. “For example, someone might have occasion to travel up to 14000 ft (4267.2 m) and then return to a more normal elevation. You would not want the unit to blow a seal, therefore your device testing needs to consider that pressure differential.”
Richard Ray, a senior engineer at the Elkton, Maryland, USA, plant of Terumo Medical Corp., www.terumomedical.com) says that testing may not only serve to check for leakage, but also to simultaneously ensure against occlusions and fluid restrictions. “This is especially the case in a blood-related device where problems of this type could cause cytolysis,” says Ray. “Testing these devices must ideally involve inflation, deflation, leakage and checking for proper flow. On some of our parts, for example, we have a pressure decay test and also a flow test. The latter would be to check parts that were bonded to ensure that nothing got in to cause any occlusions or restrictions during the assembly process.”
To a great extent, modern leak test equipment can mitigate these challenges by combining more than one procedure in a single tester and utilising sequential or step programming.
Product size
Another issue to consider is the fact that medical devices range from the extremely small, because of their invasive nature, to the relatively large. Any tester must offer a flexible filling circuit to accommodate volumes ranging from a fraction of a cubic centimetre to a litre or more.
John Elsberry, Manufacturing Engineer at Synovis Interventional Solutions (Lino Lakes, Minnesota, USA, www.synovismed.com) is involved with development of a device that will be used for thrombectomy procedures. Elsberry says it is important to perform multiple leak detection related tests on each part of the device, which the company is producing as a contract manufacturer. “The product is a small hypotube with a balloon on the end of it that is inflated during the thrombectomy. The overall device is 9.5 ft (2.8956 m) long with an i.d. of approximately 0.013 in. (0.330 mm), Elsberry explains. “There are several transitions along the line, which is composed of tube sections, and testing is done on a 100% basis to make certain that these tubes‘ flow tests are flow tested correctly. One three hole tube is especially critical because the holes are 0.015 in. (0.381 mm) diameter. Any little bit of debris, including hand oils or other substances rubbed over the holes, could occlude the orifices. So, it is critical that the leak testing equipment detects problems of this type and deciphers a good part versus one that should be rejected.”
Large products, for example intravenous bags, fluid and solid collection bags such as colostomy bags, and even the foil or plastic laminate pouches that package medical products involve the added difficulty that they are often elastic and demand even more flexibility in the testing equipment.
Part material
When incorporated into medical devices, polymers and plastics can occasionally behave in unanticipated ways. Elasticity can make repeatable testing difficult because some parts may continue to expand after reaching the desired test pressure and take a long time to stabilise, an effect referred to as “compliance.” In extreme cases, the parts could effectively seal themselves through inherent elasticity or under pressure against the fixture. One way to minimise the effect of elasticity is using a tester that allows accurate fixturing or securing of the part to minimise movement. As another option, some fixtures feature a porous surface or are textured so that air may escape from the product into a small cavity built into the fixture.
Product geometry
Frequently, medical devices feature unusual geometry. Some may incorporate tubes, valves, stopcocks, transducers, closures and components within components, which emphasises the importance of correct fixturing. Any mistakes can possibly abrade or otherwise damage the product.
“I would say that fixturing is one thing that medical device manufacturers often do not take into proper consideration; they think they can do it themselves, but when they go to test the product there is something they did not foresee,” explains Ray, “That’s where experience comes in: letting the leak test equipment manufacturers control the fixturing. There are a lot of things they already know that help manufacturers incur less downtime and less development time, so they are saving money in the long run and adding to productivity, too. Also, leak test equipment vendors are going to ensure that their equipment complies with the specifications and requirements given to them. It’s their responsibility to deliver that to the manufacturer.”
Leak volumes
Ideally, leak volumes should be near zero. Life threatening risks of even the tiniest leak include cross contamination of blood, bacterial seeding and insufficient or excessive medication dosages. At a minimum, leakage can jeopardise patient comfort. For example, with personal long term care products the repeatability of the testing helps verify that critical thresholds are met. Unlike reliance on the memory of an operator to initiate a sequence of tests, the automation of leak detection in some testing equipment helps improve repeatability because the testing process becomes nonsubjective. The ability to automatically compensate for temperature changes is built-in to some leak detection equipment and this also helps ensure accuracy and repeatability in testing.
Application specificity
One of the shortest means to accomplish many of the requirements of testing medical devices is to find a tester specifically designed to handle each individual product. Given that this is an unrealistic approach in that a medical manufacturer may offer more than 100 different products, any tester should come as close as possible to adapting to each application.
“We used to build all of our test equipment from scratch and, although doing this worked for us, we needed to shorten our project time frame,” says Brian Yutzy, a design engineer at Marshall Gas Controls (San Marcos, Texas, USA, www.marshallgascontrols.com) a manufacturer of high-pressure and adjustable regulators, hose assemblies and pigtails. “We went looking for an alternative solution and the vendor worked with us to develop a system that fits our needs. It built-in the necessary input/output (I/O) cards, tools and transducers.”
A testing equipment vendor can further maximise the potential of test equipment to fit a manufacturer’s specific needs through a redesign of the tester or by reconfiguring it to integrate within the manufacturer’s production system.
Manufacturing environment
“You have to meet your customer’s requirement that the products are as clean and defect-free as possible, and are in no way going to endanger or create discomfort to the patient,” adds Ray at Terumo Medical. “Typically, our clean rooms are at least Class 100000 and many areas are Class 10000 or lower.”
Any leak tester used in a clean room environment must comply with the conditions. Components should be of noncorrosive material. Fittings that require sealant should not allow any out-gassing into the part being tested or the room.
Cycle time
Ideally, test equipment must support semiautomatic or fully automated leak detection systems that streamline product delivery, sealing, clamping, testing and marking. Timesaving features such as infills, which reduce volumes and allow for varying product sizes to be tested in the same chamber with minimum changeover time and expense; programmable logic controller connectivity; and remote start input can greatly speed the testing process. In addition, the latest multichannel testers, some of which can operate as many as 10 channels, automatically cycle through all the tests at the push of a button.
The use of independently started test channels can also improve cycle time and throughput. One leak tester can then be shared by several operators, each of whom may start their own test or sequence independently. “At our Elkton, Maryland plant, it used to take us 4.5 minutes to cycle through a test,” says Ray. “With our automated leak testers we have it down to just 1.5 minutes, improving throughput by 200%.”
Human-machine interface
A leak tester, no matter how good its performance, is nothing if the human–machine interface lacks ready comprehension. Because medical device leak testing is often labour intensive, the tester should be designed so that it is easy to use to minimise operator fatigue.
Programming should be simplified by software with preformatted test configurations that can be easily modified to each application. Results should be nonsubjective and clearly displayed to minimise the occurrence of false rejection or acceptance.
Leak testers should also lend themselves to instant, intuitive operation. Additional considerations include touch screen input, large graphical displays, selectable engineering units, built-in diagnostics and remote troubleshooting.
Documentation
In the realm of medical devices, malfunctions extend far beyond the domain of product liability lawsuits and can enter the universe of wrongful death actions, hence proper documentation is paramount. At a minimum, a leak tester should be able to input leak rates and other results directly into a database or spreadsheet for archival purposes.
Other industry options for data downloading include RJ-45 Ethernet connectors, RS-232 serial ports, PCMCIA card slots, digital I/O cards and 24-volt reference outputs. Also helpful are screens that display results at the control unit, for interrogation by a supervisor as required. Marking capabilities by ink, percussion or laser also speed and reinforce the documentation process.
For more information, contact Martin Bryant, Vice President, Sales and Marketing at Uson LP, 8640 North Eldridge Parkway, Houston, Texas 77041, USA, tel. +1 281 671 2000, e-mail: info@uson.com, www.uson.com.
Uson provides online tables that offer quick calculations for hole diameter to flow rate calculation; leak correlation for one gas relative to another; leak rate to pressure drop relationship; flow rate conversion; conversion for volume; conversion for pressure; and hole/size/flow rate approximation.
