Hermetic seals are an integral to increasing the biocompatibility and longevity of an implantable device. However, they are only as effective as the integrity of their seal. For medical implants used inside the body, organic material can affect the device or pose a threat to patient safety. Therefore, leak tests can assess the potential of microscopic leaks that cause unwanted moisture or contaminants to enter the device. For miniature hermetically sealed medical devices, their tests require micro-sized parts for both device development and testing fixtures. This article delves into fine and gross leak tests for miniature hermetically sealed medical implants and provides parameters for these tests.
Fine Leak Testing
When it comes to fine leak testing, helium gas is the standard choice, due to its unique molecular properties. But, why helium? If you are testing microscopic leaks, especially for miniaturized implants, you want the gas that has the smallest molecular size. At first, hydrogen seems ideal since it has the lowest atomic mass. However, helium’s position as a noble gas causes its atomic radius to be 11% smaller than hydrogen. Furthermore, while hydrogen is highly reactive, helium is an inert gas and not flammable, making it the ideal choice. The ASTM E493/E493M − 11 standard highlights two methods of performing helium leak testing: bombing and backfilling.
Method A: Bombing
In bombing, the implant is hermetically sealed and externally pressurized with helium to detect gas leakage into the device. First, the implant is exposed to helium tracer gas in a vacuum chamber under a usual minimum pressure of 4 atm for about 1-4 hours. It is then transferred to a different chamber where a mass spectrometer is used to detect the amount of helium that leaked into the device. During this transfer to the test chamber, there is a time limit constraint of 30 minutes according to the FDA to prevent loss of helium. Acceptable leak rates depend on device volumes: for miniaturized implants, these leak rates are exponentially lower and require rigorous testing. A typical pass criteria for bombing testing is 10-7 Pa·m 3/s to 10-9 Pa·m3/s. However, engineers typically reserve this testing method for practical, less sensitive applications with fewer critical requirements.
Method B: Backfilling
In backfilling, the implant is filled with helium and helium leakage out of the device is determined. It is preconditioned in a vacuum chamber to evacuate all the remaining air inside the device. While the implant is inside the chamber, it is typically filled with helium gas at a pressure of 1 to 5 atm and hermetically sealed. After it is sealed, the implant is removed from the chamber and a detector quantifies how much helium leaked out. The pass criteria for this ultra-fine leak testing method is 10-9 Pa·m 3/s to 10-11 Pa·m3/s. Due to the complexity of filling implants with helium and hermetically sealing them inside a vacuum chamber, engineers use this method for the most critical applications. These include long-term implantable devices like pacemakers, cochlear implants, and neurostimulators, where even microscopic leaks can lead to device failure over time.
Gross Leak Testing
Although helium leak testing is highly sensitive, fine leak tests cannot detect gross leaks. Therefore, engineers use a separate set of tests to identify gross leaks, including pressure flow and pressure decay tests. The typical acceptance criteria for moderate and large leaks are 10-2 Pa·m 3/s to 10-5 Pa·m3/s and 10-5 Pa·m 3/s to 10-7 Pa·m3/s, respectively.
Pressure Flow Test
Pressure flow tests check for gross leaks by filling the object with pressurized gas, letting it stabilize, and measuring leakage. For miniature devices, a 2 to 10 psi pressure is typically used for 10 to 60 seconds for ultra-sensitive detection. If too much gas flows out, it means there’s a large leak. Pressure flow testing is quick, reliable, and sometimes non-destructive, making it useful for automated quality checks in manufacturing. (Refer to ASTM F1140 for further information)
Pressure Decay Test
In a differential pressure decay test, the implant is placed in a chamber while pressure is applied through the feedthrough or internal cavity. The chamber is kept at atmospheric, lower, or vacuum pressure, creating a difference between the inside of the device and its surroundings. If it is hermetically sealed, the internal pressure remains stable, but if there is a leak, gas will escape and cause a change in internal pressure. Because the timing, pressure, and volume of the implant is known, the decay rate of the pressure can be calculated (same parameters as a pressure flow test). Examples of applications for this method are for pacemakers, neurostimulators, and drug pumps, where feedthroughs must prevent unwanted fluid flow. (Refer to ASTM F2095 for further information)
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