T. Kim Parnell
The United States Food & Drug Administration (FDA) requirements for Class III implantable medical devices requires a 10 year life, or approximately 400 million cycles of service.
High cycle testing of complete prototype devices to 400 million cycles can take months to complete. If the design does not survive, the test cycle repeats, resulting in significant delays. Crucial information is obtained in far less time by combining accelerated testing with FEA of the component. This approach saves time by performing the design iterations and comparisons using FEA. The refined design now has a very high probability of success in the initial round of high-cycle tests on the full component. While fatigue testing gives only pass/fail data, combining test data with the analysis results provides information on the design margin. The margin shows how much the design exceeds the minimum requirements and represents a factor of safety above the required life.
Real-World Examples
The development of implantable, medical devices typically has all of the considerations described in the prior section. A cardiovascular stent provides a good case study to illustrate these ideas. A cardiologist can use a balloon angioplasty procedure to restore full blood flow in an artery that is partially blocked by arterial plaque. Plaque reduces the blood flow because it decreases the flow area in the artery. In angioplasty, the physician tries to restore the flow area in the artery by compressing the plaque to alleviate the obstruction. A stent is often deployed as a sort of scaffolding to exert pressure on the plaque, thus keeping it compressed against the arterial wall.
After manufacture, a balloon expandable stent is squeezed down onto an angioplasty balloon and then inserted into a delivery catheter. Reducing the overall diameter of the stent/balloon combination is desirable since this reduces the crossing profile and makes it easier to maneuver the stent/balloon through the tortuous passages of the vascalature.
For a stent made of 316L stainless steel, the size reduction from the manufactured condition to the delivery configuration in the catheter results in plastic strain. After positioning the stent in the desired location, the balloon is pressurized to expand the stent securely against the arterial wall. The maximum inflation diameter is typically larger than the arterial diameter to establish good contact. The expansion step from the stent on the balloon in the delivery configuration to the maximum dimension at full pressure increases the stent diameter by approximately 400 percent, and causes additional plastic deformation in the device. Next, the balloon is deflated, and the stent unloads elastically, typically reducing its diameter by 5-10 percent from the maximum inflation diameter. Now that the stent is deployed in the artery, the in-vivo loads from the body are applied.
Because of the plastic deformation, this sequence of loading steps leads to a state of residual stress on the stent following deployment. The residual stress distribution after deployment is static with respect to time, and therefore does not change. The in-vivo loads produce elastic cyclic stresses that are superposed on the mean stress (residual stress) field. The FDA requires at least a 10-year life of the device when subjected to the cyclic in-vivo loads.
There are several possible ways to get test results for comparison with the analysis. The stent diameter change as a function of the balloon inflation pressure provides one such comparison. After inflation to the maximum diameter, the balloon pressure is removed, and the stent recoils elastically. Then compare the measured elastic recoil against the value obtained in analysis. For a third type of test, relevant to the in-vivo loading, the stent is deployed in a very thin elastic membrane, and vacuum pressure is applied internally. Because the membrane is very flexible, it is not capable of supporting the vacuum pressure loads. All of the vacuum loading is effectively applied to the stent. This test is conceptually simple, but difficult to measure accurately in practice.
The analysis results provide an important link to understanding the expected life of the device when subject to high-cycle loading. Hairpin or U-Bend specimens removed from the complete device may be cyclically loaded at high speed, generally employing much higher cyclic rates than is possible for the full stent. Pulsation testing of the full stent generally requires elaborate testing equipment with the stents deployed in tubes that are then cyclically pressurized in a saline environment. Approximately three months are required to test a full stent to 400 million cycles in saline. In contrast, the subspecimens may be cycled in air much faster and with higher loads, bringing the time required for a test down to less than a few days. Performing a series of these tests to failure for varying cyclic displacement amplitudes produces a device-specific S/N curve (cyclic displacement amplitude versus the number of cycles to failure). The FEA results provide the link between the complete device and the high cycle tests on the subdevice. Analysis provides the stress level that is associated with a given cyclic displacement level. With this information, the S/N data is recast into the form of cyclic stress data.
This approach is extremely useful for design by dramatically compressing the development time. The accelerated approach does not eliminate the need for testing full devices in saline. However, it moves the long time testing out of the initial development cycle and gives a very high probability of success.