This benchtop test provides empirical evidence for the continued structural integrity of the devices when subjected to mechanical fatigue replicating in vivo conditions. The test is designed to simulate the device radial fatigue due to expansion and contraction of the surrounding vessel. Physiological strain of a healthy vessel at normal heart rate is first modeled using mock arteries implanted with the device. The test is then accelerated to obtain results in less time than physiological rates would allow. The test is conducted under simulated physiological conditions with saline at 37°C.
The testing objective is to meet the requirements for in vitro mechanical fatigue testing stated in ASTM F2477-07 “Standard Test Methods for in vitro Pulsatile Durability Testing of Vascular Stents” as well as ISO 25539-1, 7198:1998, and 99128. The test demonstrates the integrity of the device under mechanical fatigue for a minimum of 10 years post-implantation. A device failure is defined by the device manufacturer (for example, any broken or cracked strut visible at 30x magnification during or at the end of the test). The device manufacturer will also examine the devices post-testing against the defined acceptance and/or failure criteria.
MDT uses ElectroForce 9100 Series Stent Testers which operate on the principle of fluid displacement. The Model 9120 Series Test Instruments owned by MDT were designed with straight AAA devices in mind. They accommodate larger mock artery diameters and lengths and will push greater fluid volumes to achieve radial strain levels of up to 9%. Electrodynamic linear motors on each end of the tester apply a sinusoidal action which forces fluid into and out of mock arteries causing them to expand and contract. A laser micrometer measures the radial strain by scanning the outside diameter of the mock arteries at 1200 scans per second.
In addition to standard bifurcated setups, MDT also has the ability to simulate bending strains often involved in AAA indications. The drawing (right) shows a system that has been modified to perform pulsatile fatigue in a bent condition. This system performs radial pulsatile fatigue on a specimen that has been bent 180 degrees.
|* Frequency is both size and strain dependent.|
|Device OD (Aortic)||Mock Artery #||Frequency*|
|20mm - 40mm||4-6||20Hz - 40Hz|
Not unlike peripheral vascular devices, repeated bending strains can be more dominant than radial fatigue strains. In these cases, special fixturing may be needed to impart the strains due to bending. The drawing (right) shows a test fixture developed by MDT engineers that can be fitted to either a Bose ElectroForce 3300 or an Instron ElectroPuls Test Instrument. The fixture features a multi-station lower platen outfitted with the appropriate aortic fittings, as well as an upper manifold outfitted with the appropriate iliac fittings. The testing fixture is lowered into an environmental chamber filled with temperature-controlled saline. The fixture can accommodate up to 12 specimens and can be run at frequencies up to 40Hz. Another variation of this fixture (right, lower) allows MDT to perform compression/tension testing. In addition to providing information on stent frame durability, both tests may also provide useful information on the strength of AAA aortic wall attachments(e.g. hooks or tines).
MDT tests are conducted in accordance with ANSI/ISO/IEC 17025-2005. Testing schedules and resource needs are determined during project kickoff meetings with MDT engineers and the customer. Services are performed in accordance to standard operating procedures and test protocols specific to the project. For feasibility or non-standard tests, MDT will develop a method and protocol with the customer's input and will review the final process with the customer prior to initiation of the test.
MDT offers several inspection methods including:
The test equipment software monitors the cycle count, displacement, and pressure throughout the duration of the test. MDT engineers monitor tests daily and record temperature and pressure for the system, as well as the outside radial strain at predetermined positions for each device being tested. MDT sends weekly updates to ensure customers are apprised of the status of their test. Updates include raw data collected throughout the week for the customer’s analysis.
MDT tests are conducted in accordance with ANSI/ISO/IEC 17025-2005. In compliance with this standard, MDT test reports include comprehensive documentation and summary of the test protocol and results. All reports are GLP compliant and can be submitted directly to regulatory agencies. Any test method deviations, additions, or exclusions are documented and reviewed with the customer prior to beginning the test. Information within the report includes the specific test conditions, statements of compliance with test protocol and the estimated measurement uncertainty. The reported results only relate to the items tested. Any opinions and interpretations provided will be clearly marked as such in the test report. Additional information required by customer-specific methods will also be noted. At the conclusion of the test, MDT provides the customer with a Microsoft Windows formatted CD containing copies of all raw data, inspection results, and other information collected during the test (see example test reports).
Please take a few moments to tell us more about your device and test requirements in our short questionnaire, and we will be happy to provide you with a quote.