These benchtop tests provide empirical evidence for the continued structural integrity of percutaneous and transcatheter heart valves when subjected to mechanical fatigue replicating in vivo conditions. The tests are designed to simulate the physiological strain due to the expansion and contraction of the surrounding vessel or the movement of the leaflet. The test is accelerated to obtain results in less time than physiological rates would allow. Two primary test methods are used to simulate these strains:
Stent Strut and/or Simulated Strut Fixture Fatigue (SN Curve Generation)
The stent strut fatigue tests the high strain portion of the stent or valve support structure. The test specimen geometry is determined using Finite Element Analysis. A strut apex or other representative geometry is placed on an axial fatigue tester using special fixturing to apply cyclic tension/compression. The test fixture is often customized to accommodate the specimen geometry and typically 12 to 15 samples are tested simultaneously. Additionally, load monitoring is often added to ensure that the specimen sees the desired load throughout the test. Load monitoring is also used to detect specimen failures.
Full Device Fatigue
This test is designed to simulate the device radial fatigue due to expansion and contraction of the surrounding vessel. The device is implanted in a physiologically-relevant mock vessel (or within a model of the physiological application site). The applied physiological strain of the healthy vessel/device combination is measured at a normal heart rate and slightly elevated blood pressure (normally 72 bpm and 150/50mmHg). The test is then accelerated to obtain results in less time than physiological rates would allow. The test is conducted under the observed physiological strains 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 15 years post-implantation. A device failure is defined by the device manufacturer (for example, loss of continuity for the stent strut test or any broken or cracked strut visible at 30x magnification during or at the end of the full device test). The device manufacturer will also examine the devices post-testing against the defined acceptance and/or failure criteria.
For the stent strut fatigue test, MDT uses the ELectroForce® 3000 Series test instruments, which operate using a linear motor. For the full device test, MDT uses ElectroForce 9100 Series Stent Testers which operate on the principle of fluid displacement. 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 arteries at 1200 scans per second.
MDT tests are conducted in accordance with ANSI/ISO/IEC 17025-2005. 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.
To satisfy recent regulatory body requests, MDT provides custom mock arteries for percutaneous heart valves in a range of sizes, geometries, and materials. Mock arteries may be fabricated in either latex or silicone, and their design is determined at the time of order.
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 the test daily and record the 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).
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