Gripping Medical Devices for Tensile Testing
Medical devices such as leads and sutures are typically subject to tensile testing to characterize their mechanical properties under tension. Gripping these devices during testing provides additional challenges and can require custom fixturing to obtain meaningful data.
Tensile testing is a common method that assesses the ultimate tensile strength, yield strength, yield point, elongation, Young’s modulus, and other mechanical properties of medical devices. Securing wire samples on tensile testing equipment can be challenging depending on the device. If slippage in the grips occurs during testing, the data becomes less reliable. For some devices, samples also tend to fail at the grips, indicating gripping issues that need to be addressed.
Custom grips for medical device tensile testing
A well-designed medical device tensile test will closely evaluate the component properties and include suitable grips that prevent sample slippage, allowing them to fail between the grips rather than at the grip-sample interface.
Having tested the tensile properties of medical devices for several decades, we have developed a full arsenal of gripping fixtures that we continue to expand with new device designs. This article features examples of some of our custom tensile testing grips for different devices to provide you with some design ideas that may be useful for your testing.
Medical tubing tensile testing
The device shown in Figure 1 is a small diameter tubing inserted into a collet-type grip that prevents tubing collapse during testing. To secure the sample, slip a plastic crimping tube over each end of the specimen, insert a copper mandrel into each specimen end, load the first crimping tube into the spinning collet grip, and tighten the collet. Then mount the loaded collet grip on the bottom shaft of the tensile tester, and the unloaded collet grip on the top shaft of the tensile tester. Raise the top collet grip to allow space for loading the other end of the specimen, insert the crimping tube until it is flush with the grip, and tighten the collet.
The sample is immersed in heated saline solution and pulled to failure on an Instron tensile tester.
Nitinol wire tensile testing
Nitinol wires, or suture materials, have a variety of applications within the medical device industry, including vascular, neurovascular, orthopedic and dental. ASTM F2516-18, Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials, specifies the methods for determining upper plateau strength, lower plateau strength, residual elongation, tensile strength, and elongation of nitinol wires.
In the fixture shown in Figure 2, the nitinol wire is wrapped around a grooved round part and then secured in place with a hand-tightened compression clamp. Because the wire is wrapped around the grooved fixture, friction takes over as the primary constraining force over a larger surface area of the device under test. This helps limit any premature or non-representative failures at the grip site from mechanical damage or clamping. Another application of this fixture is for suture testing as it allows for looping the suture material around the grooved fixture and securing it in place to prevent slipping.
Neurostimulator lead tensile testing
Neurostimulator lead tensile testing, shown in Figure 3, often takes place after cyclic loading to characterize the maximum load, maximum elongation and failure method for each lead. Lead anchors are potted and mounted on the grips, and care is taken to prevent electrode crushing. Continuity measurements are taken prior to test start and once the test is completed. Typically, a given increase in DC resistance or complete discontinuity will constitute a failure. In addition to the mechanical properties of the lead, dielectric withstand can be performed post-test to ensure the integrity of the electrical insulation of the lead.
Other medical device tensile test setups
Some devices have a metal wire encased in an outer sheath of a different material, typically a plastic. The outer sheaths are often coated for different device applications. The graph below, Figure 4, shows a typical load vs. strain curve highlighting the ultimate strength and yield point. In this example, the tensile properties of samples with three different coatings, C1-3, were compared against the uncoated sample.
A variation of a tensile setup used for measuring a device’s coefficient of friction is shown below in Figure 5. In this case, the coated outer sheath is clamped at the top with alligator-style grips and secured at the bottom of the bath. To give it stiffness, a mandrel is inserted into the sheath. Along the sample length, the device is placed between two Delrin® pads and pulled upwards and downwards for a total of 15 runs. The test is then repeated for varying pull rates and coatings. Average load and average peak load vs. run are captured during each test. Peak loads represent the maximum force values which occur at the onset of each advancement and retraction (change in direction). Thus, the peak loads represent “break loose” forces as the samples transition from being stationary to being in motion. This allows for easy comparison between coating options for a device.
The Element advantageOur Engaged Experts excel at medical device mechanical testing and have worked with many challenging device designs and test setups over the last several decades. Contact us to discuss how we can help with your test project.
Element has one of the most expansive medical device testing scopes in the world ranging from orthopedics and cardiovascular implants testing to EMC/EMI/product safety testing, and biological and packaging evaluations. We strive to meet all your medical device testing needs in the most expedient, efficient and responsive way.