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Catheters are used for a wide variety of interventional procedures that most commonly target issues within a person’s arterial network but are also used for urological conditions. The issues being addressed could include diagnostic and therapeutic tasks such as clogged arteries, irregular heartbeats or compromised urological pathways. In a typical procedure, the catheter is used to deliver a medical device (e.g. stent) and treat the disease of concern. It can even be used to deliver devices like artificial heart valves percutaneously through an incision in the groin all the way to the heart. Needless to say, the robustness and reliability of both the devices and the delivery catheter are of critical importance.

These products, therefore, have to undergo a battery of physical testing that will determine physical properties and characteristics to meet their specified capabilities. Although there are several other types of tests that must be considered, we will explore the three main physical tests conducted in laboratories around the world and their respective potential equipment solutions. This trifecta of solutions includes results for tensile testing, lubricity testing, and pushability/trackability testing.

Tensile Testing (No. 1 of 3)

Tensile testing is foundational for the determination of core material properties of extruded catheter tubing and materials for guidewires and stents, for example. Whether we look at large urological catheter materials made of polyethylene or ultra-small tubing targeting neurovascular applications, the desired results and the test systems required to deliver them are quite similar. Typical required test parameters and results include wall thickness (and the reference inner & outer diameters), peak or break load (and resulting stress when including wall thickness and total specimen area), breaking load (stress) and sometimes modulus. Occasionally, testing at body temperature (37 ⁰C or 98 ⁰F) may be necessary to get a more well-rounded view of the catheter function either in air or in liquid media such as saline.

Typical tensile tests are performed on a universal testing system. First, you must consider the overall travel that your sample or interventional product will require during testing. Ideal systems are available in several heights to create flexibility in configuration. Next, pneumatic grips with special line contacting rubber finishes ensure that test slippage is minimized during testing. Lastly, a long travel extensometer provides high resolution information so calculations of strain at break as well as modulus can be performed automatically. Optionally, the entire system can be encapsulated in a 37 ⁰C temperature chamber to provide all results at room or elevated temperatures.

All forces, speeds, displacements, strains, and temperature should be calibrated on an annual basis in accordance with ASTM E4. The actual test can be performed by using state-of-the-art testing software that can be configured to meet regulatory requirements mandated by the 21 CFR Part 11 standard.

Lubricity Testing (No. 2 of 3)

One common characteristic of medical tubing is hydrophilic coatings, which dramatically reduce the friction of the catheter as they are deployed throughout tortuous atrial and venous networks as well as urological pathways. These high-performance coatings are typically evaluated by clamping onto one end of the catheter and pulling it through two small plates or pads on the bottom end. With new technology evolving almost daily—involving surface modifications with plasma treatments, for example—performing an accurate and repeatable lubricity friction test is critical. 

Testing for the measurement of the frictional resistance or sliding ability can be performed on the exact same system as the tensile testing application mentioned above. Similarly, the testing must be performed at body temperature and products are stored in a heated water bath to more precisely simulate their actual usage. The configuration that creates the most repeatable and reproducible results includes automatically applied deadweights that provide the exact same clamping resistance force for each test. Testing machine manufacturers can provide a tall frame to accommodate the very long catheter test specimens that are commonly used. The configuration includes a heated water bath so that the catheter can be tested under body-like conditions. The rubber pads through which the catheter is pulled should be examined and replaced on a regular basis to ensure optimal long-term consistency in test results. A bonus capability is the ability to perform cyclic testing, which simulates aging of an interventional device or changes of the hydrophilic characteristics and procedures.

Once again, the lubricity testing system should have all system elements calibrated annually. The same testing software would be utilized for testing and configured for 21 CFR Part 11 compliancy needs.

Trackability Testing (No. 3 of 3)

To complete the trifecta of solutions, we must look at the most complex of the three testing applications for catheters. With these applications the catheter is often used in conjunction with other elements of the delivery system (guidewires, stents, etc.) and is inserted into and withdrawn from anatomical models that simulate the vascular system of interest. The models can be 2D or 3D in design and are used to simulate interventional delivery procedures. The output of this process would include insertion forces, tracking forces, product deployment and product retrieval. Ultimately, the test is providing both qualitative and quantitative performance characteristics of the products in an “as-close-as-possible” to real-world environment. This testing is typically performed in both wet and dry environments to evaluate the products’ capabilities.

Testing for these products is usually designed to best mimic how the surgeon would actually be utilizing the delivery system. First, it is routinely performed in a horizontal direction since patients undergoing the procedures are lying down. Next, the actual deployment of any device is not completed in a single motion since the catheter must weave throughout a long tortuous path which can ultimately be longer than 8 feet. Existing horizontal test system employ submersible pneumatic action grips which work sequentially to grab and push the catheter, let go, return, grab and repeat the process—emulating a surgeon pushing the device—via the moving test machine crosshead. Having a configuration to handle the long length of catheters without an operator manually feeding them into the test is a critical feature of the systems.

Leading testing machine manufacturers offer comprehensive support with all aspects of trackability testing. This ranges from the preparation of user requirements and customer-specific specifications to design qualification, project implementation, and the final installation qualification and operational qualification processes. Manufacturers also offer qualification services that cover all facets of trackability testing, where customers can choose between standardized qualification documents or individualized documents tailored to their individual user requirement specifications. Lastly, the right horizontal system, similar to the tensile and lubricity solutions, utilizes testing software and can be made compliant with all 21 CFR Part 11 requirements.