Put to the test: Addressing catheter testing challenges

Alan Thomas, Zwick Roell discusses catheter testing and how to address any challenges.

Increasing regulatory pressure is challenging medical device manufacturers to meet strict requirements in design, testing, and production. This trend is reflected in catheter systems that are used for vascular surgery along with coronary stents.

There are many different types of catheters including those used to transport various small instruments or repair devices to a particular site in the body. Some catheters have small integrated tools to enable a surgeon to repair a problem in the patient’s body without having to perform open heart surgery. Other catheters deliver drugs directly to an infected area, while some can deploy a stent that will keep an artery open. 

For surgeons, challenges associated with such procedures include feeding catheters into femoral arteries, navigating sharp turns and advancing the device without damaging surrounding tissue. Manufacturers of catheter systems work closely with surgeons to develop standard and custom catheters, stents, and guide-wires to meet these challenges.

Catheter manufacturers continually monitor their product by physically testing the complete units, as well as individually testing the component parts. To effectively test such devices, catheter manufacturers need to replicate the condition of a patient lying on an operating table and a surgeon inserting a catheter. Meeting such needs requires a horizontal testing machine with a range of special adaptations to facilitate simulated operational procedures being carried out. 

Stents are tested both in compression and flexure modes and the frictional behaviour of the complete catheter is measured as it is pushed through a simulated artery known as a “tortuous path.”

To facilitate this action, test equipment engineers at Zwick in Germany, have developed a system that controls the test machine crosshead and automated pneumatic grips. The horizontal machine is essentially a test bed that incorporates a workspace to accommodate 3-D models and water baths. Even large delivery devices can be mounted and tested throughout their full range of functionality.

In a typical test, the testing machine pushes the catheter into the tortuous path for a designated distance before the pneumatic grip, which replicates the hand of a surgeon, then releases the catheter and the crosshead moves back to the original start point. The pneumatic grips close onto the catheter and the crosshead moves forward once again. 

This sequence is repeated until the catheter has been fully inserted into the simulated artery. This test procedure, which is fully automated, can easily accommodate different configurations of tortuous paths and the machine software typically enables the following range of results to be calculated.

• Track force. Measures the force needed to advance a catheter, interventional device or guide wire through a tortuous path.
• Push efficiency. Uses the proximal and distal load cell to measure the amount of force the distal tip of the product sees when a known force is being applied to the product on the proximal end.
• Insertion force measurement. Measures the force used to advance through the introducer sheath.
• Guide-wire movement. Measures the force needed to advance a guide wire though catheter, guide catheter or other interventional device.
• Flexibility. Measure of a catheter tips ability to track over a specified bend in a guide wire, such as 90 degrees.
• Guide-wire and catheter lubricity track measurement. Comparative test using the track test data to determine if coatings affect the force required to advance product through a tortuous path.

The results can be calculated with high precision, as the extremely stiff load frame with digital control, ensures that forces measured during the test originate from the sample under test and not from within the machine itself. The machine control system has such a high degree of accuracy and resolution that it can position the crosshead of the machine to less than 1µm, and read forces to an accuracy of better than 0.5% down to values of less than 0.1mN/0.02lbsf. 

The machine software platform controls all of the test parameters including the safety features of the machine. At the same time, it can acquire and process the raw test data in real-time which is stored as individual data points and as calculated result data. A standard video camera to be connected to the system to record the test sequence and the incoming video signal is automatically synchronised with the force and displacement data. It allows a more complete record of the test to be saved or transmitted to interested third parties.

Stringent regulatory environments require the ability to maintain accurate records of the testing procedures and results, and the software includes the functions ‘electronic records and signatures’ for complete digital documentation of all safety critical tests.