ASK THE EXPERT
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Jim Ritchey is director of medical devices and biomaterials markets at Instron Corp. (Norwood, MA). With 23 years of experience at the company in sales, service, and marketing positions, he focuses on account management, technology development, collaboration, and acquisition activities. Ritchey received a BS in materials science and engineering from Cornell University (Ithaca, NY) and an MBA from Cleveland State University (Cleveland, OH).
Meredith Platt is product manager at Instron Corp. She has been with the company for five years as both an applications engineer in the biomedical market and as a product manager for Bluehill Software and Instron's line of video extensometry equipment. Platt received a BS in biomedical engineering from Worcester Polytechnic Institute (Worcester, MA), an MS in biomechanics from Pennsylvania State University (State College, PA), and an MBA from Northeastern University (Boston).
What is the role of mechanical testing in ensuring end product device quality?
Ritchey, Platt: Product quality depends on mechanical testing in three important areas of the product development and manufacturing process. First, testing of raw materials determines the materials' static and dynamic properties. In turn, these properties are used as inputs to finite element analysis models that help predict the behavior of new medical device designs. Second, during production, materials often undergo a transformative process before being incorporated into a medical device. For example, plastic is extruded into tubing before it is used to build parts for a catheter. Testing is performed at this stage to verify that the material's properties still meet the performance requirements of the product. Third, devices are assembled from a variety of different materials and components. A stent delivery system, for example, is composed of tubing, wires, connectors, and the actual stent. Testing is performed to ensure that the connectors perform properly and that the stents are deployed appropriately under interventional conditions.
In all three of these phases, a variety of tests is performed to ensure product quality during patient use: fatigue testing for gauging stent life, torsion testing for determining connector strength and integrity, hardness testing for ascertaining the quality of metals processing and heat-treating processes, impact testing for qualifying repetitive-use surgical instruments, and other tests for determining such characteristics as peel/adhesion strength, kink resistance, and friction glide force.
What new demands have been placed on testing and inspection equipment by the growth of minimally invasive surgical techniques and the increasing role of stents, catheters, and balloons?
Ritchey, Platt: The deployment of ever-shrinking stents for procedures involving smaller and smaller neural and less-accessible peripheral arteries requires instrumentation to measure very small forces in tension, torsion, and radial testing modes. Based on the use of electromechanical and electrodynamic equipment, such tests are critical for demonstrating product performance and durability. Additionally, as products shrink, fatigue characterization, gripping techniques, and load issues will become increasingly challenging. To meet this challenge, new grips and load cells will be required to accommodate very small parts.
What equipment breakthroughs are improving medical device testing?
Ritchey, Platt: Two recent instrument breakthroughs are improving the quantity and quality of the information being collected through medical device testing: video extensometry and electrodynamic systems for both fatigue and static tests on materials and components.
The introduction of video extensometry allows for direct noncontact measurement of strain during testing, providing accurate data on the level of strain a part is subjected to when it reaches the breaking point. In addition, the method improves reproducibility, which can be affected by the wear and tear exhibited by traditional contacting extensometers. Video extensometry also enables users to test stent materials such as nitinol wires in body-temperature baths, yielding detailed results such as residual elongation, upper plateau strength, and lower plateau strength.
The introduction of electrodynamic test systems provides fatigue testing with a total displacement of up to 60 mm. While older technology accommodates only smaller products, these systems can perform research into overlapping stents and large stent grafts, which often require larger displacements.
In light of recent performance problems with medical devices such as drug-eluting stents and increased scrutiny by government regulators, are testing practices changing or becoming more rigorous?
Ritchey, Platt: Testing practices have advanced throughout the development and evolution of stent products. However, instances of cracks or premature failures in drug-eluting stents are prompting an intensified search for appropriate testing techniques.
Current regulations focus on test-to-success criteria, in which whole parts are tested and simulations are performed that mimic how parts will function in the body. However, some experts are examining a variety of methods known as testing modes and combination testing modes, which are more representative of clinical conditions. In contrast to test-to-success methods, testing modes and combination testing modes are based on materials science. For example, they seek to understand the fibers of a mesh to determine when they will crack. These modes involve enhanced scrutiny of medical device materials using advanced tensile testing as well as the examination of end products using tensile fatigue, torsion fatigue, bending fatigue, and other test methods.
However, when all is said and done, there is no sure-fire way to determine through testing when a product will deteriorate. It takes millions of test cycles to determine product breakdown points.
How are testing methods incorporating the use of equipment and software that simulate body conditions and movements?
Ritchey, Platt: The development of testing techniques that simulate body conditions is a perplexing challenge. Because human anatomy varies so greatly from one body to the next, simulating standard body conditions is difficult to achieve or predict. For example, one abdominal aortic aneurysm may have a single bulging characteristic, while another may have a long series of thin-walled features. Can we design a single standard test and instrument that meets both of these disease states? Probably not.
The most important step in the medical device testing field has been the movement toward solid scientific principles of fracture mechanics and finite element analysis, techniques which have been employed successfully in the automotive and aerospace industries for decades. Additionally, with the implementation of a test-to-fracture mentality rather than a test-to-success mentality, the industry is moving quickly to establish safer and more effective devices.
Do you have a general or application-specific question on another topic that one of our experts could answer in a future column? If so, e-mail MPMN managing editor Bob Michaels at bob.michaels@cancom.com.
