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Program Area: Mechanics of Materials and Structures
Scope: Medical device performance and safety requires reliable and safe use of materials. The synthesis, processing, and fabrication of materials affect the molecule structure, phases, and ultimately the physical, chemical, and mechanical properties, and biocompatibility of devices used in medical applications. Failure can result from improper material selection, inadequate stress analysis during device design, manufacturing errors, or misuse/abuse of devices. The Shiley heart valve weld failures, silicone breast implant membrane ruptures, and urethane pacemaker lead cracks are all examples of prominent material integrity issues. Degradation of materials can not only affect performance, it can also produce toxic substances which can cause serious injury or death to the patient. However, degradation is not always undesirable. It may be by design as with resorbables. Thus materials characterizations must always be done keeping the context of end use in mind.
The Mechanics of Materials and Structures program is structured to help CDRH understand materials issues of concern in both pre-market evaluations and post-market reported adverse events. The materials of interest include synthetics like metals and polymers, materials of biological origin, and those used in tissue engineered medical products (TEMPs). We have the capabilities to measure mechanical properties ranging from the tensile strength of sutures and medical glove materials, to the fatigue strength of total joint prostheses. Besides purely mechanical characterizations, our measurement capabilities for TEMPs constructs and scaffolds include quantification of phenotypic stability and the histomorphology of TEMPs relevant cell types. The combined output of this effort includes improved critical review of manufacturers' claims and data, test method development, material and methods standards development, and publications related to the public health impact of medical device materials design, fabrication, or failure.
Background: Activities in this program may be triggered within any phase of the product life cycle. In general, the activities of this group are directed not only towards resolving the specific issue that provided the trigger, but also in finding ways to apply the knowledge gained to future device problems. Since the inception of the FDA Medical Device program, this group has maintained a heavy involvement with voluntary device standards organizations, such as ASTM International. Their participation in these standards activities has leveraged Agency’s resources with industry and academia to create lasting consensus solutions to these regulatory issues once the laboratory studies have been completed. A few examples of these activities are provided in the following paragraphs.
Compatibility issues involving magnetic resonance imaging (MRI) systems and implants or support equipment have existed since this imaging technology was introduced. CDRH has received reports of adverse events through its post-market monitoring system and the scientific literature describing deaths, burns, and other injuries from dislodged aneurysm clips, failed pacemakers, hurtling oxygen bottles, and brain stimulators. In addition, pre-market clearance of devices likely to be exposed to MRI has been a continuing problem. Some implants can be used near the magnet but not in the magnet. Other implants cease to function temporarily in the magnet but restart when the device is removed. Other devices fail completely in MRI. Other devices interfere with imaging but are immune from damage. And, in some cases the device can produce RF heating when placed within the MRI system, resulting in serious burns. In response, OSEL scientists did a number of experiments to support their leading the development of 4 ASTM International standards on MRI compatibility, that are now utilized in pre-market reviews.
As a result of the recent new healthcare industry practices to reuse single use devices (SUDs), OSEL scientists first evaluated the post-market device performance of balloon angioplasty catheters after single use at area cardiology centers. As a result of these and other studies, the issues of reuse have become an integral part of the pre-market review of reprocessed SUDs. Results of OSEL investigations provided vital information used in formulating Agency guidance on SUDs and opened but not used (OBNU) devices and has been used to develop training for field inspectors.
A potential problem was detected during pre-market review when an ODE reviewer observed that a plasma spray coating on total hip could be scraped off with a credit card. Because there were no reliable tests or acceptance criteria for abrasion resistance, all devices of this type were subjected to required postmarket surveillance. Industry responded by improving the quality of the coatings. OSEL put together a research team which developed a test method, directed and participated in a round robin, and wrote an ASTM standard (F1978) for abrasion testing of thermal sprayed coatings. An OST, OSB and ODE team was assembled to develop a guidance document for rescinding the required postmarket surveillance. The companies used the method to document the improved abrasion resistance and the surveillances were rescinded. Pre-market concerns in ODE also recognized the need to standardize the characterization of the alginate, chitosan and collagen materials used in TEMPs as scaffolds. Staff in this program area led the standards development effort which, to date, has resulted in approval of 3 standards for characterizing these materials. This also has led to lab and standards development to characterize natural materials after exposure to cells.
As technology advances in the medical materials arena, it is critical for OSEL scientists and programs to maintain the expertise in these areas. The field of nanotechnology is presenting exciting challenges in composite materials. TEMPs present a variety of material issues as well as cellular response issues. To address the broad scope of materials, we have also worked with other FDA centers (CFSAN, CDER, and CBER) on a diverse range of products, such as blood filters, imaging agents, adhesives and packaging materials, as well as the decontamination of instruments that may have contacted Creutzfeldt-Jakob Disease (CJD). We are also piloting some laboratory work on the effects of repeated sterilization on resorbable polymers which we hope to develop in the near future into a full project.
Project Descriptions:
The following projects in Mechanics of Materials and Structures are planned
for FY04.
Relevance to FDA’s And CDRH’s Mission, Program, and Public Health Impact:
The broad based nature of the mechanics and materials expertise has helped the Center in its mission in every phase of the TPLC. We have worked with ODE, OSB and OC to develop guidance documents and a substantial number of standards. We have worked with OSB and OC in MDR, PMA inspection and Compliance actions. Numerous concerns raised in CDRH have been resolved by simply relying on laboratory experience of OSEL scientists. In other situations, consultations have led to longer duration laboratory studies. The horizontal nature of the program is such that work initiated to address the problems within one branch or division has often been extended to common problems within another. Mechanical and corrosion studies which were initiated by the ODE branch responsible for coronary stents have led to consults and guidance to ODE branches reviewing peripheral, endovascular, biliary and esophageal stents. In the TEMPs arena, products are currently in use for artificial skin for wound and burn repair and for regeneration of cartilage. Many more products and uses for products are under development for TEMPs and other medical devices.
This laboratory supports the Center’s mission to assure the mechanical safety and effectiveness of medical devices. It develops new or improved techniques for measuring wear, abrasion, strength, degradation, and fatigue of materials, and durability of devices. The group works actively to identify biologically relevant parameters, to test and evaluate regulated devices, to assess established and proposed measurement protocols, and to participate in the development and support of voluntary consensus standards and guidance documents.
The laboratory has a broad spectrum of mechanical testing capabilities including corrosion testing, fatigue and abrasion testing, and metallography. It hast has uniaxial and biaxial (tension-torsion) servohydraulic testing machines as well as a collection of universal testing machines suitable for a wide range of load and displacement levels and rates. The laboratory also has the capability to perform a variety of morphological measurements using an array of instruments that include an analytical TEM microscope, SEM with EDAX, atomic force microscope, small angle X-ray scattering, fluorescent laser scanning confocal microscope, and a number of photomicroscopes.
Five Year Objectives:
Materials issues will continue to play a major role in the overall safety and effectiveness of medical devices. History will repeat itself; things will continue to degrade, wear, and break. In addition, we must be able to anticipate new areas of development and areas where problems may arise. New materials and new technologies, such as nanophase composites, hydrogels, biointeractive surfaces and TEMPs will see future applications in the universe of new medical devices. In addition, challenges presented by custom designed components and the development of ever smaller-scale minimally invasive and nanodevices will create a need for more sensitive and miniaturized methods. The features that limit the usefulness of these materials in these applications need to be identified to prevent injuries, and also insure that post-market problems are handled correctly.
It is essential to maintain the laboratory capabilities, both in manpower and equipment, in such a way that we can continue to react effectively to unanticipated mechanical and materials problems with devices as they arise. The mechanical quality of new device materials must be assured by the appropriate pre-market testing and post market surveillance. The appropriate test methods and measurements, and their limitations need to be identified. We need to incorporate these methods into national and international standards, which will result in the use of uniform, described and accepted methods, as well as to increase efficiency, quality and uniformity of product reviews. The goal of the mechanics of materials and structures program is to develop the regulatory science base to meet these new challenges.
Program areas on the horizon may include new issues. Over the next five years, we must continue to maintain a well-rounded laboratory program of proactive research and on-demand problem solving. To this end we must maintain a program of continuing education to ensure that our laboratory staff is familiar with current practices in medical technology, with a carefully directed addition of new members to replace recent retirees who are essential to our continuing ability to achieve the group's mission of protecting the public health.
Updated January 27, 2005
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