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U.S. Department of Health and Human Services

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FY 2001 Materials Stability and Degradation Mechanisms

The ability of a medical device to perform its intended function is ultimately related to the performance of the materials used under the particular service conditions to which the device is subjected. Thus, investigating the failure mechanism of medically relevant materials under a variety of service conditions becomes critical to CDRH's mission of assuring the medical devices are safe and effective. In order to achieve this goal, this program area focuses on the development of predictive, laboratory-based methods for determining the long-term stability of materials in contact with a variety of service environments. Included in this determination is identifying critical parameters, either in the device fabrication history, storage, or in the service environment, which can lead to failure through material related causes. These service environments vary by site of implantation, application conditions (oxygen rich atmospheres for example), or storage conditions of devices and components that could affect shelf life.

The tasks undertaken within this program area vary depending on Center needs. The following narratives describe several program elements that have recently been completed and the effects of the laboratory undertakings on the regulatory decisions made by the Center.

As the final step towards the prevention of adverse effects brought on by excessive aging of dialysis cartridges used in hemodialyzers, an experimental and a Monte Carlo computer modeling study of aged dialyzers was completed and published. The results, based on dialyzers as old as 13 years, supported an acceptable shelf life of more than three years. This confirms ODE acceptance of the 2-year manufacturer's expiration date. Also, working with ASTM D11.40 and the Akron Rubber Development Laboratory, an approach and, ultimately a standard for accelerated aging of gloves, to determine shelf life has been devised. This will help make the determination of shelf life less burdensome in that short-term data will be used to predict long-term behavior. A study of the effect of nonoxynol-9 (N-9) on polyurethanes used to make condoms in response to concerns from ODE and Compliance about the effect of N-9 on the shelf life of such products has been completed. Effects on mechanical properties were found to be short-term and compensation by adjustment of initial properties can be done. Biological modeling and animal experiments in cooperation with CFSAN are underway to assess the effect of ingesting bisphenol A. Bisphenol A is a component of food can coatings and dental sealants and leaches from these materials in very small amounts. This work is intended to help resolve arguments about the level of no effect. If this level were to be changed the public health impact could be very great. OST used its expertise in analyzing device performance to develop, in conjunction with NASA laboratories, test methods that helped evaluate the potential for certain oxygen regulators, used in home health care and emergency medical situations, to catch fire. The resultant fires could cause serious injury to the patients and health care providers. As a result of this work, manufactures are changing their designs to be more robust and less likely to ignite. The work on the reuse of single use medical devices described below lead the effort in reclassifying these types of devices, as well as drafting new regulations. Further materials research related to Materials Stability and Degradation is discussed in the section on reuse.

Shelf Life of Medical Gloves

Key words: shelf life, gloves, covalently cross-linked, accelerated aging, creep, modulus

OST has been working with the expiration dating working group of ASTM (D11.40) to develop an accelerated testing method to allow manufacturers to provide a tentative but accurate shelf-life level on glove packages. Since the thermal and/or oxidative degradation properties of covalently cross-linked materials (e.g., latex) often disobey simple Arrhenius or Q10 rule, studies of temperature aging on latex glove fingers are underway to determine the acceptability of various accelerated tests. Creep and modulus are being measured under forces comparable to those developed during actual use. The methodology developed in this project can serve as a platform to evaluate the completeness of the data, and thus, the appropriateness of a manufacturer’s claim to predict a safe shelf life for devices (not just those made of latex) that challenge Arrhenius.

Determining the shelf life of materials is essential to the safe use of medical devices. This gives manufacturers a means for determining a shelf life in a short period of time.

Explant Pathology of Cardiovascular Devices

Key words: decellularized carotid artery, extracellular,

The purpose of this study was to evaluate the morphologic findings in small-diameter freeze-dried, decellularized carotid artery grafts implanted in goats as carotid artery interposition grafts for 6 to 7 months. Unimplanted decellularized carotid artery grafts did not contain intact cells; however, remnants of smooth muscle cells were present in the media. The extracellular matrix was well preserved. All decellularized grafts were patent at explant, without significant dimensional changes or aneurysm formation. Their luminal surfaces were lined by a thin neointima, consisting of myofibroblasts, collagen, and a discontinuous layer of endothelial cells. Histologic evidence of calcification was not observed; however, electron microscopy showed calcification of minute remnants of cell membranes. Inflammatory cells were not present in the graft wall. Host cell migration was greatest in the adventitia along the length of the graft. The migration of host cells into the media was more apparent close to the anastomoses, forming cellular nests rich in extracellular proteoglycans, while cell migration into areas subjacent to the lumen was minimal. Ingrowth of host blood vessels was not observed. These results demonstrate satisfactory structural and morphologic features of a decellularized carotid artery small diameter graft implanted for up to 7 months.

Assessing the Threat of Damage to Medical Devices and Packaging from Irradiation by the US Postal Service

Key words: Gamma sterilization, e-beam irradiation, polymer degradation, device packaging, radiation effect, free-radical generation

The Office of Compliance has been tasked by ORA/ORO to examine the impact of radiation sterilization of medical devices shipped via the mail. OST has accepted the responsibility to find and assemble published data regarding the radiation damage thresholds – for both the electron beam and gamma sources – for the various polymers used in either the packaging or the devices themselves. The collection of available literature, lab reports, or contractor lab reports will become the starting point for deciding what studies will be initiated to fill in the gaps in the matrix of material vs. the radiation source. These complications will be shared with AAMI’s Committee on Radiation Sterilization as well as other affected trade associations.

These changes in mail security have become a new issue with the assurance of medical device safety and effectiveness under the responsibility of CDRH.

Mechanical Properties of Polyurethane Film Exposed to Solutions of NONOXYNOL-9 Surfactant and Polyethylene Glycol

Key words: polyurethane, nonoxynol 9 (N9)

Polyetherurethane condom properties (swelling, annealing shrinkage, tensile strength, strain to failure, modulus, creep, Tg, diffusion kinetics and absorption) were studied after exposure to solutions of N9 spermicide in PEG-400 for various times. As the N9 concentration increased, large amounts of N9 were absorbed and the mechanical properties decreased due to plasticization rather than hard segment domain disruption. Shelf life is not an issue since most of the absorption and decrease in mechanical properties occurred within the first 20 hours after soaking. Anisotropy in properties suggests that the condom should be cut to optimize these property-orientation relationships.

The methods and results, unavailable in either the literature or in document submissions, have been used by ODE in requests for additional information in reviews and will be used in evaluating new condom materials and lubricants.