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U.S. Department of Health and Human Services

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FY 2000 Prediction Of Materials Stability And Identification Of Degradation Mechanisms


This program 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 the identification of critical parameters, either in the device fabrication history or in the service environment, which can lead to failure through material related causes. These service environments can be as varied as site of implantation, application conditions (oxygen rich atmospheres for example), or storage conditions of devices and components. 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.

Abrasion Resistance of Medical Glove Materials
Key words: gloves, abrasion resistance, tear strength

With an increasing demand for non-latex medical gloves, there is a need to assess the durability of alternative glove materials. The durability characteristics of specimens cut from the back and palm area of five varieties of medical gloves were explored via abrasion resistance to a smooth, stainless steel abrading bar. The five glove materials evaluated included natural rubber latex and four non-latex alternatives: nitrile, neoprene, vinyl, and a styrene-ethylene/butylene-styrene block copolymer ("SEBS"). Most specimens were obtained from surgical gloves except for vinyl specimens that were obtained from examination gloves. Results indicated that abrasion resistance is dependent on glove material and not necessarily on thickness of the material.

The Body's Response to Deliberate Implants: Phagocytic Cell Responses to Large Substrata versus Small Particles
Key words: biocompatibility, cell spreading, implants

It is important to characterize possible inflammatory responses to small particles and to clearly separate these effects from responses to larger objects nearby. This research monitors the intermediate production of inflammation-related reactive oxygen and morphological alterations of human monocyte-derived macrophages interacting with the walls of nonpolar and polar polystyrene cuvettes and with small particles of Teflon, polyethylene, Co-Cr-Mo alloy, titanium and alumina. The two types of polystyrene substrata represent the nonpolar "bacterial" (as produced) and the polar "tissue culture" (gas plasma treated) materials widely used in biological testing. The spreading of macrophage during contact with the higher surface energy, polar substratum further suppressed "oxidative bursts" than from rounded cells in contact with the lower-energy, non-polar substratum. Particulate matter engulfed by both rounded and spread cells did not significantly enhance intermediate oxygen production beyond levels observed for no-particle controls. Biocompatibility of some implants might be related to cell-spreading, induced suppression of reactive oxygen intermediate production, improving the tissue integration of gas plasma treated implants.

Cleaning and Performance of Single Use Devices Project
Key words: single use, resterilization, gastrointestinal devices, cleaning protocols

OST continues studies on the effects of reprocessing single use devices for interventional cardiology and gastrointestinal procedures continue. Material and degradation studies focused on developing a variety of mechanical and chemical characterization methods. Interventional cardiology devices were subjected to repeated resterilization with ethylene oxide to study the effect on balloon compliance and catheter resistance to twisting and bending. Evidence of chemical degradation was also investigated with FTIR. Corrosion test methods were utilized to examine the effects of material composition and device design on degradation of metallic components of gastrointestinal devices. Scientists examined microstructural and chemical compositional changes by scanning electron microscopy and x-ray analysis. These methods were also used in developing a protocol for chemically tagging various biological soils for the microscopic examination of the effectiveness of cleaning protocols.

Use of Physiologically-Based Pharmacokinetic Models (PBPK) in Risk Assessment
Key words: PBPK, risk assessment, D4

PBPK models provide a physiological representation of the biological system and describe the relationship between exposure and system responses leading to adverse health effects. Predicting tissue doses or accumulations as a function of exposure indicates where adverse effects would occur, as most are proportional to dose. Because the PBPK model incorporates physiological, biochemical information specific to the chemical as well as to the species tested (animal or human), it offers a method to extrapolate from high to low doses, one route to another, and especially from one species to another. This reduces uncertainty compared to risk assessments based only on exposure levels. Recent PBPK models developed in OST have predicted bioaccumulation of octamethylcyclotetrasiloxane (D4) in fatty tissues (e.g. breasts), the absorption, distribution, metabolism, and excretion (ADME) of toluene diamine from polyurethane implants, and the ADME of methylene dianiline leached from hemodialyzers.

An In-Vitro Study of the Effect of In-Folds on Durability of Mammary Implants
Key words: mammary implant, fatigue, in vitro testing, silicone polymer.

This project examined whether the presence of an "in-folding" in a filled mammary shell could lead to shortening of the time-to-failure, under tensile fatigue. Specimens, cut from pristine shells, were prepared and mounted in `S’-folded, creased and unfolded configurations. A characteristic change occurring in a transmembrane capacitance AC signal, used to monitor working of each specimen, was chosen as a marker that preceded frank shell perforation. Subjecting all specimens to controlled conditions led to estimates of time-to-failure that demonstrated an order-of-magnitude reduction in typical lifetime for the folded specimens, when compared with that for pair-matched, unfolded specimens. Creases also reduced the fatigue lifetime but not as much as folds. Observations based on the experimental behavior of folded shell material have provided possible mechanisms for the development of in-folds clinically, as well as for the development of silicone-on-silicone abrasive wear and generation of debris.

Shelf-Life of Non-Latex Condoms
Key words: shelf life, condoms, mechanical properties

Assessing the shelf life of these products is important because generally a long shelf life is desired. Various environmental agents, lubricants, and additives may affect the shelf life of condoms made of new materials where a history of use has not been well established. OST has studied the effect of various solutions of the spermicide nonoxynol-9 in polyethylene glycol on the properties of various polyetherurethanes used to make condoms. Breaking strength was inversely proportional to the concentration of nonoxynol-9, but stretching the material to failure did not show any trend. Mass uptake and swelling occurred in the first 20 hours of exposure and then approach limiting values. Most effects showed a variation with the orientation referring to the roll direction for films. Adjusting the initial mechanical properties to allow for some decrease by the additives is likely to be sufficient.

Shelf-Life of Medical Gloves
Key words: shelf life, gloves, accelerated test

Manufacturers desire to have an accelerated test to establish a tentative shelf life before all the real data is acquired. Studies of temperature aging on latex glove fingers are underway in an effort to better understand the process and thus determine the acceptability of various accelerated tests. Creep and modulus are measured under forces comparable to those developed during actual use. Their behavior in short-term tests at higher temperatures is different from that in longer-term, low-temperature tests. This may explain why attempts to use the Arrhenius or the Q10 relationship with latex materials do not work very well. OST has been working with an ASTM D11:40 glove subcommittee on developing acceptable accelerated tests.

Shelf-Life of Cellulose Acetate Hemodialyzers
Key words: cellulose acetate, degradation, simulation

A previous investigation in an OST laboratory linked cellulose acetate (CA) membrane degradation with adverse health effects in hemodialysis patients. A molecular population model was developed to track CA degradation of a dialyzer membrane during storage. The model used a random number to select individual polymer molecules out of a population, and then another to select a site on the molecule for the degradation reaction to occur. The resulting molecular fragments were then redistributed into the population. Molecular weight averages and acetyl content were recalculated as the reaction simulation proceeded. The model was validated with experimental measurements, including the molecular weight distribution, on dialyzers stored up to 13.3 years. It was found that the degradation reactions can be accurately modeled as random events and that the reaction events occur at constant rates.