Gas-phase detonation propagation in mixture composition gradients.

The propagation of detonations through several fuel-air mixtures with spatially varying fuel concentrations is examined numerically. The detonations propagate through two-dimensional channels, inside of which the gradient of mixture composition is oriented normal to the direction of propagation. The simulations are performed using a two-component, single-step reaction model calibrated so that one-dimensional detonation properties of model low- and high-activation-energy mixtures are similar to those observed in a typical hydrocarbon-air mixture. In the low-activation-energy mixture, the reaction zone structure is complex, consisting of curved fuel-lean and fuel-rich detonations near the line of stoichiometry that transition to decoupled shocks and turbulent deflagrations near the channel walls where the mixture is extremely fuel-lean or fuel-rich. Reactants that are not consumed by the leading detonation combine downstream and burn in a diffusion flame. Detonation cells produced by the unstable reaction front vary in size across the channel, growing larger away from the line of stoichiometry. As the size of the channel decreases relative to the size of a detonation cell, the effect of the mixture composition gradient is lessened and cells of similar sizes form. In the high-activation-energy mixture, detonations propagate more slowly as the magnitude of the mixture composition gradient is increased and can be quenched in a large enough gradient.

How does a beta -hairpin fold/unfold? competition between topology and heterogeneity in a solvable model.

We study the competition between topological effects and sequence inhomogeneities in determining the thermodynamics and the un/folding kinetics of a beta-hairpin. Our work utilizes a new exactly solvable model that allows for arbitrary configurations of native contacts. In general, the competition between heterogeneity and topology results in a crossover of the dominant transition state. Interestingly, near this crossover, the single reaction coordinate picture can be seriously misleading. Our results also suggest that inferring the folding pathway from unfolding simulations is not always justified.

The legal and scientific basis for FDA's assertion of jurisdiction over cigarettes and smokeless tobacco.

On August 28, 1996, the US Food and Drug Administration (FDA) asserted jurisdiction over cigarettes and smokeless tobacco under the Federal Food, Drug, and Cosmetic Act. Under this Act, a product is a "drug" or "device" subject to FDA jurisdiction if it is "intended to affect the structure or any function of the body." The FDA determined that nicotine in cigarettes and smokeless tobacco does "affect the structure or any function of the body" because nicotine causes addiction and other pharmacological effects. The FDA then determined that these pharmacological effects are "intended" because (1) a scientific consensus has emerged that nicotine is addictive; (2) recent studies have shown that most consumers use cigarettes and smokeless tobacco for pharmacological purposes, including satisfying their addiction to nicotine; and (3) newly disclosed evidence from the tobacco manufacturers has revealed that the manufacturers know that nicotine causes pharmacological effects, including addiction, and design their products to provide pharmacologically active doses of nicotine. The FDA thus concluded that cigarettes and smokeless tobacco are subject to FDA jurisdiction because they contain a "drug," nicotine, and a "device" for delivering this drug to the body.

Reported complications of silicone gel breast implants: an epidemiologic review.

PURPOSE: To review the range of local and systemic complications attributed to silicone breast implants and to evaluate the epidemiologic literature on these complications. DATA SOURCES: Epidemiologic studies of the potential risks of silicone breast implants identified by MEDLINE search and literature review. STUDY SELECTION: Epidemiologic studies with cohort, case-control, and cross-sectional designs. When epidemiologic studies were unavailable (as for estimates of local complications), case series were reviewed. DATA EXTRACTION: Epidemiologic studies evaluated for methodologic quality, including such characteristics as study design, sample size and selection, determination of silicone exposure and outcome variables, and duration of follow-up. DATA SYNTHESIS: The epidemiologic literature on the potential complications of silicone breast implants has concentrated primarily on connective tissue disorders and cancer. Estimated of the true incidence of local complications, such as rupture, capsular contracture, and breast pain, are unavailable. Studies of scleroderma and other defined connective tissue diseases suggest that implant recipients have no substantially increased risk for these disorders; however, the epidemiologic literature is insufficient to rule out an association between breast implants and connective tissue disease-like syndromes. Overall, the rate of breast cancer does not seem to be increased in women with silicone breast implants. However, the risk to women as they reach the postmenopausal years in not yet known. CONCLUSIONS: Information is insufficient to adequately advise women who currently have or are seeking to obtain breast implants about the overall risk of these devices. No epidemiologic study has indicated that the rate of well-defined connective tissue disease or breast cancer has greatly increased in women with silicone breast implants, but no studies have ruled out a moderately increased risk for these diseases. No studies have adequately addressed the crucial issue of local complications such as rupture and capsular contracture, although evidence increasingly points to a higher risk for rupture as implants age.

Effects of fibrin sealant on the fixation of porous titanium and pyrolytic carbon implants.

The authors investigated the potential role the fibrin sealant system may portray in the fixation of osseous implants. The application of a layer of fibrin did not interfere with the fixation of osseous implants of either pyrolytic carbon or Biolite-coated porous titanium. A greater percentage of tissue ingrowth was observed in the porous titanium implants in the presence of the fibrin sealant system; however, no significant difference in the ultimate interfacial shear stress was observed.

Food safety: revising the statute.

There is increasing recognition that federal food safety laws and policies need to be revised. Congressional debate on proposed amendments to the Food, Drug, and Cosmetic Act has generated several different perspectives on how the food safety laws should be changed. Before a consensus can be reached, scientists, regulators, the food industry, and consumers will have to review such complex and controversial issues as the level of acceptable risk, the value of risk-benefit analysis, the proper role of independent scientific review, and the reliability of quantitative risk assessment.

Mast cells and tumor angiogenesis.

Tumor angiogenesis factor (TAF) elicits a strong vasoproliferative response when implanted upon the chorioallantoic membrane (CAM) of the chick embryo. This response is first observed stereomicroscopically 2-3 days after implantation. A 40-fold increase in mast cell density is observed within the vicinity of this implant by 24 h. Mast cells that have been isolated from retired breeder Sprague-Dawley rats fail to evoke a vascular reaction when implanted on the CAM. An intermediate role for the mast cell in tumor angiogenesis is suggested.