THE HYSTERESIS LOOP 3. STRESS-DIP EXPERIMENTS

Abstract— A stress-dip technique for the determination of an effective stress in cyclic straining has been improved. The critical stress drop, Δσ_C, for which zero strain relaxation rate is observed can be obtained from experiments. The significance of this quantity is discussed in terms of the statistical theory of the hysteresis loop. The measurements of the effective stress at different temperatures and along the hysteresis loop are reported. The observed asymmetry of stress in cyclic straining is ascribed to asymmetry of effective stress.     

Purification and characterization of a troponin C-like phosphodiesterase activator from bovine thyroid

A troponin C-like phosphodiesterase activator from bovine thyroid has been purified to homogeneity. The overall purification was about 9,800-fold with a yield of 8%. Bovine thyroid activator protein is identical in biologic properties to that isolated from bovine brain. They have the same specific activity regarding stimulation of bovine brain cyclic nucleotide phosphodiesterase. Both proteins form a Ca2+-dependent complex with heart muscle troponin I which is stable in 6M urea-polyacrylamide gel and which is similar, but not identical, to the troponin C-troponin I complex. The physiochemical properties of bovine thyroid activator protein are identical with those of bovine brain and other phosphodiesterase activator proteins and are similar to heart muscle and skeletal muscle troponin C as follows: (A) they bind 3-4 exchangeable calcium ions/mol with dissociation constants between 10(-5) and 10(-6) M, (B) they are highly acidic with a high content of aspartic and glutamic acids and isoelectric points of approximately 4.1, (C) these proteins have an unusual ultraviolet absorption spectrum with six discrete maxima between 250 and 284 nm which are characteristic of phenylalanine and tyrosine, and (D) these proteins have a low content of cysteine, histidine, tyrosine and proline. The tryptic peptide maps of bovine thyroid and brain activator protein are very similar. However, despite a very similar amino acid composition, the peptide map of bovine heart muscle troponin C is significantly different from that of the other two proteins. The molecular weight of thyroid and brain activator protein is 16,500, while that of heart troponin C is 18,500. Thyroid and brain activator protein, as well as heart troponin C, appear to undergo significant Ca2+-dependent conformational changes, as measured by the difference in the circular dichroism spectrum and electrophoretic mobility observed in the presence and absence of calcium ion.     

Diatom bionanotribology--biological surfaces in relative motion: their design, friction, adhesion, lubrication and wear

Tribology is the branch of engineering that deals with the interaction of surfaces in relative motion (as in bearings or gears): their design, friction, adhesion, lubrication and wear. Continuous miniaturization of technological devices like hard disc drives and biosensors increases the necessity for the fundamental understanding of tribological phenomena at the micro- and nanoscale. Biological systems show optimized performance also at this scale. Examples for biological friction systems at different length scales include bacterial flagella, joints, articular cartilage and muscle connective tissues. Scanning probe microscopy opened the nanocosmos to engineers: not only is microscopy now possible on the atomic scale, but even manipulation of single atoms and molecules can be performed with unprecedented precision. As opposed to this top-down approach, biological systems excel in bottom-up nanotechnology. Our model system for bionanotribological investigations are diatoms, for they are small, highly reproductive, and since they are transparent, they are accessible with different kinds of optical microscopy methods. Furthermore, certain diatoms have proved to be rewarding samples for mechanical and topological in vivo investigations on the nanoscale. There are several diatom species that actively move (e.g. Bacillaria paxillifer forms colonies in which the single cells slide against each other) or which can, as cell colonies, be elongated by as much as a major fraction of their original length (e.g. Ellerbeckia arenaria colonies can be reversibly elongated by one third of their original length). Therefore, we assume that some sort of lubrication of interactive surfaces is present in these species. Current studies in diatom bionanotribology comprise techniques like atomic force microscopy, histochemical analysis, infrared spectrometry, molecular spectroscopy and confocal infrared microscopy.     

Is tubal sterilization associated with an increased risk of subsequent hysterectomy but a decreased risk of ovarian cancer? A review of recent literature

Recent epidemiologic studies have suggested that tubal sterilization (TS) may lead to an increased incidence of subsequent hysterectomy but a decreased risk of ovarian cancer. This review evaluates the nature and magnitude of these two relationships, which should be of great concern and interest to women, clinicians and the administrators of family planning programs. The positive relationship between TS and subsequent hysterectomy is more likely to be of a motivational rather than biological nature, and a considerable number of unnecessary hysterectomies after TS could be avoided by changing the attitudes of physicians and women. The inverse relationship between TS and ovarian cancer appears causal, although the exact biological mechanisms remain to be clarified. Theoretically, this non-contraceptive beneficial effect of TS could be used as a primary preventive measure to curb the incidence of the highly fatal ovarian cancer. However, a number of medical, ethical, and economic questions attending use of a generally irreversible contraceptive procedure as a preventive measure must first be answered. The issue of whether TS is associated with any long-term sequelae, and, if so, whether the association is of a cause-and-effect nature or a by-product of time passage and aging of the woman, should be addressed by well-designed studies.     

哈佛研制出具有宏观电势的纳米光伏电池

Charles Lieber博士领导哈佛大学的一个研究小组开发了一种约300nm厚的共轴硅纳米线,可以制作用来驱动小型电路和纳米机器的光伏（PV）电池。