Reliability demonstration for safety-critical systems

This paper suggests a new model for reliability demonstration of safety-critical systems, based on the TRW Software Reliability Theory. The paper describes the model; the test equipment required and test strategies based on the various constraints occurring during software development. The paper also compares a new testing method, Single Risk Sequential Testing (SRST), with the standard Probability Ratio Sequential Testing method (PRST), and concludes that: • SRST provides higher chances of success than PRST • SRST takes less time to complete than PRST • SRST satisfies the consumer risk criterion, whereas PRST provides a much smaller consumer risk than the requirement

A bounded-search iterated greedy algorithm for the distributed permutation flowshop scheduling problem

As the interest of practitioners and researchers in scheduling in a multi-factory environment is growing, there is an increasing need to provide efficient algorithms for this type of decision problems, characterised by simultaneously addressing the assignment of jobs to different factories/workshops and their subsequent scheduling. Here we address the so-called distributed permutation flowshop scheduling problem, in which a set of jobs has to be scheduled over a number of identical factories, each one with its machines arranged as a flowshop. Several heuristics have been designed for this problem, although there is no direct comparison among them. In this paper, we propose a new heuristic which exploits the specific structure of the problem. The computational experience carried out on a well-known testbed shows that the proposed heuristic outperforms existing state-of-the-art heuristics, being able to obtain better upper bounds for more than one quarter of the problems in the testbed.Ministerio de Ciencia e Innovación DPI2010-15573/DP

In Vivo Corrosion of Two Novel Magnesium Alloys ZEK100 and AX30 and Their Mechanical Suitability as Biodegradable Implants

In magnesium alloys, the components used modify the alloy properties. For magnesium implants in contact with bone, rare earths alloys are commonly examined. These were shown to have a higher corrosion resistance than other alloys and a high mechanical strength, but their exact composition is hard to predict. Therefore a reduction of their content could be favorable. The alloys ZEK100 and AX30 have a reduced content or contain no rare earths at all. The aim of the study was to investigate their in vivo degradation and to assess the suitability of the in vivo μCT for the examination of their corrosion. Implants were inserted in rabbit tibiae. Clinical examinations, X-rays and in vivo μCT scans were done regularly. Afterwards implants were analyzed with REM, electron dispersive X-ray (EDX), weighing and mechanical testing. The in vivo μCT is of great advantage, because it allows a quantification of the corrosion rate and qualitative 3D assessment of the corrosion morphology. The location of the implant has a remarkable effect on the corrosion rate. Due to its mechanical characteristics and its corrosion behavior, ZEK100 was judged to be suitable, while AX30, which displays favorable degradation behavior, has too little mechanical strength for applications in weight bearing bones

Physiologic compliance in engineered small-diameter arterial constructs based on an elastomeric substrate.

Compliance mismatch is a significant challenge to long-term patency in small-diameter bypass grafts because it causes intimal hyperplasia and ultimately graft occlusion. Current engineered grafts are typically stiff with high burst pressure but low compliance and low elastin expression. We postulated that engineering small arteries on elastomeric scaffolds under dynamic mechanical stimulation would result in strong and compliant arterial constructs. This study compares properties of engineered arterial constructs based on biodegradable polyester scaffolds composed of either rigid poly(lactide-co-glycolide) (PLGA) or elastomeric poly(glycerol sebacate) (PGS). Adult baboon arterial smooth muscle cells (SMCs) were cultured in vitro for 10 days in tubular, porous scaffolds. Scaffolds were significantly stronger after culture regardless of material, but the elastic modulus of PLGA constructs was an order of magnitude greater than that of porcine carotid arteries and PGS constructs. Deformation was elastic in PGS constructs and carotid arteries but plastic in PLGA constructs. Compliance of arteries and PGS constructs were equivalent at pressures tested. Altering scaffold material from PLGA to PGS significantly decreased collagen content and significantly increased insoluble elastin content in constructs without affecting soluble elastin concentration in the culture medium. PLGA constructs contained no appreciable insoluble elastin. This research demonstrates that: (1) substrate stiffness directly affects in vitro tissue development and mechanical properties; (2) rigid materials likely inhibit elastin incorporation into the extracellular matrix of engineered arterial tissues; and (3) grafts with physiologic compliance and significant elastin content can be engineered in vitro after only days of cell culture

In Vivo Degradation Behavior of the Magnesium Alloy LANd442 in Rabbit Tibiae

In former studies the magnesium alloy LAE442 showed promising in vivo degradation behavior and biocompatibility. However, reproducibility might be enhanced by replacement of the rare earth composition metal "E" by only a single rare earth element. Therefore, it was the aim of this study to examine whether the substitution of "E" by neodymium ("Nd") had an influence on the in vivo degradation rate. LANd442 implants were inserted into rabbit tibiae and rabbits were euthanized after 4, 8, 13 and 26 weeks postoperatively. In vivo μCT was performed to evaluate the in vivo implant degradation behaviour by calculation of implant volume, density true 3-D thickness and corrosion rates. Additionally, weight loss, type of corrosion and mechanical stability were appraised by SEM/EDS-analysis and three-point bending tests. Implant volume, density and true 3-D thickness decreased over time, whereas the variance of the maximum diameters within an implant as well as the corrosion rate and weight loss increased. SEM examination revealed mainly pitting corrosion after 26 weeks. The maximum bending forces decreased over time. In comparison to LAE442, the new alloy showed a slower, but more uneven degradation behavior and less mechanical stability. To summarize, LANd442 appeared suitable for low weight bearing bones but is inferior to LAE442 regarding its degradation morphology and strength

The Influence of Intergovernmental Organizations on Main Determinants of the Open Systems Model with Correlation Analysis Method Application.

The paper aims at analyzing the nature of relations between Intergovernmental Organizations and International Corporations. In first instance the study concentrates on identifying the key determinants of Intergovernmental Organizations behaviours. The following section is a description of business environment of International Companies based on the Open Systems Model. Three levels of interrelations have been mentioned, including the Operating Environment, the Host – Country Environment and the Global Environment. The solution proposal provides an analysis of the influence of determinants of Intergovernmental Organizations behaviours on the determinants of business environment of International Corporations with use of correlation analysis method

Dynamic energy budget approach to evaluate antibiotic effects on biofilms

Quantifying the action of antibiotics on biofilms is essential to devise therapies against chronic infections. Biofilms are bacterial communities attached to moist surfaces, sheltered from external aggressions by a polymeric matrix. Coupling a dynamic energy budget based description of cell metabolism to surrounding concentration fields, we are able to approximate survival curves measured for different antibiotics. We reproduce numerically stratified distributions of cell types within the biofilm and introduce ways to incorporate different resistance mechanisms. Qualitative predictions follow that are in agreement with experimental observations, such as higher survival rates of cells close to the substratum when employing antibiotics targeting active cells or enhanced polymer production when antibiotics are administered. The current computational model enables validation and hypothesis testing when developing therapies.Comment: to appear in Communications in Nonlinear Science and Numerical Simulatio

Rates of Performance Loss and Neuromuscular Activity in Men and Women During Cycling: Evidence for A Common Metabolic Basis of Muscle Fatigue

The durations that muscular force and power outputs can be sustained until failure fall predictably on an exponential decline between an individual’s 3-s burst maximum to the maximum performance they can sustain aerobically. The exponential time constants describing these rates of performance loss are similar across individuals, suggesting that a common metabolically based mechanism governs muscle fatigue; however, these conclusions come from studies mainly on men. To test whether the same physiological understanding can be applied to women, we compared the performance-duration relationships and neuromuscular activity between seven men [23.3 ± 1.9 (SD) yr] and seven women (21.7 ± 1.8 yr) from multiple exhaustive bouts of cycle ergometry. Each subject performed trials to obtain the peak 3-s power output (Pmax), the mechanical power at the aerobic maximum (Paer), and 11–14 constant-load bouts eliciting failure between 3 and 300 s. Collectively, men and women performed 180 exhaustive bouts spanning an ~6-fold range of power outputs (118–1116 W) and an ~35-fold range of trial durations (8–283 s). Men generated 66% greater Pmax (956 ± 109 W vs. 632 ± 74 W) and 68% greater Paer (310 ± 47 W vs. 212 ± 15 W) than women. However, the metabolically based time constants describing the time course of performance loss were similar between men (0.020 ± 0.003/s) and women (0.021 ± 0.003/s). Additionally, the fatigue-induced increases in neuromuscular activity did not differ between the sexes when compared relative to the pedal forces at Paer. These data suggest that muscle fatigue during short-duration dynamic exercise has a common metabolically based mechanism determined by the extent that ATP is resynthesized by anaerobic metabolism