Ballistic nanofriction

Sliding parts in nanosystems such as Nano ElectroMechanical Systems (NEMS) and nanomotors, increasingly involve large speeds, and rotations as well as translations of the moving surfaces; yet, the physics of high speed nanoscale friction is so far unexplored. Here, by simulating the motion of drifting and of kicked Au clusters on graphite - a workhorse system of experimental relevance -- we demonstrate and characterize a novel "ballistic" friction regime at high speed, separate from drift at low speed. The temperature dependence of the cluster slip distance and time, measuring friction, is opposite in these two regimes, consistent with theory. Crucial to both regimes is the interplay of rotations and translations, shown to be correlated in slow drift but anticorrelated in fast sliding. Despite these differences, we find the velocity dependence of ballistic friction to be, like drift, viscous

In Vitro Interactions between Bacteria, Osteoblast-Like Cells and Macrophages in the Pathogenesis of Biomaterial-Associated Infections

Biomaterial-associated infections constitute a major clinical problem that is difficult to treat and often necessitates implant replacement. Pathogens can be introduced on an implant surface during surgery and compete with host cells attempting to integrate the implant. The fate of a biomaterial implant depends on the outcome of this race for the surface. Here we studied the competition between different bacterial strains and human U2OS osteoblast-like cells (ATCC HTB-94) for a poly(methylmethacrylate) surface in the absence or presence of macrophages in vitro using a peri-operative contamination model. Bacteria were seeded on the surface at a shear rate of 11 1/s prior to adhesion of U2OS cells and macrophages. Next, bacteria, U2OS cells and macrophages were allowed to grow simultaneously under low shear conditions (0.14 1/s). The outcome of the competition between bacteria and U2OS cells for the surface critically depended on bacterial virulence. In absence of macrophages, highly virulent Staphylococcus aureus or Pseudomonas aeruginosa stimulated U2OS cell death within 18 h of simultaneous growth on a surface. Moreover, these strains also caused cell death despite phagocytosis of adhering bacteria in presence of murine macrophages. Thus U2OS cells are bound to loose the race for a biomaterial surface against S. aureus or P. aeruginosa, even in presence of macrophages. In contrast, low-virulent Staphylococcus epidermidis did not cause U2OS cell death even after 48 h, regardless of the absence or presence of macrophages. Clinically, S. aureus and P. aeruginosa are known to yield acute and severe biomaterial-associated infections in contrast to S. epidermidis, mostly known to cause more low-grade infection. Thus it can be concluded that the model described possesses features concurring with clinical observations and therewith has potential for further studies on the simultaneous competition for an implant surface between tissue cells and pathogenic bacteria in presence of immune system components

Continuous deep sedation and homicide: an unsolved problem in law and professional morality

When a severely suffering dying patient is deeply sedated, and this sedated condition is meant to continue until his death, the doctor involved often decides to abstain from artificially administering fluids. For this dual procedure almost all guidelines require that the patient should not have a life expectancy beyond a stipulated maximum of days (4–14). The reason obviously is that in case of a longer life-expectancy the patient may die from dehydration rather than from his lethal illness. But no guideline tells us how we should describe the dual procedure in case of a longer life-expectancy. Many arguments have been advanced why we should not consider it to be a form of homicide, that is, ending the life of the patient (with or without his request). I argue that none of these arguments, taken separately or jointly, is persuasive. When a commission, even one that is not itself life-shortening, foreseeably renders a person unable to undo the life-shortening effects of another, simultaneous omission, the commission and the omission together should be acknowledged to kill her. I discuss the legal and ethical implications of this conclusion