Charging capacitors from thermal fluctuations using diodes.

We theoretically consider a graphene ripple as a Brownian particle coupled to an energy storage circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, the approach to equilibrium may become ultraslow. Detailed balance is temporarily broken as current flows between the two diodes and charges storage capacitors. The energy harvested by each capacitor comes from the thermal bath of the diodes while the system obeys the first and second laws of thermodynamics.

Additional direct medical costs associated with nosocomial infections after head and neck cancer surgery: a hospital-perspective analysis.

The clinical impact of surgical site infections (SSI) and postoperative pneumonia (PP) after head and neck cancer surgery has been assessed in the past, but little is known about their economic impact. The present study was designed to evaluate costs related to SSI and PP after head and neck cancer surgery with opening of mucosa. The incidence of SSI and PP was measured in a prospective cohort of 261 patients who had undergone head and neck cancer surgery. The additional direct medical costs related to these infections from the hospital perspective were determined based on postoperative length of stay. The mean direct hospital costs for patients with and without SSI or PP were compared. Of the 261 patients, 81 (31%), 21 (8%) and 13 (5%) developed SSI, PP or both, respectively. The additional lengths of stay attributable to SSI, PP or both were 16, 17 and 31 days, respectively, and additional direct medical costs related to these conditions were 17,000, 19,000 and 35,000 Euros. Nosocomial infections after head and neck cancer surgery significantly increase patients' length of stay and therefore generate additional direct medical costs. These results support the application of preventive interventions to reduce nosocomial infections in this setting.

Igniting homogeneous nucleation.

Transient homogeneous nucleation is studied in the limit of large critical sizes. Starting from pure monomers, three eras of transient nucleation are characterized in the classic Becker-Döring kinetic equations with two different models of discrete diffusivity: the classic Turnbull-Fisher formula and an expression describing thermally driven growth of the nucleus. The latter diffusivity yields time lags for nucleation which are much closer to values measured in experiments with disilicate glasses. After an initial stage in which the number of monomers decreases, many clusters of small size are produced and a continuous size distribution is created. During the second era, nucleii are increasing steadily in size in such a way that their distribution appears as a wave front advancing towards the critical size for steady nucleation. The nucleation rate at critical size is negligible during this era. After the wave front reaches critical size, it ignites the creation of supercritical clusters at a rate that increases monotonically until its steady value is reached. Analytical formulas for the transient nucleation rate and the time lag are obtained that improve classical ones and compare very well with direct numerical solutions.

Three eras of micellization.

Micellization is the precipitation of lipids from aqueous solution into aggregates with a broad distribution of aggregation number. Three eras of micellization are characterized in a simple kinetic model of Becker-Döring type. The model assigns the same constant energy to the (k-1) monomer-monomer bonds in a linear chain of k particles. The number of monomers decreases sharply and many clusters of small size are produced during the first era. During the second era, nuclei are increasing steadily in size until their distribution becomes a self-similar solution of the diffusion equation. Lastly, when the average size of the nuclei becomes comparable to its equilibrium value, a simple mean-field Fokker-Planck equation describes the final era until the equilibrium distribution is reached.

Response of a single cell to an external electric field.

The response of a cell to an external electric field is investigated using dimensional analysis and singular perturbation. The results demonstrate that the response of a cell is a two-stage process consisting of the initial polarization that proceeds with the cellular time constant (< 1 microseconds), and of the actual change of physiological state that proceeds with the membrane time constant (several milliseconds). The second stage is governed by an ordinary differential equation similar to that of a space-clamped membrane patch but formulated in terms of intracellular rather than transmembrane potential. Therefore, it is meaningful to analyze the physiological state and the dynamics of a cell as a whole instead of the physiological states and the dynamics of the underlying membrane patches. This theoretical result is illustrated with an example of an excitation of a cylindrical cell by a transverse electric field.

Effective boundary conditions for syncytial tissues.

This study derives effective boundary conditions for potentials and currents on the interface between syncytial tissue and a surrounding volume conductor. The derivation is based on an idealized representation of the syncytium as a network of interconnected cells arranged periodically in space. The microscopic model of an interface assumes that the extracellular fluid is in direct contact with the outside volume conductor and that the inside of the cells is separated from the outside by the membrane. From this microscopic model, a homogenization process and boundary layer analysis derive effective boundary conditions applicable to macroscopic volume-averaged potentials. These effective boundary conditions call for the extracellular potential and current density to be continuous with the potential and current density in the volume conductor, and for the intracellular current to vanish. Hence, the long-debated appropriate boundary conditions for the bidomain model are established.

Homogenization of syncytial tissues.

This article derives a continuum representation of a multicellular, syncytial tissue directly from its microstructure and basic physical principles. The final equations for the homogenized syncytium contain the bidomain model as a special case. The derivation begins with an idealized, periodic representation of the tissue microstructure. Potentials inside the cells and in the extracellular fluid are governed by the Laplace equation. Electrical properties of the membrane separating those two regions are represented by the boundary conditions on the potentials. An homogenization process based on a two-scale asymptotic expansion converts this microscopic, pointwise description into an averaged, continuum representation by two reaction-diffusion equations. The same process also yields formulas for the effective conductivities of the tissue in terms of its microstructure and specific conductivities of cytoplasm and extracellular fluid. The validity of the homogenized syncytium model is assured deep in the tissue for autonomous processes, such as propagation, and in the presence of external fields that are nearly uniform and limited in strength. The derived model is not formally valid on the surface of tissue, in the proximity of sources, and under strong or rapidly changing electrical fields.

Asymptotic analysis of a viscous cochlear model.

A model of cochlear macromechanics, involving a viscous cochlear fluid, is analyzed asymptotically for slowly varying membranes. The results exhibit the effect of viscosity on the amplitude, wavelength, and damping rate, and confirm the conclusion that viscosity is not important. However the method can be used on other models, such as those of cochlear micromechanics, where viscosity is more important.

Synergy of mecillinam, a beta-amidinopenicillanic acid derivative, combined with beta-lactam antibiotics.

Mecillinam, a beta-amidinopenicillanic acid derivative, was combined with ampicillin, amoxicillin, carbenicillin, cephalothin, cefamandole, and cefoxitin and tested against most members of the Enterobacteriaceae and Pseudomonas. Synergy was demonstrated with selected isolates of most of the organisms tested. Isolates highly susceptible to mecillinam (minimum inhibitory concentration, <0.8 mug/ml) were not synergistically inhibited by addition of another beta-lactam antibiotic. Synergy of mecillinam and a beta-lactamase-resistant penicillin, cloxacillin, was demonstrated. In media of osmolality >10 mOsm or of conductivity >6 mS, mecillinam and beta-lactam antibiotics showed synergy in most instances, whereas at low osmolality and conductivity the activity of mecillinam is so great that synergy cannot be demonstrated. The proportion of mecillinam to beta-lactam antibiotic that will be synergistic ranged from 100:1 to 1:1 to 1:100. Mecillinam did not increase the activity, minimum inhibitory concentration or minimum bactericidal concentration values, of beta-lactam compounds against streptococci, staphylococci, clostridia, listeria, or bacteroides. Synergy was not demonstrated with combinations of mecillinam and aminoglycosides (kanamycin, gentamicin, tobramycin, amikacin), chloramphenicol, tetracycline, or polymyxins.