The viscosity of polyelectrolyte solutions and its dependence on their persistence length, concentration and solvent quality.

In this work, a comprehensive study of the influence on shear viscosity of polyelectrolyte concentration, persistence length, salt concentration and solvent quality is reported, using numerical simulations of confined solutions under stationary Poiseuille flow. Various scaling regimes for the viscosity are reproduced, both under good solvent and theta solvent conditions. The key role played by the electrostatic interactions in the viscosity is borne out when the ionic strength is varied. It is argued that these results are helpful for the understanding of viscosity scaling in entangled polyelectrolyte solutions for both rigid and flexible polyelectrolytes in different solvents, which is needed to perform intelligent design of new polyelectrolytes capable of fine tuning the viscosity in complex fluids.

Modeling of Branched Thickening Polymers under Poiseuille Flow Gives Clues as to How to Increase a Solvent's Viscosity.

The viscosity enhancement of a solvent produced by the addition of thickening branched polymers is predicted as a function of polymer concentration, branch length and persistence length, and strength of the covalent bonding interactions. Nonequilibrium, stationary-state Poiseuille numerical simulations are performed using the dissipative particle dynamics model to obtain the viscosity of the fluid. It is found that the clustering of the polymers into aggregates increases the viscosity and that it is more strongly affected by the strength of the bonding interactions. General scaling relationships are found for the viscosity as a function of the variables studied, which are expected to be useful for the design and synthesis of new viscosifying polymers. It is argued that our results can be applied to aqueous thickeners, of importance for colloidal fluids such as paints and coatings and also for nonpolar fluids such as supercritical CO2, which is a promising nonhydraulic fracking fluid also useful in enhanced oil recovery.

Scaling features of the tribology of polymer brushes of increasing grafting density around the mushroom-to-brush transition.

Nonequilibrium coarse-grained, dissipative particle dynamics simulations of complex fluids, made up of polymer brushes tethered to planar surfaces immersed in a solvent yield nonmonotonic behavior of the friction coefficient as a function of the polymer grating density on the substrates, Γ, while the viscosity shows a monotonically increasing dependence on Γ. This effect is shown to be independent of the degree of polymerization, N, and the size of the system. It arises from the composition and the structure of the first particle layer adjacent to each surface that results from the confinement of the fluid. Whenever such layers are made up of as close a proportion of polymer beads to solvent particles as there are in the fluid, the friction coefficient shows a minimum, while for disparate proportions the friction coefficient grows. At the mushroom-to-brush transition (MBT) the viscosity scales with an exponent that depends on the characteristic exponent of the MBT (6/5) and the solvent quality exponent (ν=0.5, for θsolvent), but it is independent of the polymerization degree (N). On the other hand, the friction coefficient at the MBT scales as µâˆ¼N^{6/5}, while the grafting density at the MBT scales as Γ∼N^{-6/5} when friction is minimal, in agreement with previous scaling theories. We argue these aspects are the result of cooperative phenomena that have important implications for the understanding of biological brushes and the design of microfluidics devices, among other applications of current academic and industrial interest.

Genetic structure of West Greenland populations of lumpfish Cyclopterus lumpus.

In this study, 11 microsatellite markers were used to determine the structure of West Greenlandic lumpfish Cyclopterus lumpus populations across six spawning locations spanning >1500 km and compared with neighbouring populations in Canada and Iceland. To evaluate whether data allow for identification of origin of C. lumpus in Greenlandic waters, genetic assignment analysis was performed for 86 C. lumpus sampled on a feeding migration. Significant structuring with isolation by distance was observed in the West Greenland samples and two major subpopulations, north and south, were suggested. Based on FST values, closer relationships were observed between Greenland and Canada, than Greenland and Iceland. Surprisingly, the North Greenland population showed more similarities with Canadian samples, than did the geographically closer south-west Greenland population. Origin could be assigned for a high proportion of non-spawning fish and demonstrated a marked east-west spatial separation of fish of Greenlandic and Icelandic genotypes.

Multiscale Modeling of the Effect of Pressure on the Interfacial Tension and Other Cohesion Parameters in Binary Mixtures.

We study and predict the interfacial tension, solubility parameters, and Flory-Huggins parameters of binary mixtures as functions of pressure and temperature, using multiscale numerical simulation. A mesoscopic approach is proposed for simulating the pressure dependence of the interfacial tension for binary mixtures, at different temperatures, using classical dissipative particle dynamics (DPD). The thermodynamic properties of real systems are reproduced via the parametrization of the repulsive interaction parameters as functions of pressure and temperature via molecular dynamics simulations. Using this methodology, we calculate and analyze the cohesive energy density and the solubility parameters of different species obtaining excellent agreement with reported experimental behavior. The pressure- and temperature-dependent Flory-Huggins and repulsive DPD interaction parameters for binary mixtures are also obtained and validated against experimental data. This multiscale methodology offers the benefit of being applicable for any species and under difficult or nonfeasible experimental conditions, at a relatively low computational cost.

Hyperscaling relationship between the interfacial tension of liquids and their correlation length near the critical point.

Interfaces involving coexisting phases in condensed matter are essential in many phenomena: wetting, nucleation, morphology, phase separation kinetics, membranes, phase coexistence in nanomaterials, etc. The majority of analytical theories available use concepts derived from mean field artifacts which do not describe adequately these systems. Satisfactory numerical simulation of interfaces at the atomistic to mesoscopic scale is still a challenge. In the present work, the interfacial tension between mixtures of organic solvents and water is obtained from mesoscopic computer simulations. The temperature dependence of the interfacial tension is found to obey a scaling law with an average critical exponent µ = 1.23. Additionally, we calculate the evolution of the correlation length, defined as the thickness of the interface between the immiscible fluids, as a function of temperature and find that it obeys also a scaling law with an average critical exponent being ν = 0.67. Finally, we show that the comparison of µ and ν for these binary mixtures constitutes the first test of Widom's hyperscaling relationship between these exponents in 3d, expressed as µ = ν (d - 1). Based on these values and those for the 3d Ising model it is argued that both systems belong to the same universality class, which opens up the way for the calculation of new scaling exponents.

Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in a theta solvent.

We undertake the investigation of sheared polymer chains grafted onto flat surfaces to model liposomes covered with polyethylene glycol brushes as a case study for the mechanisms of efficient drug delivery in biologically relevant situations, for example, as carriers for topical treatments of illnesses in the human vasculature. For these applications, specific rheological properties are required, such as low viscosity at high shear rates, to improve the transport of the liposomes. Therefore, extensive non-equilibrium, coarse-grained dissipative particle dynamics simulations of polymer brushes of various lengths and shear rates are performed to obtain the average viscosity and the friction coefficient of the system as functions of the shear rate and polymerization degree under theta-solvent conditions, and we find that the brushes experience considerable shear thinning at large shear rates. The viscosity (η) and the friction coefficient (µ) are shown to obey the scaling laws η ∼ γ dot above (-0.31) and µ ∼ γ dot above (0.69) at high shear rates (γ dot above) in a theta solvent, irrespective of the degree of polymerization of brushes. These results confirm recent scaling predictions and reproduce very well trends in measurements of the viscosity at a high shear rate (γ dot above) of red blood cells in a liposome containing medium.

Modeling the temperature dependent interfacial tension between organic solvents and water using dissipative particle dynamics.

The interfacial tension between organic solvents and water at different temperatures is predicted using coarse-grained, mesoscopic Dissipative Particle Dynamics (DPD) simulations. The temperature effect of the DPD repulsive interaction parameters, aij, for the different components is calculated from the dependence of the Flory-Huggins χ parameter on temperature, by means of the solubility parameters. Atomistic simulations were carried out for the calculation of the solubility parameters for different organic compounds at different temperatures in order to estimate χ and then the aij coefficients. We validate this parametrization through the study of the interfacial tension in a mixture of benzene and water, and cyclohexane and water, varying the temperature. The predictions of our simulations are found to be in good agreement with experimental data taken from the literature, and show that the use of the solubility parameter at different temperatures to obtain the repulsive DPD parameters is a good alternative to introduce the effect of temperature in these systems.

Study of interfacial tension between an organic solvent and aqueous electrolyte solutions using electrostatic dissipative particle dynamics simulations.

The study of the modification of interfacial properties between an organic solvent and aqueous electrolyte solutions is presented by using electrostatic dissipative particle dynamics (DPD) simulations. In this article, the parametrization for the DPD repulsive parameters a(ij) for the electrolyte components is calculated considering the dependence of the Flory-Huggins χ parameter on the concentration and the kind of electrolyte added, by means of the activity coefficients. In turn, experimental data were used to obtain the activity coefficients of the electrolytes as a function of their concentration in order to estimate the χ parameters and then the a(ij) coefficients. We validate this parametrization through the study of the interfacial tension in a mixture of n-dodecane and water, varying the concentration of different inorganic salts (NaCl, KBr, Na(2)SO(4), and UO(2)Cl(2)). The case of HCl in the mixture n-dodecane/water was also analyzed and the results presented. Our simulations reproduce the experimental data in good agreement with previous work, showing that the use of activity coefficients to obtain the repulsive DPD parameters a(ij) as a function of concentration is a good alternative for these kinds of systems.

Dental school deans' and dentists' perceptions of infection control and HIV/AIDS patient care: a challenge for dental education in Mexico.

HIV/AIDS patients face unique oral diagnostic and treatment challenges. The aim of this investigation among dental school deans (DSD) and graduate school applicants (GSA) who had qualified from 30 different dental schools was to assess their perceptions on dental education relevant to infection control (IC) and HIV/AIDS patient care. The questionnaire included Likert-type scale evaluations of agreement with statements. Of 158 questionnaires, 23 DSD (68% response rate), and 123 GSA (100% response rate) returned valid questionnaires. Fifteen (65%) DSD and 89 (72%) GSA ranked as "very strong" their perception that infection control prevents the transmission of blood borne viruses. However, the perception prevailed, among DSD and GSA, that HIV infection was a "very strong" to "strong" occupational hazard. Special reprocessing of instruments used on HIV patients was frequently reported. Many considered "very strong" to "strong" that HIV/AIDS patients must be treated in specialized clinics, and nearly half (48%) of the DSD and one third (35%) of the GSA stated that their school does refer HIV/AIDS patients to dental treatment in specialized clinics. These results indicate that many dental schools in Mexico must provide better education on IC and HIV/AIDS patient care to enhance attitudes toward HIV/AIDS patients.

Mexican dental school deans' opinions and practices regarding oral cancer, 2007.

Dentists must be trained in oral cancer (OC) screening and counseling. However, educational gaps exist in OC prevention worldwide. The objective of this investigation was to assess self-reported perceptions and practices relevant to OC education among Mexican dental school deans. At a leadership meeting in 2007, deans were given a questionnaire containing Likert-type scale evaluations of agreement with statements. Associations between variables were analyzed with Pearson's chi-square test. Of thirty-four deans attending, twenty-three (68 percent response rate) answered the questionnaire in full. Among the respondents, 83 percent believed "very strongly" that dentists must look for OC, but only 52 percent believed "very strongly" that OC screening must be adopted as a standard practice. Fifty-two percent ranked dentists' responsibility in looking after their patients' overall health as "very strong." The deans indicated less support for dentists' roles to intervene in tobacco and alcohol cessation. Participant deans lead institutions that provide education for over 12,000 dental students; their low awareness on OC screening and counseling may hinder the establishment of routine standardized screening and health promotion that help save human lives.

[Magnetic resonance imaging and magnetic resonance spectroscopy methods for measuring intra- and extra-cellular pH: clinical implications]. / Espectroscopía e imagen del pH intra y extracelular mediante métodos de resonancia magnética. Implicaciones clínicas.

We review the different methods for measuring pH by magnetic resonance imaging and magnetic resonance spectroscopy and discuss their potential diagnostic repercussions. We begin with a brief description of intra- and extra-cellular pH regulation in physiological and pathological conditions. Then we present the main 31P or 1H magnetic resonance spectroscopy procedures, which are based on the dependence of the pH on the chemical displacements of the intrinsic intracellular inorganic phosphate or of the H2 proton of imidazole in extrinsic indicators. Finally, we describe the procedures that use magnetic resonance imaging, whose main tool is the dependence of the pH (i) on the relaxivity of certain paramagnetic contrast agents, or (ii) on the processes of magnetic transference between diamagnetic molecules (DIACEST) or paramagnetic molecules (PARACEST) and the free water in the tissues. We briefly illustrate the potential clinical applications of these new procedures.

Espectroscopía e imagen del pH intra y extracelular mediante métodos de resonancia magnética. Implicaciones clínicas / Magnetic resonance imaging and magnetic resonance spectroscopy methods for measuring intraand extra-cellular pH: clinical implications

Se revisan los diversos métodos de medida de pH que utilizan espectroscopía e imagen por resonancia magnética y su potencial repercusión diagnóstica. La revisión comienza con una breve descripción de la regulación del pH intra y extracelular en condiciones fisiológicas y patológicas. Posteriormente se presentan los principales procedimientos basados en espectroscopía de resonancia magnética de 31P ó 1H, basados en la dependencia del pH de los desplazamientos químicos del fosfato inorgánico intracelular intrínseco o del protón H2 del imidazol en indicadores extrínsecos. Finalmente se describen los procedimientos que utilizan imagen por resonancia magnética, empleando como herramienta principal la dependencia del pH de (i) la relajatividad de determinados agentes de contraste paramagnéticos o (ii) de los procesos de transferencia de magnetización entre moléculas diamagnéticas (DIACEST) o paramagnéticas (PARACEST) y el agua libre de los tejidos. Se ilustran brevemente las potenciales aplicaciones clínicas de estos nuevos procedimientos

We review the different methods for measuring pH bymagnetic resonance imaging and magnetic resonance spectroscopy and discuss their potential diagnostic repercussions. We begin with a brief description of intra- and extracellular pH regulation in physiological and pathological conditions. Then we present the main 31P or 1H magnetic resonance spectroscopy procedures, which are based on the dependence of the pH on the chemical displacements of the intrinsic intracellular inorganic phosphate or of the H2 proton of imidazole in extrinsic indicators. Finally, we describe theprocedures that use magnetic resonance imaging, whosemain tool is the dependence of the pH (i) on the relaxivity of certain paramagnetic contrast agents, or (ii) on the processes of magnetic transference between diamagnetic molecules (DIACEST) or paramagnetic molecules (PARACEST) and the free water in the tissues. We briefly illustrate the potential clinical applications of these new procedures

Tsallis' q index and Mori's q phase transitions at the edge of chaos.

We uncover the basis for the validity of the Tsallis statistics at the onset of chaos in logistic maps. The dynamics within the critical attractor is found to consist of an infinite family of Mori's q -phase transitions of rapidly decreasing strength, each associated with a discontinuity in Feigenbaum's trajectory scaling function sigma. The value of q at each transition corresponds to the same special value for the entropic index q, such that the resultant sets of q-Lyapunov coefficients are equal to the Tsallis rates of entropy evolution.

Phase-field model of Hele-Shaw flows in the high-viscosity contrast regime.

A one-sided phase-field model is proposed to study the dynamics of unstable interfaces of Hele-Shaw flows in the high viscosity contrast regime. The corresponding macroscopic equations are obtained by means of an asymptotic expansion from the phase-field model. Numerical integrations of the phase-field model in a rectangular Hele-Shaw cell reproduce finger competition with the final evolution to a steady-state finger.

[Chlamydia pneumoniae and arteriosclerosis]. / Chlamydia pneumoniae y arteriosclerosis.

INTRODUCTION: The relationship between Chlamydia pneumoniae and atherosclerosis has been assessed in several studies. Development and conclusions. Their results suggest a possible role of Chlamydia pneumoniae in the origin and development of the disease, while the actual link between them are still unknown. However, the presence of Chlamydia pneumoniae in patients with atherosclerosis could be just a coincidence.

[The white coat effect in primary care. Analysis of patients with newly diagnosed arterial hypertension]. / El efecto de bata blanca en atención primaria. Análisis en los pacientes con hipertensión arterial de nuevo diagnóstico.

BACKGROUND: In a non-selected group of hypertensive patients with a new diagnosis at primary care, blood pressures obtained in the office (oBP) are compared to ambulatory ones (aBP). White coat hypertension (WCH) and white coat phenomenon (WCP) are estimated to evaluate a white coat effect on such population. PATIENTS AND METHODS: An ambulatory blood pressure monitoring (ABPM) was performed to 91 hypertensive patients (55 females) from 21 to 70 years-old, with consecutive diagnosis in 4 office rooms in an health center (oBP mean > or = 140 mmHg for systolic blood pressure and/or 90 mmHg for diastolic blood pressure during three measures at least). WCH was defined by systolic aBP < or = 135 mmHg and diastolic < or = 85 mmHg, simultaneously, and WCP by oBP-aBP differences > or = 20 and/or 10 mmHg (for respective systolic and diastolic blood pressure). RESULTS: oBP systematically exceeded aBP with wide variations for each subject (difference mean +/- SD: 15 +/- 13/7 +/- 9 mmHg for systolic/diastolic), sex (female: 19 +/- 12/11 +/- 9, male 8 +/- 11/2 +/- 9) and depending on the observer who made the measure. WCH was detected in 27 patients (22 females), and WCP in 47 (37 females), so 30% (confidence interval [CI]: 21-40%) and 52% (CI: 41-62%) respective prevalences are supposed. CONCLUSIONS: Within primary care, white coat effect causes a substantial hypertension sobrediagnosis as quantitative as qualitatively. Confirming measures made by nurses and additionally, ABPM for women, seem to lessen this effect.

Air evacuation of critically burned patients.

We analyze the repercussions of air evacuation on the physiopathology of the critically burned patient based on the experience on 63 patients evacuated by air. Clinical repercussions are due to accelerations, vibrations, noise, and, primarily, to altitude. Accelerations are important during take-off and landing, and vibrations may be important in helicopter evacuations in the presence of craniofacial trauma. The noise, especially in helicopters, can interfere with diagnostic and therapeutic maneuvers in-flight. The altitude modifies atmospheric pressure, partial pressure of oxygen, and water concentration in inhaled air. In the aircraft we use, atmospheric pressure is between 550 and 532 mm Hg at the normal flight altitudes. This situation determines the expansion of body gases. Hypoxia seriously worsens any respiratory insufficiency, primarily in the presence of smoke inhalation. The decrease of water concentration in the inhaled air compels the increase of fluid perfusion. Pre-flight and in-flight measures are analyzed, especially with regard to smoke inhalation, pneumothorax, and parenteral perfusion.

Experimental model for local administration of nerve growth factor in microsurgical nerve reconnections.

An experimental model for local administration of neuronotrophic substances at the site of peripheral nerve lesion is presented. The model consists of a subcutaneously located silicone reservoir and a connecting tube with its distal end fixed in the proximity of the severed and repaired nerve. The results of the preliminary tests of the model are presented. Sixty Sprague-Dawley rats were divided into two groups: control (saline-treated) (n = 30) and NGF-treated (n = 30). After axotomy of the sciatic nerve, an epineurial repair is performed. NGF or saline is injected daily into the subcutaneous reservoir during the first 3 weeks after axotomy and a single dose in the fourth week. The regenerated nerve observed in the NGF-treated group after four weeks of treatment presents a greater percentage of myelinated axons, thicker myeline sheaths, and more mature endoneurial layers. This experimental model provides a reliable and quantitative way to deliver neuronotrophic substances in site and at different administration rates.