Pressure responsive gating in nanochannels coated by semiflexible polymer brushes.

We study by coarse-grained molecular-dynamics simulations the liquid flow in a slit channel with the inner walls coated by semiflexible polymer brushes. The distance between walls is close enough such that polymers grafted to opposing walls interact among each other and form bundles across the channel in poor solvent conditions. The solvent is simulated explicitly, including particles that fill the interior of the channel. The system is studied in equilibrium and under flow, by applying a constant body force on each particle of the system. A non-linear relation between external force and flow rate is observed, for a particular set of parameters. This non-linear response is linked to a morphological change of the polymer brushes. For large enough forces, the bundle structures formed across the channel break as the chains lean in the direction of the flow, and clear the middle of the channel. This morphological alteration of the polymer configurations translates in a sudden increase in the flow rate, acting as a pressure-responsive gate. The relation between flow and external force is investigated for various parameters, such as grafting density, quality of the solvent and polymer bending rigidity. We observe a non-monotonic dependence of the flow as a function of the polymer rigidity, and find an optimum value for the persistence length. We also find that the force threshold at which the morphological changes happen in the polymer brush, depends linearly on the grafting density. These findings can lead to new flow control techniques in micro and nano-fluidic devices.

Droplet Transport in a Nanochannel Coated by Hydrophobic Semiflexible Polymer Brushes: The Effect of Chain Stiffness.

We study the influence of chain stiffness on droplet flow in a nanochannel, coated with semiflexible hydrophobic polymers by means of nonequilibrium molecular dynamics simulations. The studied system is then a moving droplet in the slit channel, coexisting with its vapor and subjected to periodic boundary conditions in the flow direction. The polymer chains, grafted by the terminal bead to the confining walls, are described by a coarse-grained model that accounts for chain connectivity, excluded volume interactions and local chain stiffness. The rheological, frictional and dynamical properties of the brush are explored over a wide range of persistence lengths. We find a rich behavior of polymer conformations and concomitant changes in the friction properties over the wide range of studied polymer stiffnesses. A rapid decrease in the droplet velocity was observed as the rigidity of the chains is increased for polymers whose persistence length is smaller than their contour length. We find a strong relation between the internal dynamics of the brush and the droplet transport properties, which could be used to tailor flow properties by surface functionalization. The monomers of the brush layer, under the droplet, present a collective "treadmill belt" like dynamics which can only be present due the existence of grafted chains. We describe its changes in spatial extension upon variations of polymer stiffness, with bidimensional velocity and density profiles. The deformation of the polymer brushes due to the presence of the droplet is analyzed in detail. Lastly, the droplet-gas interaction is studied by varying the liquid to gas ratio, observing a 16% speed increase for droplets that flow close to each other, compared to a train of droplets that present a large gap between consecutive droplets.

Brushes of semiflexible polymers in equilibrium and under flow in a super-hydrophobic regime.

We performed molecular dynamics simulations to study the equilibrium and flow properties of a liquid in a nano-channel with confining surfaces coated with a layer of grafted semiflexible polymers. The coverage spans a wide range of grafting densities from essentially isolated chains to dense brushes. The end-grafted polymers were described by a bead spring model with a harmonic potential to include the bond stiffness of the chains. We varied the rigidity of the chains, from fully flexible polymers to rigid rods, in which the configurational entropy of the chains is negligible. The brush-liquid interaction was tuned to obtain a super-hydrophobic channel, in which the liquid did not penetrate the polymer brush, giving rise to a Cassie-Baxter state. Equilibrium properties such as brush height and bending energy were measured, varying the grafting density and the stiffness of the polymers. We also studied the characteristics of the brush-liquid interface and the morphology of the polymer chains supporting the liquid for different bending rigidities. Non-equilibrium simulations were performed, moving the walls of the channel in opposite directions at constant speed, obtaining a Couette velocity profile in the bulk liquid. The molecular degrees of freedom of the polymers were studied as a function of the Weissenberg number. Also, the violation of the no-slip boundary condition and the slip properties were analyzed as a function of the shear rate, grafting density and bending stiffness. At high grafting densities, a finite slip length independent of the shear rate or bending constant was found, while at low grafting densities a very interesting non-monotonic dependence on the bending constant is observed.

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.

Mean properties and free energy of a few hard spheres confined in a spherical cavity.

We use analytical calculations and event-driven molecular dynamics simulations to study a small number of hard sphere particles in a spherical cavity. The cavity is also taken as the thermal bath so that the system thermalizes by collisions with the wall. In that way, these systems of two, three, and four particles, are considered in the canonical ensemble. We characterize various mean and thermal properties for a wide range of number densities. We study the density profiles, the components of the local pressure tensor, the interface tension, and the adsorption at the wall. This spans from the ideal gas limit at low densities to the high-packing limit in which there are significant regions of the cavity for which the particles have no access, due the conjunction of excluded volume and confinement. The contact density and the pressure on the wall are obtained by simulations and compared to exact analytical results. We also obtain the excess free energy for N = 4, by using a simulated-assisted approach in which we combine simulation results with the knowledge of the exact partition function for two and three particles in a spherical cavity.

Mixed brush of chemically and physically adsorbed polymers under shear: inverse transport of the physisorbed species.

We study mixed brushes under shear flow by molecular dynamics simulation with an explicit solvent. The primary brush is formed by chemically grafting polymers to a solid substrate, the secondary brush is comprised of shorter, physically end-adsorbed molecules that can laterally diffuse. By virtue of the immobility of the grafted end-points of the primary brush, its individual macromolecules perform a cyclic motion. If there is a well defined solvent-brush interface, this cyclic motion of the primary brush molecules will collectively result in the reversal of the flow inside of the primary brush. This backflow, linear in the shear rate, gives rise to the transport of the shorter, physically end-adsorbed molecules in the opposite direction of the solvent flow. We discuss which conditions are necessary to observe this counter-intuitive phenomenon. Comparing Poiseuille and Couette flow we demonstrate that the magnitude of the local shear rate at the brush-liquid interface dictates the cyclic motion and concomitant inversion of transport but that these universal effects are independent of the type of driving the flow.

Measuring the composition-curvature coupling in binary lipid membranes by computer simulations.

The coupling between local composition fluctuations in binary lipid membranes and curvature affects the lateral membrane structure. We propose an efficient method to compute the composition-curvature coupling in molecular simulations and apply it to two coarse-grained membrane models-a minimal, implicit-solvent model and the MARTINI model. Both the weak-curvature behavior that is typical for thermal fluctuations of planar bilayer membranes as well as the strong-curvature regime corresponding to narrow cylindrical membrane tubes are studied by molecular dynamics simulation. The simulation results are analyzed by using a phenomenological model of the thermodynamics of curved, mixed bilayer membranes that accounts for the change of the monolayer area upon bending. Additionally the role of thermodynamic characteristics such as the incompatibility between the two lipid species and asymmetry of composition are investigated.

Liquid and Droplet Transport in Brush-Coated Cylindrical Nanochannels: Brush-Assisted Droplet Formation.

We study, by coarse-grained molecular dynamics simulations, equilibrium and flow properties of a liquid in cylindrical nanochannels, coated with polymer brushes. The parameters of the interaction potential model confer a chemical incompatibility between brush monomers and liquid particles. First, we study cylindrical channels whose radii are larger than the brush height and a continuous column of liquid forms at the center of the channel. These results are contrasted to the limiting case in which the radius of the cylinder is comparable to the brush height. In this second case, the grafted polymers interact across the channel and "close" it. We observe a train of droplets as the stable liquid morphology. The droplet size is comparable to the cylinder radius. By applying a constant body force onto the liquid, we induce a Poiseuille-like flow and investigate the morphology and flow rate as a function of driving force. Upon increasing the driving force, we encounter a nonequilibrium transition from a closed channel with slowly moving droplets to a flowing liquid thread at the center. The switching between these two states is reversible.

Frictional forces between strongly compressed, nonentangled polymer brushes: molecular dynamics simulations and scaling theory.

By means of molecular dynamics simulations and scaling theory we study the response of opposing polymer brushes to constant shear motion under good solvent conditions. Model systems that contain explicit solvent molecules (Lennard-Jones dimers) are compared to solvent-free systems while varying of the distance between the grafted layers and their molecular parameters, chain length and grafting density. Our study reveals a power-law dependence of macroscopic transport properties on the Weissenberg number, W, beyond linear response. For instance, we find that the kinetic friction constant scales as mu approximately W(0.57) for large values of W. We develop a scaling theory that describes our data and previous numerical data including recent experiments.

Comparison of dissipative particle dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems.

In this work we compare and characterize the behavior of Langevin and dissipative particle dynamics (DPD) thermostats in a broad range of nonequilibrium simulations of polymeric systems. Polymer brushes in relative sliding motion, polymeric liquids in Poiseuille and Couette flows, and brush-melt interfaces are used as model systems to analyze the efficiency and limitations of different Langevin and DPD thermostat implementations. Widely used coarse-grained bead-spring models under good and poor solvent conditions are employed to assess the effects of the thermostats. We considered equilibrium, transient, and steady state examples for testing the ability of the thermostats to maintain constant temperature and to reproduce the underlying physical phenomena in nonequilibrium situations. The common practice of switching off the Langevin thermostat in the flow direction is also critically revisited. The efficiency of different weight functions for the DPD thermostat is quantitatively analyzed as a function of the solvent quality and the nonequilibrium situation.

Fluctuation-induced forces between rings threaded around a polymer chain under tension.

We characterize the fluctuation properties of a polymer chain under external tension and the fluctuation-induced forces between two ring molecules threaded around the chain. The problem is relevant in the context of fluctuation-induced forces in soft-matter systems, features of liquid interfaces, and to describe the properties of polyrotaxanes and slide-ring materials. We perform molecular-dynamics simulations of the Kremer-Grest bead-spring model for the polymer and a simple ring-molecule model in the canonical ensemble. We study transverse fluctuations of the stretched chain as a function of chain stretching and in the presence of ring-shaped threaded molecules. The fluctuation spectra of the chains are analyzed in equilibrium at constant temperature, and the differences in the presence of two-ring molecules are compared. For the rings located at fixed distances, we find an attractive fluctuation-induced force between the rings, proportional to the temperature and decaying with the ring distance. We characterize this force as a function of ring distance, chain stretching, and ring radius, and we measure the differences between the free chain spectrum and the fluctuations of the chain constrained by the rings. We also compare the dependence and range of the force found in the simulations with theoretical models coming from different fields.

Case report: imaging of a bilobed gallbladder.

Imaging of the gallbladder demonstrates a wide range of anatomical variants, including anomalies in location, number and shape. Duplication anomalies are quite rare and are characterized by a large variety of configurations depending on the size and degree of fusion of the two lobes, and on the number and disposition of the cystic ducts. We present a case of a deeply cleft, bilobed gallbladder imaged by computed tomography (CT), ultrasonography (US) and oral cholecystography (OCG). The anomaly consisted of complete duplication of the body and fundus into two distinct and separated lobes both of which entered a single infundibulum. Awareness of congenital gallbladder variants may help in recognizing and correctly classifying gallbladder abnormalities, thus preventing misdiagnoses.

[Screening for abdominal aortic aneurysms in the general population. Preliminary results]. / Screening per gli aneurismi dell'aorta addominale nella popolazione generale. Risultati preliminari.

Between March 1991 and April 1993, 1188 subjects aged 65-75 years, out of 2734 invited, underwent abdominal ultrasound for screening of asymptomatic abdominal aortic aneurysms (AAA). For each patient the maximal anteroposterior and transverse diameters of the suprarenal and infrarenal aorta were measured. According to the literature data an AAA is defined as an aortic dilatation > 29 mm. AAA < 40 mm are followed by ultrasound every 6 months and the AAA > 39 mm are considered for surgical repair after complete clinical work-up. In addition any aortic dilatation ranging 26-29 mm is followed too, using the same criteria. The aorta was normal in 1112 patients (95.12%), an infrarenal aortic dilatation was found in 21 patients (1.79%), an AAA < 40 mm in 15 patients (1.28%) and an AAA > 39 mm in 21 patients (1.79%). The global prevalence of AAA > 29 mm was 3.07% (0.3% for the females and 6.8% for the males), similar to that reported by other authors. The statistically significant (p < 0.01) risk factors were: smoking, alcohol consumption, coronary disease and chronic lung obstruction. Hypertension and dyslipidaemia were not significant (but HDL-cholesterol and Apo-B), according to a different etiology of the aneurysms. In addition 38.5% of the patients had total cholesterol > 240 mg/dl but only 34.9% of these was under medical treatment and/or on a diet. Our preliminary data confirm the results of similar studies in other countries: screening for AAA is worthwhile on the general population and, looking to a better cost-benefit rate, it might be focused only on males.

[Transgastric lymphadenography]. / La linfoadenografia transgastrica.

The transgastric lymphadenography technique carried out by the endoscopic route in 52 patients at the Digestive Endoscopy Centre of the 3rd Surgical Clinic of Rome University is described. On the basis of personal results it is considered that transgastric lymphadenography represents an interesting technique for the study of perigastric lymphnodes in normal and pathologic conditions. It is thought that it could also be useful in the study of the abdominal lymphatics although further verification and a larger series are necessary.

Advanced practice nursing role: nurse practitioner.

Nurse Practitioners are advanced practice nurses (APNs) who provide primary and acute care to individuals in many settings. The NP diagnoses and treats medical and surgical conditions that require acute, short-term management and chronic, long-term treatment. States vary in regulating processes regarding collaborative agreements, prescriptive authority, medical staff privileges, and insurance/third party reimbursement.

Health promotion for the elderly: issues and program planning.

Orthopaedic nurses have opportunities to educate the elderly regarding health-promoting lifestyle changes. Although there are many such changes to consider, the focus here is on exercise, injury prevention, smoking and alcohol cessation, immunization, and health screening. Since the elderly present a challenge to traditional education methods, specific strategies are suggested for elder health education.