Ionic transport in lipid bilayer membranes

The current-voltage relationships of model bilayer membranes have been measured in various phospholipid systems, under the influence of both a gradient of potential and an ionic concentration, in order to describe the ion translocation through hydrated transient defects (water channels) across the bilayer formed because of lipid structure fluctuations and induced by temperature. The results have been analyzed in the light of a statistical rate theory for the transport process across a lipid bilayer, recently proposed by Skinner et al. (1993). In order to take into account the observed I-V curves and in particular the deviation from an ohmic behavior observed at high potential values, the original model has been modified, and a new version has been proposed by introducing an additional kinetic process. In this way, a very good agreement with the experimental values has been obtained for all of the systems we have investigated (dimyristoylphosphatidyl ethanolamine bilayers and mixed systems composed by dimyristoylphosphatidyl ethanolamine/dimyristoylphosphatidylcholine mixtures and dimyristoylphosphatidyl ethanolamine/phosphatidic acid dipalmitoyl mixtures). The rate constants governing the reactions at the bilayer interfaces have been evaluated for K+ and Cl- ions, as a function of temperature, from 5 to 35 degrees C and bulk ionic concentrations from 0.02 to 0.2 M. Finally, a comparison between the original model of Skinner and the modified version is presented, and the advantages of this new formulation are briefly discussed.     

A new blood compatible and permselective hollow fiber membrane for hemodialysis

The authors have prepared a blood compatible and highly permselective hemodialysis membrane composed of polyether segmented nylon. This block copolymer was synthesized by polycondensation of bis-3-aminopropyl-poly(tetramethylene oxide) (PTMO) and poly(imino-1,3-bismethyl-cyclohexyl-iminoisophtharoyl) (NyBl) prepolymer obtained by polycondensation of 1,3-bis(aminomethyl)cyclohexane (B) and isophthalic acid (I). The molecular weight (MW) calculated from the number of end-groups was 16,000-21,000. In vitro blood compatibility was evaluated in terms of platelet adhesion onto the surface. PTMO-NyBl surfaces showed excellent platelet adhesion preventing properties. The PTMO-NyBl hollow fiber membrane was obtained by a dry-wet spinning process. The membranes had higher permeability coefficients for macromolecules ranging from MW 10,000 to 20,000 than polysulfone hollow fiber membrane (PS membrane), and had acceptably low albumin permeability for use as a dialysis membrane. The ex vivo blood compatibilities of PTMO-NyBl membrane and PS membrane were investigated by extracorporeal circulation in a pig model. The PTMO-NyBl membrane gave excellent results when assessing hemodialysis leukopenia, oxidative burst, and free platelet count decrease.     

Development of a silicone hollow fiber membrane oxygenator for ECMO application

A new silicone hollow fiber membrane oxygenator for extracorporeal membrane oxygenation (ECMO) was developed using an ultrathin silicone hollow fiber, with a 300 microm outer diameter and a wall thickness of 50 microm. The hollow fibers were mechanically cross-wound on the flow distributor to achieve equal distribution of blood flow without changing the fiber shape. The housing, made of silicone coated acryl, was 236 mm long with an inner diameter of 60 mm. The surface area was 1.0 m2 for prototype 211, and 1.1 m2 for prototype 209. The silicone fiber length was 150 mm, and the silicone membrane packing density was 43% for prototype 211 and 36% for prototype 209. Prototype 211 has a priming volume of 208 ml, and prototype 209 has a priming volume of 228 ml. The prototype 211 oxygenator demonstrates a gas transfer rate of 120 +/- 5 ml/min (mean +/- SD) for O2 and 67 +/- 12 ml/min for CO2 under 2 L of blood flow and 4 L of O2 gas flow. Prototype 209 produced the same values. The blood side pressure drop was low compared with the silicone sheet oxygenator (Avecor, 1500ECMO). These results showed that this new oxygenator for ECMO had efficiency similar to the silicone sheet oxygenator that has a 50% larger surface area. These results suggest that the new generation oxygenator using an ultrathin silicone hollow fiber possesses sufficient gas transfer performance for long-term extracorporeal lung support.     

Model experiments on mass transfer in ladle metallurgy

Model experiments on mass transfer in gas-stirred ladles were carried out in reactors of different geometric dimensions. The model system consists of: water, cyclohexan as model slag, iodium as element to be extracted from water into slag phase, and compressed air as stirring gas. The experimental results show that when using an eccentric bottom nozzle, rate constants of mass transfer are always smaller than with centric gas injection. Centric stirring leads to comparatively larger increases of rate constants if a certain gas flow rate is exceeded. Both results can be explained by different emulsification conditions of slag phase. Theoretical calculations of residence times show that mainly the emulsification of small droplets taken along by the recirculation flow is responsible for accelerations of mass transfer in gas-stirred ladles.     

Effects of ultrafiltration on solute clearances in hollow fiber artificial kidneys

Although solute clearances in artificial kidney coils increase with ultrafiltration (UF), we have previously shown that increases are usually less than UF rate (most likely because of decreases in diffusive transport with UF coils and, for larger solutes, molecular sieving). The present studies demonstrate the effects of UF on clearances of Na and bromsulphalein (BSP) (mol. wt. 838) in hollow fiber dialyzers. Clearances were measured at increasing transmembrane hydrostatic pressures at perfusion rates of 200 and 500 ml. per minute. Fractions of total clearance attributable to diffusion as compared to solvent drag forces were calculated. Sieving coefficients were determined in studies where diffusion was minimized and clearance was primarily by solvent drag. Clearance increases were less than UF rate only for BSP; molecular sieving most likely accounts for the difference at high perfusion rates. Only at 200 ml. per minute was slight decrease of diffusion with UF suggested. Thus, in contrast to coils, there is minimal or no decrease in diffusion with UF in hollow fiber dialyzers.     

Development of a membrane oxygenator for ECMO using a novel fine silicone hollow fiber

One of the limitations of conventional silicone hollow fiber oxygenators compared with microporous membrane oxygenators is poor gas permeability. However, the silicone hollow fiber is free from plasma leakage, which is the major life limiting factor of the microporous membrane oxygenator. It has been difficult to fabricate a fine, thin hollow fiber for reduction of resistance to gas permeability because of the poor mechanical strength of conventional silicone materials. The authors developed a novel silicone material with sufficient mechanical strength, and a fine silicone hollow fiber with a diameter of 30 microns and wall thickness of 50 microns, which is approximately half that of a conventional silicone hollow fiber. Using this newly developed silicone hollow fiber, the authors developed a compact extracapillary flow membrane oxygenator. The oxygenator consists of fine silicone hollow fibers inserted in a housing made of polycarbonate. The housing is a cylindrical case, 20 cm long and 55 mm in inside diameter. The hollow fibers are cross-wound. The surface area of the membrane is 2.0 m2, and priming volume is 230 ml. Gas transfer performance of the newly developed oxygenator was evaluated by in vitro experiments. Oxygen and carbon dioxide transfer rates were 195 ml/min and 165 ml/min, at a blood flow rate 3 L/min. The novel silicone membrane oxygenator developed in this study can be used for extended duration in such applications as extracorporeal membrane oxygenation.     

Temperature-jump experiments on thin lipid membranes in the presence of valinomycin

Temperature jump relaxation experiments on planar lipid membranes in the presence of valinomycin were performed using the absorption of a strong light flash as an energy source for the generation of the T-jump. The relaxation of the current carried by valinomycin/Rb+ complexes was measured. The results were interpreted on the basis of a transport model which was also analyzed by voltage jump relaxation experiments. The study shows that the application of the T-jump technique provides valuable information about transport kinetics as well as the dynamics of the membrane structure. At the given experimental conditions the relaxation of the current is believed to reflect a temperature-dependent transition of the membrane to a new conformational state of low order. The relaxation could be resolved with the present technique only at low temperatures and for membranes of high microviscosity.     

Analysis of fiber debonding and sliding experiments in brittle matrix composites

The use of a recent analysis of fiber debonding and sliding in brittle matrix composites to interpret the results of fiber pulling and pushing experiments is examined. The stress-displacement relations are expressed in normalized forms that are convenient for curve fitting to experimental measurements and the analysis is extended to provide stress-displacement relations for cyclic loading in addition to monotonic loading. The ranges of some of the important elastic parameters and their influence on the stress-displacement relations are examined. Differences between single and multiple fiber pulling and between pushing and pulling experiments are assessed.     

Hybrid-Cells-in-Series Model for Solute Transport in a River

A hybrid-cells-in-series (HCIS) model has been conceptualized to simulate transport of a conservative solute in a river. The concentration graph of the effluent from the first hybrid unit follows a skewed concentration-time profile more close to reality. When the linear size Δx of the hybrid unit is more than 4?DL/u, where DL = longitudinal dispersion coefficient, u = mean flow velocity, the concentration graphs at nΔx, n = 1,2,3,… are approximately equal to that predicted by Ogata and Banks’ model. The model parameters α, T1, and T2, which are the times of residence of solute in the plug flow zone, and in the first and second thoroughly mixed reservoir respectively, can be estimated from a time-concentration graph using: (1) partial moments, (2) three characteristics of the graph, i.e., time to peak, peak concentration and the partial first moment, and (3) least square optimization. The performance of the HCIS model has been verified using the data of field tests conducted in rivers. The model parameters estimated using one concentration graph simulate the concentration graphs observed at other location downstream with reasonable accuracy.     

Nitric oxide prevents human platelet adhesion to fiber membranes in whole blood

During cardiopulmonary bypass or long-term extracorporeal life support, foreign surface induced platelet deposition in the oxygenator causes deterioration of gas exchange. In this study, the authors evaluated the effectiveness of nitric oxide (NO) in reducing the adhesion of platelets in whole blood to the surface of hollow fiber membranes. For this purpose, a test chamber was designed consisting of a gas exchanger with ten mitsubishi multi-layered composite hollow fibers (MHF: 257 mm OD; 203 mm ID; 70 mm length) and a polypropylene tube (16 mm OD; 100 mm length). Pure N2 (control) or nitric oxide (NO) (100 ppm, 200 ppm in N2) were delivered into the test chamber previously filled with 13 ml human whole blood. Platelet counts and platelet factor 4 (PF4), as a measure of platelet activation, were measured before and after either 1 or 2 hr of testing, and fibers were observed under scanning electron microscopic study (SEM) after each experiment. In the control and 100 ppm NO groups, platelet counts decreased and the level of PF4 increased during the 1 hr period. In the 200 ppm NO group, almost no platelet deposition could be observed on the surface of fibers under SEM. In conclusion, NO flow through hollow fiber membranes can markedly reduce platelet adhesion. Additional quantitative studies should define the optimal concentration for this effect and determine if this finding could improve oxygenator function, especially under conditions of long-term support.     

A mass transport theory for interphase precipitation with application to vanadium steels

A mass transport theory is developed for interphase precipitation reactions in binary and pseudobinary systems where constant concentrations of solute are maintained at the interphase boundary. The theory presents exact solutions to the problem of diffusion-controlled growth involving the coupled motion of two boundaries. It is generally applicable to interphase pre-cipitation in both solid-solid and solid-liquid systems. Aspects of the theory are discussed with reference to the interphase precipitation reaction in model iron-carbon-vanadium alloys.     

Estimation of peritoneal mass transport by three-pore model in children

BACKGROUND: Computerized modeling is increasingly used to optimize the efficacy of peritoneal dialysis (PD). The Personal Dialysis Capacity (PDC) test is a new tool to model PD efficacy based on the three-pore model of peritoneal mass transport. We sought to evaluate (i) whether the PDC test is applicable to children on chronic PD, and (ii) whether the physiological mass transport coefficients defined in the three pore model are dependent on age or body size in childhood. METHODS: A validation study was performed in 32 pediatric chronic PD patients. Twenty tests were performed using a standard CAPD regimen, and 22 tests using a simplified automated PD (APD) protocol. Test accuracy and precision were evaluated by comparison of predicted with measured 24-hour dialysate clearances of urea, creatinine, beta2-microglobulin and albumin and ultrafiltration rates. Long-term reproducibility was assessed in 16 patients by repeated clearance studies after a median time interval of 10 weeks. RESULTS: While daily clearances of urea and creatinine were predicted with good precision and accuracy with both test protocols (concordance correlation coefficients 0.90 to 0.98, mean difference predicted-calculated -0.6 to +0.6 ml/min/1.73 m2), ultrafiltration rates were predicted more closely by the APD (r = 0.97) than by the CAPD test (0.80). Middle and large molecule clearances were predicted less precisely in both test settings (r = 0.48 to 0.83). Re-test reproducibility was slightly lower than the predictive precision observed in the original test (r = 0.80 to 0.91). The calculated total peritoneal pore area increased in absolute terms, decreased with body size when standardized to weight, and was independent of body size when normalized to body surface area. The body size-normalized fluid reabsorption rate was slightly increased in young infants compared to older children or adults. CONCLUSIONS: The PDC test permits to model peritoneal solute and water transport with remarkable precision in children of all age groups. While the peritoneal pore area is a linear function of body surface area, fluid reabsorption appears to be slightly increased in young infants.     

激光表面合金化快速凝固条件下的溶质分配系数

本文在充分考虑激光表面合金化过程高温度梯度产生的Soret效应和液相金属强制性对流的作用,在Aziz模型基础之上,从理论上分析了激光表面合金化非平衡凝固过程溶质在界面的分布,并推导出新的溶质分配系数公式。