High Hydraulic Permeability Research Articles

Dialysis membranes in convective treatments.

substitution fluid. No dialysis fluid is present. Thistechnique offers the advantage of excellent clearancesHaemodialysis is basically divided into two categories: for large molecules. The treatment, however, has onlylow-flux dialysis and high-flux dialysis. Low-flux dia- been applied in Europe because of the need for largelysis includes the standard haemodialysis technique in quantities of substitution fluid in commercially pre-which dialysers with low hydraulic permeability are pared bags. (ii) Haemodiafiltration is a form of therapyutilized. Blood and dialysate flow rates are at 250 and in which diffusion and convection are conveniently500 ml/min, respectively and the average duration is combined. Dialysate is circulated countercurrent to4 h. When the dialyser surface area is increased, and blood but the amount of ultrafiltration exceeds theflow rates of blood and dialysate are increased up to programmed patient weight loss and therefore the final400 and 800 ml/min, respectively, the treatment is fluid balance is achieved by the infusion in the venousdefined as high efficiency haemodialysis [1]. In this line of appropriate amounts of substitution fluid [4–6].treatment the clearance of small molecules is remark- Due to the high costs related to replacement solutions,ably increased and the treatment time generally can be new approaches have been proposed in Europe forshortened, but the clearance for middle to large molec- performing haemofiltration and haemodiafiltration [7].ules is still very low due to the permeability character- Part of the fresh dialysate is diverted from its line toistics of the membrane. the dialyser, it is filtered and it finally is used as aClassic cellulosic membranes such as cuprophan are replacement solution. These on-line treatments havetypically low-flux membranes i.e. permeability to water the advantage of reduced costs, unlimited productionis in the range of 5–6 ml/h/mmHg/m2, they are strongly of substitution fluid and no need for tons of commer-hydrophilic and remarkably thin to permit an optimal cially produced fluid in bags. On the other hand, theutilization in diffusive treatments such as microbiological and chemical purity of the on-linehaemodialysis. prepared fluid may represent a concern since no on-lineOn the other hand, classic high-flux synthetic mem- controls can be made. (iii) High-flux dialysis is abranes originally were fully hydrophobic, strongly technique in which highly permeable dialysers areasymmetric and with a thick wall. They were employed utilized in conjunction with an accurate ultrafiltrationprincipally in convective therapies such as haemofiltra- control of the dialysis machine. Because of the naturetion due to their high hydraulic permeability, i.e. of the hollow fibre dialyser, the correct water balance30–40 ml/h/mmHg/m2, and their high sieving coeffi- is achieved thanks to an internal filtration in thecients [2]. proximal part and a backfiltration in the distal part ofOnly recently has it been possible to design synthetic the filter [8]. As a consequence, some of the advantagesmembranes with reduced wall thickness and a mixed of removing larger molecules are maintained becausehydrophobic–hydrophilic structure. This has permitted of the internal convection, while the need for replace-their use in new techniques such as high-flux haemodia- ment solution is avoided by a significant amount oflysis and haemodiafiltration in which diffusion and backfiltration of fresh dialysate. In this way, someconvection are conveniently combined [3]. of the advantages of haemodiafiltration are maintainedWhen high-flux membranes are utilized, three differ- with a simpler and easier layout of the technique [8].ent approaches can be chosen: (i) haemofiltration is a Again, the dialysate purity is a key issue in thisfully convective treatment in which large amounts of treatment where bacterial products could be back-ultrafiltrate are produced and replaced by a sterile transported into the blood together with the reverseflux of water. This may affect the entire concept ofbiocompatibility of the system. Concerning this aspect,

Stability and leakiness: Opposing challenges to the glomerulus

The complex architecture of the glomerular tuft is stabilized by several mechanisms. The basic system consists of the GBM and the mesangium maintaining the branching pattern of the capillary network. Superimposed are the podocytes, which appear to take effect by two mechanisms. First, podocytes contribute to the stabilization of the capillary folding pattern by supporting the angles between neighboring capillaries. Second, podocyte foot processes fixed to the outer aspect of the GBM probably function as contractile patches counteracting the elastic distension of the GBM. Simultaneously, the pattern of foot process interdigitation underlies the elaboration of a filtration slit and is thus pivotal for the high hydraulic permeability and the specificity of the glomerular filter. The loss of this pattern-commonly termed "foot process effacement" or "foot process fusion"-is frequently found in pathological situations and results in a decrease in permeability and impairment in specificity. On the other hand, foot process effacement is associated with prominent hypertrophy of the contractile apparatus of podocytes, suggesting an increased ability to generate forces counteracting capillary expansion. Thus, foot process effacement appears as an adaptive change in podocyte phenotype giving priority to the support function of podocytes for the prize of reducing the specific permeability.

Stable isotope geochemistry of bentonites from the island of Milos (Greece)

Smectites from bentonite deposits from the island of Milos were analyzed for their oxygen and hydrogen isotopic ratios. Based on published isotopic fractionation data the results argue for a main meteoric water origin of the fluids responsible for the transformation of the pyroclastic rocks to bentonites. Alteration temperatures were estimated to be in the range from 30° to 90°C. In addition, δ 18O and δ 13C ratios were also measured on calcite separated from the bentonites. Dissolution of carbonates in the overlying marl and reprecipitation in the bentonite/pyroclastic rocks is considered as the origin of most of the calcites. In consequence of this polyphase history, δ 13C-values indicate a marine carbon source while δ 18O ratios suggest equilibrium of calcite with a fluid phase at elevated temperatures. Isotopic data of Smectites and calcite point to similar conditions for the formation of both minerals. It is proposed that an aquifer in the NE of Milos was heated during volcanic activity in the early Quaternary and that heated groundwater circulated through pyroclastic rocks with high hydraulic permeability to form extensive bentonite deposits.

Recombinant human erythropoietin and high flux haemodiafiltration

Since 1982, 32 uraemic patients were treated in our institution by high flux haemodiafiltration (H-HDF) in order to shorten significantly the dialytic treatment session. H-HDF used a high surface area filter (1.4-1.9 m2) with high hydraulic permeability (polyacrylonitrile and polysulfone), at high blood flow (450 ml/min) and high rates of reinfusion of substitution fluid (22 l/session). In this way the dialytic session was shortened to 140 +/- 19 min, maintaining a good cardiovascular stability and high dialytic efficiency (Kt/V > 1.1). Human recombinant erythropoietin rHuEpo introduced in the therapy of this group in 1987 has resulted in an improvement of renal anaemia, but also a prolongation of the time of dialytic treatment due to a decrease in the efficiency of filters. During the period of the study, the treatment time increased from 140 +/- 19 min to 168 +/- 25 min with a concomitant increase of haematocrit and haemoglobin (from 24% to 36% and from 7.9 to 10.5 g/dl, respectively). H-HDF maintains a noticeable increase in dialytic efficacy with good cardiovascular stability, but the goal of a significant reduction in the time of treatment can no longer be obtained.

Technical and Clinical Evaluation of a New Asymmetric Polysulfone Membrane (Biosulfane®)

First generation asymmetric polysulfone membranes had high hydraulic permeability (kf = 40 ml/h/mmHg/sqm) but a low diffusive permeability due to the hydrophobic nature and wall thickness of 75-100 microns. We have tested a new polysulfone membrane with a wall thickness of 40 microns in a series of in vitro and in vivo dialysis session experiments. The new "Biosulfane" membrane presented a Kf of 45.8 with constant performance up to 240 mins. The koA was 760 and the clearance value at 350 ml/min of Qb in hemodiafiltration was 255 ml/min for urea, 210 for creatinine, 225 for phosphate, 76 for inulin. In high flux dialysis the clearances were similar except for inulin which was 32% lower due to the lower convection amount. Beta-2 microglobulin clearance was 22 ml/min in high flux dialysis and 37 in hemodiafiltration. Solute sieving coefficients were close to 1 for the majority of the studied solutes in a wide range of molecular weights and slight variations were observed for charged solutes due to Donnan's effect. The sieving for Inulin was 0.96 while that for Beta-2 microglobulin was not measurable due to a large molecule adsorption on the inner structure of the fibres. The good performances of this membrane are probably due to reduced wall thickness and a consequent improvement in diffusive permeability to small size solutes.

Operational Instability in Extracorporeal Filtration of Blood

After encountering recurrent flow stasis in continuous arteriovenous hemofiltration (CAVH) clinical systems, a combined experimental and theoretical investigation was undertaken of hydraulic operational behavior of extracorporeal systems operated with blood of high hematocrit. Theoretical analyses were developed for selected modalities of extracorporeal blood purification. These modalities include hemofiltration, volume-controlled hemofiltration, CAVH, and volume-controlled CAVH. These analyses have revealed that steady state hydraulic operation may not always be possible when the venous return blood hematocrit becomes elevated. For all modalities, except volume-controlled hemofiltration, one would expect to encounter a departure from steady state operation at a critical hematocrit specific to that system. One would typically not expect to encounter this operational instability phenomenon, unless venous hematocrit was greater than 50. When operational instability is encountered, progressive hemoconcentration of blood and elevation of hemofilter blood pressure occur despite no further perturbation of the system. For modalities in which blood is pumped, the pressure rises until such time as mechanical integrity of the circuit is breached. For the arteriovenous modalities, the pressure rises, and blood flow and uhrafiltration ultimately cease. Laboratory experiments were performed to validate this theory. Blood was perfused through hemofilters of high hydraulic permeability. The ultrafiltrate pressure was varied stepwise from a positive pressure sufficient to prevent net uhrafiltration (or backfiltration) to approximately – 50 mm Hg. Examination of hemofilter pressures and uhrafiltration rates in twenty-four experiments showed a return to steady state operation after most perturbations in ultrafiltrate pressure. In eleven of these experiments (only those in which the venous hematocrit was greater than 47), a subsequent small perturbation in ultrafiltrate pressure was followed by a large upward pressure excursion in the pre-and posthemofilter pressures such that the experiment had to be terminated to avoid disruption of tubing or membrane. No clotting occurred, and the experiments were repeatable with the same or different hemofilters. The experimental results are in reasonably good agreement with the theoretical analysis for pumped uhrafiltration. Hematocrit values at which this operational instability occurs can be predicted based on the shape of the apparent viscosity versus hematocrit curve and system design parameters, in particular hemofilter hydraulic permeability and venous catheter diameter. These hematocrit values may define an upper limit of postfilter hematocrit at which some blood purification systems can operate.

Reverse osmosis and fine structure of decrystallized acrylic acid grafted nylon 6 membranes

AbstractNylon 6 film and acrylic acid grafted nylon 6 (GN) membrances were reacted with paraformaldehyde and lactic acid in the presence of acid catalysts. Decrystallized nylon 6 (DN) and decrystallized grafted nylon 6 (DGN) membranes were thus obtained. The cross sections of GN and DGN membranes were observed by a transmission electron microscope. Branch poly(acrylic acid) penetrated toward the center of the membrane and deposited homogeneously within the membrane as the extent of grafting exceeded about 100%. The reverse osmosis of DN and DGN membranes was investigated. The water permeability through the membranes was improved by the decrystallization reaction. DGN membranes with more than 100% grafting show high values of the salt rejection (Rs) as compared with DN and DGN membranes with about 50% grafting, especially at the region of the high hydraulic permeability coefficient of water (K). The relationships among Rs, K, and the volume fraction of water (H) are discussed by considering the results of the decrystallization reaction and electron microscopy.

A New Operational Principle for Blood Treatment with Highly Permeable Membranes

Conventional membrane devices for blood treatment have traditionally been operated either as hemofilters or as dialyzers. This article discusses the possibility of combining both aspects in one single device, using a single membrane with a high hydraulic permeability. In fact, the combined function is found to be the only possible mode of operation, using very permeable membranes, but this apparently has so far not been recognized in actual application.

Permeabilities of coagulated cellulose acetate dialysis membranes

AbstractThe diffusive flux of NaCl and the hydraulic flux of pure water through coagulated cellulose acetate membranes are examined. Coagulated cellulose acetate membranes (without densification by heat treatment or drying) possess higher permeability than what may be expected from the permeabilities of the dry polymer. Their overall hydraulic permeability (ultrafiltration rate of water) is greatly dependent upon the membrane casting conditions and the resulting asymmetry of the membrane. On the other hand, the asymmetry of a membrane does not play as great a role in the diffusive permeability of a solute. With homogeneous membranes, higher diffusive flux is always accompanied by higher hydraulic permeability. With asymmetric membranes, this is not always true. The diffusive permeability of NaCl and the hydraulic permeability of water through coagulated cellulose acetate membranes can be controlled nearly independently. Consequently, high diffusive (NaCl) permeability with low hydraulic water permeability and vice versa can be obtained by varying the casting conditions and also by partially saponifying the denser portion of the membrane.