Biocompatibility issues in hemodialysis

Electrospun Membranes Based on Quaternized Polysulfones: Rheological Properties-Electrospinning Mechanisms Relationship

Composite membranes based on a polymer mixture solution of quaternized polysulfone (PSFQ), cellulose acetate phthalate (CAP), and polyvinylidene fluoride (PVDF) for biomedical applications were successfully obtained through the electrospinning technique. To ensure the polysulfone membranes' functionality in targeted applications, the selection of electrospinning conditions was essential. Moreover, understanding the geometric characteristics and morphology of fibrous membranes is crucial in designing them to meet the performance standards necessary for future biomedical applications. Thus, the viscosity of the solutions used in the electrospinning process was determined, and the morphology of the electrospun membranes was examined using scanning electron microscopy (SEM). Investigations on the surfaces of electrospun membranes based on water vapor sorption data have demonstrated that their surface properties dictate their biological ability more than their specific surfaces. Furthermore, in order to understand the different macromolecular rearrangements of membrane structures caused by physical interactions between the polymeric chains as well as by the orientation of functional groups during the electrospinning process, Fourier transform infrared (FTIR) spectroscopy was used. The applicability of composite membranes in the biomedical field was established by bacterial adhesion testing on the surface of electrospun membranes using Escherichia coli and Staphylococcus aureus microorganisms. The biological experiments conducted establish a foundation for future applications of these membranes and validate their effectiveness in specific fields.

Recent advancement challenges with synthesis of biocompatible hemodialysis membranes

The focus of this study is to enhance the protein fouling resistance, hydrophilicity, biocompatibility, hemocompatibility and ability of the membranes and to reduce health complications like chronic pulmonary disease, peripheral vascular disease, cerebrovascular disease, and cardiovascular disease after dialysis, which are the great challenges in HD applications. In the current study, the PSF-based dialysis membranes are studied broadly. Significant consideration has also been provided to membrane characteristics (e.g., flowrate coefficient, solute clearance characteristic) and also on commercially available polysulfone HD membranes. PSF has gained a significant share in the development of HD membranes, and continuous improvements are being made in the process to make high flux PSF-based dialysis membranes with enhanced biocompatibility and improved protein resistance ability as the major issue in the development of membranes for HD application is biocompatibility. There has been a great increase in the demand for novel biocompatible membranes that offer the best performances during HD therapy, for example, low oxidative stress and low change ability of blood pressure.

Blood-incompatibility in haemodialysis: alleviating inflammation and effects of coagulation

Blood-incompatibility is an inevitability of all blood-contacting device applications and therapies, including haemodialysis (HD). Blood leaving the environment of blood vessels and the protection of the endothelium is confronted with several stimuli of the extracorporeal circuit (ECC), triggering the activation of blood cells and various biochemical pathways of plasma. Prevention of blood coagulation, a major obstacle that needed to be overcome to make HD possible, remains an issue to contend with. While anticoagulation (mainly with heparin) successfully prevents clotting within the ECC to allow removal of uraemic toxins across the dialysis membrane wall, it is far from ideal, triggering heparin-induced thrombocytopenia in some instances. Soluble fibrin can form even in the presence of heparin and depending on the constitution of the patient and activation of platelets, could result in physical clots within the ECC (e.g. bubble trap chamber) and, together with other plasma and coagulation proteins, result in increased adsorption of proteins on the membrane surface. The buildup of this secondary membrane layer impairs the transport properties of the membrane to reduce the clearance of uraemic toxins. Activation of complement system-dependent immune response pathways leads to leukopenia, formation of platelet–neutrophil complexes and expression of tissue factor contributing to thrombotic processes and a procoagulant state, respectively. Complement activation also promotes recruitment and activation of leukocytes resulting in oxidative burst and release of pro-inflammatory cytokines and chemokines, thereby worsening the elevated underlying inflammation and oxidative stress condition of chronic kidney disease patients. Restricting all forms of blood-incompatibility, including potential contamination of dialysis fluid with endotoxins leading to inflammation, during HD therapies is thus still a major target towards more blood-compatible and safer dialysis to improve patient outcomes. We describe the mechanisms of various activation pathways during the interaction between blood and components of the ECC and describe approaches to mitigate the effects of these adverse interactions. The opportunities to develop improved dialysis membranes as well as implementation strategies with less potential for undesired biological reactions are discussed.

Surface Topography and Surface Elemental Composition Analysis of Helixone®, a New High-Flux Polysulfone Dialysis Membrane

Modern dialysis membranes need to fulfil two basic requirements. Firstly, the membrane structure, defined in terms of the size, structure and distribution of the pores at the inner separating layer of the membrane must be such that uraemic solutes of a defined molecular-weight range are selectively removed. Secondly, the physical and chemical properties of the blood-contacting surface must be such that minimal blood-material interactions take place that could either affect the functioning of the membrane, or, cause adverse reactions for the patient.

A new polysulfone dialysis membrane, Helixone®, has been developed specifically for the elimination of larger uraemic toxins using convective therapy modalities such as haemodiafiltration. The membrane is characterised by the nanoscale modulation of the innermost surface structures that lead to significantly increased sieving coefficients for molecules such as β2-microglobulin, while maintaining the extremely low albumin removal property of the high-flux Fresenius Polysulfone® membrane. A recent publication (Ronco C, Bowry SK. Nanoscale modulation of the pore dimensions, size distribution and structure of a new polysulfone-based high-flux dialysis membrane. Int J Artif Organs 2001; 24: 726–35) described the characterisation of the membrane of Helixone® in terms of the membrane wall structure- and permeation-related parameters.

In this paper, we describe the analysis of membrane surface parameters that influence the biocompatibility as well as the functioning of a membrane. The degree of roughness and the type of chemical groups of a blood-contacting surface are two of the main determinants of the biocompatibility characteristics of a membrane. The surface elemental composition of Helixone® was determined using electron spectroscopy for elemental analysis (ESCA) while the surface topography of the membrane was evaluated using atomic force microscopy (AFM). The analysis showed that Helixone® has an improved, smoother blood-contacting surface and retains the essential surface chemistry, and therefore the acknowledged biocompatibility profile, of the Fresenius Polysulfone® membrane.

Assessment of the association between increasing membrane pore size and endotoxin permeability using a novel experimental dialysis simulation set-up

Background

Membranes with increasing pore size are introduced to enhance removal of large uremic toxins with regular hemodialysis. These membranes might theoretically have higher permeability for bacterial degradation products. In this paper, permeability for bacterial degradation products of membranes of comparable composition with different pore size was investigated with a new in vitro set-up that represents clinical flow and pressure conditions.

Methods

Dialysis was simulated with an AK200 machine using a low-flux, high-flux, medium cut-off (MCO) or high cut-off (HCO) device (n = 6/type). A polyvinylpyrrolidone-solution (PVP) was recirculated at blood side. At dialysate side, a challenge solution containing a filtrated lysate of two water-borne bacteria (Pseudomonas aeruginosa and Pelomononas saccharophila) was infused in the dialysate flow (endotoxin ≥ 4EU/ml). Blood and dialysate flow were set at 400 and 500 ml/min for 60 min. PVP was sampled before (PVP_pre) and after (PVP_post) the experiment and dialysate after 5 and 55 min. Limulus Amebocyte Lysate (LAL) test was performed. Additionally, samples were incubated with a THP-1 cell line (24 h) and IL-1β levels were measured evaluating biological activity.

Results

The LAL-assay confirmed presence of 9.5 ± 7.4 EU/ml at dialysate side. For none of the devices the LAL activity in PVP_pre vs. PVP_post was significantly different. Although more blood side PVP solutions had a detectable amount of endotoxin using a highly sensitive LAL assay in the more open vs traditional membranes, the permeability for endotoxins of the 4 tested dialysis membranes was not significantly different but the number of repeats is small. None of the PVP solutions induced IL-1β in the THP-1 assay.

Conclusions

A realisitic in vitro dialysis was developed to assess membrane translocation of bacterial products. LAL activity on the blood side after endotoxin exposure did not change for all membranes. Also, none of the PVP_post solutions induced IL-1β in the THP-1 bio-assay.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Spallation of Small Particles From Peristaltic Pump Tube Segments

In most extracorporeal filtration devices such as hemodialysis a peristaltic pump is used to circulate blood. Pump function requires the repeated compression of an elastomeric tube from which particles may be shed into the circulatory system, a process called spallation. Earlier studies are likely to have missed the large number of small particles (<2 µm in diameter) that appear. The present study uses more modern equipment that detects and sizes particles down to 0.6 µm. As polyvinyl chloride (PVC) tubing is commonly used for this process, a series of studies was conducted on three different types to study its spallation characteristics, along with a co-extruded PVC/polyurethane tubing known for its enhanced biocompatibility properties. For all types of PVC tubes, the average size of the spallated particles was 0.83 ± 0.03 µm; for the PVC/polyurethane tubing the average size of the spallated particles was approximately twice that reported for PVC tubing. For PVC tubes with equal inner diameter, those with less plasticizer released fewer particles; for tubes with the same Shore hardness, tubes with larger internal diameters released fewer particles. It was also shown that PVC tubes operating at a slower flow rate does not reduce the total number of particles released per volume pumped. The total number of particles spallated from the PVC/polyurethane tubing was 10 times lower than from the lowest spallating PVC tubing.

Is there a future for electrochemically assisted hemodialysis? Focus on the application of polypyrrole–nanocellulose composites

This work summarizes the various aspects of using electrochemically assisted solute removal techniques in hemodialysis with a focus on blood electrodialysis and electrochemically controlled uremic retention solute removal using polypyrrole. In particular, the feasibility of using highly porous conductive polypyrrole–Cladophora cellulose membranes for hemodialysis are overviewed as a part of our dedicated research efforts during the past 4 years. The potential benefits and the current limitations associated with using the electrochemically controlled uremic retention solute removal techniques are discussed in detail.

Theoretical Application of Irreversible (Nonequilibrium) Thermodynamic Principles to Enhance Solute Fluxes across Nanofabricated Hemodialysis Membranes

Objective. Nanotechnology has the potential to improve hemodialysis membrane technology. Thus, a major objective is to understand how to enhance toxic solute fluxes across these membranes. The aim of this concept building study is to review the application of irreversible thermodynamic (IT) to solute fluxes. Methods. We expanded the application of the Nernst-Planck equation to include the Kedem-Katchalsky equation, pH, membrane thickness, pore size, and electric potential as variables. Results. (1) Reducing the membrane's thickness from 25 μm to 25 nm increased the flux of creatinine, β₂-microglobulin, and tumor necrosis factor-α (TNF-α) by a thousand times but prevented completely albumin flux, (2) applying an electric potential of 50–400 mV across the membrane enhanced the flux of the respective molecules by 71.167 × 10⁻³, 38.7905 × 10⁻⁸, and 0.595 × 10⁻¹³ mol/s, and (3) changing the pH from 7.35 to 7.42 altered the fluxes minimally. Conclusions. The results supported an argument to investigate the application of IT to study forces of fluxes across membranes. Reducing the membrane's thickness—together with the application of an electrical potential—qualities achievable by nanotechnology, can enhance the removal of uremic toxins by many folds. However, changing the pH at a specific membrane thickness does not affect the flux significantly.

Comparison of the hemodynamics in 6mm and 4-7 mm hemodialysis grafts by means of CFD

The aim of our study is to investigate with computational fluid dynamics (CFD) whether different arterial anastomotic geometries result in a different hemodynamics at the arterial (AA) and venous anastomosis (VA) of hemodialysis vascular access grafts. We have studied a 6mm graft (CD) and a 4-7 mm graft (TG). A validated three-dimensional CFD model is developed to simulate flow in the two graft types. Only the arterial anastomosis (AA) geometry differs. The boundary conditions applied are a periodic velocity signal at the arterial inlet and a periodic pressure wave at the venous outlet. Flow rate is set to 1,000 ml/min. The time dependent Navier-Stokes equations are solved. Wall shear stress (WSS), wall shear stress gradient (WSSG) and pressure gradient (PG) are calculated. Anastomotic flow is asymmetric although the anastomosis geometry is symmetric. The hemodynamic parameters, WSS, WSSG and PG, values at the suture line of the arterial anastomosis of the TG are at least twice as much as in the CD. Comparing the parameters at the two AA indicate that little flow rate increase introduces the risk of hemolysis in the TG whereas the CD is completely free of hemolysis. The hemodynamic parameter values at the venous anastomosis of the CD are 24 till 35% higher compared to the values of the TG. WSS values (> 3 Pa) in the VA are in the critical range for stenosis development in both graft geometries. The zones where the parameters reach extreme values correspond to the locations where intimal hyperplasia formation is reported in literature. In all anastomoses, the hemodynamic parameter levels are in the range where leucocytes and platelets get activated. Our simulations confirm clinical results where TG did not show a better outcome when compared to the CD.

Verification of the chemical composition and specifications of haemodialysis membranes by NMR and GPC-FTIR-coupled spectroscopy

The chemical composition of a dialysis membrane is decisive towards determining its physical and biochemical properties--two fundamental determinants of the success of therapy offered to patients suffering from chronic renal failure. From the vast variety of synthetic polymers available, only a few are suitable for the manufacture of dialysis membranes that have to conform to the diverse demands of modern haemodialysis and related therapies. Recently, a membrane labelled as polyamide (Polyamide S) has caused some confusion to end-users in that the product specification for the membrane is given as 'polyarylethersulfone' or simply as Polyamide S membrane. As the chemical and physical properties of these two polymer types are distinctly different, it is unclear whether the functional characteristics of Polyamide S are to be attributed to polyamide, polyarylethersulfone, or, to both polymers. We therefore undertook investigations to ascertain the exact chemical nature of the Polyamide S membrane using a series of chemical analytical tools and an appropriate polyamide reference. The analytical techniques were conventional gel permeation chromatography (GPC), GPC-FTIR coupled spectroscopy using dimethyl acetamide and hexafluoroisopropanol as solvents and nuclear magnetic resonance spectroscopy. Glass transition temperature measurements and quantitative elemental analysis were also carried out. None of the analytical techniques used showed any traces of polyamide in Polyamide S; no aliphatic or aromatic polyamide chemical entities were detected in any of the samples tested. The Polyamide S dialysis membrane thus comprises, solely, of polyarylethersulfone, which is also known as polyethersulfone.

In vitro evaluation of the hydraulic permeability of polysulfone dialysers

An in vitro set-up has been designed to study the hydraulic permeability of hollow fiber dialysers. Forward and reverse dialysate ultrafiltration were determined using both sterile dialysers and samples with a protein layer settled on the membrane (Fresenius F6, F8, F60 and F80). The ultrafiltration coefficient KUF (ml/h.mmHg) was calculated as the ratio of volumetrical flow (QUF) and transmembrane pressure (TMP) measurements. The protein layer on the membrane was induced either by recirculating human plasma through the dialysers (in vitro) or by a standard hemodialysis session (in vivo). KUF is largely independent of TMP up to 600mmHg (low flux) and 60mmHg (high flux) for forward and reverse flow In sterile dialysers, backfiltration yields a significantly different KUF except for the F80. An in vitro induced protein layer on the membrane decreases KUF15-30% (forward) and 4-12% (backward) in low flux and 45-70% (forward) and 65-73% (backward) in high flux dialysers.

Anticoagulant potential of regioselective derivatized cellulose

Regioselective derivatization was carried out introducing sulfate, phosphate or quaternary ammonia groups in C2-, C3- and C6-position of the anhydroglycose unit of cellulose. The anticoagulant potential of these derivatives was estimated with common clotting assays, such as thrombin time and partial thromboplastin time. It was found that a pronounced anticoagulant activity of cellulose derivatives could be achieved if the degree of substitution (DS) with sulfate was above 1.0. The anticoagulant activity was maximal at a DS of about 1.5 and then decreased again. Further, it was detected that particularly sulfation in C2-position resulted in a pronounced anticoagulant activity of cellulose derivatives. Development and application of assays specific for thrombin and factor Xa indicated that the anticoagulant potential of these cellulose derivatives was mainly due to anti-thrombin activity. The comparison of cellulose sulfates and cellulose derivatized with phosphate and quaternary ammonium groups demonstrated that the negative charge and type of the substituent is an important prerequisite for the anticoagulant activity of cellulose derivatives. Indeed, derivatization with sulfate produced superior activity in comparison with phosphate.

Surface modification of chitosan membranes by complexation-interpenetration of anionic polysaccharides for improved blood compatibility in hemodialysis

Chitosan membrane surface was modified by complexation and interpenetration of anionic polysaccharides--heparin and dextran sulfate--for improved blood compatibility in hemodialysis. Electron spectroscopy for chemical analysis results showed a characteristic sulfur (S) and sodium (Na) peaks after modification with dextran sulfate. The sulfur/carbon (S/C) atomic composition ratio increased from 0.03 to 0.08 when the bulk dextran sulfate concentration used for modification was increased from 2.5 to 10 mg ml-1. The permeability of urea and creatinine did not change significantly upon modification with heparin or dextran sulfate. Surface modification, however, did decrease the permeability coefficients of glucose, vitamin B-2, and vitamin B-12. Unlike Cuprophan, chitosan and surface-modified chitosan membranes did not significantly activate the complement system as measured by the serum iC3b concentration. Compared to forty and sixty fully-activated platelets present on control surfaces, surface modification with heparin and dextran sulfate significantly reduced the number of adherent platelets per 25,000 microns 2 area and the extent of platelet activation. Surface modification with anionic polysaccharides, however, did significantly shorten the plasma recalcification time leading to fibrin clot formation. The results of this study show that chitosan membrane surface can be modified by complexation-interpenetration of anionic modifying agents. The modified membranes do resist complement activation and platelet adhesion and activation.

Permeability and blood compatibility properties of chitosan-poly(ethylene oxide) blend membranes for haemodialysis

Chitosan-poly(ethylene oxide) (PEO) blend membranes, using different molecular weights of PEO, were developed for improved permeability and blood compatibility. The equilibrium hydration increased from 44.7% for chitosan to 62.5% for chitosan-PEO blend membranes when the molecular weight of PEO was 10,000 (10K) or higher. An increase in the hydration of PEO blend membranes was due to intermolecular association between PEO and chitosan chains. Scanning electron microscopy showed that chitosan-PEO membranes were highly porous with size ranging from 50 to 80 nm in diameter observed in membranes made with PEO10K. Electron spectroscopy for chemical analysis suggested an increase in PEO on the membrane surface with increasing molecular weight in the blend. The permeability coefficient of urea increased from 5.47 x 10(-5) cm2 min-1 in chitosan to 9.86 x 10(-5) cm2 min-1 in chitosan-PEO10K membranes. The increase in permeability coefficient could be either due to an increase in the hydrophilicity or the high porosity of the membranes. Although chitosan-PEO membranes did not prevent serum complement activation, platelet adhesion and activation were significantly reduced. Chitosan-PEO blend membranes, therefore, appear to be beneficial in improving the permeability of toxic metabolites and in reducing the thrombogenicity for haemodialysis.

Technology and clinical application of large-bore and implantable catheters

The access of choice for patients receiving regular dialysis therapy remains the surgically created arteriovenous fistula. In a number of such patients as well as those requiring treatment for acute renal failure, plasmapheresis or hemoperfusion alternative techniques of access are used. Of the techniques used subclavian and femoral catheterization are preferred for short-term vascular access while in patients requiring long term vascular access the use of a surgically inserted central venous line is favoured. The technology performance and clinical aspects of the use of large bore and implantable catheters are reviewed.