Focusing on membranes

The scientific principles and technological determinants of haemodialysis membranes

In most biological or industrial (including medical) separation processes, a membrane is a semipermeable barrier that allows or achieves selective transport between given compartments. In haemodialysis (HD), the semipermeable membrane is in a tubular geometry in the form of miniscule pipes (hollow fibres) and separation processes between compartments involve a complex array of scientific principles and factors that influence the quality of therapy a patient receives. Several conditions need to be met to accomplish the selective and desired removal of substances from blood in the inner cavity (lumen) of the hollow fibres and across the membrane wall into the larger open space surrounding each fibre. Current HD membranes have evolved and improved beyond measure from the experimental membranes available in the early developmental periods of dialysis. Today, the key functional determinants of dialysis membranes have been identified both in terms of their potential to remove uraemic retention solutes (termed ‘uraemic toxins’) as well subsidiary criteria they must additionally fulfill to avoid undesirable patient reactions or to ensure safety. The production of hundreds of millions of kilometres of hollow fibre membranes is truly a technological achievement to marvel, particularly in ensuring that the fibre dimensions of wall thickness and inner lumen diameter and controlled porosity—all so vital to core solute removal and detoxification functions of dialysis—are maintained for every centimetre length of the fragile fibres. Production of membranes will increase in parallel with the increase in the number of chronic kidney disease (CKD) patients expected to require HD therapies in the future. The provision of high-quality care entails detailed consideration of all aspects of dialysis membranes, as quality cannot in any way be compromised for the life-sustaining—like the natural membranes within all living organisms—function artificial dialysis membranes serve.

Fabrication of macroporous cryogels as potential hepatocyte carriers for bioartificial liver support

Two different cryogels composed of copolymer of acrylonitrile (AN) and N-vinyl-2-pyrrolidone (NVP) (poly(AN-co-NVP)) and interpenetrated polymer networks (IPN) of chitosan and poly(N-isopropylacrylamide) (poly(NiPAAm)-chitosan) were fabricated by gelation at sub-zero temperatures. The two cryogels possess an interconnected network of macropores of size 20-100 μm and efficient transport properties as determined by physiochemical analysis. Both cryogels support in vitro growth and function of fibroblasts (COS-7) and human liver hepatocarcinoma cells (HepG2). The cryogels are hemocompatible as demonstrated by low albumin adsorption and platelet adherence. Furthermore, in vivo implantation of poly(NiPAAm)-chitosan cryogel in mice shows its biocompatibility with the surrounding tissue. Primary rat hepatocytes grown on poly(NiPAAm)-chitosan cryogel for 96 h formed cellular aggregates and maintained their functions in terms of, ammonia removal, ureagenesis and drug detoxification. Cryogel-based closed continuous bioreactor systems could maintain HepG2 cells at high density for 7 days. Off-line clinical evaluation of these cryogel-based bioreactors showed the ability of immobilized cells to detoxify circulating plasma obtained from patients with acute on chronic liver failure (ACLF). Altogether, the presented data suggests cryogels as a potential bioreactor matrix for bio-artificial liver support system.

On the tissue compatibility of poly(ether imide) membranes: an in vitro study on their interaction with human dermal fibroblasts and keratinocytes

Recently we have developed a novel type of membrane based on poly(ether imide) (PEI) which is considered for biomedical application. To improve its physical and biological performance it was modified by blending with poly(benzimidazole) (PBI). In the present study both membranes were characterized in terms of their physicochemical properties and in vitro tissue compatibility using human dermal fibroblasts and keratinocytes. The modified membrane (PEI*) was more hydrophilic, less porous and had an increased surface (zeta) potential. We further found that blending with PBI tends to promote cell contact, at least initially, as indicated by the improved overall cell morphology, adhesion and spreading of fibroblasts, and the development of focal adhesion complexes. The effects of fibronectin (FN) and serum coating were also beneficial when compared to pure PEI and tissue culture polystyrene (TCP), which correlates to a higher adsorption of both FN and vitronectin detected by ELISA. However, a clear tendency for homotypic cellular interaction particularly of keratinocytes was obtained in contact with membranes, which was much stronger pronounced on PEI*. Although the initial adhesion was greater on PEI*, a surprising decrease in cell growth was observed at later stages of incubation, which may be explained with the membrane-promoted cellular aggregation leading to an easier detachment from the substratum. Thus, membranes based on blends of PEI with PBI could provide a tissue compatible scaffold with lowered adhesive properties, which might be a useful tool for the transfer of cells, for example, to in vitro engineered tissue constructs.

Tethering poly(ethylene glycol)s to improve the surface biocompatibility of poly(acrylonitrile-co-maleic acid) asymmetric membranes

To improve the surface biocompatibility, asymmetric membranes fabricated from poly(acrylonitrile-co-maleic acid)s (PANCMAs) synthesized by water-phase precipitation copolymerization were tethered (or immobilized) with poly(ethylene glycol)s (PEGs) by esterification reaction. Chemical changes on the membrane surface were characterized by Fourier transform infrared spectroscopy and elemental analysis to confirm the immobilization of PEG onto the PANCMA membranes. The hydrophilicity and blood compatibility of the PEG-tethered PANCMA membrane were investigated by water contact angle, water absorption, protein adsorption, plasma platelets adhesion and cell adhesion measurements, and the results were compared with the corresponding PANCMA membranes. It was found that, after the tethering of PEG, the hydrophilicity of the membrane can be improved significantly, and the protein adsorption, platelets adhesion and macrophage attachment on the membrane surface are obviously suppressed. Furthermore, not only the content of maleic acid in PANCMA, which influences the tethering density of PEG, but also the molecular weight of PEG has great effect on the surface modification of PANCMA membranes for biocompatibility.

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.

Membranes for biohybrid liver support systems--investigations on hepatocyte attachment, morphology and growth

The biological properties of four different membranes were studied regarding their possible application in biohybrid liver support systems. Two of them, one made of polyetherimide (PEI), and a second based on polyacrylonitrile-N-vinylpyrollidone co-polymer (P(AN-NVP)), were recently developed in our lab and studied for the first time. Together with pure polyacrylonitrile (PAN) membranes, the three preparations were characterised as ultra-filtration membranes. Their ability to support cell attachment, morphology, proliferation and function of human hepatoblastoma C3A cells was studied. The role of surface morphology for the interaction with hepatocytes was highlighted using a commercial, moderately wettable polyvinylidendifluoride (PVDF) membrane with micro-filtration properties. Comparative investigations showed strongest interaction of C3A cells with PAN membranes, as the focal adhesion contacts were more expressed and cell growth was also high. However, the functional activity in terms of albumin synthesis was reduced. Very similar results were obtained with the most hydrophobic PEI membrane. In contrast, the most hydrophilic membrane P(AN-NVP) was found to provoke stronger homotypic adhesion (E-cadherin expression) of C3A cells and less substratum attachment (focal adhesions), but enhanced albumin secretion. However, proliferation of C3A cells was lowered. Micro-porous PVDF membrane showed very good initial attachment, but the resulting cell material and cell-cell interaction were relatively poor developed. Among four membranes tested, PEI seems to be the most attractive membrane for biohybrid liver devices, as it provides good surface properties for hepatocytes interaction, but in addition it is highly thermostable, which would permit steam sterilisation. No simple relationship, however, between the wettability of the membranes and their ability to support hepatocyte adhesion and function was found in this study.

Dialysis membranes today

In recent years, hemodialytic therapies have evolved from the simple, diffusion-dependent removal of small molecular weight substances from blood to advanced therapy modalities involving the convective removal of larger uremic sloutes. The clinical benefits of removal of substances such as beta2-microglobulin (beta2-m) have been reported by several authors: elimination of large-molecular weight "uremic toxins" is now widely accepted as being beneficial to the overall quality of life of patients. This trend would not have been possible without parallel technical developments, especially that of new membranes having more open pore structures resulting in higher sieving coefficients and increased hydraulic permeability. Not all polymer types are suitable for the manufacture of high-flux membranes required for convective therapies in which large fluid volumes are exchanged. Amongst the more important criteria are: the selected polymer must be able to undergo steam sterilisation, have high endotoxin retention capabilities, be versatile for the fabrication of a range of hydraulic permeabilities and, of course, have high blood compatibility. The aim of this paper is, firstly, to review the major membrane development phases over the last quarter of a century. Secondly, the suitability of current membrane materials to meet the aforementioned requirements will be examined. Thirdly, in view of the recent, rapid proliferation of polysulfone-based membranes, dialysis membranes of the polysulfone 'family' are placed under scrutiny; membranes of this class represent a significant portion of the product portfolio of dialyser manufacturers today, yet, few end-users are able to distinguish between the salient features of the respective products because of a combination of confusing membrane nomenclature, classification, tradenames and product claims.

Polyetherimide: a new membrane-forming polymer for biomedical applications

Membranes for biohybrid organs such as the biohybrid liver support system have to face 2 different environments, namely blood and tissue cells. Accordingly, the respective membrane surfaces must have optimal properties in terms of biocompatibility for blood or tissue. Flat membranes prepared by a phase inversion process from polyetherimide were modified by binding of tris-(hydroxymethyl)-aminomethane to obtain a surface with hydroxyl groups by binding of polyethylene imine to attach a hydrophilic macromolecule with amine groups useful as a spacer for later bonding of further ligands and by attachment of heparin. The binding of the different ligands was successful as monitored by different physicochemical methods. The blood response of plain polyetherimide was comparable to that of polyacrylonitrile, and it could be further improved by the binding of heparin. The tissue compatibility of polyetherimide and its different modifications was compared with commercial cell culture substrate membranes (Millicell) and found to be comparable for polyetherimide and even better after the modification with tris-(hydroxymethyl)-aminomethane. In conclusion, polyetherimide seems to be an interesting material for the production of membranes for application in biohybrid organ systems.

Quo vadis dialysis membrane?

The development of dialysis membranes is closely related to the development of dialysis as a routine therapy for patients with kidney failure. Without having membranes and dialyzers available as commodity products, the treatment of more than 1 million uremic patients worldwide would be impossible. Several transition periods can be identified: a change in membrane geometry from flat sheet to capillaries, a shift in market appreciation from cellulose to synthetic polymers, and from low-flux to high-flux dialyzers. This shift is supported by the notion that convective therapies using high-flux membranes allow the removal of large-molecular-weight solutes. From a historical background, three eras of perception can be identified for both membrane and dialysis development. First, the period of survival when nephrologists had to focus on techniques for blood access and availability of membranes. Second, the period of issues dedicated to rather specific features of membranes and dialysis therapy such as dose of dialysis, reuse, sterilization, and membrane biocompatibility. And third, the period of quality tops this sequence with a complicated approach: the principal area of interest from the medical community has switched to issues such as quality of life, morbidity, mortality, therapy standards, and cost-effectiveness. New membrane developments should focus on this situation.

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.

Heparins and blood polymorphonuclear stimulation in haemodialysis: an expansion of the biocompatibility concept

BACKGROUND

At the concentrations used in haemodialysis and in a dose-dependent way, unfractionated heparin (UFH) and, to a lesser degree, a low-molecular-weight heparin (LMWH) stimulate polymorphonuclear cells (PMN) in vitro, and could act in synergy with the stimulatory effect of dialysis membranes in vivo. To examine this hypothesis, we studied the effects of different heparin types and regimens on blood PMNs during haemodialysis sessions.

METHODS

Ten haemodialysed patients were studied during regular dialysis sessions on a cellulose triacetate membrane (CT 110 G; 1.10 m(2); Baxter), with four different random heparin protocols: one high-UFH regimen (HHR) at 90 IU/kg body-weight (b.w.) and one low-UFH regimen (LHR) at 50 IU/kg b.w., and with a LMWH (nadroparin calcium) at 85 (HHR) or 45 (LHR) IU/kg b.w. Blood granulocytes, platelet counts, and plasma granulocyte degranulation products (elastase, lactoferrin) were measured serially during 4 h dialysis sessions.

RESULTS

After 10 min, the reduction in PMNs with UFH was 29.5% for HHR (P<0.01) and 28.5% for LHR (P<0.01), and only 16.8 and 18.6% with LMWH (NS), significantly higher for HHR with UFH than with LMWH (P<0.01). At 60 min, the elastase increase with HHR was greater, 61% with UFH (P<0.01) and 37.8% with LMWH (P<0.01), significantly higher than LHR for UFH (P<0.05) or LMWH (P<0.05). The overall decrease in platelets (with LMWH P<0.01) and the overall increase in lactoferrin (P<0.001) were not different between heparinization procedures.

CONCLUSION

Under a conventional heparin regimen, the PMN variation during the course of the dialysis session suggests a more biocompatible effect of LMWH over UFH. In addition, the variation of elastase favours the lower dose, whatever the type of heparin. Heparin type and dose should therefore be considered in studies addressing biocompatibility in haemodialysis: a low dose of LMWH may be viewed as a better biocompatible treatment with regard to leukocyte stimulation.

Development of membranes for the cultivation of kidney epithelial cells

The development of biohybrid organs (BHO) will benefit from improved membranes regarding transport and cell contacting properties. Here we describe in a first study the development and testing of membranes made of polyacrylonitrile (PAN) and polysulfone (PSU) for the immobilisation of kidney epithelial cells. Comparative investigations on overall polymer toxicity tested with 3T3 fibroblasts, and morphology and proliferation of Madin-Darby canine kidney (MDCK) cells cultured on the membranes could show that these materials have comparable cell contacting properties like Millicell membranes. Since PAN and PSU have superior membrane forming properties with regard to membrane geometry, i.e. for the preparation of hollow fibres, and porosity, i.e. for immuno isolation, both materials or modifications thereof seem to be suitable for the application in BHO such as biohybrid kidney.

Dialyzer performance in the clinic: comparison of six low-flux membranes

The aim of this study is to assess the clinical performance of 6 different low-flux dialysis membranes under steady-state conditions in terms of urea and phosphate clearances. Ten stable hemodialysis patients were examined. The following dialyzers were studied, all in 1.5- to 1.6-m2 format: cuprammonium, cellulose acetate, cellulose diacetate, hemophane, polysulfone (low-flux), and polysynthane. The following parameters were examined: urea reduction ratio, phosphate reduction ratio, "instantaneous dialyzer clearance" for urea and phosphate, and total amount of urea and phosphate removed in the dialysate over a 1-week (three dialyses) period. Although there were differences between the membranes, all produced results within a narrow range. There was no one membrane that produced superior clearances in all categories. The cellulose acetate membrane was the least satisfactory membrane. Phosphate clearances were at best one third that of urea clearances. When choosing a low-flux dialysis membrane, urea and phosphate clearances are so similar amongst different membranes that other criteria are likely to have a greater influence on the choice of membrane.

Immunological effects of therapeutic immunoadsorption with respect to biocompatibility

The activation of the complement system leading to generation of anaphylatoxins and the membrane attack complex depends on the chemical nature of the adsorptive system and the anticoagulation used. The method of the primary separation determines the presence of cell debris in the plasma as well as the extent of platelet activation. The particular role of anticoagulation and its properties to prevent/reduce complement activation on immunadsorption material is discussed and the combined use of citrate and heparin is proposed. The quality of the reinfused plasma--as discussed on the example of LDL-apheresis--is therefore influenced by the amount of the activated split products. This determines finally the extent of cellular activation during therapeutic immunadsorption when receptor-dependent activation of cells by C3a(desarg) and C5a(desarg) can occur.

Contact activation of plasmatic coagulation on polymeric membranes measured by the activity of kallikrein in heparinized plasma

Kallikrein is involved in the generation of bradykinin during extracorporal circulation, that is believed to play an important role in cases of anaphylactic shock during hemodialysis. Therefore, a method for the assessment of kallikrein generation was developed, based on the chromogenic substrate S-2302. Comparison of kallikrein-like activity on glass using citrate or heparinized plasma demonstrated enhanced activity in the presence of heparin. The applicability of the assay, and the time course of kallikrein generation was demonstrated with glass and cuprophan. Membranes based on pure polyacrylonitrile, or its copolymers differing in their content of acrylic acid, 2-hydroxyethyl acrylate, and allylsulphonate were investigated with respect to kallikrein-like activity, and physicochemical surface properties. It was found that high content in 2-hydroxyethyl acrylate, and acrylic acid caused a substantial activation of the contact system while low content in allylsulphonate (less than 2 mol%) did not result in enhanced kallikrein-like activity. The activating materials were characterized to be highly wettable, and had the most negative zeta potentials.

Impact of ultrafiltration on back-diffusion in hemodialyzer

Ultrafiltration of water from blood to dialysate decreases the rate of back-diffusion of solutes from dialysate to blood. Therefore, back-clearance (bK) of hemodialyzers may be expressed as bK = bK0--bTrQu, where bK0 is the diffusive back-clearance, bTr is the "back-"transmittance coefficient, and Qu is the net ultrafiltration rate. A formula for bK was derived from the one-dimensional theory of hemodialyzer, and bTr was described as a function of bK0 and the Staverman reflection coefficient. The transport parameters, bK0 and bTr, for creatinine and vitamin B12 were measured in two types of hemodialyzers with negligible back-filtration, using water solutions, and compared with the transport parameters, K0 and Tr, for the case of both diffusion and ultrafiltration from blood to dialysate. bK0 was in general equal to K0. bTr was not different from Tr for creatinine whereas bTr was lower than Tr for vitamin B12. Experimental values of bTr for vitamin B12 were in general agreement with theoretical predictions. However, experimental values of bTr for creatinine were lower than predicted values. We conclude that the impact of ultrafiltration on back-clearance for slowly diffusing solutes is weaker than on their clearance.