Hemodiyaliz amaçlı poliakrilonitril membranlar hazırlanması

Preparation of polyacrylonitrile membranes for hemodialysis application

Tez No: 66820
Yazar: GAMZE BEHMENYAR
Danışmanlar: DOÇ. DR. BİRGÜL TANTEKİN (ERSOLMAZ)
Tez Türü: Yüksek Lisans
Konular: Kimya Mühendisliği, Chemical Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1997
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Kimya Mühendisliği Ana Bilim Dalı
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 69

Özet

ÖZET Hemodiyaliz akut veya kronik böbrek yetmezliğitedavisinde en çok kullanılan yöntemlerden birisidir. Ülkemizde hemodiyaliz tedavisi görmekte olan çok sayıda hasta mevcuttur. Bu hastalara diyaliz hizmeti verebilecek kuruluşların yetersiz olmasının yanısıra, hemodiyaliz tedavisinin pahalı olması çok sayıda hastanın yeterli tedaviyi görememesine sebep olmaktadır. Bu tedavinin pahalı olmasının en büyük nedeni hemodiyaliz cihazlarında kullanılan ve kandaki toksik maddeleri ayıran bir membran ihtiva eden diyalizörün yurt dışından ithal ediliyor olmasıdır. Bu çalışmanın amacı hemodiyaliz amaçlı poliakrilonitril (PAN) membranlar hazırlanması, hazırlanan membranların üre ve kreatinin geçirgenlikleri ölçülerek karakterize edilmesi ve membran hazırlama şartlarının membran özelliklerine etkilerinin belirlenmesidir. Bu amaçla önce geçirgenlik ölçümlerinin gerçekleştirilebilmesi için arasına membranın yerleştirildiği iki tane cam hazneden oluşan bir diyaliz hücresi tasarlanmıştır. Hücrede membranın sol tarafındaki hazneye üre veya kreatinin çözeltisi koyulurken sağ tarafındaki haznede saf su sirküle edilmiştir. Deney esnasında membranın sol tarafındaki hazneden belirli zaman aralıklarında örnekler alınarak üre ve kreatinin konsantrasyonları UV spektrofotometre yardımı ile belirlenmiştir. Deneylerde kullanılan PAN membranlar faz dönüşümü yöntemine göre hazırlanmıştır. Membranın geçirgenliğine etki eden dört önemli hazırlama parametresinin membranın ayırma özellikleri üzerindeki etkileri incelenmiştir. Bu parametreler döküm çözeltisindeki polimer konsantrasyonu, banyo sıcaklığı, çöktürme banyosundaki çözücü (DMF) konsantrasyonu, banyoda çöktürme işleminden önce uygulanan evaporasyon süresidir. Hazırlanan tüm membranlar için üre ve kreatinin membran yüzeyinde adsorplandığı gözlenmiştir. Bu adsorpsiyon davranımının PAN ile üre veya kreatinin arasındaki elektrostatik etkileşimden kaynaklandığı ve hazırlanan membranların (-) yüklü olduğu düşünülmektedir. Seçilen membran hazırlama parametrelerinin üre ve kreatinin geçirgenliklerini çok etkilediği ve istenen ayırma özelliklerinde diyaliz membranları hazırlanabilmesi için membranın yapısı ile performansı arasındaki ilişkilerin çıkarılmasının gerektiği sonucuna varılmıştır. iv

Özet (Çeviri)

SUMMARY PREPARATION OF POLYACRYLONITRILE MEMBRANES FOR HEMODIALYSIS APPLICATION Hemodialysis is used in the treatment of patients which suffer from chronic or acute kidney failure to remove toxic metabolic wastes like urea, uric acid, and creatinine from blood. During a hemodialysis run blood is passed over one side of the membrane, while a dialysate solution containing a buffered and isotonic mixture of dextrose and salts is circulated on the other side. The composition of the dialysate is set to maintain the correct ionic balance in the blood. The concentration difference across the membrane between the blood and dialysate streams causes small solutes to diffuse through the membrane while larger molecules like proteins and blood cells are rejected. An additional aim of hemodialysis is the removal of excess body fluid, and therefore the process is also driven by a small pressure difference, typically of 0.2 bar. This pressure difference is high enough to produce 1-4 liter urine which is not produced by the kidney of the patient. People suffering from chronic kidney failure should preferably have their blood cleaned continuously. For practical and economical reasons, dialysis is performed only two or three times a week. A typical hemodialysis treatment lasts from 4 to 6 hours. Approximately 460000 kidney failure patients are treated with hemodialysis in the world and the number of patients is increasing by 7 to 8 percent annually. During hemodialysis, two physical processes are in operation simultaneously. The first process, diffusion, describes the movement of solutes, such as urea, from the blood compartment to the dialysate across a semipermeable membrane, and the movement of substances such as calcium and bicarbonate from the dialysate into the blood. The driving force for this movement is the concentration gradient across the membrane. The amount of material that diffuses across the semipermeable membrane is a function of the concentration gradient, the surface area and the diffusivity of the membrane. The other process operating during dialysis is convection or ultrafiltration. Fluid moves under hydrostatic pressure from the blood to the dialysate compartment. The quantity of fluid ultrafiltered depends on the pressure difference between the blood and dialysate compartments. This transmembrane pressure (TMP) can be controlled by varying the pressure in the dialysate or blood compartments. Decreasing dialysate pressure will increase ultrafiltration. The rate of ultrafiltration is dependent on the pressure gradient across the membrane, the surface area of the membrane and the ultrafiltration properties of the membrane. Each membrane has a unique ultrafiltration coefficient which depends on the structure of the membrane and the thickness. The transport of large molecular weight substances increase with increasing ultrafiltration rate but the main purpose of ultrafiltration is to remove excess body fluid.The primary purpose of the dialysis treatment is to remove the toxins which are normally eliminated by the kidney and to maintain mineral and water balance. There are a large number of subtances which accumulate during renal failure other than urea, creatinine and uric acid which may be important to remove but not known at this time. In the past there was a large search for 'uremic' toxin but now it appears evident that the uremic syndrome is probably not caused by the accumulation of a single toxic subtance, but is most likely the result of presence of a number of toxic agents. There are currently two views: Babb and Schribner at the Universty of Washington feel that the most important uremic toxins have a molecular weight in the range of 1000-2000, so-called“middle molecules,”while many others feel that the major toxins are much smaller, being in the range of urea (100-200). Most useful and important characteristics of a dialyzer are clearance and ultrafiltration coefflcent. Clearance describes the amount of blood that can be completely cleared of a given solute in unit time. Thus, if 100 ml of blood per minute is completely cleared of urea as it passes through the dialyzer, the dialyzer is said to have a urea clearance of 100 ml/min. Urea clearance is a surrogate for the clearance of small molecules and vitamin B12 clearance is a surrogate for the clearance of middle molecules. In general, synthetic membrane dialyzers have higher clearance of middle and larger molecules than cellulose based dialyzers. The ultrafiltration coefficient (KUf) is the number of milliliters of fluid transfered across the membrane per hour when 1mm Hg TMP is applied. For the patient who tends to gain large amounts of weight between treatments, it is necessary to use a dialyzer with an ultrafiltration capability high enough to allow the removal of fluid gained. In renal failure therapy membranes are used today in three different operating modes. These are hemodialysis, hemofiltration and hemodiafiltration. Hemofiltration is a filtration process where the driving force for mass transport is the hydrostatic pressure difference rather then the concentration difference as in hemodialysis. Hemofiltration leads to a volume change of the bloodstraem, which has to be replaced by a proper solution either before, or after the filtration procedure. Hemodiafiltration process is a combination of hemodialysis and hemofiltration. The dialyser membrane is the main determinant of what and how much is removed during dialysis. As the most important component of the dialyzer, dialysis membranes are generally classified into cellulosic and noncellulosic types. The most commonly used cellulosic membranes are cellulose acetate, cuprophan, hemophan and other modified cellulose. Noncellulosic polymers are synthetic polimers the monomer of which can not be found in nature. Examples of synthetic polymers are polyacrylonitrile, polysulfone, polyamide, polycarbonate, polypropylene and polymethylmethacrylates. Synthetic membranes have higher ultrafiltration rates and in general more biocompatible than cellulosic membranes. They have increased diffusive permeabilities, especially for higher molecular weight subtances (middle molecules), however, they are more expensive. There are three dialyzer configurations in current use: coil dialyzers, flat plate dialyzers and hollow fiber dialyzers. The main advantage of coil dialyzers is their low cost but they have relatively low solute clearance and ultrafiltration rates. Plate VIdialyzers have a higher surface area for the amount of extracorporeal blood volume than coil dialyzers and therefore higher solute clearance and ultrafiltration rates. The most commonly used dialyzers are hollow fiber dialyzer$. They have the highest clearance and ultrafiltration rates. The major disadvantage of the hollow fiber dialyzers is that they tend to have higher residual blood volumes, which can aggravate anemia in patients with chronic renal failure. They also are technically more difficult to manufacture, which results in somewhat higher cost. The structures of membranes with optimal performance in hemodialysis, hemofiltration and hemodiafiltration can be defined as follows : (1) The active membrane layer should be as thin as possible to obtain high transmembrane fluxes. (2) The porosity of the membrane at the surface as well as in the matrix should be as high as posible to provide high transmembrane fluxes. (3) The diameter of pores should not exceed a certain value to eliminate leakage of larger essential molecules. (4) The pore size distribution should be as narrow as possible to obtain a sharp molecular weight cut-off. (5) The membrane structure should guarantee a certain minimum mechanical strength. (6) The diffusion coefficient in the membrane should be high. (7) The membrane protein adsorption should be limited. (8) The membrane should have good blood compatibility. (9) All materials of the final membrane have to be nontoxic and chemically inert. The goal of this study is preparation and characterization of polyacrylonitrile membranes for hemodialysis applications and investigation of the effect of preparation parameters on membrare properties and structure. For this purpose a dialysis test cell has been designed to study the permeabilities of urea and creatinine through polyacrylonitrile membranes. The dialysis cell is made from two detachable glass compartments and the membrane under investigation is placed between the two compartments. Urea or creatinine solution is placed in the left-hand side of the cell and distilled water was circulated in the right hand-side, using a peristaltic pump. During the experiments samples (0,5-1 cm3) from the left-hand side of the cell were taken out periodically. For the estimation of urea and creatinine concentration a UV spectrophotometer was used. All experiments were carried out at 37 °C in a waterbath and before the start of each experiment, urea and creatinine solutions and the membrane was preconditioned at the required temperature. The membranes used in the experiments were prepared from polyacrylonitrile (PAN) by the phase inversion technique. Casting solutions were prepared by dissolving appropriate amounts of PAN in dimethylformamide. Then the solution was cast in the form of a thin film of a certain thickness on a glass plate using a casting knife built for this purpose and finally the cast films were immersed in a nonsoh/ent bath to obtain asymmetric structure. In order to investigate the effect of evaporation time prior to immersion into the bath, on the membrane structure some of the membranes were prepared by allowing the solvent to evaporate in nitrogen atmosphere for a fixed time Vlland then immersing in to the nonsolvent bath. Distilled water was used as the nonsolvent. The effect of four significant preparation parameters on the permeability of membranes have been investigated: polymer concentration in the casting solution, precipitation temperature, the concentration of solvent (DMF) in the precipitation bath and the evaporation time allowed prior to precipitation procedure. For all membranes, it was seen that urea and creatinine were adsorbed on the surface of the membrane. Adsorption of urea and creatinine to membranes can be explained by electrostatic intreactions between membranes and the molecules. PAN membranes seems to be negatively charged since urea and creatinin are to be known positively charged. Membrans exhibited higher permeabilities for urea than creatinine, as expected. Because the movement of molecules by difiiisive transport is inversely proportional to their molecular weight urea due to it's lower molecular weight should have a faster permeation rate than creatinine. When urea and creatinine permeabilities of the membranes prepared from casting solutions with three different polymer concentrations (10%, 14% and 18% by weight) were measured, both urea and creatinine permeabilities decrased as the polymer concentration in the casting solution were increased. It was observed that permeabilities of membranes prepared from casting solutions containing 10% and 14% PAN were close to each other for both urea and creatinine diffusion. When the solvent in the cast films are allowed to evaporate for a certain period of time prior to immersion into the nonsolvent bath, membranes obtained are expected to exhibit different seperation properties. Membranes prapered from 14% PAN casting solution exhibited lower creatinine permeabilities as the evaporation time increases. An evaporation step prior to the bath step cause increased polymer concentrations close to the surface of the film on the glass plate and hence a dense layer is formed at the surface. Increasing evaporation time will increase the thickness of this dense layer and both the diffusion of the solvent into the nonsolvent bath and the diffusion of the nonsolvent into the film will become more difficult. In this, case, a spongy structure is expected in the membrane cross-section and the permeabilities should decrease. The results in case of casting solution with 14% PAN concentration met these expectations. An interesting observation was that the amount of creatinine adsorbed on the surface of the membranes have decreased as the evaporation time was increased. When the evaporation time was increased in case of membranes prepared from 10% PAN casting solution, results were not as expected. Creatinine permeabilities increased slightly as the evaporation time was increased. Since the viscosity of the polymer solution is lower in case of 10% PAN solution, the time it takes for the surface to density is much longer resulting in no significant effect of evaporation prior to immersion bath and higher permeabilities compared to membranes cast for 14% solution. Effect of addition of solvent (DMF) into the precipitation bath was investigated by preparing membranes from 14% PAN casting solution and precipitating in water bath containing 0, 20 and 40% DMF. As the DMF concentration in the bath increased, VI 11urea and creatinine permeabilities of the membranes decreased. This behaviour is in accordance with the expection of small pore structure due to the decreased precipitation rate when solvent is added into the precipitation bath. It was also observed that creatinine adsorption on the membrane surface was less in membranes prepared in precipitation bath containing more DMF. Membranes prepared from a 14% PAN casting solution when precipitated at two different precipitation bath temparetures (25°C, 45°C) exhibited decreases in urea permeabilities as the precipitation bath temperature increased. Solubility of the polymer in the film increases as the temperature increases and this is expected to cause a spongy structure for membrane cross-section and a drop in permeability. Almost no creatinine adsorption was observed on the membrane surface in the case of the membrane prapered at 45°C bath temperature. This seems to be the cause of increased creatinine permeability as the precipitation bath temperature was increased from 25°C to 45°C. There is not enough imformation at this point to relate this phenomena to membrane structure. In conclusion, urea and creatinine permeation characteristics of PAN membranes are greatly affected by the preparation parameters. More work needs to be done on the relationship between the membrane preparation parameters and membrane structure and performance, in order to prepare PAN membranes with desired seperation properties for hemodialysis applications. IX

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