Biomed. Eng.-Biomed. Tech. 2015; aop
Qinglei Zhang, Xiaolong Lu*, Lihua Zhao, Juanjuan Liu and Chunfeng Wu
Research on polyvinylidene fluoride (PVDF) hollow-fiber hemodialyzer Abstract: In this study, polyvinylidene fluoride (PVDF) hollow-fiber hemodialysis membranes were prepared by non-solvent-induced phase separation. The PVDF hollowfiber hemodialyzers were prepared by centrifugal casting. The results showed that the PVDF membrane had better mechanical and separation properties when the membrane wall thickness was 40 μm and the N,N-dimethylacetamide in the core was 70 Vol%. Compared with commercial polysulfone hemodialysis membrane (Fresenius F60S membrane), the PVDF membrane had better mechanical property and ultrafiltration (UF) flux of pure water. The PVDF dialyzer’s removal efficiency for middle molecules was proven to be much higher than that of the F60S dialyzer. The UF coefficient of a high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas F60S is 42.5 ml/h/mm Hg, which can promote clearance for middle molecules. Keywords: hemodialysis membrane; hemodialyzer; performance; PVDF; structure. DOI 10.1515/bmt-2014-0190 Received December 26, 2014; accepted February 13, 2015
Introduction Hemodialysis has been extensively applied as a lifesustaining treatment for patients with end-stage renal disease [5]. The core aim for hemodialysis is to remove “middle-” and “small-”molecule toxin, such as β2microglobulin (β2-MG) and urea nitrogen. Cellulose membranes are widely used for hemodialysis because of their hydrogel structure and small thickness, which provide a
*Corresponding author: Xiaolong Lu, Institute of Biological and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China; and State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, China, E-mail: [email protected] Qinglei Zhang, Lihua Zhao and Chunfeng Wu: Institute of Biological and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China Juanjuan Liu: Tianjin Third Central Hospitals, Tianjin 300170, China
very effective removal for small solutes such as urea and creatinine. However, these membranes provide relatively little clearance for middle molecules and cause complement activation upon contacting with blood [12]. To improve the clearance for “middle” molecules and the blood compatibility of membranes, synthetic polymeric materials were developed to prepare hemodialysis membranes, such as polymethylmethacrylate, polyethylene, polypropylene, polyacrylonitrile, polyvinyl alcohol, polysulfone (PSF), and polyethersulfone membranes [8, 20, 25, 26]. However, the clearance for “middle”-molecule toxin and ultrafiltration (UF) flux were not ideal. The clearance for the solute mainly depended on blood, hemodialyzer, and dialysate. A number of research have been done on the effect of diffusion and convection on solute clearance [1, 6, 7, 9, 11, 14–17, 19, 21], whereas there are only few research on polyvinylidene fluoride (PVDF) hollowfiber hemodialyzer. PVDF has recently received great attention as a membrane material because of its outstanding properties, such as high mechanical strength, thermal stability, anti-ultraviolet radiation, smooth surface, and low protein adsorption, compared to other polymeric materials. PVDF membranes have been extensively applied in UF and micro-filtration for general separation purposes and are currently being explored as potential candidates in membrane contactor and membrane distillation applications [2, 4, 10, 18, 22]. PVDF has gained worldwide attention in biomedical research owing to its excellent property. Bouaziz et al. considered PVDF as one of the artificial vascular materials [3]. PVDF has a promising future application in the hemodialysis field. Our previous study prepared PVDF hollow-fiber hemodialysis membranes. The results showed that the PVDF membranes were prepared by blending with polyethylene glycol (PEG) polymers, which had good mechanical performance and biocompatibility but worse separation properties, especially the UF flux of pure water. The UF flux of pure water was only 64.3 l/h/m2, which was lower than that of the Fresenius F60S membrane [23, 24]. In this study, an attempt was made to improve the separation properties of PVDF hollow-fiber membranes (HFMs) by optimizing the membranes’ morphology and structure. At the same time, the dialysis performance evaluation of PVDF dialyzer was also studied.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
2 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer
Materials and methods Materials PVDF (1010, Solvay, Lyon, France), polyvinylpyrrolidone (PVP), PEG (Sigma-Aldrich Trading, Shanghai, P.R. China), Fresenius F60S membranes (Fresenius Medical Care, Frankfurter, Germany), lysozyme (LZM) and urea (Beijing Pubo Xin Biological Technology, Beijing, P.R. China), bovine serum albumin (BSA; Shanghai Biomedical Engineering Technical Service, Shanghai, P.R. China), N,N-dimethylacetamide (DMAc) (Samsung, Seoul, South Korea). All the reagents used in the study were of reagent grade.
Preparation of PVDF HFMs and hemodialyzer The PVDF HFMs were prepared by non-solvent-induced phase separation (NIPS) through a spinning equipment. Casting dopes were prepared by adding PVDF, PEG (6000 Da), and 1,4-diethylene dioxide into DMAc, followed by stirring at 70°C until the solution became homogeneous. 1,4-Diethylene dioxide and PEG worked as pore-forming agents to produce porous structures in the membrane. The inner diameter of PVDF HFMs was 200 μm and wall thicknesses were 30, 40, and 50 μm, respectively. The PVDF hemodialyzers were prepared by centrifugal casting. The parameters of PVDF and F60S hemodialyzers are shown in Table 1.
Characterization of the HFMs Morphology, maximum pore size, and porosity: Morphology studies of PVDF HFMs were carried out using a scanning electron microscope (Hitachi S-4800; Hitachi, Tokyo, Japan). Measurements were carried out on fiber soaked in ethanol for 15 min. At room temperature, the membrane was immersed in ethanol, and then, nitrogen can be pressurized into the inside. The bubble point pressure, P, was reached when the first string of bubbles came from the walls of the membrane. The maximum pore size can be calculated according to Eq. 1 [13]: r=
0.06378 , 2P
(1)
Where r is the pore radius (μm), P is bubble point pressure (MPa), and the ethanol surface tension is 22.3 mN/m. The membrane porosity, ε, was measured by soaking the membrane in pure water for 2 h, and then, the membrane surface was dried by filter paper. The membrane was weighed before and after absorption of pure water. The porosity was calculated using Eq. 2: ε=
( Ww -Wd ) ρW ( Ww -Wd ) ρW +Wd ρp
× 100%,
(2)
Where ε is the porosity of the membrane (%), Ww is the mass of the wet membrane, Wd is the mass of the dry membrane, ρW is the density of water (1.0 g/cm3), and ρp is the density of the membrane (1.78 g/cm3). Mechanical properties: The mechanical performance of the membranes was measured using an electronic single-yarn strength tester at room temperature. Each sample was clamped at both ends and unidirectionally stretched at a constant elongation rate of 500 mm/min with an initial length of 10 cm. Specimens were selected randomly and tested from each batch of the dried hollow-fiber sample. The tensile elongation and tensile strength at break were determined. At least five measurements were performed for each experiment and the average data were reported. Bursting pressure is a mechanical performance parameter of membranes. The membrane will be damaged when the pressure reaches bursting pressure. The value of bursting pressure was measured using the same equipment as the maximum pore size. The UF flux of pure water: Self-assembly widgets were made with 20 pieces of PVDF HFMs by epoxy resin cast. The inner diameter and wall thicknesses of PVDF membranes were 200 and 40 μm, respectively. The length of membranes was about 20 cm. The surface area of membranes was 25 cm2. UF flux was measured using the internal pressure method, with the pure water having a feed rate of 200 ml/min. The membranes were preloaded under 0.2 MPa for about 20 min. After adjusting the test temperature (25°C), The UF flux was measured with the inlet pressure (0.102 MPa) and the outlet pressure (0.098 MPa). The UF flux was calculated using Eq. 3: J=
V S⋅t
,
(3)
Where J is the UF flux of pure water (l/h/m2), V is the volume of the permeate flow (l), S is effective membrane area (m2), and t is sampling time (h).
Table 1: Parameters of PVDF and F60S hemodialyzers. Name
Shell Material Membrane Material Number/root Inner diameter (μm) Wall thicknesses (μm) Effective length (mm) Effective area (m2)
Dialysis performance test
Specifications and parameters
Polycarbonate
Polycarbonate
PVDF 6000 200 40 22 0.8
PSF 6000 200 40 22 0.8
For ease of studying, β2-MG and human serum albumin were replaced by LZM and BSA. LZM and β2-MG have similar performance. The molecular weight of LZM is 14 kDa, while β2-MG is 11.8 kDa. Their molecules are both positively charge and spherical molecules. At the same time, their isoelectric points are relatively close (LZM, 4.6; β2-MG, 5.7). The molecular weight of BSA is 67 kDa, whereas that of human serum albumin is 69 kDa. For this study, urea was chosen to characterize dialysis performance in the removal of small molecules. The standard dialysis solution comprises pure water, urea, LZM, and BSA. The concentration of urea, LZM, and BSA were 2000, 35, and
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 3
1000 mg/l, respectively. The flow rate of the simulation fluid was 300 ml/min. Sieving coefficients: Sieving coefficients were measured with a feed rate of 300 ml/min. The membranes were pre-flushed for about 30 min under 37°C±1°C. The sieving coefficient was measured using a transmembrane pressure (TMP) of 113±13 kPa. The sieving coefficient was calculated using Eq. 4: SC =
CF , CI
(4)
where SC is the sieving coefficient (%) and CF and CI are the concentration of filtered and inlet solution, respectively (mg/l). UF coefficient: The UF coefficient was defined as the volume of flow water through the dialysis membrane under certain TMP, which reflects the water capacity of dialyzer. The membranes were preflushed for about 30 min under 37°C±1°C. The UF coefficient was measured with different filtration flow rates (QF) and TMPs. Clearance for urea and LZM: The standard dialysis solution comprises pure water, urea, LZM, and BSA. The flow rate of simulation and dialysis fluid was 300 ml/min. The concentrations of simulation and dialysis fluid were measured with UV-Vis spectrophotometer (TU-1810; Purkinje, Beijing, China). The clearance (C) was calculated using Eq. 5:
CHF = QF ⋅SC ,
(5)
where CHF is the clearance (ml/min), QF is the filtration flow rate (ml/min), and SC is the sieving coefficient (%).
Results and discussion Membrane thickness effect on structure and performance Membrane thickness effect on structure The PVDF membranes were prepared by NIPS. A series of modified PVDF membranes were prepared by blending with PEG. The PVDF content was 22 wt% and the PEG content was 18.8 wt%. The cross-sectional scanning electron micrographs (SEMs) of the PVDF membranes are M-30, M-40, and M-50 with membrane thickness of 30, 40, and 50 μm, respectively. The cross-sectional SEM morphologies of different PVDF membranes are shown in Figure 1. The results exhibited that
Figure 1: The cross-sectional SEM images of different thicknesses. The inner diameter is 200 μm and the wall thicknesses of the PVDF HFMs are 30, 40, and 50 μm, respectively.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
4 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer
the membranes have an asymmetric structure with a dense skin layer outside, an intermediate layer with a finger-like structure, and a bottom layer of fully developed pores. This structure is formed via NIPS, especially by instantaneous phase separation. This asymmetric structure is suitable for biomedical application, as it not only can provide high mechanical strength but also prevent leakage.
Table 3: Solute removal performance of PVDF hemodialyzers. Membrane label M-30 M-40 M-50
Membrane thickness effect on performance In view of the potential practical applications in the biomedical field, it is essential for PVDF HFMs to retain good mechanical strength. To evaluate the mechanical properties of PVDF membranes, the tensile properties of all samples are exhibited in Table 2. It can be seen that the bursting pressure increases with an increase in membrane thickness. The tensile elongation of membranes has little change with an increase in thickness, which is mainly due to the similar structure of the membranes. The UF flux of pure water decreases from 107.1 to 74.5 l/h/m2 with increase in membrane thickness, which can be explained by the increase in transmembrane resistance as membrane thickness increases. The solute removal performance of PVDF hemodialyzer is shown in Table 3. Table 3 shows that the SC for BSA, LZM, and urea decreases as the PVDF membrane thickness increases. The PVDF dialyzers with membrane thickness of 40 and 50 μm have better SC for BSA and LZM than the PVDF dialyzer with membrane thickness of 30 μm. The reason is that the maximum pore size can affect the permeation performance of the BSA and LZM. The SC for urea of different PVDF dialyzers is very approximate, which can be explained by the smaller molecular weight of urea, i.e., it can easily go through the membrane. The clearance for LZM and urea decreases as the PVDF membrane thickness increases. The PVDF dialyzers with membrane thickness of 30 and 40 μm have higher clearance for LZM and urea than the PVDF dialyzer with membrane thickness of 50 μm, which is mainly due to the increase in transmembrane resistance as membrane thickness increases.
Property
BSA
LZM
Urea
SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min)
11.5±1.2 7.9±0.6 6.6±0.4
70.1±2.4 140.2±2.6 67.1±1.6 134.2±1.7 56.8±1.2 113.6±1.4
91.2±3.0 182.4±4.2 90.7±2.6 181.4±3.1 81.3±1.4 162.6±1.7
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
Composition of core liquid effect on structure and performance Composition of core liquid on structure A series of modified PVDF membranes were prepared with the same casting dopes and outer coagulation bath (pure water) but with different inner coagulation bath (pure water and DMAc). The cross-sectional SEM micrographs of PVDF membranes are M-0, M-1, M-2, and M-3 with DMAc 0, 35, 70, and 80 Vol%, respectively. The inner diameter and wall thicknesses of PVDF HFMs are 200 and 40 μm, respectively. The cross-section and surface SEM morphologies of PVDF membranes are shown in Figure 2. Figure 2 shows that the composition of the core liquid has an important impact on membrane structure. The finger-like structure of the cross section becomes increasingly larger and the inner surface of membrane becomes increasingly denser as the volume of DMAc decreases. This typically asymmetric structure is formed via NIPS, especially by instantaneous phase separation, which can be due to the rapid precipitation resulting in finger-like pores and the slow precipitation giving the sponge-like structure. It is easy to form a large finger-like structure of cross section when the core liquid is pure water. Composition of core liquid effect on performance
Table 2: Mechanical and separation performance of different PVDF membranes. Membrane label
M-30 M-40 M-50
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
UF flux of pure water (l/h/m2)
14.8 11.6 9.9
382 374 333
0.625 0.645 0.695
0.075 0.074 0.072
107.1 98.2 74.5
Table 4 shows that the PVDF membranes with different core fluid have obvious difference in mechanical and separation performance. As DMAc in the core fluid increases, the mechanical properties and the UF flux of pure water increase significantly. Tensile elongation at the break increases from 71% to 426%, and bursting pressure increases from 0.465 to 0.680 MPa. At the same time, the UF flux increases from 45.2 to 142.5
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 5
Figure 2: The cross-sectional and surface SEM images of different PVDF membranes. The PVDF membranes are M-0, M-1, M-2, and M-3, with DMAc of 0, 35, 70, and 80 Vol%, respectively. The inner diameter and wall thicknesses of PVDF HFMs are 200 and 40 μm, respectively.
l/h/m2. Figure 2 shows that the number of finger-like pores are obviously reduced as DMAc in the core fluid increases, which can improve mechanical performance. The maximum pore size increases as DMAc in the core fluid increases. The solute removal performance of different core liquid PVDF HFMs are shown in Table 5. The SC and clearances for big- and middle-molecule substances such as BSA and LZM are different, especially for the middle
molecules. However, The SC and clearances for small-molecule substance such as urea is appropriate. The clearance for LZM increases from 74.2 to 145.0 ml/min, whereas the SC for BSA decreases from 3.9% to 15.6% as DMAc in the core fluid increases, which can be explained by the different distributions and forms of membrane pore size. In general, the clearance for small-molecule urea is higher than LZM. The molecular weight of urea is small, which can easily go through the membrane.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
6 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer Table 4: Mechanical and separation performance of different PVDF membranes. Label
M-0 M-1 M-2 M-3
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
UF flux of pure water (l/h/m2)
10.6 11.2 11.6 12.2
71 200 374 426
0.465 0.505 0.645 0.680
0.067 0.070 0.074 0.081
45.2 64.3 98.2 142.5
Table 5: Solute removal performance of PVDF hemodialyzer for simulated solution. Membrane label
Property
BSA
LZM
Urea
M-0
SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min)
3.9±0.4 5.6±0.6 7.9±0.6 15.6±1.4
37.1±1.6 74.2±1.2 53.5±1.5 107.0±2.4 67.1±1.6 134.2±1.7 72.5±1.5 145.0±2.6
85.7±2.6 171.4±2.3 87.2±2.6 174.4±2.5 90.7±2.6 181.4±3.1 92.4±2.6 184.8±2.5
M-1 M-2 M-3
Contrastive study of PVDF and commercial Fresenius F60S membrane Morphology and structure The cross-sectional and outer-surface SEMs of PVDF and Fresenius F60S membrane are shown in Figure 3. Results show that there are few finger-like pores in PVDF and Fresenius F60S membranes. The structure not only can provide high mechanical strength but also affect separation performance.
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
Mechanical and separation performance Table 6 shows that the PVDF membrane has better mechanical performance and higher UF flux of pure water than the Fresenius F60S membrane. The tensile strength and elongation of PVDF membrane are 11.6 MPa and 374%,
Figure 3: Cross-sectional and outer-surface SEM micrographs of PVDF and Fresenius F60S membranes.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 7
Table 6: Mechanical performance and UF flux of PVDF and Fresenius F60S membranes.
Table 7: Dialysis performance of PVDF and F60S dialyzers. Label Property
Materials
PVDF F60S
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
11.6 8.9
374 66
0.645 0.475
0.074 0.079
UF flux of pure water (l/h/m2) 98.2 82.5
respectively, whereas those of the F60S membrane are 8.9 MPa and 66%, respectively. The PVDF membrane and the Fresenius F60S membrane have different mechanical performance, which mainly depends on different materials. The UF flux of PVDF is 98.2 l/h/m2, whereas that of the F60S membrane is 82.5 l/h/m2. The reason is that there are PEG additives staying in the membranes that can increase the hydrophilicity of PVDF membranes.
Dialysis performance of PVDF and F60S dialyzers Table 7 shows that the F60S dialyzer SC for BSA can reach 1.3% and LZM and urea are 48.2% and 93.4%, respectively. The PVDF dialyzer SC for LZM and urea are 67.1% and 90.7%, respectively. It can be concluded that the PVDF dialyzer’s removal efficiency for middle molecules is much higher than that of the F60S dialyzer. The UF coefficient of the high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas that of the F60S dialyzer is 42.5 ml/h/mm Hg, which can promote clearance for middle molecules. However, the PVDF dialyzer SC for BSA is about 7.9%, which is not ideal.
Conclusions Core fluid composition and membrane thickness had an important effect on membrane mechanical and separation properties. The mechanical properties and the UF flux of pure water increase significantly with increase in DMAc in the core fluid. The SC and clearances for BSA, LZM, and urea decrease as the membrane thickness increases. The SC and clearances for BSA, LZM, and urea decrease as the volume of DMAc increases. When membrane thickness is 40 μm and DMAc in the core is 70 Vol%, the PVDF membrane has better mechanical properties, higher UF flux, and solute clean property. Compared with the Fresenius F60S hemodialysis membrane, the PVDF membrane had better mechanical performance and UF flux of pure water. The tensile elongation of PVDF membrane was 374%, whereas that of the Fresenius F60S membrane was 66%. The UF coefficient of
PVDF F60S
BSA
LZM
Urea UF coefficient (ml/h/mm Hg)
SC (%) 7.9±0.6 67.1±1.6 90.7±2.6 CHF (ml/min) 134.2±1.7 181.4±3.1 SC (%) 1.3±0.4 48.2±2.3 93.4±4.2 CHF (ml/min) 96.4±1.9 186.8±3.5
62.6 42.5
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
the high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas that of F60S is 42.5 ml/h/mm Hg. It was proven that the removal efficiency of PVDF dialyzer for middle molecules is much higher than that of the F60S dialyzer. The PVDF dialyzer SC for LZM was 67.1%, whereas that of the Fresenius F60S dialyzer was 48.2%. Acknowledgments: This work was supported by the National Natural Science Foundation of China (21106100, 21176188, and 51278336) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20111201110004).
References [1] Allen R, Frost TH, Hoenich NA. The influence of the dialysate flow rate on hollow fiber hemodialyzer performance. Artif Organs 1995; 19: 1176–1180. [2] Bottino A, Capannelli G, Comite A. Novel porous poly (vinylidene fluoride) membranes for membrane distillation. Desalination 2005; 183: 375–382. [3] Bouaziz A, Richert A, Caprani A. Vascular endothelial cell responses to different electrically charged poly (vinylidene fluoride) Supports under static and oscillating flow conditions. Biomaterials 1997; 18: 107–112. [4] Chae SR, Yamamura H, Ikeda K, et al. Comparison of fouling characteristics of two different poly-vinylidene fluoride microfiltration membranes in a pilot-scale drinking water treatment system using precoagulation/sedimentation, sand filtration, and chlorination. Water Res 2008; 42: 2029–2042. [5] Dahe GJ, Teotia RS, Kadam SS. The biocompatibility and separation performance of antioxidative polysulfone/vitamin ETPGS composite hollow fiber membranes. Biomaterials 2011; 32: 352–365. [6] Eloot SD, Asseler YD, Debondt P, et al. Combining SPECT medical imaging and computational fluid dynamics for analyzing blood and dialysate flow in hemodialyzers. Int J Artif Organs 2005; 28: 739–749. [7] Huang Z, Klein E, Li B, et al. A new method to evaluate the local clearance at different annular rings inside hemodialyzers. ASAIO J 2003; 49: 692–697.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
8 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer [8] Ishihara K, Hasegawa T, Watanabe J, et al. Protein adsorptionresistant hollow fibers for blood purification. Artif Organs 2002; 26: 1014–1019. [9] John KL, Alfred KC, Lawrence YA, et al. Hemodialyzer mass transfer area coefficients for urea increase at high dialysate flow rates. Kidney Int 1997; 51: 2013–2017. [10] Khayet M, Khulbe KC, Matsuura T. Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process. J Membr Sci 2004; 238: 199–211. [11] Leypoldt JK, Cheung AK. Increases in mass transfer-area coefficients and urea Kt/V with increasing dialysate flow rate are greater for high-flux dialyzers. Am J Kidney Dis 2001; 38: 575–579. [12] Li LL, Cheng C, Xiang T, et al. Modification of polyethersulfone hemodialysis membrane by blending citric acid grafted polyurethane and its anticoagulant activity. J Membr Sci 2012; 405–406: 261–274. [13] Lu XL, Discuss about the measure methods for performance of hollow fiber porous membranes. Membr Sci Technol 2011; 32: 1–6. [14] Lucchi L, Fiore GB, Guadagni G, et al. Clinical evaluation of internal hemodiafiltration (I HDF): a diffusive-convective technique performed with internal filtration enhanced high flux dialyzers. Int J Artif Organs 2004; 27: 414–419. [15] Meyer TW, Leeper EC, Bartleet DW, et al. Increasing dialy-sate flow and dialyzer mass transfer area coefficient to increase the clearance of protein-bound solutes. J Am Soc Nephrol 2004; 15: 1927–1935. [16] Meyer TW, Peattie JW, Miller JD, et al. Increasing the clearance of protein-bound solutes by additon of a sorbent to the dialysate. J Am Soc Nephrol 2007; 18: 868–874.
[17] Osuga T, Obata T, Ikehira H. Detection of small degree of nonuniformity in dialysate flow in hollow fiber dialyzer using proton magnetic resonance imaging. Magn Reson Imaging 2004; 22: 417–420. [18] Park HH, Deshwal BR, Jo HD, et al. Absorption of nitrogen dioxide by PVDF hollow fiber membranes in a G-L contactor. Desalination 2009; 243: 52–64. [19] Poh CK, Hardy PA, Liao Z, et al. Effect of flow baffles on the dialysate flow distribution of hollow fiber hemodialyzers. J Biomech Eng 2003; 125: 48148–48149. [20] Ran F, Nie SQ, Zhao WF, et al. Biocompatibility of modified polyethersulfone membranes by blending amphiphilic triblock copolymer of poly(vinyl pyrrolidone)b-poly(methylmethacrylate)-b-poly(vinylpyrrolidone). Acta Biomater 2011; 7: 3370–3381. [21] Ronco C, Scabardi M, Goldon M, et al. Impact of spacing filaments external and dialyzer performance. Int J Artif Organs 1997; 20: 211–216. [22] Tan X, Tan SP, Teo WK, Li K. Polyvinylidene fluoride (PVDF) hollow fiber membranes for ammonia removal from water. J Membr Sci 2006; 271: 59–68. [23] Tian KP, Xiang HL, Li YJ. Biocompatibility of modified polyvinylidene fluoride membranes. Biomed Eng Clin Med 2010; 14: 207–211. [24] Zhang QL, Lu XL, Zhao LH. Preparation of polyvinylidene fluoride (PVDF) hollow fiber hemodialysis membranes. Membranes 2014; 4: 81–95. [25] Zhao CS, Liu T, Lu ZP, et al. Evaluation of polyethersulfone hollow fiber plasma separator by animal experiments. Artif Organs 2001; 25: 60–63. [26] Zhao CS, Liu XD, Nomizu M, et al. Blood compatible aspects of DNA modified polysulfone membrane – protein adsorption and platelet adhesion. Biomaterials 2003; 24: 3747–3755.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Qinglei Zhang, Xiaolong Lu*, Lihua Zhao, Juanjuan Liu and Chunfeng Wu
Research on polyvinylidene fluoride (PVDF) hollow-fiber hemodialyzer Abstract: In this study, polyvinylidene fluoride (PVDF) hollow-fiber hemodialysis membranes were prepared by non-solvent-induced phase separation. The PVDF hollowfiber hemodialyzers were prepared by centrifugal casting. The results showed that the PVDF membrane had better mechanical and separation properties when the membrane wall thickness was 40 μm and the N,N-dimethylacetamide in the core was 70 Vol%. Compared with commercial polysulfone hemodialysis membrane (Fresenius F60S membrane), the PVDF membrane had better mechanical property and ultrafiltration (UF) flux of pure water. The PVDF dialyzer’s removal efficiency for middle molecules was proven to be much higher than that of the F60S dialyzer. The UF coefficient of a high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas F60S is 42.5 ml/h/mm Hg, which can promote clearance for middle molecules. Keywords: hemodialysis membrane; hemodialyzer; performance; PVDF; structure. DOI 10.1515/bmt-2014-0190 Received December 26, 2014; accepted February 13, 2015
Introduction Hemodialysis has been extensively applied as a lifesustaining treatment for patients with end-stage renal disease [5]. The core aim for hemodialysis is to remove “middle-” and “small-”molecule toxin, such as β2microglobulin (β2-MG) and urea nitrogen. Cellulose membranes are widely used for hemodialysis because of their hydrogel structure and small thickness, which provide a
*Corresponding author: Xiaolong Lu, Institute of Biological and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China; and State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, China, E-mail: [email protected] Qinglei Zhang, Lihua Zhao and Chunfeng Wu: Institute of Biological and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China Juanjuan Liu: Tianjin Third Central Hospitals, Tianjin 300170, China
very effective removal for small solutes such as urea and creatinine. However, these membranes provide relatively little clearance for middle molecules and cause complement activation upon contacting with blood [12]. To improve the clearance for “middle” molecules and the blood compatibility of membranes, synthetic polymeric materials were developed to prepare hemodialysis membranes, such as polymethylmethacrylate, polyethylene, polypropylene, polyacrylonitrile, polyvinyl alcohol, polysulfone (PSF), and polyethersulfone membranes [8, 20, 25, 26]. However, the clearance for “middle”-molecule toxin and ultrafiltration (UF) flux were not ideal. The clearance for the solute mainly depended on blood, hemodialyzer, and dialysate. A number of research have been done on the effect of diffusion and convection on solute clearance [1, 6, 7, 9, 11, 14–17, 19, 21], whereas there are only few research on polyvinylidene fluoride (PVDF) hollowfiber hemodialyzer. PVDF has recently received great attention as a membrane material because of its outstanding properties, such as high mechanical strength, thermal stability, anti-ultraviolet radiation, smooth surface, and low protein adsorption, compared to other polymeric materials. PVDF membranes have been extensively applied in UF and micro-filtration for general separation purposes and are currently being explored as potential candidates in membrane contactor and membrane distillation applications [2, 4, 10, 18, 22]. PVDF has gained worldwide attention in biomedical research owing to its excellent property. Bouaziz et al. considered PVDF as one of the artificial vascular materials [3]. PVDF has a promising future application in the hemodialysis field. Our previous study prepared PVDF hollow-fiber hemodialysis membranes. The results showed that the PVDF membranes were prepared by blending with polyethylene glycol (PEG) polymers, which had good mechanical performance and biocompatibility but worse separation properties, especially the UF flux of pure water. The UF flux of pure water was only 64.3 l/h/m2, which was lower than that of the Fresenius F60S membrane [23, 24]. In this study, an attempt was made to improve the separation properties of PVDF hollow-fiber membranes (HFMs) by optimizing the membranes’ morphology and structure. At the same time, the dialysis performance evaluation of PVDF dialyzer was also studied.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
2 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer
Materials and methods Materials PVDF (1010, Solvay, Lyon, France), polyvinylpyrrolidone (PVP), PEG (Sigma-Aldrich Trading, Shanghai, P.R. China), Fresenius F60S membranes (Fresenius Medical Care, Frankfurter, Germany), lysozyme (LZM) and urea (Beijing Pubo Xin Biological Technology, Beijing, P.R. China), bovine serum albumin (BSA; Shanghai Biomedical Engineering Technical Service, Shanghai, P.R. China), N,N-dimethylacetamide (DMAc) (Samsung, Seoul, South Korea). All the reagents used in the study were of reagent grade.
Preparation of PVDF HFMs and hemodialyzer The PVDF HFMs were prepared by non-solvent-induced phase separation (NIPS) through a spinning equipment. Casting dopes were prepared by adding PVDF, PEG (6000 Da), and 1,4-diethylene dioxide into DMAc, followed by stirring at 70°C until the solution became homogeneous. 1,4-Diethylene dioxide and PEG worked as pore-forming agents to produce porous structures in the membrane. The inner diameter of PVDF HFMs was 200 μm and wall thicknesses were 30, 40, and 50 μm, respectively. The PVDF hemodialyzers were prepared by centrifugal casting. The parameters of PVDF and F60S hemodialyzers are shown in Table 1.
Characterization of the HFMs Morphology, maximum pore size, and porosity: Morphology studies of PVDF HFMs were carried out using a scanning electron microscope (Hitachi S-4800; Hitachi, Tokyo, Japan). Measurements were carried out on fiber soaked in ethanol for 15 min. At room temperature, the membrane was immersed in ethanol, and then, nitrogen can be pressurized into the inside. The bubble point pressure, P, was reached when the first string of bubbles came from the walls of the membrane. The maximum pore size can be calculated according to Eq. 1 [13]: r=
0.06378 , 2P
(1)
Where r is the pore radius (μm), P is bubble point pressure (MPa), and the ethanol surface tension is 22.3 mN/m. The membrane porosity, ε, was measured by soaking the membrane in pure water for 2 h, and then, the membrane surface was dried by filter paper. The membrane was weighed before and after absorption of pure water. The porosity was calculated using Eq. 2: ε=
( Ww -Wd ) ρW ( Ww -Wd ) ρW +Wd ρp
× 100%,
(2)
Where ε is the porosity of the membrane (%), Ww is the mass of the wet membrane, Wd is the mass of the dry membrane, ρW is the density of water (1.0 g/cm3), and ρp is the density of the membrane (1.78 g/cm3). Mechanical properties: The mechanical performance of the membranes was measured using an electronic single-yarn strength tester at room temperature. Each sample was clamped at both ends and unidirectionally stretched at a constant elongation rate of 500 mm/min with an initial length of 10 cm. Specimens were selected randomly and tested from each batch of the dried hollow-fiber sample. The tensile elongation and tensile strength at break were determined. At least five measurements were performed for each experiment and the average data were reported. Bursting pressure is a mechanical performance parameter of membranes. The membrane will be damaged when the pressure reaches bursting pressure. The value of bursting pressure was measured using the same equipment as the maximum pore size. The UF flux of pure water: Self-assembly widgets were made with 20 pieces of PVDF HFMs by epoxy resin cast. The inner diameter and wall thicknesses of PVDF membranes were 200 and 40 μm, respectively. The length of membranes was about 20 cm. The surface area of membranes was 25 cm2. UF flux was measured using the internal pressure method, with the pure water having a feed rate of 200 ml/min. The membranes were preloaded under 0.2 MPa for about 20 min. After adjusting the test temperature (25°C), The UF flux was measured with the inlet pressure (0.102 MPa) and the outlet pressure (0.098 MPa). The UF flux was calculated using Eq. 3: J=
V S⋅t
,
(3)
Where J is the UF flux of pure water (l/h/m2), V is the volume of the permeate flow (l), S is effective membrane area (m2), and t is sampling time (h).
Table 1: Parameters of PVDF and F60S hemodialyzers. Name
Shell Material Membrane Material Number/root Inner diameter (μm) Wall thicknesses (μm) Effective length (mm) Effective area (m2)
Dialysis performance test
Specifications and parameters
Polycarbonate
Polycarbonate
PVDF 6000 200 40 22 0.8
PSF 6000 200 40 22 0.8
For ease of studying, β2-MG and human serum albumin were replaced by LZM and BSA. LZM and β2-MG have similar performance. The molecular weight of LZM is 14 kDa, while β2-MG is 11.8 kDa. Their molecules are both positively charge and spherical molecules. At the same time, their isoelectric points are relatively close (LZM, 4.6; β2-MG, 5.7). The molecular weight of BSA is 67 kDa, whereas that of human serum albumin is 69 kDa. For this study, urea was chosen to characterize dialysis performance in the removal of small molecules. The standard dialysis solution comprises pure water, urea, LZM, and BSA. The concentration of urea, LZM, and BSA were 2000, 35, and
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 3
1000 mg/l, respectively. The flow rate of the simulation fluid was 300 ml/min. Sieving coefficients: Sieving coefficients were measured with a feed rate of 300 ml/min. The membranes were pre-flushed for about 30 min under 37°C±1°C. The sieving coefficient was measured using a transmembrane pressure (TMP) of 113±13 kPa. The sieving coefficient was calculated using Eq. 4: SC =
CF , CI
(4)
where SC is the sieving coefficient (%) and CF and CI are the concentration of filtered and inlet solution, respectively (mg/l). UF coefficient: The UF coefficient was defined as the volume of flow water through the dialysis membrane under certain TMP, which reflects the water capacity of dialyzer. The membranes were preflushed for about 30 min under 37°C±1°C. The UF coefficient was measured with different filtration flow rates (QF) and TMPs. Clearance for urea and LZM: The standard dialysis solution comprises pure water, urea, LZM, and BSA. The flow rate of simulation and dialysis fluid was 300 ml/min. The concentrations of simulation and dialysis fluid were measured with UV-Vis spectrophotometer (TU-1810; Purkinje, Beijing, China). The clearance (C) was calculated using Eq. 5:
CHF = QF ⋅SC ,
(5)
where CHF is the clearance (ml/min), QF is the filtration flow rate (ml/min), and SC is the sieving coefficient (%).
Results and discussion Membrane thickness effect on structure and performance Membrane thickness effect on structure The PVDF membranes were prepared by NIPS. A series of modified PVDF membranes were prepared by blending with PEG. The PVDF content was 22 wt% and the PEG content was 18.8 wt%. The cross-sectional scanning electron micrographs (SEMs) of the PVDF membranes are M-30, M-40, and M-50 with membrane thickness of 30, 40, and 50 μm, respectively. The cross-sectional SEM morphologies of different PVDF membranes are shown in Figure 1. The results exhibited that
Figure 1: The cross-sectional SEM images of different thicknesses. The inner diameter is 200 μm and the wall thicknesses of the PVDF HFMs are 30, 40, and 50 μm, respectively.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
4 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer
the membranes have an asymmetric structure with a dense skin layer outside, an intermediate layer with a finger-like structure, and a bottom layer of fully developed pores. This structure is formed via NIPS, especially by instantaneous phase separation. This asymmetric structure is suitable for biomedical application, as it not only can provide high mechanical strength but also prevent leakage.
Table 3: Solute removal performance of PVDF hemodialyzers. Membrane label M-30 M-40 M-50
Membrane thickness effect on performance In view of the potential practical applications in the biomedical field, it is essential for PVDF HFMs to retain good mechanical strength. To evaluate the mechanical properties of PVDF membranes, the tensile properties of all samples are exhibited in Table 2. It can be seen that the bursting pressure increases with an increase in membrane thickness. The tensile elongation of membranes has little change with an increase in thickness, which is mainly due to the similar structure of the membranes. The UF flux of pure water decreases from 107.1 to 74.5 l/h/m2 with increase in membrane thickness, which can be explained by the increase in transmembrane resistance as membrane thickness increases. The solute removal performance of PVDF hemodialyzer is shown in Table 3. Table 3 shows that the SC for BSA, LZM, and urea decreases as the PVDF membrane thickness increases. The PVDF dialyzers with membrane thickness of 40 and 50 μm have better SC for BSA and LZM than the PVDF dialyzer with membrane thickness of 30 μm. The reason is that the maximum pore size can affect the permeation performance of the BSA and LZM. The SC for urea of different PVDF dialyzers is very approximate, which can be explained by the smaller molecular weight of urea, i.e., it can easily go through the membrane. The clearance for LZM and urea decreases as the PVDF membrane thickness increases. The PVDF dialyzers with membrane thickness of 30 and 40 μm have higher clearance for LZM and urea than the PVDF dialyzer with membrane thickness of 50 μm, which is mainly due to the increase in transmembrane resistance as membrane thickness increases.
Property
BSA
LZM
Urea
SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min)
11.5±1.2 7.9±0.6 6.6±0.4
70.1±2.4 140.2±2.6 67.1±1.6 134.2±1.7 56.8±1.2 113.6±1.4
91.2±3.0 182.4±4.2 90.7±2.6 181.4±3.1 81.3±1.4 162.6±1.7
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
Composition of core liquid effect on structure and performance Composition of core liquid on structure A series of modified PVDF membranes were prepared with the same casting dopes and outer coagulation bath (pure water) but with different inner coagulation bath (pure water and DMAc). The cross-sectional SEM micrographs of PVDF membranes are M-0, M-1, M-2, and M-3 with DMAc 0, 35, 70, and 80 Vol%, respectively. The inner diameter and wall thicknesses of PVDF HFMs are 200 and 40 μm, respectively. The cross-section and surface SEM morphologies of PVDF membranes are shown in Figure 2. Figure 2 shows that the composition of the core liquid has an important impact on membrane structure. The finger-like structure of the cross section becomes increasingly larger and the inner surface of membrane becomes increasingly denser as the volume of DMAc decreases. This typically asymmetric structure is formed via NIPS, especially by instantaneous phase separation, which can be due to the rapid precipitation resulting in finger-like pores and the slow precipitation giving the sponge-like structure. It is easy to form a large finger-like structure of cross section when the core liquid is pure water. Composition of core liquid effect on performance
Table 2: Mechanical and separation performance of different PVDF membranes. Membrane label
M-30 M-40 M-50
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
UF flux of pure water (l/h/m2)
14.8 11.6 9.9
382 374 333
0.625 0.645 0.695
0.075 0.074 0.072
107.1 98.2 74.5
Table 4 shows that the PVDF membranes with different core fluid have obvious difference in mechanical and separation performance. As DMAc in the core fluid increases, the mechanical properties and the UF flux of pure water increase significantly. Tensile elongation at the break increases from 71% to 426%, and bursting pressure increases from 0.465 to 0.680 MPa. At the same time, the UF flux increases from 45.2 to 142.5
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 5
Figure 2: The cross-sectional and surface SEM images of different PVDF membranes. The PVDF membranes are M-0, M-1, M-2, and M-3, with DMAc of 0, 35, 70, and 80 Vol%, respectively. The inner diameter and wall thicknesses of PVDF HFMs are 200 and 40 μm, respectively.
l/h/m2. Figure 2 shows that the number of finger-like pores are obviously reduced as DMAc in the core fluid increases, which can improve mechanical performance. The maximum pore size increases as DMAc in the core fluid increases. The solute removal performance of different core liquid PVDF HFMs are shown in Table 5. The SC and clearances for big- and middle-molecule substances such as BSA and LZM are different, especially for the middle
molecules. However, The SC and clearances for small-molecule substance such as urea is appropriate. The clearance for LZM increases from 74.2 to 145.0 ml/min, whereas the SC for BSA decreases from 3.9% to 15.6% as DMAc in the core fluid increases, which can be explained by the different distributions and forms of membrane pore size. In general, the clearance for small-molecule urea is higher than LZM. The molecular weight of urea is small, which can easily go through the membrane.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
6 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer Table 4: Mechanical and separation performance of different PVDF membranes. Label
M-0 M-1 M-2 M-3
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
UF flux of pure water (l/h/m2)
10.6 11.2 11.6 12.2
71 200 374 426
0.465 0.505 0.645 0.680
0.067 0.070 0.074 0.081
45.2 64.3 98.2 142.5
Table 5: Solute removal performance of PVDF hemodialyzer for simulated solution. Membrane label
Property
BSA
LZM
Urea
M-0
SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min) SC (%) CHF (ml/min)
3.9±0.4 5.6±0.6 7.9±0.6 15.6±1.4
37.1±1.6 74.2±1.2 53.5±1.5 107.0±2.4 67.1±1.6 134.2±1.7 72.5±1.5 145.0±2.6
85.7±2.6 171.4±2.3 87.2±2.6 174.4±2.5 90.7±2.6 181.4±3.1 92.4±2.6 184.8±2.5
M-1 M-2 M-3
Contrastive study of PVDF and commercial Fresenius F60S membrane Morphology and structure The cross-sectional and outer-surface SEMs of PVDF and Fresenius F60S membrane are shown in Figure 3. Results show that there are few finger-like pores in PVDF and Fresenius F60S membranes. The structure not only can provide high mechanical strength but also affect separation performance.
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
Mechanical and separation performance Table 6 shows that the PVDF membrane has better mechanical performance and higher UF flux of pure water than the Fresenius F60S membrane. The tensile strength and elongation of PVDF membrane are 11.6 MPa and 374%,
Figure 3: Cross-sectional and outer-surface SEM micrographs of PVDF and Fresenius F60S membranes.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
Q. Zhang et al.: PVDF hollow-fiber hemodialyzer 7
Table 6: Mechanical performance and UF flux of PVDF and Fresenius F60S membranes.
Table 7: Dialysis performance of PVDF and F60S dialyzers. Label Property
Materials
PVDF F60S
Tensile strength (MPa)
Tensile elongation (%)
Bursting pressure (MPa)
r (μm)
11.6 8.9
374 66
0.645 0.475
0.074 0.079
UF flux of pure water (l/h/m2) 98.2 82.5
respectively, whereas those of the F60S membrane are 8.9 MPa and 66%, respectively. The PVDF membrane and the Fresenius F60S membrane have different mechanical performance, which mainly depends on different materials. The UF flux of PVDF is 98.2 l/h/m2, whereas that of the F60S membrane is 82.5 l/h/m2. The reason is that there are PEG additives staying in the membranes that can increase the hydrophilicity of PVDF membranes.
Dialysis performance of PVDF and F60S dialyzers Table 7 shows that the F60S dialyzer SC for BSA can reach 1.3% and LZM and urea are 48.2% and 93.4%, respectively. The PVDF dialyzer SC for LZM and urea are 67.1% and 90.7%, respectively. It can be concluded that the PVDF dialyzer’s removal efficiency for middle molecules is much higher than that of the F60S dialyzer. The UF coefficient of the high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas that of the F60S dialyzer is 42.5 ml/h/mm Hg, which can promote clearance for middle molecules. However, the PVDF dialyzer SC for BSA is about 7.9%, which is not ideal.
Conclusions Core fluid composition and membrane thickness had an important effect on membrane mechanical and separation properties. The mechanical properties and the UF flux of pure water increase significantly with increase in DMAc in the core fluid. The SC and clearances for BSA, LZM, and urea decrease as the membrane thickness increases. The SC and clearances for BSA, LZM, and urea decrease as the volume of DMAc increases. When membrane thickness is 40 μm and DMAc in the core is 70 Vol%, the PVDF membrane has better mechanical properties, higher UF flux, and solute clean property. Compared with the Fresenius F60S hemodialysis membrane, the PVDF membrane had better mechanical performance and UF flux of pure water. The tensile elongation of PVDF membrane was 374%, whereas that of the Fresenius F60S membrane was 66%. The UF coefficient of
PVDF F60S
BSA
LZM
Urea UF coefficient (ml/h/mm Hg)
SC (%) 7.9±0.6 67.1±1.6 90.7±2.6 CHF (ml/min) 134.2±1.7 181.4±3.1 SC (%) 1.3±0.4 48.2±2.3 93.4±4.2 CHF (ml/min) 96.4±1.9 186.8±3.5
62.6 42.5
QF, filtration flow rate (ml/min); CHF, clearance (ml/min); SC, sieving coefficient (%); *TMP, transmembrane pressure (kPa). Test conditions: QF = 200 ml/min; *TMP = 113±13 kPa (n = 4).
the high-flux PVDF dialyzer is 62.6 ml/h/mm Hg, whereas that of F60S is 42.5 ml/h/mm Hg. It was proven that the removal efficiency of PVDF dialyzer for middle molecules is much higher than that of the F60S dialyzer. The PVDF dialyzer SC for LZM was 67.1%, whereas that of the Fresenius F60S dialyzer was 48.2%. Acknowledgments: This work was supported by the National Natural Science Foundation of China (21106100, 21176188, and 51278336) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20111201110004).
References [1] Allen R, Frost TH, Hoenich NA. The influence of the dialysate flow rate on hollow fiber hemodialyzer performance. Artif Organs 1995; 19: 1176–1180. [2] Bottino A, Capannelli G, Comite A. Novel porous poly (vinylidene fluoride) membranes for membrane distillation. Desalination 2005; 183: 375–382. [3] Bouaziz A, Richert A, Caprani A. Vascular endothelial cell responses to different electrically charged poly (vinylidene fluoride) Supports under static and oscillating flow conditions. Biomaterials 1997; 18: 107–112. [4] Chae SR, Yamamura H, Ikeda K, et al. Comparison of fouling characteristics of two different poly-vinylidene fluoride microfiltration membranes in a pilot-scale drinking water treatment system using precoagulation/sedimentation, sand filtration, and chlorination. Water Res 2008; 42: 2029–2042. [5] Dahe GJ, Teotia RS, Kadam SS. The biocompatibility and separation performance of antioxidative polysulfone/vitamin ETPGS composite hollow fiber membranes. Biomaterials 2011; 32: 352–365. [6] Eloot SD, Asseler YD, Debondt P, et al. Combining SPECT medical imaging and computational fluid dynamics for analyzing blood and dialysate flow in hemodialyzers. Int J Artif Organs 2005; 28: 739–749. [7] Huang Z, Klein E, Li B, et al. A new method to evaluate the local clearance at different annular rings inside hemodialyzers. ASAIO J 2003; 49: 692–697.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
8 Q. Zhang et al.: PVDF hollow-fiber hemodialyzer [8] Ishihara K, Hasegawa T, Watanabe J, et al. Protein adsorptionresistant hollow fibers for blood purification. Artif Organs 2002; 26: 1014–1019. [9] John KL, Alfred KC, Lawrence YA, et al. Hemodialyzer mass transfer area coefficients for urea increase at high dialysate flow rates. Kidney Int 1997; 51: 2013–2017. [10] Khayet M, Khulbe KC, Matsuura T. Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process. J Membr Sci 2004; 238: 199–211. [11] Leypoldt JK, Cheung AK. Increases in mass transfer-area coefficients and urea Kt/V with increasing dialysate flow rate are greater for high-flux dialyzers. Am J Kidney Dis 2001; 38: 575–579. [12] Li LL, Cheng C, Xiang T, et al. Modification of polyethersulfone hemodialysis membrane by blending citric acid grafted polyurethane and its anticoagulant activity. J Membr Sci 2012; 405–406: 261–274. [13] Lu XL, Discuss about the measure methods for performance of hollow fiber porous membranes. Membr Sci Technol 2011; 32: 1–6. [14] Lucchi L, Fiore GB, Guadagni G, et al. Clinical evaluation of internal hemodiafiltration (I HDF): a diffusive-convective technique performed with internal filtration enhanced high flux dialyzers. Int J Artif Organs 2004; 27: 414–419. [15] Meyer TW, Leeper EC, Bartleet DW, et al. Increasing dialy-sate flow and dialyzer mass transfer area coefficient to increase the clearance of protein-bound solutes. J Am Soc Nephrol 2004; 15: 1927–1935. [16] Meyer TW, Peattie JW, Miller JD, et al. Increasing the clearance of protein-bound solutes by additon of a sorbent to the dialysate. J Am Soc Nephrol 2007; 18: 868–874.
[17] Osuga T, Obata T, Ikehira H. Detection of small degree of nonuniformity in dialysate flow in hollow fiber dialyzer using proton magnetic resonance imaging. Magn Reson Imaging 2004; 22: 417–420. [18] Park HH, Deshwal BR, Jo HD, et al. Absorption of nitrogen dioxide by PVDF hollow fiber membranes in a G-L contactor. Desalination 2009; 243: 52–64. [19] Poh CK, Hardy PA, Liao Z, et al. Effect of flow baffles on the dialysate flow distribution of hollow fiber hemodialyzers. J Biomech Eng 2003; 125: 48148–48149. [20] Ran F, Nie SQ, Zhao WF, et al. Biocompatibility of modified polyethersulfone membranes by blending amphiphilic triblock copolymer of poly(vinyl pyrrolidone)b-poly(methylmethacrylate)-b-poly(vinylpyrrolidone). Acta Biomater 2011; 7: 3370–3381. [21] Ronco C, Scabardi M, Goldon M, et al. Impact of spacing filaments external and dialyzer performance. Int J Artif Organs 1997; 20: 211–216. [22] Tan X, Tan SP, Teo WK, Li K. Polyvinylidene fluoride (PVDF) hollow fiber membranes for ammonia removal from water. J Membr Sci 2006; 271: 59–68. [23] Tian KP, Xiang HL, Li YJ. Biocompatibility of modified polyvinylidene fluoride membranes. Biomed Eng Clin Med 2010; 14: 207–211. [24] Zhang QL, Lu XL, Zhao LH. Preparation of polyvinylidene fluoride (PVDF) hollow fiber hemodialysis membranes. Membranes 2014; 4: 81–95. [25] Zhao CS, Liu T, Lu ZP, et al. Evaluation of polyethersulfone hollow fiber plasma separator by animal experiments. Artif Organs 2001; 25: 60–63. [26] Zhao CS, Liu XD, Nomizu M, et al. Blood compatible aspects of DNA modified polysulfone membrane – protein adsorption and platelet adhesion. Biomaterials 2003; 24: 3747–3755.
Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 4:56 PM
