Study of Dialyzer Membrane (Polyflux 210H) and Effects of Different Parameters on Dialysis Performance

doi:10.4236/ojneph.2013.33029

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Open Journal of Nephrology, 2013, 3, 161-167

http://dx.doi.org/10.4236/ojneph.2013.33029 Published Online September 2013 (http://www.scirp.org/journal/ojneph)

Study of Dialyzer Membrane (Polyflux 210H) and Effects

of Different Parameters on Dialysis Performance

Md Shihamul Islam*, Jerzy Szpunar

Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Canada

Email: *[email protected]

Received May 22, 2013; revised June 20, 2013; accepted July 13, 2013

bution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly

cited.

ABSTRACT

Problems frequently encountered in kidney malfunction include abnormal fluid levels in the body, increased acid levels,

abnormal levels of Urea, Glucose, Endothelin, β2-Microglobulin and Complement Factor D. Parameters characterizing

the structure of dialyzers are very important because they decide overall clearance of toxin molecules and at the same

time should allow retaining useful molecules in the blood. In this paper, a cross sectional image of the dialyzer mem-

brane with details of the porosity is presented. A multilayered membrane model with different porosity for each layer,

describes the actual structure of Polyflux 210H membrane. This model is developed using Finite Element Software—

COMSOL Multiphysics 4.3. A blood flow with substances like—Urea, Glucose, Endothelin, β2-Microglobulin, Com-

plement Factor D and Albumin is introduced. For a certain blood flow rate, the toxins diffuse through the membrane

and on the other side of the membrane a dialysate flow removes the toxins. Here, different parameters, such as flow rate

of blood and dialysate, length and radius of the fiber are changed to simulate how these changes affect toxin clearance

and the removal of useful molecules.

Keywords: Simulation, Dialysis; Axisymmetric Model; 3-Layered Membrane; Effective Diffusivity; Parameters;

COMSOL Multiphysics 4.3

1. Introduction

A substantial amount of research has been performed

regarding homogeneous dialyzer membranes. These ho-

mogeneous dialyzer membranes are mostly made of cel-

lulose. These membranes have a uniform pore structure

from the inner to the outer side of the membrane. The

surface of such membranes was not very biocompatible,

because exposed hydroxyl groups would activate com-

plement in the blood passing by the membrane. There-

fore, these days dialyzer membranes are made from syn-

thetic materials, using polymers such as polyarylether-

sulfone, polyamide, polyvinylpyrrolidone, polycarbonate

and polyacrylonitrile. These synthetic membranes acti-

vate complement to a lesser degree than cellulose mem-

branes [1]. They have a structure which is known as

asymmetric. This actually means that the shape of a pore

gradually changes from inner to outer surface of the

membrane. Synthetic membranes can be made in either

low- or high-flux configuration, but most are high-flux.

Polyflux 210H is an example of such high flux asymmet-

ric polymer membrane. Sakai et al. [2] studied and cap-

tured the images of inner and outer surface images of

such an asymmetric membrane. They calculated the sur-

face porosity based on those photomicrographs. These

give an idea of the surface porosity. But to what extend

that porosity continues from one surface to another is not

sure. Taking cross sectional image of the membrane and

calculating the porosity from that image allow a better

description of it. Experimental studies [3,4] have been

done to determine whether increasing the dialysate or

blood flow rate leads to better clearance or not. But these

studies did not consider the structure of the membrane.

Also these studies were limited to clearance of Urea only.

A reasonable concern for doctors, in these days, is

whether the necessary elements like Albumin are dif-

fused through the membrane during the dialysis or not.

Most of the experimental studies are done with dialyzers

which are commercially available. So they actually give

a comparison between different dialyzers. But which

parameters of dialysis process are really important to

have a better overall clearance of toxin molecules and at

the same time retain useful molecules is not clear. Also

*Corresponding author.

M. S. ISLAM, J. SZPUNAR

162

how the change of those parameters will affect the clear-

ance is important and will be investigated in this work.

2. Polyflux 210H Dialyzer Membrane

In one dialyzer of Polyflux 210H, there are approxi-

mately 12,000 fibers. These fibers are made of Polyamix

which is a blend of Polyarylethersulfone, Polyvinylpyr-

rolidone and Polyamide [5]. Polyflux 210H fiber has a

porous structure. A single fiber has a length of 270 mm.

The fibers used in this research were fixed using a

mixture of 2.5% glutaraldehyde and 2% paraformalde-

hyde in a 0.1 M phosphate buffer. A series of mixtures of

pure ethanol and distilled water in different ratios were

used to dehydrate the fibers. To investigate the internal

surface of the fibers, some fibers were placed on a carbon

double sided adhesive scanning electron microscopy

(SEM) tape glued from the other side to an SEM stub.

The SEM stub with the fibers glued to its surface was

transferred for viewing under the stereoscopic optical

microscope. The fiber walls were dissected longitudi-

nally using a surgical scalpel. Then a 250 Å layer of gold

(purity 99.991%) is applied to render the surfaces of the

capillaries conductive for observation under the Scanning

Electron Microscope (SEM) and the Field Emission

Scanning Electron Microscope (FESEM). The SEM used

here is a Joel-6010LV and the FESEM is a SU6600 Hitachi.

Figure 1. Longitudinal image of a fiber.

At first the longitudinal image (Figure 1) of the fiber

is taken from the outside.

A cross section of the fiber is showed in Figure 2.

Then images of the outer and inner side of the mem-

brane are taken to get an idea of the pore size and poros-

ity. The outer surface has a range of pore size which var-

ies between 0.45 μm to 20.40 μm (Figure 3).

But for the inner surface the range is between 34 nm

and 45 nm (Figure 4). Then to get a better idea of the

porosity changes, cross sectional images of the mem-

brane are taken. This gives a clear idea of the porosity

changes across the thickness.

Figure 2. Cross section of the fiber.

In Figure 5, a dialyzer membrane having a total thick-

ness of 45 μm is divided into three different layers to

calculate the porosity for each of the layer. The first layer

(thickness of 8 μm) has a porosity of around 0.1, second

layer (thickness of 12 μm) has a porosity of around 0.27

and third layer (thickness of 25 μm) has a porosity of

around 0.4. Several photomicrographs from each of the

layers are taken to calculate the porosity and then an av-

erage value of porosity is introduced for each layer.

3. Method of Developing Model

The configuration of a modern hollow fiber dialysis as-

sembly can be seen in Figure 6 . The blood flows through

the fibers while the dialysate flows over the capillaries in

a counter-current manner similar to a shell and tube heat

exchanger. In this application the flow is laminar. Figure 3. Outer surface of the dialyzer membrane.

M. S. ISLAM, J. SZPUNAR 163

Figure 4. Inner surface of the dialyzer membrane.

Figure 6. Diagram of dialysis hollow-fiber dialyzer.

Figure 5. Cross section of the membrane.

Figure 7 shows the three sections of the axisymmetri-

cal domain: the domain on the left represents the blood

flow in the fiber, the small domain in the middle with

three layers represents the membrane, and the domain on

the right represents the outer dialysate flow in the shell.

4. Equations

The following simplified PDE (Partial Differential Equa-

tion) describes the convective and diffusion processes in

the blood and the dialysate [6].



ii i

Dc cu 



(1)

where ci denotes the concentration of the toxin (mol/m3)

in the respective phase, D denotes the diffusion coeffi-

cient (m2/s) in the liquid phases and u denotes the veloc-

ity (m/s) in the respective liquid phase.

For different toxins the diffusion coefficient is differ-

ent and it can be calculated from [7] Figure 7. Model Geometry with R1, R2, R3.

M. S. ISLAM, J. SZPUNAR

164

Table 1. Six molecules with their molecular weight, diame-



0.552

1.62 10DMW



 (2) ter and diffusion coefficient.

where MW is the molecular weight of the respective

toxin. Molecule Molecular Diameter (nm) Diffusion

(

Both the blood and dialysate flow is considered fully

developed laminar flow. For an inlet velocity of blood

along the axial direction [8],





























ei i

Dc 

(3)

where QB is the volume flow rate of blood, R1 is the in-

ner radius of the hollow fiber, r is the radial coordinate

and n is the number of fibers in a dialyzer which is

12,000 for Polyflux 210H dialyzer.

For the phenomenon of diffusion through the mem-

brane, the following equation has been used-



(4)

where ci denotes the concentration of the molecules

(mol/m3) in the respective phase, D denotes the diffusion

coefficient (m2/s) of the molecules and De denotes the

effective diffusion coefficient of the molecules in the

porous media.

The term effective diffusivity is defined [9] as



DDfqSA (5)

 (6)



1 2.10502.08651.70680.72603

1 0.758

qq q

fq q

 





(7)



S (8)

Where D is the diffusion coefficient of molecule, f(q) the

friction coefficient, SD the steric hindrance factor at the

pore inlet in diffusion, Ak the membrane porosity, q is the

ratio of solute radius rs to pore radius rp. Here, the poros-

ity values of the three layers of the membrane are used

for Ak.

The six molecules that are considered in this paper are

listed in Table 1 with their molecular weight [10], di-

ameter [11] and diffusion coefficient.

5. COMSOL Multiphysics 4.3

weight (Da) coefficient

x 10-8 cm2/s)

Urea 60 0.48 1690.35

Glucose 180 1.0 921.73

E 4

β2- in

ndothelin282.8 2.60 160.25

Microglobul11800 3.88 91.59

Complement

Factor D

Albumin

24000 5.12 61.89

66000 7.8 35.41

and boundary conditions, simulations are done for chang-

e rate of Urea at different blood flow

be concluded that the clearance

7. Results

of Blood Flow Rate on Clearance

For d flow rate of QB = 300, 400 and 500 ml/min

, with the increasing

7.2. Effects of Dialysate Flow Rate on Clearance

The flow rate, QB= 400 ml/min is kept constant

7.3. Effects of Length of the Dialyzer Fiber on

The er fiber is varied from 270 to 540

es of different parameters. A post processing result is

showed in Figure 8.

6. Validation

ncAt first, the cleara

rate is compared with the experimental results provided

by the manufacturer [5].

From Figure 9, it can

te of Urea at different blood flow rate is in good

agreement with the data provided by the Polyflux 210H

manufacturer.

7.1. Effects

Rate

the bloo

and dialysate flow rate of QD = 500ml/min, the clearance

rate for six molecules is calculated.

As it can be seen from Figure 10

ood flow rate, the clearance rate of both Urea and Glu-

cose increase rapidly. Specially, for Urea, when the

blood flow rate increases from 300 to 500 ml/min, the

clearance rate almost gets doubled. And from Figure 11,

it is evident that the clearance rate of Albumin remains

almost constant.

Rate

blood

and the dialysate flow rate, QD is gradually increased.

From Figures 12 and 13, it can be said that at a co

ant blood flow rate, the increasing dialysate flow rate

ensures further clearance of Urea and Glucose.

COMSOL Multiphysics 4.3 is used for developing and

simulating the model of Polyflux 210H dialyzer. COM-

SOL Multiphysics is a finite element analysis, solver and

simulation software / FEA Software package for various

physics and engineering applications, especially coupled

phenomena or multiphysics.

Clearance Rate

length of the dialyz

mm when QB = 300 ml/min and QD = 500 ml/min. After developing the model with necessary inlet, outlet

M. S. ISLAM, J. SZPUNAR 165

Figure 8. Concentration of Urea at both blood and dialysate

side along the membrane (axisymmetric).

Figure 9. Clearance of Urea for both experimental and

simulation cases at blood flow rate, QB = 300, 400 and 500

ml/min whereas dialysate flow rate, QD = 500 ml/min.

Figure 11. Clearance rate of Endothelin, β2-Microglobulin,

Complement Factor D and Albumin at different blood flow

rates when QD = 500 ml/min.

Figure 12. Clearance of Urea and Glucose at different

Dialysate flow rates when QB = 400ml/min.

Figure 13. Clearance of Endothelin, β2-Microglobulin, Com-

plement Factor D and Albumin at different Dialysate flow

rates when QB = 400 ml/min.

Figure 10. Clearance rate of Urea and Glucose at different

blood flow rates when QD = 500 ml/min.

M. S. ISLAM, J. SZPUNAR

166

From Figure 14, it can be seen that if the length of the

7.4. Effects of Radius of the Dialyzer Fiber on

The r fiber is increased from 0.1 mm

alyzer fiber is increased, the clearance rate of Glucose

increases more rapidly than the clearance rate of Urea.

For Endothelin and β2-Microglobulin (Figure 15) the

clearance rate increases twice compared to the initial

condition. Meanwhile, the clearance rate of Albumin

does not change that much.

Clearance Rate

radius of the dialyze

to 0.2 mm when QB = 300 ml/min and QD = 500 ml/min.

From Figures 16 and 17 it is evident that the effect of

creasing radius of dialyzer fiber is similar to that of

increasing the length of the dialyzer fiber. However, if a

case is considered where the clearance rate of Albumin is

similar for length of 450 mm with a radius of 0.1 mm and

radius of 0.17 mm with a length of 270 mm dialyzer fiber.

From Table 2, it is evident that for the same level of

earance rate of Albumin (or loss of Albumin), the dia-

Figure 14. Clearance rate of Urea and Glucose at QB =

300ml/min and QD = 500 ml/min.

Figure 16. Clearance rate of Urea and Glucose at QB = 300

ml/min and QD = 500 ml/min.

Figure 17. Clearance of Endothelin, β2-Microglobulin, Com

able 2. Clearance rate of different molecules for two dia-

Length = 270 mm,

plement Factor D and Albumin at QB = 300 ml/min and QD

= 500 ml/min.

lyzer fibers consisting length = 450 mm, radius = 0.1 mm

and length = 270 mm, radius = 0.17 mm.

Length = 450 mm,

Radius = 0.1 mm Radius = 0.17 mm

Ura e281.1 271.3

β2-in

lucose 234.1 221

ndothelin 67.21 61.8

Microglobul37.97 35.05

omplement Factor D24.04 22.92

Albumin 10.67 10.7

zer fiber with relatively higher length and lower radius ly

shows better clearance of Urea, Glucose, Endothelin,

β2-Microglobulin and Complement Factor D than the

dialyzer fiber with relatively higher radius and lower

length.

Figure 15. Clearance of Endothelin, β2-Microglobulin, Com-

plement Factor D and Albumin at QB = 300 ml/min and QD

= 500 ml/min.

M. S. ISLAM, J. SZPUNAR

167

clusion

of Urea at different blood flow rate is

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The clearance rate

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