**2.1.1 Improve biocompatibility**

In our recent study (Su BH, et al., 2008), larger molecular weight PVP was used to blend with PES to prepare hollow fiber hemodialysis membrane, and the performance was evaluated in vitro and in vivo. The biocompatibility profiles of the membranes showed slight neutropenia and platelet adhesion at the initial stage of the hemodialysis. The clearance and the reduction ratio after the hemodialysis of small molecules (urea, creatinine, phosphate) for the PES membrane were higher in vitro than that in vivo.

Barzin et al. (Barzin et al., 2004) prepared two kinds of PES hollow fiber membranes for hemodialysis by blending two ratios of PES to PVP (PES/PVP = 18/3 and 18/6 by weight). It was observed that the water flux of the hollow fiber increased significantly when heattreated in water, while decreased when heat-treated in air. On the other hand, the molecular weight cutoff of the hollow fiber increased slightly when heat-treated in water, while decreased drastically when heat-treated in air. SEM images also showed that the surface morphology of the membranes was different before and after heat-treatment. The performance data of the hollow fiber heated in air at 150 C was found to be the most appropriate for hemodialysis application. It was also found that the hollow fiber membrane prepared from the blend ratio of PES/PVP = 18/3 showed slightly higher flux than that

Polyethersulfone Hollow Fiber Membranes for Hemodialysis 69

HFM-20-1.2

HFM-20-1.6

HFM-20-0

Fig. 3. Time-dependent flux of PMMA-AA-VP modified PES membranes during the

For the membranes: HFM-20-1.2 (The amounts of PES and the terpolymer are 20 and 1.2 wt.%, respectively); HFM-20-1.6 (The amounts of PES and the terpolymer are 20 and 1.6

Many other methods can also be used for the modification of PES hollow fiber, the following reviewed the methods. Though not all of them are discussed for hemodialysis membranes, some of the methods can be used for the modification of PES hemodialysis membranes.

Torto and coworkers (Torto et al., 2004) provided a method for the in situ modification of hollow fiber membranes used as sampling units for microdialysis probes. The method consisted of adsorption-coating of high-molecular-weight PEI onto membranes, already fitted on microdialysis probes. Modification of membranes was designed to specifically explore the so-called Andrade effects and thus enhance the interaction of membranes with enzyme. Such a procedure can be modified and employed to either promote or reduce membrane-protein interaction for hollow fibers used as microdialysis sampling units or

To modify PES membranes, photochemical surface technique is attractive, and has several advantages. Mild reaction conditions and low temperature may be applied; and high selectivity is possible by choosing the reactive groups or monomers and respective excitation wavelength; and it can be easily incorporated into the end stages of a

ultrafiltration process.

**2.2 Other methods** 

**2.2.1 Surface-coating** 

wt.%, respectively); HFM-20-0 (20wt.% PES).

(From reference, Zou et al., 2010)

other similar membrane applications.

**2.2.2 Photo-induced surface grafting** 

PBS solution: 0–30 and 95–120 min; BSA solution: 40–90 min. n=3.

prepared from a solution with PES/PVP ratio of 18/6. Of course, PVP could also be used to modify PES hollow fiber membranes for hemofiltration (Yang et al., 2009). Gholami et al. (Gholami et al.; 2003) found that the hollow fiber membranes shrank by heat treatment, as evidenced by a decrease in flux and an increase in solute separation, although there was no visible change in the hollow-fiber dimension. However, for flat-sheet PES membranes, the membrane surface altered, and surface parameters (such as surface roughness) have been changed after non-contact heating (microwave irradiation) (Mansourpanah et al., 2009). Erlenkotter and coworkers (Erlenkotter et al, 2008) evaluated the dialysis membrane hemocompatibility. In order to compare different polymers used in the manufacturing of dialysis membranes, a set of the following hemocompatibility parameters was assessed and assembled to an overall score: generation of complement factor 5a, thrombin-antithrombin III-complex, release of platelet factor 4, generation and release of elastase from polymorphonuclear granulocytes, and platelet count. By blending polyarylate with PES, the membrane hemocompatibility improved. They also provided a score model to facilitate the selection of membrane polymers with an appropriate hemocompatibility pattern for dialysis therapy.

### **2.1.2 Improve antifouling property**

Membrane fouling is still a crucial problem for hollow fiber membrane. When fouling takes place on membrane surfaces, it causes flux decline, leading to an increase in production cost due to increased energy demand. Qin et al. (Qin et al., 2004) selected solvent-resistant hollow-fiber UF membranes by measurement of fiber swelling and treatability studies on spent solvent cleaning rinse. The results indicated that the membranes made of both cellulose acetate (CA) and polyacrylonitrile (PAN) materials could tolerate the solvent present and were suitable or treating the spent solvent rinses, whereas PES and PSF membranes were not suitable. The CA membrane had the lowest fouling tendency when treating the spent solvent rinse. Nakatsuka et al. (Nakatsuka et al., 1996) also found that the permeate flux for the hydrophilic CA membranes was much higher than that of the hydrophobic PES membrane, a phenomenon which was explained by membrane fouling due to the adsorption of substances in raw water on and in the pores of the membranes. Xu et al. (Xu et al., 2009) observed that the fouling layer grew faster on the inside surface of the PES hollow fiber at a lower flow rate than that at a higher flow rate due to the lower shear stress. These results suggested that PES hollow fiber membrane should be modified to improve antifouling property by increasing hydrophilicity.

Arahman et al. (Arahman et al., 2009) modified PES hollow-fiber membrane by blending with hydrophilic surfactant Tetronic 1307. The fouling of the PES membrane with blending Tetronic 1307 was lower than that of the original PES membrane in the case of BSA filtration. A functional terpolymer of poly (methyl methacrylate–acrylic acid–vinyl pyrrolidone) (PMMA-AA-VP) was synthesized via free radical solution polymerization using DMAC as the solvent in our recent study (Zou et al., 2010). The terpolymer can be directly blended with PES using the solvent to prepare modified PES hollow fiber membrane. The hydrophilicity of the blended membranes increased, and the membranes showed good protein antifouling property. The antifouling property is always expressed as the timedependent flux during the ultrafiltration process (PBS solution and BSA solution alternatively switched), as shown in Figure 3.

Fig. 3. Time-dependent flux of PMMA-AA-VP modified PES membranes during the ultrafiltration process.

For the membranes: HFM-20-1.2 (The amounts of PES and the terpolymer are 20 and 1.2 wt.%, respectively); HFM-20-1.6 (The amounts of PES and the terpolymer are 20 and 1.6 wt.%, respectively); HFM-20-0 (20wt.% PES).

PBS solution: 0–30 and 95–120 min; BSA solution: 40–90 min. n=3. (From reference, Zou et al., 2010)
