Ultraviolet UV modification

UV irradiation is an effective technique for membrane surface modification. It maybe the most appropriate of all surface modification techniques because of certain features: the low cost of operation, mild reaction conditions, it is highly surface-selective, and because it alters the material surface with facile control of the chemistry (Ma et al. 2000a). Several studies have been carried out using photoinitiators (Kita et al. 1994; Richey et al. 2000; Ulbricht et al. 1998) or specially synthesized molecules with UV-sensitive end groups (Taniguchi et al. 2003; Thom et al. 2000). Figure 5 shows a typical two-step process for UV-induced polymerization (Ma et al. 2000b).

Xu and coworkers (2004) modified PPMMs using phospholipid-analo-gous polymers (PAPs) (Fig. 6). N,N-dimethylaminorthyl methacrylate (DMAEMA) was grafted to the membrane surface by photoinduced graft polymerization and then reacted with 2-alkyloxy-2-oxo-1,3,2-dioxaphos-pholanes. Five 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes, containing oc-tyloxy, dodecyloxy, tetradecyloxy, hexadecyloxy, and octadecyloxy groups in their molecular structure, were used to fabricate the PAP-modified PP-MMs. The influence of grafting degree on BSA adsorption is shown in Fig. 7. It can be seen that the adsorbed BSA on the membrane decreased with increasing grafting degree for both poly(DMAEMA)-grafted and PAP-modified membranes at the studied range. The five PAP-modified membranes had better protein resistance than the unmodified polypropylene membrane and poly(DMAEMA)-grafted membranes. Nevertheless, as the number of carbon atoms was increased from 14 to 16 and 18, BSA adsorption on the membrane surface was clearly suppressed. The reason for this was probably the mimetic characteristics of the PAP-modified membranes; the principal components of a biomembrane are lipids, proteins, and carbo-

Modified Pap
Fig. 5. Schematic diagram of a novel two-step process for photoinduced graft polymerization
Modified Pap
Fig. 6. Schematic representation for the fabrication of phospholipid-analogous polymer (PAP)-modified PPMMs. DMAEMA N,N-dimethylaminorthyl methacrylate

1000

1000

100 -\—i—i—i—i—i—i—i—i—i—|—i—|—i—|—i—|—i—|—i—|—i— 0123456789 10 11 Grafting degree (wt.%)

Fig. 7. BSA adsorption on poly(DMAEMA)-grafted and PAP-modified membranes hydrates. However, the amount of proteins and carbohydrates is relatively small. Most of the lipids are phospholipids such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidic acid, phos-phatidylinositol, and phosphatidylglycerol. As part of their structure, these phospholipids all have two long alkyloxy groups. Two layers of phospho-lipid molecules are facing each other, burying the hydrophobic moieties inside the membrane. The zwitterionic species cover the membrane surface and there is a reorientation of the PAP to put the zwitterions at the surface of PAP-modified PPMMs. During the protein adsorption measurements, the PAP-modified PPMMs were immersed in BSA solution for 24 h. In that case, the membranes were in an aqueous environment and should present the zwitterions at the surface. Thus, in these conditions, the membrane surfaces were hydrophilic and were relatively similar to those of the biomembranes. The hydrophilic surfaces normally facilitate the reduction of protein adsorption on the membrane (Ma et al. 2000a; Steen et al. 2002; Wavhal and Fisher 2003; Xu et al. 2003). Furthermore, with an increase of the number of carbon atoms of the long alkyloxy groups, the impact of the original membrane surface on protein adsorption could be reduced, which leads to a decrease in protein adsorption on the PAP-modified membranes. Therefore the membranes exhibit excellent protein resistance.

N-Vinyl-2-pyrrolidone is a hydrophilic and nonionic monomer that can be easily initiated through radical, thermal, or photo irradiation (Senogles and Thomas 1975). Poly(N-vinyl-2-pyrrolidone) (PVP) is a polymer with great potential applications in different areas of biomedicine due to its ex-

N-Vinyl-2-pyrrolidone (NVP)

N-Vinyl-2-pyrrolidone (NVP)

Modified Pap
Fig. 8. Molecular structure of N-vinyl-2-pyrrolidone (NVP) and ultraviolet (UV)-induced grafting polymerization
160 140120100 8060 4020

A

B

__V--

-

-

--V

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Concentration of BSA, g/L

Induced Grafting

Fig. 9. a Effect of grafting degree (GD) on BSA adsorption in PVP-modified PPMMs. • nascent PPMM; O PVP-modified PPMM GD = 1.49 wt. % and A PVP-modified PPMM GD = 4.08 wt. %. b Static contact angle of PVP-modified PPMMs with different grafting degrees

10 12 14 16 18 20 Grafting degree (wt.%)

Fig. 9. a Effect of grafting degree (GD) on BSA adsorption in PVP-modified PPMMs. • nascent PPMM; O PVP-modified PPMM GD = 1.49 wt. % and A PVP-modified PPMM GD = 4.08 wt. %. b Static contact angle of PVP-modified PPMMs with different grafting degrees cellent biocompatibility with living tissues and extremely low cytotoxicity (Wetzels and Koole 1999). Due to its outstanding performance, PVP has also been used widely as a surface-modification agent. Liu et al. (2004) used PVP as modifier to improve the surface properties of PPMMs by UV-induced graft polymerization of N-vinyl-2-pyrrolidone, as shown in Fig. 8. After grafting PVP to the membrane surface, as been expected, protein adsorption was markedly reduced with increasing grafting degree (Fig. 9). Usually, materials processing hydrophilic surface show relatively low nonspecific adsorption for proteins or cells. Therefore, the reduction in BSA adsorption could be ascribed mainly to the improvement in hydrophilicity effected by the introduction of PVP chains on the PPMM surface.

Yang et al. (2005) grafted a novel glycopolymer containing linear sugar moieties, D-gluconamidoethyl methacrylate (GAMA) (Fig. 10), to the PPMMs by UV-induced graft polymerization and obtained a hydrophilic surface with low protein fouling and relatively high flux recovery (Table 2). After surface modification, the antifouling property was evaluated by filtrating a 1 g/L BSA solution through the membranes. Pure water flux after cleanout was also examined to confirm flux recovery. The ethanol-wetted nascent membrane showed the largest loss of flux within the measurement time, which suggests that a large amount of BSA protein had been deposited on the surface. However, the reduction in flux was suppressed by grafting of GAMA, indicating that the GAMA polymer layer effectively prevented the

OH OH

OH OH

Fig. 10. Molecular structure of GAMA

Fig. 10. Molecular structure of GAMA

Table 2. Permeation and antifouling properties of D-gluconamidoethyl methacrylate (GAMA) grafted polypropylene microporous membrane (PPMMs). JW Flux of pure water, JP flux of 1 g/l bovine serum albumin solution, JR flux after cleanout, RFR relative flux reduction as (1 - JP/JW) x 100%, FRR flux recovery ratio as JR/JW x 100%

Table 2. Permeation and antifouling properties of D-gluconamidoethyl methacrylate (GAMA) grafted polypropylene microporous membrane (PPMMs). JW Flux of pure water, JP flux of 1 g/l bovine serum albumin solution, JR flux after cleanout, RFR relative flux reduction as (1 - JP/JW) x 100%, FRR flux recovery ratio as JR/JW x 100%

Membrane

Jw (kg/m2 h)

Jp (kg/m2 h)

(kg/m2 h)

RFR

FRR

Ethanol-wetted

340 ±7

93 ± 22

219 ± 22

73

64

2.23 wt% GAMA grafted

466±26

187±24

390±26

60

84

3.5 wt% GAMA grafted

608±21

348±37

488±38

57

80

4.58 wt% GAMA grafted

729±14

452±38

646±30

38

89

5.47 wt% GAMA grafted

762±22

434±32

658±33

57

86

6.03 wt% GAMA grafted

764±28

452±25

649±35

59

85

adsorption of BSA. Moreover, the recovery flux increased significantly with increasing grafting degree, and a relatively high flux recovery ratio (> 80%) was achieved even at lower grafting degree. All of these improvements in surface properties can be ascribed to the highly hydrophilic nature of the linear sugar moieties in the grafted chains.

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