Biomimetic Modification

A potential technique for reducing protein adsorption on polymeric membranes is to mimic a biologic surface in nature, commonly named bio-mimetic modification. For example, the red blood cell plasma membrane, unlike synthetic polymer membranes, naturally resists protein fouling. This property may be attributed to the unique phospholipid bilayer structure of the biomembrane. Therefore, great efforts have been made to immobilize phospholipid molecules onto the polymer surface to build a biocompatible surface. To improve the physical and chemical stability, phospholipid molecules with polymerizable groups were synthesized. Researchers have synthesized many kinds of phospholipid-analogous polymers and developed various methods to immobilize them.

We have developed a PAN-based membrane containing phospholipid moieties to improve its protein resistance and hemocompatibility. Two methods were adopted to build the biomimetic surface containing phos-pholipid moieties. As illustrated in Figs. 15 and 16, one is copolymeriz-ing acrylonitrile with a polymerizable monomer containing phospholipid moieties, the other is to introduce phospholipid moieties onto the acry-lonitrile/HEMA copolymer (PANCHEMA) membrane surface by chemical reactions. The resultant membrane containing phospholipid moieties was denoted as a PMANCP membrane. Results from Fourier transfer infrared spectroscopy, x-ray photoelectron spectroscopy, and proton/phosphor nuclear magnetic resonance confirmed the chemical structure of the membrane containing phospholipid moieties.

Water contact angle was used to characterize the improvement of hy-drophilicity by the introduction of phospholipid moieties. Static contact angles as a function of contact time on the PAN, PANCHEMA, and PMANCP membranesareshowninFig. 17. Itcanbeseenthatthewatercontactangles on PAN and PANCHEMA membranes decreased slightly with time, whereas those on the PMANCP membrane decreased sharply. Furthermore, the water on the PMANCP membrane surface extended out in a few minutes. This was due to the interaction between the water and the polar group of zwitterion moieties on the membrane surface. Advancing and receding water contact angles are presented in Fig. 18. As reported by Ulbricht et al. (1998), grafting hydrophilic polymers on the PAN-based membrane surface only

Protein Adsorption
Fig. 15. Schematic representation of the synthesis of PAN containing phospholipid moieties. THF Tetrahydrofuran

had a slight effect on the contact angle; similar results were obtained from the measurements. However, the hydrophilicity was effectively improved by introducing the phospholipid moieties onto the PAN-based membrane, it can be seen from Fig. 18 that the dynamic contact angles on the phos-pholipid-moiety-modified membranes were obviously lower than on the PANCHEMA membranes.

According to the platelet-adhesion experiments, the hemocompatibil-ity of the PAN membrane was remarkably improved by the introduction of phospholipid moieties. This was also confirmed by the static protein adsorption, which is shown in Fig. 19. It was found that the amount of BSA adsorbed on the PAN and PANCHEMA membranes increased almost linearly with the increase in the concentration of BSA. However, differing

Schematic Protein Adsorption
Fig. 16. Schematic representation for introducing phospholipid moieties onto the acrylo-nitrile/2-hydroxyethyl methacrylate (HEMA) copolymer (PANCHEMA) membrane surface
Pan Membrane Modification

Fig. 17. Water contact angle as a function of contact time on the PAN-based membranes. The HEMA mole fraction in the PANCHEMA membrane is 0% (a), 6.4% (b), 9.3% (c), and 17.8% (d). The mole fraction of phospholipid moiety on the PANCHEMA membrane containing phospholipids (PMANCP) membrane surface is 6.09% (e), 9.19% (f), and 17.1% (g)

Fig. 17. Water contact angle as a function of contact time on the PAN-based membranes. The HEMA mole fraction in the PANCHEMA membrane is 0% (a), 6.4% (b), 9.3% (c), and 17.8% (d). The mole fraction of phospholipid moiety on the PANCHEMA membrane containing phospholipids (PMANCP) membrane surface is 6.09% (e), 9.19% (f), and 17.1% (g)

Fig. 18. Static (SCA, black columns), advancing (ACA, hatched columns), and receding (RCA, white columns) contact angles on the PAN-based membranes. The HEMA mole fraction in thePANCHEMAmembraneis0%(a), 6.4% (b), 9.3% (c), and 17.8% (d). The mole fraction of phospholipid moiety on PMANCP membrane surface is 6.09% (e),9.19% (f), and 17.1% (g)

Fig. 18. Static (SCA, black columns), advancing (ACA, hatched columns), and receding (RCA, white columns) contact angles on the PAN-based membranes. The HEMA mole fraction in thePANCHEMAmembraneis0%(a), 6.4% (b), 9.3% (c), and 17.8% (d). The mole fraction of phospholipid moiety on PMANCP membrane surface is 6.09% (e),9.19% (f), and 17.1% (g)

from those on the PAN and PANCHEMA, the adsorbance of BSA onto the PMANCP membranes increased slightly and stayed at a certain low level regardless of further increases in BAS concentration. All samples were immersed in the aqueous medium during the BSA adsorption measurement; in general, the relatively high level of free water fraction on the phos-pholipid-modified membrane surface might effectively suppress protein adsorption and platelet adhesion.

In addition to the aforementioned approaches, other methods such as blending can be used to obtain protein-resistant PAN-based membranes. To sum up, the successful development of a protein-resistant microporous membrane is an interesting issue for both water treatment membranes and blood-contacting membranes.

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