The surface of a material is the phase boundary that resides between the bulk material and the outer environment. The performance of materials relies largely upon the properties of the boundaries in many applications. For biological/biomedical applications, polypropylene membranes often suffer from grievous problems such as hydrophobicity, being chemical inert, and lacking functional groups. These disadvantages cause nonspecific protein adsorption and make PPMMs incapable ofenzyme immobilization. However, as we can see, graft polymerization of some polymers carrying various functional groups (such as -OH, -NH3, and -OOH) to the membrane surface is an effective way to conquer these shortcomings. We can choose different modifiers to confer upon the membrane different specialties for diverse applications. Hydrophilic polymers, such as poly(ethylene glycol), polyHEMA, and PAA) are usually used to reduce nonspecific protein adsorption because that protein adsorption is mediated mainly by hydrophobic interactions between the protein and the membrane surface. Nevertheless, it is more intricate for enzyme immobilization due to the complexity of the enzyme itself. Lipase has an affinity for hydrophobic surfaces; however, enzymes that catalyze redox reactions benefit from semiconducting materials. Modifiers must be adapted to a given enzyme in order to maintain its activity.

Acknowledgements. Financial support from the National Natural Science Foundation of China (Grant no. 20474054 and 20074033) and the National Basic Research Program of China (Grant no. 2003CB15705) are gratefully acknowledged. The authors thank Dr. Zhen-Mei Liu, Dr. Hong-Tao Deng, and Dr. Rui-Qiang Kou very much for their contribution.

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