that confines the biological medium and isolates it from the rest of the system, and allows the separation of chemical entities either downstream from the membrane for the recovery of products, which maybe considered as valuable chemicals (recovery), or upstream in order to avoid parasitic inhibiting effects on the maintenance of the active biomass.
Work in this field is moving increasingly toward the evaluation of membranes that are able to fulfill two functions, namely efficient selective separation and catalytic reactivity, whereby the membrane catalyzes chemical reactions, the objective being to convert input products into output products that are either less toxic or have a greater value.
The biocatalysts, in this case the enzymes, have a place of choice in this prospective work, which profits from the contributions of other fields of biotechnology such as biocompatibility studies for the development of membranes for hemodialysis (Eltsefon et al. 1988; Ishihara et al. 1999), artificial organs (Schmidt 1996), or biosensors (Cosnier et al. 1999).
Various routes for enzyme immobilization will be discussed herein, as well as an evaluation of their respective performances when grafted by more or less soft chemical reactions (Green and Hill 1984) or attached by molecular recognition (Rao et al. 1999).
It has been suggested that the two essential criteria for discrimination are the specific activity of the enzymes and the lifetime of their activity compared to that of the same active enzymes in homogeneous medium (batch processing). Some work has shown that fibrous systems are good candidates for the development of systems that have properties that are comparable to those of catalytic membranes embedded with enzymes, and properties similar to those of filtration membranes (Freitas dos Santos et al. 1997). With this intention, it is necessary to have materials bearing chemical sites that are favorable for the attachment of enzymes. The sites involved in this chemistry are generally carboxylic acid, hydroxyls, and amino or quaternary ammonium groups, which are created on the surface of these porous materials by various means such as direct chemical surface treatment, plasma, or ultraviolet (UV) activation. The reactive sites thus created allow the attachment of the enzymes by using coupling reagents such as tosyl chloride, dicyclohexylcarbodiimide, and glutaralde-hyde. Approaches aiming at creating biocompatible environments consist of modifying the surface of polymeric filtration membranes by grafting of functional groups like sugars and polypeptides (Deng et al. 2004) and then to adsorb enzymes like lipase, the activity of which is preserved in such environment.
Another method of creating a biocompatible environment is biomimetic inspiration, which was shown to be effective for enzyme attachment and was derived from protein analysis techniques (enzyme-linked immunosorbent assay) and then used for the development of biosensors (Iqbal et al.
2000; Vo-Dinh and Cullum 2000). It utilizes the very strong and specific interaction between the small protein avidin for biotin. Because of its tetrameric structure, avidin has four sites of recognition that are accessible to biotin. Various proteins and enzymes are easily biotinylated, and this mode of enzyme grafting has already been used for electrode production as well as for membranes that are made up of conducting fibers. Fibers that are covered with a polypyrrole electron-conducting polymer layer (deposited by electropolymerization), similarly to bioenzymatic electrodes such as glucose oxidase biosensors (Coche-Guerente et al. 1994), are stabilized, thus increasing the lifetime of the molecular-recognition-type enzymes fixed by this immobilization procedure.
Another approach that was inspired by the biomimetic method consists of utilizing neutral and ion-exchanging textiles as a support for immobilization. The main advantage of these textiles is primarily their availability and the diversity of the reactor structures that are accessible. This approach will be discussed in the following review, where two cases will be presented: enzyme immobilization on insulating textiles and enzyme attachment on electron-conducting textiles. In the latter case, an electric conductor is used as the supporting material for the enzymes. The ability to introduce these new catalytic materials in a chain of coupled processes, especially electrocatalysis, will be emphasized.
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