Encapsulation [Microencapsulation ; Membrane Confinement]

2.4. Encapsulation [Microencapsulation ; Membrane Confinement]

Encapsulation or microencapsulation or membrane confinement is another versatile and ef- fective approach in the field of enzyme immobilization specifically by entrapping technique. In this method the enzyme molecules, invariably taken up in an aqueous medium, may be strategically confined very much within a semipermeable membrane that ideally permits an almost absolute ‘free movement’ of the enzymes in either direction to the products and substrates but fails to allow their migration and escape, eventually as depicted in Fig. 5.10(a) and (b). In other words, the enzyme immobilization pre- vailing in encapsulation method predominantly occurs well within the microcapsules meticulously prepared from organic polymers in order that the ensuing enzymes are prevented from the ‘great escape’ ; and, of course, the comparatively low molecular weight products and substrates do have a tendency to either enter or leave the ‘capsule’ by diffusion via the membrane sooner or later.

E E SEMIPERMEABLE MEMBRANE

MEMBRANE CONFINEMENT [OR MICROENCAPSULATION]

ENCAPSULATION

Fig. 5.10. Enzyme Immobilization : (a) Encapsulation ; (b) Membrane confinment (or Microencapsulation).

In actual practice, there are two well-known general methods for preparing the membranous capsules for enzyme entrapping, namely :

(a) Phase Separation : Membranes are usually made by adopting the process of phase-separa- tion, that essentially bears a close resemblance to homogenization of water in oil. In this particular instance one phase is obviously not miscible with the other but eventually gives rise to a droplet with the other phase upon adequate mixing. Thus, ultimately the ‘enzyme’ gets entrapped right within this droplet, and

(b) Chemcical Polymerization : The chemical polymerization aids in the preparation of the specific water-insoluble membrane, and thus the enzyme in question gets duly entrapped during this on-going phenomenon of polymerization.

Examples : The various typical examples are as follows : (i) Semipermeable collodion or nylon membranes in the shape of spheres (round beads) are

invariably utilized for the microencapsulation of an enzyme. These materials are also

PHARMACEUTICAL BIOTECHNOLOGY

(ii) Fibres of celluclose triacetate may also be employed for the entrapment of enzymes within this synthetic material. However, these fibres may be either woven into a suitable fabric or packed into the columns carefully.

Choice of Method : The choice of method exclusively depends upon the degree of immobilization corresponding to the specific enzyme involved, and its subsequent application. In fact, a rather broad spectrum of variation has been duly cited in the literature with respect to the actual level of retention of the ensuing enzyme activity (%) upon the immobilization to various commercially available support materials as summarized under :

Enzyme

Support

Method of Enzyme

Retention of Enzyme

Activity (%) Aminocyclase

Immobilization

AE-Cellulose

0.6 CN Br-Activated Sephadex Glutaraldehyde

Cross-linked with

Covalent Bonding

CM-Sephadex C-50*

Ionic Bonding

55 DEAE-Sephadex A-59

53 *Iodoacetyl Cellulose

Entrapment

[Adapted From : Fermentation Technology Today, 1972]

A plethora of viable, feasible, and tangible strategies have been judiciously used for carrying out the phenomenon of encapsulation (membrane confinement) that may be enumerated briefly as under :

(1) Bioreactor or ‘reaction vessel’ may be partitioned into two separate compartments by means of a semipermeable membrane ; while one chamber contains the ‘enzyme’ and the other has either the substrate or the product.

(2) Hollow fibre membrane units mostly contain the enzyme strategically located in their lu- men or hollow space, and are adequately submerged in the ensuing substrate. Importantly,

this sequence of events caters for an extremely large surface area per unit volume viz., greater than 20 m 2 .L –1 , but is found to be quite beneficial significantly for such substrates which are definitely much smaller in size and dimension in comparison to the corresponding enzyme molecules. Nevertheless, the hollow fibres are quite expensive, and also may be employed with a variety of enzymes even the so-called coenzyme-regenerating systems.

(3) The desired and selected enzymes may be skillfully packed right into the ‘microcapsules’ duly formed but the aid of a typical polymerization reaction*, such as : usage of 1, 6-

diaminohexane.

In conclusion, one may critically observe that each immobilization strategy does possess certain strengths and weaknesses that may be adequately summarized in Table 5.1. below :

* Besides, these enzymes may be entrapped within liposomes which are nothing but small spherical materials composed of concentric lipid membranes.

ENZYME IMMOBILIZATION