Biomedical Engineering Reference
In-Depth Information
Immobilized Soluble Enzymes
Entrapped
Bound
Covalently Bonded
Adsorbed
(Physical
or ionic)
Matrix-
Entrapped
Membrane-
Entrapped
To Support
To Enzyme
Between
Macroscopic
Membranes
Microscopic
Encapsulated
E
E
E
E
E
E
E
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E
E
E
E
E
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E
E
E
E
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E
FIGURE 8.17
Major immobilization methods.
of the intracellular enzymes are membrane bound, immobilized enzymes provide a model
system to mimic and understand the action of some membrane-bound intracellular enzymes.
Product purity is usually improved, and effluent handling problems are minimized by
immobilization.
8.3.1. Methods of Immobilization
Major methods of immobilization are summarized in
Fig. 8.17
. The two major categories
are entrapment and surface immobilization.
8.3.1.1. Entrapment
Entrapment is the physical enclosure of enzymes in a small space. Matrix entrapment and
membrane entrapment,
including microencapsulation, are the two major methods of
entrapment.
Matrices used for enzyme immobilization are usually polymeric materials such as
Ca-alginate, agar,
-carrageenin, polyacrylamide, and collagen. However, some solid
matrices such as activated carbon, porous ceramic, and diatomaceous earth can also be
used for this purpose. The matrix can be a particle, a membrane, or a fiber. When immobi-
lizing in a polymer matrix, enzyme solution is mixed with polymer solution before poly-
merization takes place. Polymerized gel-containing enzyme is either extruded or
a template is used to shape the particles from a liquid polymer
e
enzyme mixture. Entrap-
ment and surface attachment may be used in combination in some cases.
Membrane entrapment of enzymes is possible; for example, hollow-fiber units have been
used to entrap an enzyme solution between thin, semipermeable membranes. Membranes of
nylon, cellulose, polysulfone, and polyacrylate are commonly used. Configurations, other
than hollow fibers, are possible, but in all cases, a semipermeable membrane is used to retain
high molecular weight compounds (enzyme), while allowing small molecular weight
compounds (substrate or products) access to the enzyme.
A special form of membrane entrapment is microencapsulation. In this technique, micro-
scopic hollow spheres are formed. The spheres contain the enzyme solution, while the sphere
k
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