Enzyme Immobilization on Chitosan-Based Supports - Chitosan Based Hydrogels: Functions and Applications - page 344

complex network prepared by a mixture of polycation and polyanion and the layer-by-
layer (LBL) self-assembly system formed by sequential deposition of interactive polymers
from their solutions. Self-assembly of matter is of fundamental importance in different
fields, including life sciences. It is widely used to describe the phenomenon of self-organi-
zation. LBL self-assembly is also used in the chitosan organic-inorganic hybrid system,
such as self-assembly of chitosan and nano-Au (see Section 8.5.4.3) . In addition, based on
the above sol-gel approach from ref. [15], chitosan hybrid gelling is developed by the use
of organosilane agents that are often regarded simply as cross-linkers.
It is an easy method to directly tether or coat natural macromolecules on the surface of
the support to form a biomimetic layer for enzyme immobilization. It is considered as an
effective method to increase the biocompatibility of synthesis polymers or inorganic sup-
ports, by introducing a biofriendly interface on the support surface for enzyme immobili-
zation through surface modification technologies, which may reduce some nonbiospecific
enzyme-support interactions, create a specific microenvironment for the enzyme, and
benefit the enzyme activity. In comparison with the inorganic supports, the synthesis
polymers are easier to entrust functional groups for chemical reactions. In fact, this method
has been widely used in tissue engineering very recently.
Moreover, chemical modification methods for chitosan supports have been used to improve
stability, mechanical strength, and modification of different functional groups that may be
superior for enzyme immobilization. For example, hydrophobic modification of the hydro-
philic chitosan backbone allows for the formation of micellar structures that have been
shown to be attractive for enzyme encapsulation, often increasing thermal tolerance, chemi-
cal tolerance, and long-term stability. Furthermore, one of the ways to improve these proper-
ties is the grafting of monomers onto the matrix. The properties of the resulting graft
copolymers are broadly controlled by the characteristics of the side chains, including molec-
ular structure, length, and number. To date, a number of research works have been published
that aim to study the effects of these variables on the grafting parameters and the properties
of grafted chitosan polymers. In addition, two kinds of enzyme-chitosan conjugates that can
further form gel or immobilize on the support were approximately considered as one of the
chitosan modifications and are also described in this chapter (see Section 8.4.3).
8.3 Chitosan Gel for Enzyme Immobilization
Commonly, methods of chitosan gel preparation can be divided into four groups: the sol-
vent evaporation method, the neutralization method, the chemical cross-linking method,
and the ionotropic gelation method [2]. In this section, the phase-inversion technique is
described by combining the neutralization method and solidification by a nonsolvent
such as ethanol. In addition, several new methods are also briefly presented. For example,
chitosan cryogels based on physical cross-linking were obtained by freezing-thawing
repeated treatments. Stable chitosan films were fabricated via the electrodeposition
method. Nanofibers were produced through the electrospinning technique.
8.3.1 Solvent evaporation
The method is mainly used for the preparation of membranes and films, the latter being
especially useful in preparing minute enzymatically active surfaces deposited on the tips