Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 199

R 1

O

O

NH

+R 1 NC R 2

OH

R
Carboxylic group
R = Hyaluronic acid
or gelatin

OC

R

EDC

N

R 2

R 1 R 1

O

NH NH

+ +

O C HO N RCON O C

R

N

NH

O

R 2

R 2

+ +

RCO N H 2 N R ′ RCNH 2 R ′ HO N

O

O Free amide groups

R = Gelatin or chitosan Amide cross-link

Figure 4.12
Mechanism of covalent attachment of HA to gelatin and chitosan using EDC and NHS. R 1 =-CH 2 -CH 3 ,
R 2 =-(CH 2 ) 3 -NH + -(CH 3 ) 2 C 1 . (From Liu, H. F. et al. 2004. Biomaterials 25: 3523-3530. With permission.)

show excellent cytocompatibility for fibroblast [149]. Heparin-functionalized chitosan-
based scaffolds are formed via immobilizing the heparin on the chitosan-based network
using a special cross-linker. In general, carbodiimide is used because of the existence of
carboxylic acid groups in heparin molecules. Heparin can be immobilized on the chitosan/
collagen films using the EDC/morpholinoethane sulfonic acid (MES) cross-linker system.
The addition of heparin to the EDC cross-linking solution allows the remaining amino
groups of collagen or chitosan to react with the carboxylic acid groups of heparin. The
tensile strength and Young's modulus of the chitosan/collagen/heparin network decrease
with the addition of heparin. With the introduction of heparin, more carboxyl groups are
provided to take part in the cross-linking reaction with the amino groups. Most of the
heparin molecules may not infiltrate into the inner part of the collagen/chitosan films and
cross-linking may happen only on the surface of the films. Thus incorporation of heparin
only on the surface of collagen/chitosan films can invoke adverse effects on the mechani-
cal strength of the films. However, the mechanical strength of the collagen/chitosan/hepa-
rin film is enough to withstand the forces incurred during cell culture [150].
Mi and coworkers [151] conjugated heparin into chitosan-alginate PEC scaffolds using
EDC. In the conjugation process, carboxylic acid groups of heparin are preactivated using
EDC/NHS, followed by reaction of the activated heparin with the amine groups on
(cf. h i t o is a n ( cf. Figure 4.13). bFGF is incorporated into these scaffolds through bioaffinity
with heparin. The release rate can be controlled by controlling the immobilized heparin
concentration and the released bFGF from the scaffold retains its biological activity.