Effect of dual crosslinking on physico-chemical properties of hydrogels prepared from chitosan and alginate

Hydrogels have established their utility in the field of biomedical science and technology including drug delivery and tissue engineering, among other applications. Crosslinking density critically affects the resultant
physical property of the hydrogels. Here, we have successfully synthesized carboxymethylchitosan (CMC) and
oxidized alginate (AA) from chitosan and sodium alginate, respectively. CMC and AA were used to fabricate
CMC-AA-single network (CMC-AA-SNH) and CMC-AA-double network (CMC-AA-DNH) hydrogels. Crosslinking of CMC-AA-SNH was done by dynamic covalent bonding, that is, imine bond formation, whereas CMC-AA-DNH was crosslinked via covalent imine bond and Ca2+ mediated ionic interactions. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) studies were employed to
characterize the components of the hydrogels. Effect of dual crosslinking over the single crosslinked hydrogel
was extensively analyzed by rheological studies. Scanning electron microscopy revealed that the CMC-AA-DNH
was more densely packed with interconnected structure than CMC-AA-SNH. Swelling study demonstrated that
the degree of swelling of CMC-AA-DNH was significantly less than CMC-AA-SNH due to more crosslinking density. Compressive mechanical test of the hydrogels further indicated that CMC-AA-DNH exhibits fracture stress of 79.5 kPa. These results indicate how the physical and mechanical properties of a polymeric hydrogel system can be tuned through control of crosslinking, which have important implications for the use of these gels for biomedical applications.