pH-responsive swelling behavior of collagen gels prepared by novel crosslinkers based on naturally derived di- or tricarboxylic acids

The aim of this study was to compare the physicochemical properties of alkali-treated collagen (AlCol) gels prepared using two kinds of naturally derived crosslinkers made from citric and malic acids (CAD and MAD, respectively) that we have developed. From the crosslinking reaction between active ester groups and amino groups of AlCol, we successfully obtained AlCol gels, named AlCol-CAD and AlCol-MAD, prepared using CAD and MAD, respectively. The gelation time of the AlCol solution containing CAD initially decreased with increasing CAD concentration up to 70 mM, and then increased as the CAD concentration increased further. The gelation time reached its minimum and began to increase. On the other hand, for AlCol-MAD solution, gelation occurred within 40 s at any MAD concentration. Moreover, the residual amino groups in AlCol-CAD and AlCol-MAD were found to decrease with increasing CAD or MAD concentrations, whereas increased residual carboxyl groups were detected only in the case of AlCol-CAD. The swelling ratio of AlCol-CAD significantly increased at CAD concentrations above 50 mM. On the other hand, AlCol-MAD showed little increase in swelling ratio with increasing MAD concentration. Also, AlCol-CAD was swollen when the gels were immersed in a solution with high pH. On the other hand, no significant increase in swelling ratio was observed when AlCol-MAD was immersed in a similar solution. These results suggest that the different amounts of carboxyl groups in AlCol-CAD affected the swelling behavior of gels and that this pH-responsive AlCol-CAD has potential for drug delivery systems and tissue engineering.