Freeze-concentrated layers as a unique field for the formation of hydrogels

The phase separation of ice crystals and solutes and bound water that occurs during freezing can be used as a reaction field to control a hierarchical structure of hydrogels. Here, we present a study of carboxymethyl cellulose nanofiber (CMCF) hydrogels formed using solid-quasi liquid phase separation. CMCF hydrogels were formed simply by adding citric acid to frozen CMCF and thawing the mixture. The compressive strength of the freeze crosslinked CMCF was 200,000 times higher than that of CMCF hydrogel formed by a conventional crosslinking method. It was found that rearrangement of CMCF structures via hydrogen bonding proceeds in the freeze concentration layer before the ice crystals melt. Under freeze concentration, CMCF and bound water are confined at high concentrations. The crosslinking reaction in such a unique space contributed to the formation of CMCF hydrogel with high mechanical strength. We discuss the gelation behavior and properties of freeze crosslinked CMCF hydrogels and their applications. (author)