Cyclic cryogelation: a novel approach to control the distribution of carbonized cellulose fibres within polymer hydrogels

The fabrication of conductive composite hydrogels is still challenging mainly because of the heterogeneous distribution of conductivity across the structure. Carbon substrates are mostly used to induce conductivity in the polymer hydrogels. The first step towards an effective induction of conductivity is to reach a uniform dispersion of carbon within the hosting matrix. This paper describes a new cyclic cryogelation method for the fabrication of conductive hydrogels using chitosan as the polymer matrix and conductive cellulose based fibres as the filler. This is achieved by the cyclic thermal treatment of gelatinized chitosan solution, and orderly control of the hydrogen bonding network formation before the solvent evaporation. These networks regularly limit the chain mobility of polymer across the hydrogel and restrict the agglomeration of fibres within the matrix. As a result, a significant improvement in the homogeneity of conductivity across the treated hydrogel was achieved, shown by thermal camera mapping. In addition, the thermally treated chitosan/20 wt% fibres demonstrated a tensile strength improvement of about 50% compared to that of the untreated composite. The proposed method offers a good control of fibre dispersion within the polymer matrix, and can serve as a practical design concept for 3D conductive hydrogels in biotechnological applications.