High mechanical strength gelatin composite hydrogels reinforced by cellulose nanofibrils with unique beads-on-a-string morphology

This work prepared high mechanical strength gelatin composite hydrogels reinforced by cellulose nanofibrils with unique beads-on-a-string morphology. In detail, cellulose nanofibrils (H-Cel) with unique beads-on-a-string morphology were obtained by acid hydrolysis followed by intensive sonication. The D-H-Cel nanofibrils were prepared through oxidizing part of the non-esterified hydroxyl groups on the H-Cel into aldehyde groups. D-H-Cel were then mixed with gelatin and D-H-Cel/Gel composite hydrogels were produced. During the mixing, a giant network structure was constructed through the Schiff-base reaction between the aldehyde groups on the surface of D-H-Cel nanofibrils and the primary amino groups on gelatin macromolecular chains. Since the cellulose nanofibrils were covalently bonded to gelatin, the stress could be efficiently transferred between the reinforcing agent and matrix, resulting in a composite hydrogel with drastically increased mechanical strength. The compressive strength of D-40H-Cel/Gel hydrogel reached 3.398 MPa. SEM images showed a highly porous three-dimensional structure in the hydrogel with regulated pore size. The crosslinking indices were measured with ninhydrin assay. The composite hydrogels could maintain their shape well in buffer solution. The present work shows that natural polymer-based composite hydrogels with extremely high mechanical strength could be obtained by reinforcing with surface modified cellulose nanofibrils with unique beads-on-a-string morphology.