Mussel-Inspired Adhesive Nano-Filler for Strengthening Polyacrylamide Hydrogel

Hydrogels are emerging as one of the most attractive water-containing polymeric materials for many biomedical and industrial engineering applications. Cellulose nanocrystals (CNC) are often used as filler material to overcome the drawbacks of conventional hydrogels. However, weak frictional interaction of CNC with the polymeric backbone of hydrogels in the presence of water has limited its applications. Herein, we report a mussel-inspired preparation of a new bio-based nano-filler by grafting dopamine (DOPA) over the carboxylated cellulose nanocrystal (CCN), which has been obtained by oxidizing CNC extracted from sawdust. The swelling of polyacrylamide (PAM) hydrogels has been significantly suppressed by the incorporation of the modified filler. In contrast, the mechanical properties like Young's modulus, tensile strength, and toughness of PAM hydrogels increased remarkably indicating a strong interaction of CNC-DOPA with the PAM chains. For instance, the tensile strength increased from 19 kPa for the CNC-PAM to 39 kPa for the CCN-DOPA-PAM. Interestingly, insertion of only 1 % (w/w) CCN-DOPA has led to the enhancement of the mechanical strength of PAM even more than that caused by the insertion of 4 % pristine CNC. The spectroscopic data along with the morphological analysis of the composite hydrogels suggest strong hydrogen bonding of DOPA moiety and the PAM backbone is responsible for this incredible improvement of the mechanical strength.