Mechanically strong and thermosensitive hydrogels reinforced with cellulose nanofibrils

To overcome the poor mechanical strength of thermosensitive hydrogels, cellulose nanofibril (CNF) was used as a reinforcing agent for the preparation of PEG-based thermosensitive hydrogels through radical copolymerization. The maximum bloom and rupture strength of the hydrogel were increased respectively with 37.7% and 52% at CNF content of 0.35 wt%. The dynamic rheological properties further confirmed that CNF played a significant role in the improvement of mechanical strength, especially in elasticity. Due to abundant hydroxyl groups exposed on the surface of CNFs, the obtained hydrogels exhibited a decreasing equilibrium swelling ratio (ESR) and deswelling rate in comparison with untreated hydrogels. However, the water retention ratio (WRR) increased when CNF content increased. In addition, both treated and untreated thermosensitive hydrogels possessed continuous volume phase transition with temperature ranging from 0 to 70 °C. Combined with SEM and ATR-FTIR tests, it was indicated that CNFs contributed to the formation of hydrogen bonding between the copolymer matrix and a tight physical lock by means of an entanglement effect. These composite hydrogels are considered as promising candidates in various potential applications of soft actuators, biosensors, artificial muscles and drug delivery.