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Dual responsive composite hydrogels were successfully prepared by combining cellulose nanofibril (CNF) isolated by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation with thermally responsive poly( N-isopropylacrylamide) (PNIPAAm) for drug release at 10 wt% CNF loading and − 20 °C polymerization temperature. pH responsive hydrogels were acquired by adjusting the carboxyl charge level of the CNF during TEMPO-mediated oxidation. CNF-PNIPAAm hydrogels fabricated were characterized in regards to compressive strength, functional group, low critical solution temperature (LCST), and swelling ratio of the hydrogels at different temperatures from 20 to 60 °C and pH levels from 2 to 10. Finally, the drug release behavior of these hydrogels was also investigated using methylene blue as a model drug. As the carboxylate content increases, the dual responsiveness of hydrogel improved at the expense of the compression strength. The CNF-PNIPAAm hydrogels were swollen and translucent below the LCST, and shrunken and opaque above the LCST. The Higuchi and the Krosmeyer and Peppas model was best-fitted to the drug release behavior of these hydrogels at pH 10 and pH 2, respectively. The results also indicated that a proper selection of polymerization temperature provided a way of tuning the dual-responsiveness of the hydrogels. These results also suggest that the CNF-PNIPAAm hydrogels can release drugs on demand. [ABSTRACT FROM AUTHOR]

Cellulose microfibrils (CMFs) and cellulose nanofibrils (CNFs) were isolated from hardwood pulp by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation, which produced negatively-charged carboxylate groups on their surface. First, these CMFs and CNFs were used to prepare microgels and nanogels, respectively, at different concentrations of cellulose and trivalent Al cation by inducing ionic interactions between the negatively charged carboxylates and the metal cation. Then, two other cations (i.e., divalent Ca and monovalent H were employed to understand the structure-property relationship these hydrogels. We characterized their morphology, chemical groups, mechanical properties, surface area, and pore size, and evaluated their drug-release behaviors using theophylline. Compared to the hydrogels prepared from divalent or monovalent cations, both microgel and nanogel prepared from trivalent Al showed the highest stiffness and compressive strength, which indicated that they possessed the strongest ionic cross-linking via intra- and inter-fibrillar interactions. With a decrease in the valency of the cation used, the surface area of both hydrogels decreased, while their pore radius and calculated fibril diameter increased, indicating that a higher valency cation produced a hydrogel with higher porosity and a tighter network structure. The nanogel prepared from Al also showed the highest swelling ratio and the lowest release of theophylline, while that of microgel was, in contrast, consistent. The low total drug-release behavior in nanogels was attributed to their compact and highly porous structure. The Higuchi model was the best-fit model of drug release kinetics. These results indicate that the characteristics and internal structure of hydrogel has a great impact on its properties and drug-release profile, and that it may be possible to finely tune hydrogel properties and drug release profile by altering the internal structure of hydrogels during its preparation. [ABSTRACT FROM AUTHOR]