Hydrogels are mostly used in biomedical sciences such as tissue engineering, drug delivery, and in vitro diagnostics. Nanocellulose hydrogels have multiple advantages because of their biodegradability, non-toxic properties, high water content and inherent biocompatibility. Moreover, 2,2,6,6-tetramethylpiperidinyl-1-oxyl(TEMPO)-oxidized cellulose nanofibrils (TOCNs) are completely individualized cellulose nanofibrils which have high aspect ratios, high elastic moduli, high strengths, high thermal stability, large surface areas, and large numbers of carboxylate groups on their surfaces. However, hydrogels derived from TOCNs are not robust and inherently water unstable if the TOCNs are not crosslinked or coated with a water-swellable polymer. Furthermore, the manufacturing of self-standing TOCN films is still a challenge due to the small TOCN diameter and viscosifying effect.<br/><br/>In this work, we report the TEMPO-mediated oxidation of bacterial cellulose (BC) nanopaper as a route to produce robust and water stable TOCN hydrogel without the need for additional additives or crosslinking steps, as well as self-standing TOCN films without the need for vacuum filtration or slow-drying of TOCN suspension. Pristine BC pellicle was first press-dried into a dried and well-consolidated BC nanopaper, followed by TEMPO-oxidation at various NaClO concentrations. The oxidation reaction introduced carboxylate moieties onto exposed BC nanofibrils within the nanopaper network structure. This then led to the swelling of the nanopaper into a hydrogel. A swelling ratio of up to 100 times the original thickness of BC nanopaper was observed upon TEMPO-oxidation. The water retention value of the TEMPO-oxidised BC hydrogels was also found to increase with increasing carboxylate content. These TEMPO-oxidised BC hydrogels were found to be robust and water-stable, even under prolonged (&gt;1 month) magnetic stirring in water. This greatly expands the application of nanocellulose hydrogels. We further showed that high grammage self-standing TOCN films (100 g m-2) can be fabricated as simple as press-drying a water stable TEMPO-oxidised BC hydrogel without the need of vacuum-assisted filtration or slow-drying, which is typically the rate-limiting step in the manufacturing of self-standing TOCN films. These TEMPO-oxidised BC hydrogels and self-standing TOCN films have great application prospects in the field of biomedical materials.

TEMPO-oxidised nanocellulose hydrogels and self-standing films derived from bacterial cellulose nanopaper

ACS Spring 2022

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