(690a) Cation Exchange Strategy for Producing Functional Cellulose Acetate Sulfate Hydrogels and Their Potential Applications As Bio-Inks for 3D Printing

Cellulose has been attractive as promising materials to replace petroleum-based polymers given it is biodegradable and can be obtained and produced from renewable resources including wood, forest and agricultural residues, and even cellulosic wastes. Nevertheless, the intra- and intermolecular hydrogen bonding nature between adjacent hydroxyl groups attributing to the remarkable stability of cellulose brings about challenges when processing neat cellulose into film formation for packaging applications. To overcome this limitation, the hydrogen bonding nature of cellulose must be controlled, and several different methods have been developed based on this principle. Dissolving cellulose in DMAc/LiCl destroys all the hydrogen bonds allowing transparent cellulose film formation after cellulose regeneration. Cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) have more interaction capacity with water due to their disrupted hydrogen bond structure compared to pristine cellulose by mechanical shear force and strong acid hydrolysis, respectively. Another approach is a chemical modification of cellulose hydroxyl groups resulting in cellulose derivatives that can interact with solvent more easily than pristine cellulose.

Herein, functional cellulose acetate sulfates have been proposed based on the cation exchange of ammonium cellulose acetate sulfate (NH₄-CAS) synthesized from cellulose acetate. Briefly, NH₄-CAS was synthesized using cellulose acetate (DS_acetyl 1.8) and sulfamic acid as a sulfating agent. Depending on the sulfamic acid dosage, the degree of sulfonyl group of NH₄-CAS was controllable from 0.4 to 1.

To prepare sodium cellulose acetate sulfate (Na-CAS), the work-up of synthesized NH₄-CAS was carried out using ethanolic sodium acetate solution. The cation exchange of ammonium ion to sodium ion results in water-soluble Na-CAS. After a solvent exchange to water, heating Na-CAS in water at 60 °C allowed complete dissolution. Depending on the consistency of this solution and the degree of sulfonyl group, various Na-CAS hydrogels were fabricated and their rheological properties (shear-thinning, yield stress, and thixotropy) were evaluated. Among Na-CAS hydrogels, hydrogel prepared by 4 wt% of consistency of Na-CAS (DS_acetyl 1.8 and DS_sulfonyl 0.7) was suitable as a bio-ink for 3D printing. In addtion, because of water solubility, Na-CAS hydrogel cannot maintain its 3D structure when it is exposed to excess water. Soaking Na-CAS hydrogel in an aqueous calcium chloride solution gave further cation exchange from sodium ion to divalent calcium ion, resulting in ionic cross-linked calcium cellulose acetate sulfate (Ca-CAS).

If ethanolic silver acetate solution was employed rather than sodium acetate solution during NH₄-CAS work-up, cation exchange gave brownish silver cellulose acetate sulfate (Ag-CAS). Surprisingly, Ag-CAS was converted to silver nanoparticle cellulose acetate sulfate (AgNP-CAS) during storage by auto-reduction of silver ions without external reducing agents, NaOH as AgO₂ accelerator, heat, and light. This nature-inspired in-situ AgNP generation in CAS matrix was verified by analyses of the aqueous AgNP-CAS solution (DLS and UV-vis) and AgNP-CAS films (XPS, XRD, SEM-EDS, and TEM). The antibacterial activity of AgNP-CAS against Escherichia coli and Aspergillus niger was also evaluated.