3D printing of a bio-based ink made of cross-linked cellulose nanofibrils with various metal cations.

In this work, we present an approach to cross-link cellulose nanofibrils (CNFs) with various metallic cations (Fe, Al, Ca, and Mg) to produce inks suitable for three-dimensional (3D) printing application. The printability of each hydrogel ink was evaluated, and several parameters such as the optimal ratio of M:TOCNF:HO were discussed. CNF suspensions were produced by mechanical disintegration of cellulose pulp with a microfluidizer and then oxidized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Finally, metal cations were introduced to the deprotonated TEMPO-oxidized CNF (TOCNF) suspension to cross-link the nanofibrils and form the corresponding hydrogels. The performances of each gel-ink were evaluated by rheological measurements and 3D printing. Only the gels incorporated with divalent cations Ca and Mg were suitable for 3D printing. The 3D printed structures were freeze-dried and characterized with Fourier transform infrared spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM). The better interaction of the TOCNFs with the divalent metallic cations in terms of printability, the viscoelastic properties of the inks, and the variation trends owing to various metal cations and ratios are discussed.