Polymer-Modified Cellulose Nanofibrils Cross-Linked with Cobalt Iron Oxide Nanoparticles as a Gel Ink for 3D Printing Objects with Magnetic and Electrochemical Properties

This paper presents a strategy to convert hydrophilic cellulose nanofibrils (CNF) into a highly cross-linked hydrophobic network with inorganic nanoparticles to develop a gel ink suitable for gel 3D printing. The CNF were chemically modified initially through a single-electron transfer-living radical polymerization (SET-LRP) of stearyl acrylate (SA) in the presence of the surface-modified cobalt iron oxide (CoFe2O4, CFO) nanoparticles. The modified CFO nanoparticles provide their multifunctional properties, such as magnetic and electrochemical, to the CNF hybrid network and, at the same time, act as cross-linking agents between the nanocellulose fibrils, while the grafted poly-stearyl acrylate (PSA) introduces a strong hydrophobicity in the network. A suitable gel ink form of this CNF–PSA–CFO material for gel 3D printing was achieved together with a certain solvent. Some test structure prints were directly obtained with the CNF–PSA–CFO gel and were used to evaluate the consolidation of such 3D objects through solvent exchange and freeze-drying while also keeping the magnetic and electrochemical properties of CFO in the CNF-based composite intact. The pristine CNF and CFO particles and the CNF–PSA–CFO were characterized by FTIR, SEM, XPS, TGA, VSM, and CV measurements.