High-Fidelity Direct Ink Writing of Poly(vinylphosphonate)-Reinforced Polysaccharide Inks with Tunable Properties

Herein, a concept for the structural reinforcement of nanofibrillated cellulose (NFC)/alginate (Alg) formulations for direct ink writing (DIW) applying photochemical cross-linking of poly(vinylphosphonate) copolymers is introduced. Ultraviolet (UV-A) irradiation (one second per layer) enables the covalent cross-linking of statistical copolymers consisting of diallyl vinylphosphonate (DAlVP) and diethyl vinylphosphonate (DEVP) via thiol–ene click chemistry, giving access to high shape fidelities and sophisticated structures. The polymers were obtained via rare earth metal-mediated group-transfer polymerization (REM-GTP), allowing excellent control over molecular weights and copolymer composition with narrow polydispersities. Optimized ink formulations include 9 wt % P(DEVP-stat-DAlVP) dissolved in a 3 wt % aqueous nanofibrillated cellulose (NFC) suspension with roughly 3 wt % of sodium alginate, 1,6-hexanedithiol as cross-linker, and a photoinitiator. The shear-thinning behavior of this formulation, confirmed by rotational rheology, allowed extrusion-based DIW to create various objects, demonstrating the high shape fidelity and printing stability of these inks. Specimens printed from polymer-containing NFC/Alg inks were investigated regarding water uptake and mechanical properties, revealing an improvement of the swelling shape fidelity of printed objects in CaCl2-containing physiological NaCl solution by combining chemical cross-linking and ionic cross-linking. Furthermore, the respective material properties correlated with the cross-linking density induced by the reinforcement with synthetic polymers. Finally, the pristine printed objects revealed only slight cytotoxicities toward primary human umbilical vein endothelial cells (HUVECs) and immortalized human embryonic kidney cells (HEK293 cell line).