3D Printable Hydrogel Based on TEMPO-Oxidized Cellulose Nanofibrils and Fmoc-FF for Enhanced Biological Performance and Cell Adhesion

Herein, we report a 3D printable ink made of a peptide-polysaccharide hybrid hydrogel composed of fluorenylmethyloxycarbonyl-diphenylalanine (Fmoc-FF) peptide and TEMPO-oxidized cellulose nanofibrils (ToCNF), synthesized using a pH-dependent sol–gel transition method. The ToCNF suspension is synthesized through the mechanical breakdown of a cellulose pulp using a microfluidizer, followed by its oxidation mediated with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). The properties of the hybrid inks are compared in the presence (ToCNF/Fmoc-FF-Ca²⁺) and absence (ToCNF/Fmoc-FF) of the divalent cation Ca²⁺, which acts as the cross-linker, at two optimized weight ratios (r) of ToCNF and Fmoc-FF (r = 4.5 and 6.5). The rheological measurements show that the yield strength of the ToCNF/Fmoc-FF-Ca²⁺ gel is almost double that of the hydrogel composite without Ca²⁺ ions, especially at the concentration (C) of 10 mM CaCl₂. This finding is further verified by 3D gel printing, which produced good quality prints with the cation cross-linked hydrogel. The structural analysis by Field Emission Scanning Electron Microscopy shows that the calcium ions can cross-link the ToCNF and also enhance the self-assembly of Fmoc-FF, which leads to the formation of rigid compact nanofibers even at physiological pH. The electrostatic interaction of the positively charged Ca²⁺ ions onto the negatively charged surface carboxylate groups of ToCNF and Fmoc-FF is analyzed by zeta potential (ζ) measurements. Small-angle X-ray scattering measurements give deeper structural insights into the interaction of Fmoc-FF with ToCNF. Cell responses to the hydrogels are studied in human dermal fibroblasts (NHDFs) in a direct contact test using a live/dead assay and in extract test using Alamar Blue and lactate dehydrogenase assays. The results show that high loading of Fmoc-FF decreases cell viability, while additional cross-linking with calcium reduces this cytotoxic effect.