Optimization of the FRESH 3D Printing Method Applied to Alginate – Cellulose-Based Hydrogels

In recent years, a new additive manufacturing (AM) method for three-dimensional (3D) syringe-extrusion (bio)printing of soft hydrogels has been introduced under the name of Freeform Reversible Embedding of Suspended Hydrogels (FRESH). The most common FRESH bath contains gelatin as the main compound and low concentrations of crosslinker(s) (whose nature depends on the hydrogel) for the initiation of an in-situ pre-crosslinking process during printing. In the case of sodium alginate (SA)-based hydrogels ionically crosslinked via calcium chloride (CaCl₂), the crosslinker percentage in the gelatin bath is equal to ~10 mM, usually combined with a post-crosslinking at higher concentrations. However, according to the best authors’ knowledge, no study has ever reported the combined effect of pre- and post-process crosslinker concentrations over printability using the FRESH method. Therefore, this manuscript aims to systematically investigate via a full-factorial design, the printing of two low viscous SA – cellulose-based hydrogels, containing carboxymethylcellulose (CMC) or micro-fibrillated cellulose (mFC). Different concentrations of CaCl₂ are selected for the gelatin-based FRESH support bath (10, 30, or 50 mM) and for the post-crosslinking process (0.0, 1.0, or 3.0 w/w%). A printability index Pr is chosen as the response of interest and further compared against swelling and water uptake ratios. Eventually, the overall best performances were obtained for SA-CMC at CaCl₂ (30 mM + 1.0 w/w%), with a Pr = 1.021, swelling and water uptake ratios at 48h equal to 5.99 and 88%, respectively. Therefore, this work offers new insights over the control and optimization of the crosslinker concentration in the FRESH method.