A comprehensive protocol for hydrogel-based bioink design: balancing printability, stability, and biocompatibility

Bioink design is one of the most challenging and time-consuming tasks in 3D bioprinting. This study provides a comprehensive framework balancing key factors such as printability (evaluated through rheological analysis), scaffold mechanical stability, and biocompatibility for developing inks based on alginate (Alg), carboxymethyl cellulose (CMC), and gelatin methacrylate (GelMA). A detailed protocol is presented, outlining the sequence of rheological tests, selecting appropriate parameters, and correlating them with printability indices (<i>e.g.</i>, fiber diameter and printability value) as well as printing conditions (<i>e.g.</i>, temperature, cross-linking time, and degree). Optimal formulations were identified as 4% Alg, 10% CMC, and GelMA at 8%, 12%, and 16% concentrations (4% Alg-10% CMC-<b>GelMA</b>). Rheological and printability functions were quantified, establishing them as benchmarks for bioink design. The thermo-responsive properties of GelMA allowed precise control of printability by modulating temperature and GelMA content. A mathematical model was employed to correlate the shear-thinning behavior, measured <i>via</i> shear rheology, and printing conditions. These bioinks demonstrated long-term mechanical stability (up to 21 days), superior mechanical performance, and enhanced cell proliferation at 4% Alg-10% CMC-16% GelMA. The dual curing approach (UV curing and CaCl₂ cross-linking) resulted in scaffolds with variable stiffness, showcasing their potential for gradient tissue regeneration. Notably, the protocol is adaptable to other materials and concentrations, streamlining bioink development for diverse applications in gradient tissue engineering.