Improving alginate printability with alternative cationic pre-crosslinkers: strontium-based bioinks and their impact on bioprinted cartilaginous tissues

Extrusion 3D bioprinting holds promise for a broad range of tissue engineering applications; however, the development of bioinks with optimal physical and biological properties remains an unmet challenge. Alginate is a commonly used bioink base polymer and is often prepared by pre-crosslinking with Ca 2+ to ensure shape fidelity when printing. This study explores the use of alternative divalent cations as pre-crosslinking agents. Cations with high binding affinities to alginate are found to improve the printability of bioinks. In particular, Sr 2+ pre-crosslinked bioinks support high cell viability and enhance the bioink's shape retention, while lowering the shear force required for flow, when compared to Ca 2+ . When evaluating printed constructs pre- and post-crosslinked with Sr 2+ and Ca 2+ in matched and mismatched combinations, it is determined that altering the pre-crosslinker influences the degradation rates and mechanical properties of constructs, as well as the responses of encapsulated cells. Analysis of cation retention finds that the pre-crosslinker is substantially replaced by the post-crosslinking agent, suggesting that the pre-crosslinking agent may impact the construct properties through effects that persist after substantial crosslinker substitution. The results of this study indicate that the choice of pre-crosslinker for alginate bioinks offers a simple method of tuning the properties of bioinks and bioprinted tissue constructs.