Structure, rheology, and 3D printing of salt-induced κ-carrageenan gels

κ-carrageenan is a seaweed-derived polysaccharide commonly used in the food and pharmaceutical industry as a thickening agent and rheological modifier. The biopolymer has shown promising potential as a gel-based ink for 3D printing applications. However, its application is limited due to its poor structural stability and mechanical strength. In this work, we propose to overcome the printing limitation by optimizing the ink formulation of salt-induced κ-carrageenan gels. The gelation of 0.25–2% w/v κ-carrageenan with gelling agents of potassium chloride (KCl), calcium chloride (CaCl2), and a combination of KCl and CaCl2 salts was studied using rheological and SANS measurements, and SEM and AFM imaging. A state diagram for the κ-carrageenan-salt samples reveals that the sol-gel transformation shifts to lower κ-carrageenan concentrations with the 25 mM salts. The rheological measurements show a synergistic increase in the storage modulus of κ-carrageenan by the addition of equimolar mixed salts of KCl and CaCl2. The addition of 25 mM KCl or CaCl2 significantly improved the printability of 1% w/v κ-carrageenan to form thin and continuous filaments. However, complex shapes consisting of cylinders, polygons, and multilayer structures could successfully be 3D printed with only the 1% w/v κ-carrageenan with 25 mM KCl gel ink. Live/dead assay of human lung cancer A549 cells seeded on the printed structures show the excellent biocompatibility of these gels.