3D Printing of Natural Polymer-based Hydrogel Scaffolds with Complex Geometries for Liver Regeneration

Partial or whole liver transplantation is the gold standard of treating patients with liver failure. But liver transplantation is hindered by severe limitations including availability of donor organ. In this regard, liver tissue engineering (LTE) based on 3D printing and bioprinting offers great promise for millions of patients. Extrusion 3D printing has been widely used for fabricating LTE scaffolds. However, the resolution of scaffolds constructed by extrusion 3D printing is normally low because it is highly influenced by nozzle inner diameter, platform accuracy, and ink characteristics. Improvements in extrusion 3D printing have thus mainly focused on the resolution of scaffolds and also multi-material printing. Hydrogels are important in LTE because their scaffolds can provide a suitable microenvironment for hepatic cells. Hydrogels of natural polymers such as polysaccharides are attractive as printing inks but they have poor mechanical properties. Using composite hydrogels can take advantage of constituent polymers and achieve desired mechanical properties. In this study, oxidized carrageenan, double-bond modified gelatin and carboxymethyl chitosan were mixed to create new inks for 3D printing of hydrogel LTE scaffolds. The aldehyde group of oxidized carrageenan and the amino group of gelatin and carboxymethyl chitosan formed Schiff base bonds, which are dynamic bonds that can form self-healing hydrogels. Hydrogels with self-healing ability are appealing for biomedical applications. Furthermore, the grafted double-bond gelatin enabled UV-crosslinking after 3D printing, which significantly enhanced mechanical properties of 3D printed hydrogel scaffolds. Inks of different constituent concentrations were investigated for printing the hexagonal honeycomb shape of liver micro-lobules with high precision. Detailed rheological studies of inks were performed, aiming for high printability. The structure and properties of 3D printed LTE scaffolds were characterized and assessed, and their in vitro degradation were studied. These 3D printed hydrogel scaffolds showed excellent biological properties with hepatic cells.