Gelatin-alginate hydrogel for near-field electrospinning assisted 3D and 4-axis bioprinting.

A near-field electrospinnable and three-dimensional (3D) bioprintable gelatin-alginate hydrogel was synthesized by controlling a moderate amount of alginate and a limited amount of crosslinker, tannic acid. This cytocompatible gelatin-alginate tough hydrogel exhibited excellent shape fidelity, a self-standing height exceeding 20 mm, and the capability for multilayer and four-axis 3D printing of complex scaffold shapes. The control of gel strength and rheology enables this hydrogel for successful stretching extrusion under an electric field in near-field electrospinning-induced 3D printing and four-axis printing. Nearly 74 % diameter reduction was achieved using near-field electrospinning-assisted 3D printing from a 20 mm distance, while a reduction of around 60 % was obtained in near-field electrospinning-assisted four-axis printing (with a 10 mm distance). Secondary crosslinking with Ca ions provided the hydrogel ink with enhanced mechanical properties, improved post-printing shape fidelity, and prolonged degradation or disintegration (up to 21 days) of the 3D printed scaffolds. Tannic acid release from the degraded scaffold was very low (~2 mg at the end of 72 h). The success of multilayered and four-axis printing with near-field electrospinning, the controllable mechanical properties, high cytocompatibility, and cell supportiveness of this hydrogel suggest its strong potential for diverse applications, including complex scaffolds for tissue regeneration, porous tubes, controlled drug delivery, active membranes, flexible neurotransmitters, and strain sensors.