Bio-based material formulation for extrusion printing by dityrosine crosslinking of unmodified casein

In the development of new functional bio-based materials in the field of three-dimensional (3D) printing, visible light-induced dityrosine crosslinking gains increasing interest. In this context, most current bio-based materials and ink formulations rely on previously modified chemical substances with increased tyrosine availability. In contrast, we developed and characterized a photopolymerizable ink formulation for extrusion printing based on the unmodified and naturally occurring protein casein. Manufacturability of formulations containing protein, photoinitiating system, buffer and a thickening agent turned out to be a key factor for the ink development. In total, eight different thickening agents were assessed regarding their suitability to increase the viscosity of the ink formulation to expand the fabrication window for extrusion-based 3D printing. The mechanical properties of the ink formulation and hydrogel in presence of sodium alginate were further characterized and the macroscopic fabrication of auxetic structures consisting of up to 30 layers was achieved by applying extrusion-based printing.