Characterization of the extrudability of hydrogel-based materials for 3D printing of heat-sensitive drugs

Semi-solid extrusion (SSE), one of the additive manufacturing techniques, is attracting particular attention due to its use for printing thermosensitive drugs. Among the materials used in SSE, hydrogels have received the most attention in pharmaceutical applications due to their ability to provide spatial and temporal control of the release of various therapeutic agents. 3D printing of these hydrogel-based materials requires a fundamental understanding of their non-Newtonian flow during extrusion. In this work, agar gels were subjected to extrusion tests at apparent shear rates corresponding to their printing speeds. The rheology of these gels was then studied using a laboratory setup consisting of a syringe piston pushed by varying weight and the data obtained was modelled using the Herschel–Bulkley equation to obtain the yield stress, the consistency and flow indices. The 4% (w/w) concentration of agar gel showed the best fit to the modelled data and had the optimal rheological properties. This concentration was thus used in printing cylindrical objects, and the effect of the infill density on the porosity of the objects as well as on the dissolution of a tracer was studied.