Effect of nozzle diameter and cross-linking on the micro-structure, compressive and biodegradation properties of 3D printed gelatin/collagen/hydroxyapatite hydrogel

Since the existing polymeric hydrogel inks lack printability, shape fidelity and the desired mechanical properties for bone tissue regeneration, a hydrogel comprised of gelatin (G), collagen (C), and hydroxyapatite (H) nanoparticles is utilized for extrusion-based 3D printing. The rheological characterization of the composite GCH inks was performed to evaluate their printability, while the cuboid column model scaffolds were printed at a printing speed of 8 mm/s by using different needle inner diameters (500 μm/21G, 250 μm/25G, and 200 μm/27G), followed by carbodiimide induced crosslinking for 12 or 24 h. The samples were tested for their micro-structure, swelling, compression and stiffness performance, before and after incubation in an HBSS solution for up to 14 days. The wall diameter of a 3D printed scaffold decreases and pore size increases primarily with the decreasing inner diameter of the nozzles, and secondarily by increasing the crosslinking time. This was supported with their swelling capacity and the creation of new CaP crystals on the scaffold walls' top surfaces by the time of incubation, necessary for cells’ adhesion, proliferation and growth.

The compressive modulus and stiffness of the scaffolds increases proportionally with the increase of their wall diameter and the time of crosslinking, and is inversely proportional to their pores size. The scaffold with the smaller pores provides superior modulus and stiffness, even after 14 days of incubation in the physiological solution (i.e. from ∼0.94 to ∼0.71 MPa and from 17 to 20 kPa to 5–9 kPa, respectively). This is comparative with the reported values for gelatin-based composites, and in the range for hard tissue regeneration at non-load bearing sites, as well as cartilage applications.