Rheological properties of gelatine hydrogels affected by flow- and horizontally-induced cooling rates during 3D cryo-printing
Vanja Kokol, Yasir Beeran Pottathara, Mohor Mihelčič, Lidija Slemenik Perše
Abstract
Relatively low viscosity with poor resolution and shape fidelity make soft gelatine hydrogel still challenging to use as an ink or bio-ink for 3D printing, despite its good shear-thinning behaviour and high potential in tissue engineering due to its biocompatibility and similarities to biological tissues. A horizontally-induced cooling platform has thus been proposed to accelerate the physical stabilisation of each printed deposit during such a multilayer printing by fastening its phase-change (gelation) transition, which would affect its flow (shear-thinning) and gel strength (viscoelasticity), and thus enable scaffolds to be created with more precise porosities and geometries. In order to verify the physical properties (sol-gel transitions, gelation point, gel strength) and the kinetics of gelatine solutions (5 and 10 wt%) during such printing conditions, the viscosity and oscillatory rheology as a function of kinetically different cooling / heating rates (5−48 °C/min) have been performed in the range from 37 to 0 vs. -30 °C. The gelation of 10 wt% gelatine was shown to decrease by 4 °C (to 18 °C) when the plate was cooled to 0 °C, and the cooling speed rate was changed from 5 to 42 °C/min, which increased its complex viscosity (η*) for 485 Pa (to 1135 Pa) by forming tight and in a flow direction-oriented clusters of multi/triple-helices, being confirmed with FTIR spectroscopy. The lower plate temperature (-30 °C) didn't change solutions' gelation transition significantly, while it only increased the η* (from 43 to 470 Pa) of 5 wt% hydrogels at lower cooling speed (5−12 °C/min) by forming larger and more disordered aggregates, getting immobilised by fast crystallisation of the aqueous medium. It can be concluded that, besides the gelatine concentration, the cooling speed has a major impact on its physical stabilisation by yielding macromolecularly differently structured gels, which shall affect the printability, as well as give different porous structures.