Effect of bio-ink physical properties on droplet volume in the reactive jet impingement 3D bioprinting system

Reactive jet impingement is a 3D bioprinting process which forms cell filled hydrogels through reacting droplets of polymer and crosslinker solutions. This study evaluates for the first time the relationship between the droplet volumes of the hydrogels with the viscosity and surface tension of the starting solutions. Calcium chloride, sodium alginate, thrombin, and fibrinogen solutions are characterised together with two blended solutions: collagen-alginate-fibrin and thrombin-calcium chloride, which combine to create a collagen-alginate-fibrin hydrogel. The influence of cells on bio-ink behaviour has been assessed through suspending TC28a chondrocytes within the thrombin-calcium chloride solution. Viscosity was a greater differentiator in defining print volumes than surface tension, and there is a clear relationship between droplet volume and kinematic viscosity measured at high strain rates (1000 s^-1). The addition of cells had a minimal effect on the kinematic viscosity of solutions at high strain rates and, therefore, on processing of cell filled hydrogels, meaning that processing high cell densities is possible without significant adjustments to processing parameters. Reactive jet impingement is a reliable and accurate process for creating high cell density hydrogels, and the kinematic viscosity at high strain rates is the key mechanical property in defining the relative print volumes of different inks.