Experimental study of the stable droplet formation process during micro-valve-based three-dimensional bioprinting.

Three-dimensional (3D) bioprinting offers great potential for the fabrication of complex 3D cell-laden constructs for clinical and research applications. The droplet formation process is the important first step in droplet-based 3D bioprinting, affecting the positional accuracy and printing fidelity. In this paper the drop ejection behaviour, thresholds for stable droplet generation, and formation of satellite drops are studied, under various ink properties, printing conditions, and input cell concentrations using a micro-valve-based 3D bioprinter. Three droplet ejection behaviours are identified under different conditions: an isolated stable droplet, satellites coalescing into a single droplet, and the presence of one/multiple satellites. The droplet state is represented by a phase diagram bounded by dimensionless Z number (the inverse of the Ohnesorge number) and jet Weber number, Wej, to define the printability of the utilised bioprinter. The printability range is defined as 2 < Z < 15 and 10 < Wej < 25 by considering characteristics such as stable single droplet formability and sufficient drop falling velocity. There is no fatal damage on cells within this printability range. The results show there is no strong influence of actuation system on droplet-based bioprinting printability. As the input cell concentration increases, the bioink's density and viscosity increases and surface tension decreases, which in term causes the Z number to slightly decrease. Change of cell concentration (from 0 to 1 x 107 cells/ml), within a Newtonian bioink, has negligible impact on droplet volume, falling velocity, drop ejection behaviour, breakup time, and ligament length in microvalve-based bioprinting.