Spreading behavior of cell-laden droplets in 3D bioprinting process

3D droplet-based bioprinting technology is an innovative and time-saving additive manufacturing method, which enables spatial patterning of biological materials and biochemical and living cells for multiple clinical and research applications. Understanding the criteria that control droplet spreading behavior during droplet impact is of great importance in controlling printing resolution and optimizing the printing performance. In this experimental work, the spreading of 3D printed cell-laden droplets was studied with side and bottom view images. The droplets contain 1×107 cells/ml input cell concentration and corresponding Φ=0.52% cell volume fraction and impact onto a flat hydrophilic substrate, a pre-printed droplet, and a pre-printed thin liquid film. The cell-laden droplet impact morphology, the maximum spreading factor, and the cell distribution under different printing conditions (89<We<365,174<Re<414) in a 3D bioprinting process were characterized. It was found that on the hydrophilic flat substrate, the cells homogeneously distributed into a disk structure. The maximum spreading factor, βmax, can be well described by the correlation formulas based on the energy balance and volume conservation. A power-law scaling formula was found to describe the maximum spreading in terms of the Weber number for cell-laden droplet impact on both pre-printed droplets and thin liquid films, where βmax∝We0.25. Input cell concentration, up to 1×107 cells/ml, was found to have negligible effect on the maximum droplet spreading factor in a 3D bioprinting process.