3D bioprinting is an innovative and time-saving method to precisely generate cell laden 3D structures for clinical and research applications. Good printing performance of droplet-based 3D bioprinters is characterized by: (1) stable drop generation, (2) cell output uniformity, and (3) precise droplet positioning. The aim of this thesis is to advance fundamental and practical understanding of printing performance of a novel custom-built 3D micro-valve-based bioprinter (RASTRUM TM).
 
The study was initiated by experimentally generating a printability range for stable printing performance, such as single droplet formability and sufficient drop falling velocity. Different printing parameters were varied to study the printing performance of a micro-valve-based bioprinter. Stable printing performance was found to be bounded with the jet Weber number We_j and the Z number (the inverse of the Ohnesorge number): 10 < We_j < 25 and 2 < Z < 15.
 
Maintaining cell output uniformity is one of the limitations in the droplet-based bioprinting approach. An innovative imaging system was developed, allowing direct assessment of the average printed cell number and stability from in-flight 3D printed droplets under different printing parameters. The influence on cell ejection inside in-flight droplets was characterized and discussed for uniform bioprinting performance.
Understanding the criteria that control droplet spreading behaviour and prevent droplet splashing in droplet positioning is of great importance in optimizing the 3D bioprinting performance. The dynamics of droplet positioning on flat/ droplet/ thin liquid film/ thick soft hydrogel interfaces were studied experimentally. A power-law scaling formula was found to describe the maximum droplet spreading factor for both droplet-droplet and droplet-liquid film interaction. No splashing occurred in the droplet-droplet and droplet-liquid film interaction within the splashing parameter K = We^0.5Re^0.25 < 87. The splashing threshold for droplet-hydrogel interaction was defined by K = 44.78 and L = We Re^-0.4 = 13.85 under all printing conditions.
 
In summary, new experimental analysis methods in this thesis were developed to understand how different parameters can be utilised to achieve better printing fidelity and structure integrity in micro-valve-based bioprinting. The results will be of interest to researchers working not only in microvalve-based bioprinting field but also the 3D bioprinting field in general. It is expected that these contributions will lead to new, practical methods in the 3D bioprinting applications.

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Education/Employment

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2012-2016 BEng Honours Class 1 (Mechanical Engineering), TU Dublin, Ireland
2016-2018 Master of Mechanical Engineering, UNSW, Sydney
2018-2022 PhD Mechanical Engineering (Experimental), UNSW, Sydney 
2021 APR Intern, Inventia Life Science, Sydney
2022-current Mechanical Engineer in Inventia Life Science, Sydney

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Publications

​Chen, X., O’Mahony, A. P., & Barber, T. (2021). The characterization of particle number and distribution inside in-flight 3D printed droplets using a high speed droplet imaging system. Journal of Applied Physics, 130(4), 044701.
Chen, X., O’Mahony, A. P., & Barber, T. (2022). The assessment of average cell number inside in-flight 3D printed droplets in microvalve-based bioprinting. Journal of Applied Physics, 131(22), 224701.
Chen, X., O’Mahony, A. P., & Barber, T. (2023). Spreading behavior of cell-laden droplets in 3D bioprinting process. Journal of Applied Physics, 133(1), 014701.
Chen, X., O'Mahony, A. P., & Barber, T. (2023). Experimental study of the stable droplet formation process during micro-valve-based three-dimensional bioprinting. Physics of Fluids, 35(1), 011903.
Chen, X., O’Mahony, A. P., & Barber, T. J. (2023). Effect of 3D-bioprinted droplet impact dynamics on a pre-printed soft hydrogel matrix. Experiments in Fluids, 64(3), 60.

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