Development of mist-based technologies for droplet-based and coaxial bioprinting of ionically crosslinking hydrogels

Abstract

Three-dimensional bioprinting has become one of the leading biofabrication techniques in the pursuit of creating tissue constructs, as it offers high precision and resolution of bioink deposition. Ionically crosslinking hydrogels are one of the most commonly used in threedimensional bioprinting due to their biocompatibility, ease of crosslinking, and rapid gelation. Current methods of bioprinting ionically crosslinking hydrogel bioinks rely on the use of liquid crosslinker, which impedes precise deposition of the bioinks and causes poor resolution and layer adhesion. Furthermore, current approaches involve multiple crosslinking stages and postprocessing steps, which is time consuming and could compromise the shape fidelity of the printed structure and cell viability. Therefore, an improved system to better control the gelation rate and collect the excess mist crosslinker must be developed. In this thesis, mist-based technologies are developed for the three-dimensional bioprinting of ionically crosslinking hydrogel bioinks. The mist-based crosslinking technique is implemented for two bioprinting techniques, droplet-based and coaxial bioprinting. Contrary to current approaches to droplet-based and coaxial bioprinting that utilize the crosslinker in liquid or sacrificial form to construct a scaffold, the developed technologies introduce the crosslinking agent in mist form. For the droplet-based bioprinting study, the printhead introduces the crosslinker in mist form, and features a removal mechanism that prevents crosslinker accumulation on the printbed. It is shown that the gelation rate can be controlled by adjusting the mist concentration or delivery rate. Furthermore, it is shown that the mist inlet flowrate has influences on the printing resolution and shape fidelity of the printed constructs. The printability, mechanical properties, and swelling properties of the printed constructs crosslinked using different mist delivery rates are studied. Moreover, the impacts of printing parameters, including printhead height, mist outlet pressure and printhead channel dimensions on the mist distribution within the printhead is investigated. Additionally, using high-speed imaging, the effects of mist concentration and droplet velocity on the dynamics of droplet impact onto the printing surface are characterized. Results show that the printed constructs using the developed printhead exhibit good droplet gelation and co-droplet adhesion, and high level of cell viability. For the coaxial bioprinting study, the mist-based technology is evaluated for fabricating hollow fibers in a single step. Through controlled exposure of crosslinker, the developed system prevents poor resolution and layer adhesion caused by the accumulation of liquid crosslinker on the printbed. Furthermore, it eliminates additional processing steps, such as partial crosslinking of the hydrogel prior- or removal of sacrificial material post-printing. The printability and mechanical properties of hollow fiber scaffolds printed using various mist and hydrogel concentrations are studied. It is shown that mist concentration influences the gelation rate of the hollow fiber, impacting the shape fidelity, layer adhesion, and mechanical properties of the printed structures. Moreover, the effects of printing parameters, including the mist core pressure and hydrogel flowrate, on the diameter and wall thickness of the hollow fiber are investigated. Additionally, scaffolds printed and crosslinked using mist exhibit over 90% cell viability. The developed mist-based coaxial system enables direct printing of continuous hollow fibers. The developed mist-based crosslinking technology leverages the advantages of bioprinting techniques, while providing a better control of the gelation rate and preventing the accumulation of excess crosslinker on the printbed. Furthermore, the developed mist-based technologies advance the applicability of droplet-based and coaxial bioprinting to fabricate complex and biocompatible scaffolds for tissue engineering applications.

Date	28 Apr 2023
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Ali Ahmadi (Supervisor)