Understanding quantitative process physics of 3D binderjet printing with validation in Ti-6Al-4V and inkjet bioprinting of mammalian cells
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In the field of additive manufacturing, laser or electron beam based 3D printing is widely investigated for biomedical applications. However, much less explored is the binderjet 3D printing, which allows processing of biomaterials at physiologically relevant conditions. In this context, this thesis presents a set of experimental and theoretical analysis to develop a quantitative understanding of the transient process physics of the binderjet 3D printing. In the first part, maltodextrin based aqueous binder, deployed to ‘direct’ print Ti-6Al-4V powder with the achieved mechanical properties ranged between cortical and cancellous bone are described. The adopted process induced ~99% interconnectivity in the 3D microstructure, probed using quantitative micro-CT analysis. Finite Element-based analysis was developed to predict the stress-strain response of various designed porous architectures, while assigning an ‘effective’ material property obtained from microporous models without designed porosity. In the broad second part of the thesis, the formulation of an in situ polymerisable acrylic binder/ink for printing implantable metallic biomaterials is highlighted. The modification of the printable Ti-6Al-4V powders using persulfate, allowed localised polymerisation during the 3D binderjet printing of with the on-demand deposited acrylic ink, In order to establish the theoretical perspective, Washburn’s theory was used to understand the transient kinetics of the ink/binder infiltration phenomena in ~100 µm thick powder bed layers,. The combinatorial experimental/analytical approaches enabled to predict the time required for the transient phenomena involved in binderjet printing. A pertinent combination of statistically reliable strength properties (Weibull modulus, compressive, flexural strength and compressive modulus of ~8, 222 MPa, 93 MPa and 4 GPa, respectively) of Ti-6Al-4V scaffolds was recorded along with ~98% interconnected microporous 3D microstructure. The cytocompatibility of the Ti-6Al-4V 3D architectures was established using mammalian fibroblasts and osteoblasts. In developing a better quantitative insight into ink infiltration kinetics, in the third part of this thesis, real time ink infiltration phenomena in the porous ceramic powder bed was investigated under high brilliance synchrotron X-Ray in refraction based phase contrast mode. An ethylene glycol-DI water based simulated ink was allowed to deposit ‘on-demand’ through a piezoelectric inkjet printhead and the post-impact ink infiltration behaviour through the porous alumina powder bed was captured real-time at a rate of 500 fps. Using a rigorous interactive image analysis, the real time wetting contours were extracted and penetration depth, lateral spread, transformed wetting volume was quantified. Denesuk and Holman’s models based on Washburn’s theory were adapted to develop a theoretical model of wetting volume in real time which was established to be dependent on powder bed porosity. In the fourth part of this thesis, the experimental results related to the piezoelectric inkjet bioprinting of mammalian cells and the post-printing cell functionalities are demonstrated. Cell laden ‘printable’ bioink was printed through a 60 µm orifice printhead using three driving voltages of 80, 90 and 100 V at 3k Hz frequency. Post-printing proliferation (Alamar blue) of the cells was higher, when printed using higher voltage endorsing the positive effect of smooth and uninterrupted droplet ejectionat higher voltage. Propidium iodide (PI) and Texas red conjugated dextran based hierarchical set of molecular probes (3 kDa, 10 kDa, 40 kDa, and 70 kDa) were used to probe the post-printing cell membrane permeation in real time. An interesting observation is the cell membrane damage, which persisted only for few hours after the printing operation in lower voltage (80V). Importantly, the total area of pores and the maximum pore size are found to vary in accordance with the actuating voltage. As the viability and proliferation are uncompromised in higher printing voltage, the higher lifetime of the finite sized membrane pores can be potentially useful for intra-cellular molecular transfections like gene, protein deliveries for cell engineering applications.