Chemical and Biomolecular Engineering - Theses
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Item3D printing meets colloidal processing: Hierarchically porous ceramics for bone scaffolds( 2023-08)Hierarchically porous structures in nature, such as bamboo and bone, have remarkable strength and stiffness-to-density properties. Direct Ink Writing (DIW) or robocasting, an Additive Manufacturing (AM) material extrusion technique, is able to synthetically create near-net-shaped complex geometries. In this research, DIW and colloidal processing are synergized to fabricate multiscale porous ceramic scaffolds with tailorable properties. 100-micron scale porosity is produced by the 3D printed scaffold architecture. By applying particle-stabilized emulsions or capillary suspensions as feedstock for printing, a secondary level of smaller porosity is created within the scaffold filaments via soft templating of the oil phase. A third scale of porosity of sub-micron pores is formed by partial sintering of ceramic particles. Several ceramic systems are investigated in this work, namely clay, alumina, as well as hydroxyapatite and beta-tricalcium phosphate (bio-compatible and bio-resorbable ceramics). In Chapter 3, possible criteria for obtaining suitable ceramic paste feedstocks for DIW are developed by studying the relationships between paste formulation, rheological properties and print parameters. The study demonstrates that storage modulus and apparent yield stress from visco-elastic rheological measurements could be indicators of “printability” of the feedstock. This is further strengthened by the results in Chapter 4 of a different material. The particle size, surfactant concentration, oil fraction, and mixing speed are shown to influence the rheological properties, which can be adjusted to improve printing. In Chapter 4, the primary focus is to understand how to control the pore sizes within the filament microstructure. The study shows that by increasing the oil fraction and particle size, but reducing the surfactant concentration and mixing speed, produces larger micropore sizes within the filaments. The pore morphology is also determined to morph from sphere-like pores typical of Pickering emulsions, to more elongated pores of the granular phase-inverted emulsions (referred to as capillary suspensions in later studies), by increasing the oil and/or surfactant concentrations. Next, a potential application of this customizable multi-scale porous structure is explored – as a scaffold for bone tissue regeneration purposes. By varying the two levels of macro- and microporosities, scaffolds with different properties are produced. A comprehensive study of the influence of the different levels of porosity and two micropore morphologies is undertaken. Additionally, the macropore effects from variations in print nozzle diameter and inter-filament spacing are investigated. In Chapter 5, the viability and proliferation of primary human osteoblasts (bone cells) on the scaffolds are studied in vitro, firstly on slip-cast scaffolds for their micropore morphology, then as a 3D construct for cellular interactions with both micro- and macro-porosities. In Chapter 6, the scaffolds’ physical properties, such as strength and elastic modulus under compression and bending, are investigated, as well as how they change after undergoing degradation (simulating resorption in the body). Finally, in Chapter 7, all the results from the studies are discussed as a whole, concluding that this reported process of DIW of colloidal ceramic feedstocks is a promising strategy for highly porous bone tissue engineering scaffolds. Additionally, it offers a high level of customization of mechanical as well as biological properties.