Chemical and Biomolecular Engineering - Theses

Permanent URI for this collection

Search Results

Now showing 1 - 3 of 3
  • Item
    Thumbnail Image
    3D printing meets colloidal processing: Hierarchically porous ceramics for bone scaffolds
    Chan, Sheue Lian Shareen ( 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.
  • Item
    Thumbnail Image
    Membrane ultrafiltration of skim milk and its application to cream cheese manufacture
    Wu, Qihui ( 2023-02)
    Cream cheese is an important export product for Australia; cream cheese production and consumption also continue to increase in many countries. The traditional manufacture of cream cheese generates a large volume of acid whey (~65–70%, w/w) that is often treated as wastewater, animal feed or fertilizer and can have a negative impact on the environment. Membrane ultrafiltration (UF) can be used to pre-concentrate the milk used in cream cheese production to improve the retention of whey proteins, increase productivity and reduce the generation of acid whey. The effect of UF concentration on the properties of milk and subsequent effects on the acid-coagulated gels and resulting cream cheese were investigated in this thesis. The thesis commences with a literature review of cream cheese manufacture and membrane filtration in Chapter 2. The fundamental and physico-chemical properties of cream cheese acid gels made from the addition of UF milk are then reported in Chapter 3. It was hypothesised that UF addition would impact the properties of both the milk and acid gels formed during the early stages of cream cheese production. Skim milk was concentrated to a volumetric concentration factor (VCF) of 2.5 or 5 by UF and the milk samples were standardized to a protein-to-fat ratio of ~0.23 to achieve a milk composition typical of that used for full fat cream cheese production. The UF cheese milk had a similar particle size distribution to the unconcentrated cheese milk after homogenization but increased viscosity and a slower rate of acidification, which could be improved by increasing starter culture concentration. The acid gels formed with the addition of UF retentate contained more protein and fat, resulting in a higher storage modulus, firmness and viscosity. A denser microstructure was observed in the acid gels formed with UF retentate and quantitative two- or three-dimensional analysis of confocal images found a greater volume percentage of protein and fat, decreased porosity and increased coalescence of fat. The mobility of water, as assessed by proton Nuclear Magnetic Resonance (1H NMR), was reduced in the dense UF gel networks. These insights improve our understanding of acid gel formation and can be used by manufacturers to optimize processing conditions for the use of concentrated milk and subsequent handling of firmer gels in industrial-scale cream cheese production. Chapter 3 demonstrated how calcium content increases in VCF2.5 and VCF5 retentate to ~260 mg/100 mL and ~480 mg/100 mL respectively, significantly higher than the concentration of ~120 mg/100 mL in the skim milk. The elevated calcium content that occurs in the milk concentrate after ultrafiltration is regarded as a potential cause for the defects reported in some UF-based dairy products, such as fresh cheese. In Chapter 4, to reduce calcium concentration in the UF preparations, skim milk was treated with 1% (w/v) or 2% (w/v) cation exchange resin and the treated milk then concentrated by UF to a VCF of 2.5 or 5. It was hypothesised that the removal of calcium from skim milk by cation resin would affect the properties of milk proteins as well, as the ultrafiltration process. The calcium content in the resin-treated skim milk, as well as the resulting retentates (VCF2.5 and VCF5), decreased by 20–30% compared with the non-resin treated controls. As a result of decalcification, the casein micelles partially solubilized and dissociated, which led to an increase in the soluble protein content and a lower relative turbidity for these milk samples. The decalcification of the skim milk feed also decreased the permeation flux during UF and led to a decrease in the gel concentration (or maximum concentration factor) from ~30% (w/w) solids (~6.5 fold concentration) for the control skim milk to ~24% (w/w) solids (~5.4 fold concentration) for 2% (w/v) resin treated skim milk. The average diameter of particles in skim milk was found to increase from ~160 nm to ~180 nm after calcium reduction, while the ultrafiltration process led to a decrease in particle size for the resin-treated milk samples. The zeta-potential of the calcium reduced UF retentates did not change but surface hydrophobicity increased. Analysis of the milk solids indicated that calcium depletion increased the hydration of the milk proteins to 3.3 g water per g dry pellet (2% resin, w/v), compared to the 2.2 g water per g dry pellet for the non-resin treated controls. The increase in milk protein hydration also contributed to a higher milk viscosity. Differential scanning calorimetry (DSC) showed calcium reduction decreased the denaturation temperature of alpha-lactalbumin and beta-lactoglobulin by ~3 and ~1 Celsius degree respectively. Overall, the work in Chapter 4 provides a route to produce calcium-reduced milk concentrate with potential in retentate-based dairy products with tailored functionality. The impacts of UF retentate addition and calcium reduction on the properties of the final cream cheese were then evaluated in Chapter 5. It was hypothesised that the properties of cream cheese made from UF retentate would significantly differ from those of the control cheese made from unconcentrated milk and the reduction of calcium by ion exchange would also affect the properties of cream cheese. In this work, cream cheese was made from UF concentrated milk (2.5- and 5-fold), treated with or without 2% (w/w) cation resin and the properties compared with a control cream cheese made from unconcentrated milk. The UF cheeses did not differ in protein, fat and moisture content from the control but had a higher calcium concentration if not treated with resin (~150 mg/100 g and ~230 mg/100 g for cheese made from non-calcium reduced VCF2.5 and VCF5 retentate respectively vs ~90 mg/100 g for cheese made from unconcentrated skim milk). The microstructure of the cheese made with UF without calcium reduction was more heterogeneous and porous than the control, consistent with a decreased hardness and thermal stability, providing new insights into the link between UF cream cheese microstructure and functional properties. The calcium reduction of ~20% induced by 2% (w/w) cation resin treatment prior to UF to VCF2.5 or VCF5 did not significantly affect the texture properties of the cheese formed compared to the non-resin treated counterparts but led to an increase in the size of the corpuscular structures found within the UF cheese. The concentration of free amino acids and peptides was highest in the cheese made with added UF retentate and decreased in the samples with reduced calcium, although not as low as the concentration in the control cream cheese, illustrating the potential to tune this property. This study improves our understanding of UF-produced cream cheese with differing calcium content and this knowledge may benefit future scale-up to industrial production. In conclusion, the work from this thesis broadly explored the impact of processing conditions including the concentration factor of milk, starter culture concentration and calcium concentration on the properties of the milk, acid gels and the final cream cheese product. These findings illustrate how milk solids concentration and calcium concentration can be systematically used to alter intermediate and final product properties and may be beneficial for industrial manufacturers to further optimize cream cheese production at different manufacturing stages when using concentrated milk to reduce acid whey production.
  • Item
    Thumbnail Image
    Exploring additive manufacturing and non-solvent induced phase separation for fluoropolymer membrane production
    Imtiaz, Beenish ( 2022)
    The alarming rise in scarcity of fresh and safe drinking water around the world has prompted an increase in search for new materials to fabricate more efficient water filtration membranes. Poly(vinylidene difluoride) (PVDF) is a popular membrane material however its use as long lasting membranes is hindered by fouling, compaction, and caustic damage. The overarching aim of this thesis was to integrate engineering manufacturing with membrane production in order to explore innovative methods for fabrication of flat sheet and patterned fluoropolymer membranes. The first part of this thesis reports on the bulk synthesis of dehydrofluorinated PVDF (dPVDF), catalysed by a reaction of PVDF with ethylenediamine (EDA), which results in alkene moieties along the backbone of the polymer chain, as confirmed by attenuated total reflectance – Fourier transform infrared (ATR-FTIR) and Raman spectroscopies. The second study of this thesis investigated the suitability of dPVDF solutions (prepared in N,N-dimethylacetamide (DMAc), with or without poly(vinyl pyrrolidone) (PVP)) for direct ink writing (DIW). Steady-state rheology confirmed the non-Newtonian and shear-thinning character of these solutions, whereas oscillatory rheology demonstrated superior gelling behaviour of dPVDF solutions with PVP. Additionally, the presence and changes in microstructure of the solutions were studied via modified Cole-Cole plots and Gurp-Palmen plots. Moreover, a stepwise oscillatory shear rate cycling was carried out to simulate the behaviour of the printing inks during DIW, that indicated complete recovery of microstructure in dPVDF ink post-deposition. The third study presents a hybrid process, implying DIW + ex situ non-solvent induced phase separation (NIPS), for the fabrication of flat sheet dPVDF microfiltration (MF) membranes. To produce inks for DIW, the dPVDF was dissolved in DMAc along with a pore-forming agent, PVP (at 5-30 wt%, relative to dPVDF concentration). Membranes were produced by DIW of the inks into continuous wet films - followed by NIPS in deionised (DI) water. The fabricated dPVDF membranes were more hydrophobic (water contact angle, WCA = 115 degrees) than the similarly fabricated PVDF membranes (WCA = 99 degrees) yet had greater equilibrium water content (EWC) and porosity, which correlated to the morphology of the fabricated membranes. The dPVDF membranes with 30 wt% PVP not only demonstrated stability in a caustic environment (1 M NaOH for 90 min), but also had a pure water flux of approximately 4300 LMH, within the range of commercially available PVDF membranes (approximately 6300 – 8100 LMH). Among the many physical methods of improving fouling resistance is the fabrication of patterned membranes, where the patterns provide the functionality of an integrated spacer, thus providing turbulence to the incoming feed. Despite fluoropolymers being common polymers for membrane fabrication, their commercial production into membranes remains dominated by simple casting and solvent phase separation. The final study of this thesis demonstrated a rapid and simple approach to produce patterned fluoropolymer membranes, with profiled surfaces, via immersion precipitation printing (ipP). This study utilised DIW + in situ NIPS in isopropyl alcohol (IPA), followed by ex situ NIPS in DI water. The direct phase inversion of the patterns during membrane production induces a porous morphology. Further, pure water permeability studies were performed on unpatterned membranes and membranes with patterns fabricated with a combination of different fluoropolymer materials (PVDF and synthesised dPVDF). This simple ipP approach showed potential as a viable alternative for the production of fluoropolymer membranes where the complete control over pattern height, fidelity, and shape is required. In summary, this thesis has developed a hybrid approach comprising of DIW + NIPS for membrane production, in which PVDF has been dehydrofluorinated to produce to dPVDF using EDA, and then fabricated into flat sheet membranes. These membranes have shown to be used as microfiltration (MF) membrane with improved caustic resistance relative to commercial membranes. Moreover, ipP has been utilised for the first time to fabricate patterned membranes with varying pattern shapes and heights from PVDF and/or dPVDF. This potentially points towards fabrication of membranes from different materials with variable geometries and tuneable porosities based on solvents, non-solvents, and pore-formers. The implications of this research are far-reaching as it points towards the integration of engineering manufacturing and membrane preservation. These discoveries will form the basis of future work expanding into smart membranes, bespoke membrane form factors, and new filtration applications.