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Title: Synthesis and 3D printing of hydroxyapatite scaffolds for applications in bone tissue engineering
Author: Cox, Sophie C.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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It is known that chemical and physical features of bone contribute to its functionality, reactivity and mechanical performance. This knowledge is the fundamental rationale for this project. The aim of this thesis is to study the influence of synthesis conditions on material composition and ultimately the biological performance of hydroxyapatite (HA) as well as to fabricate scaffold structures that physically emulate bone tissue. Concurrent characterisation of physiochemical properties and evaluations of in-vitro cytocompatibility, and the degree of osteoblast proliferation on CDHA substrates precipitated under different reaction conditions provides a novel contribution. Non-viability of cells seeded on substrates prepared in a solution adjusted to pH 10 (AP07) was confirmed after 1 day of culture. Dead cells were also observed after 3 days on CDHA prepared at 70°C under a controlled pH level of 11 (AP12). XRD found no discernible difference between these samples and CDHA substrates shown to be cytocompatible. The source of cytotoxicity was concluded to be the presence acidic DCPD in AP07, and positive surface charges for AP07 and AP12 that were revealed by FTIR, DTA-TGA and ZP measurements. Control of pH, increased solute concentration, the use of Toluene, and substitutions of 10mol% Mg or 2mol% Zn were shown to enhance the proliferative rate of cells seeded on CDHA synthesised at RT. CDHA prepared in a 60 Toluene: 40 DI water (% v/v) solvent system with a lower dielectric constant (AP14) exhibited marked XRD peak broadening and 20% larger surface area compared with CDHA prepared in DI water (AP09). These features are suggested to explain the enhanced proliferation of cells on AP14, which was shown to be more than double the fluorescence exhibited for AP09 after 7 days. XRF was used to confirm the presence of Sr, Mg, and Zn that were selected due to their key biological roles in bone apatite. Evidence of lattice incorporation of these divalent cations was supported by XRD analysis that demonstrated shifts of characteristic HA peaks. Mg ions inhibited the crystallisation process, which caused a 45% reduction in the crystallite size, 60% increase in particle surface area and thermal conversion to whitlockite at 600°C. The relatively low crystallinity and larger surface area of Mg and Zn doped substrates is proposed to explain the respective 80 and 40% increase in cell proliferation compared to a pure sample prepared under the same conditions. Flowability of HA:PVOH precursor materials correlated well with the mechanical stability, microstructure and porosity of 3D printed scaffolds. Anisotropic behaviour of constructs and part failure at the boundaries of interlayer bonds was highlighted by compressive strength testing. A trade-off between the ability to facilitate removal of PVOH thermal degradation products during sintering and the compressive strength of green parts was revealed. The maximum green scaffold strength of 0.85MPa was exhibited by parts that were air or vacuum dried for 6hrs. Critically, the pores of 3D printed constructs could be user designed ensuring interconnectivity and the imperfect packing efficiency of precursor powders created an inherent surface roughness and microporosity within scaffold struts. These features are known to be favourable for osteogenesis, osteoconduction and osteointegration in-vivo. This work establishes that changes to precipitation conditions cannot be deemed trivial since they may alter material composition, which ultimately determines cytocompatibility as well as the proliferative rate of cells. Due to the highly complex structure of bone there are understandably a number of on-going medical challenges and while the application of 3D printed HA bone tissue scaffolds is promising, the name apatite derived from the Greek ‘απαταο’, meaning to deceive is concluded to be very fitting.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: RC Internal medicine ; TS Manufactures