Novel robocasting and rotary evaporator routes for the preparation of sol–gel derived glass & glass ceramic scaffolds

Producing pure amorphous bioactive glasses by sol–gel from a quaternary system containing sodium is extremely difficult as they are prone to undergo readily crystallisation during heat treatment. Moreover, phase segregations and irreversible crystallisation of unwanted phases are likely to occur during the conventional xerogel process. The consequences are reduced homogeneity, excessive time and energy consumptions during the aging and slow drying steps of sol–gel derived bioglasses. A main objective of this thesis is to develop a new and expedite sol–gel synthesis process to prepare high silica quaternary bioactive glasses. Additional concepts are incorporated in the conventional sol–gel synthesis procedure to minimise the aging and drying times, while preserving the typical microstructure and properties and enhancing the degree of homogeneity. The efforts made led to the development of an innovative and rapid sol–gel method for producing bioactive glasses, which skips the ageing process. A comparative study was made between the glasses synthesised by the conventional drying method (CD) and the novel fast drying (FD). The results revealed that both methods produce stabilised bio–glasses with virtually identical behaviour and properties, but FD is 100 times faster in comparison to CD. Moreover, FD glasses exhibited improved bioactivity in simulated body fluid (SBF). The effects of some processing parameters such as calcium concentration, Ca/P ratio, types of metal salts precursors, types and concentrations of catalysts, were investigated to gain an in–depth understanding of their roles in the overall properties. The influence of agitation rate on the extent of bio– mineralisation upon immersing the samples in simulated body fluid (SBF) was also studied. The results revealed that bioactivity was enhanced by using nitrate salt precursors and citric acid as catalyst, and by increasing Ca concentration to some extent. Another aim was doping or co–doping the high silica sol–gel glass (HSSGG) selectively with Cu and La. The individual and combined effects of these doping elements on the glass structure, crystallisation, porosity, specific surface area, particle sizes, density, sintering ability, compressive strength and cytotoxicity were studied. As individual doping elements, Cu and La were found to play opposite roles in some relevant properties. On the other hand, synergistic benefits could be obtained by combining the two dopants. Cytotoxicity was significantly improved for some doping combinations, while the bio– mineralisation activity of doped samples was always very similar to that of the parent HSSGG composition. The significant intrinsic porosity fraction of sol–gel derived glass powders and the consequent uptake of any dispersing liquid medium, which will not be available for dispersion and flow, make them inappropriate for colloidal processing. These features have hindered so far the fabrication of porous scaffolds by additive manufacturing techniques such as robocasting. Overcoming this big challenge is of paramount importance for fully exploiting the specific advantages of the sol–gel derived glasses. Therefore, understanding the effects of each experimental processing variable on the properties of the powders is essential towards achieving optimised processing conditions for the fabrication of scaffolds by robocasting. Accordingly, the effects of heat treatment temperature (HTT), balls to powder ratio (BPR), and solid loading on the flow behaviour and on the viscoelastic properties of the suspensions/pastes were systematically evaluated. Suspensions with solid loadings up to 40 vol.% could be prepared from both HSSGG and Cu–La doped glasses and used to successfully fabricate scaffolds by robocasting. The formation of hydroxyapatite on the scaffold surfaces was observed after 72 h of immersion in SBF for HSSGG. These features make the scaffolds promising candidates for bone regeneration and tissue engineering applications and worthy for further in vivo investigations.