Nithyanandan, A; (2015) Template-assisted electrohydrodynamic atomisation of bioactive deposits on advanced biomaterials for orthopaedic applications. Doctoral thesis , UCL (University College London).

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Abstract

There is potential for template assisted electrohydrodynamic atomisation (TAEA) spraying, a novel ambient temperature patterning technique, to be an improved coating method compared to the current industry standard of vacuum plasma spraying metallic implants for orthopaedic applications. The reduction in temperature results in an increase in the range of coating materials and the ability to incorporate biological agents during manufacturing. This highly controllable process has been utilised to produce coatings with an ideal thickness, improves uniformity and homogeneity, and improved bond strength. The TAEA spraying of bioceramic suspension and polymer solutions in the stable cone jet mode was studied. It was found that the media concentration, the flow rate, the applied voltage and the spraying time had a significant influence on the morphology of the resultant coatings. A wide variety of predetermined topographical geometries have been achieved with a high degree of control. For the first time, TAEA has been used to pattern curved metallic surfaces with ceramics, and flat metallic substrates with polymer materials. A high degree of control over pattern line width, interline spacing and thickness were achieved. TiO2 patterns with parallel lines have been successfully produced on both convex and concave Ti substrate diameters of —25mm. Optimal results were obtained with 4 wt.% TiO2 in ethanol suspension sprayed within the stable cone jet mode window at a flow rate of 20 I.J.l.mint for 300 s at 10 kV with a collection distance of 80 mm. Nanoindentation indicates good adhesion between pattern and substrate. The effect of heat treatment to further consolidate the patterned deposits was also investigated. Hardness of the patterns was not markedly affected by heating. For TiO2 patterns at a loading rate of 1 I.J.N.sO1 and a hold time of 1 s, hardness decreases as load increases up to 7 I.J.N and remains at 0.1 GPa up to higher loads. Elastic modulus behaves in a similar way. This decrease can be attributed to the microporosity observed in the TiO2 deposits as higher loads are able to penetrate more easily into microporous structures. This work shows that TAEA is highly controllable and compatible on a range of substrate geometries. A clear correlation between the template shape and size and the achieved pattern was established, therefore the template can be chosen to reflect the specific clinical needs of the coating. Extending TAEA capabilities from flat to curved surfaces, enabling the bioactive patterning of different surface geometries, takes this technology closer to orthopaedic engineering applications.

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