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Important Findings - Titanium and Titanium-aluminium-vanadium

Titanium (Ti) and titanium-aluminium-vanadium (Ti-6Al-4V) are widely used in orthopaedic surgeries due to their impressive biocompatibility, good corrosion resistance and fatigue strengths and a low modulus and density. As it is one of the most popular current biomaterials, it is important that any technique for producing bioscaffolds enables structures to be fabricated from these materials.

Crystallography

Titanium

Titanium scaffolds were produced using titanium hydride as the precursor material. This is safe to handle and decomposes to pure Ti at temperatures above 600oC. As Ti is highly susceptible to oxidation, sintering was undertaken in an inert atmosphere. Although this halted the production of an oxide layer, XRD analysis indicated that there was the formation of titanium carbide (Ti-C) and titanium carbide nitride (Ti-C-N), which is believed to have been caused by an incomplete removal of the polymer during burnout facilitating the reaction of titanium with the carbon atoms in a nitrogen:argon atmosphere. This can be removed using corrosive chemicals, however as TiC is also biocompatible, it has not been removed in this research to avoid damaging the structure.

Titanium-aluminium-vanadium

Structures fabricated from Ti-6Al-4V exhibited similar characteristics as to those produced using Ti. There was no oxidation of the biomaterial, instead there was some carbide formation, again believed to be caused by the incomplete removal of the polymer during burnout.

XRD of powdered samples sintered to different temperaturesXRD of powdered Ti-6Al-4V scaffolds sintered to various temperatures

Porous Structure

Porous structures of Ti and Ti-6Al-4V have been successfully fabricated indicating that this method is suitable with a range of materials. This is within the prescribed limit of 200-900 μm for bone regenerative strructures and confirms that AFR can be used to produce structures with a high degree of controllability over the pore and strut size.

Ti scaffold from 45 ppi template


Varying the sintering temperature does not significantly impact upon the shrinkage of Ti scaffolds, although it can increase the stability of the constructs. Structures fabricated using the 45 ppi template exhibited a decrease in the pore size as the sintering temperature was increased, however there was no significant impact on the strut size, which would have been more significantly impacted by increased densification.

Ti scaffold from 90ppi template

Conversely, there was little effect on the pore size with scaffolds from the 90 ppi template, although there was a small decrease in strut size when the sintering temperature was increased above 1100 °C. It is thought that this could be due to the transformation from Ti to Ti-C and Ti-C-N. Similarly, varying the sintering temperature had little effect on the macrostructure of Ti-6Al-4V based constructs. Ti-6Al-4V scaffold produced from the 45ppi templateThere was a slight derease in the pore size of structures fabricated from the 45 ppi template which was not observed when the 90 ppi template was used, whilst the strut sizes were within normal experimental range regardless of the template used and final sintering temperature. Some structures exhibited some collapse at all sintering temperatures, with both templates due to the polymer burning out before the particles fully consolidated. Ti-6Al-4V scaffold produced from 90ppi templateThis was only observed if the template was coated fewer than 3 times, and so this is the minimum number of coats required to enable the green body to maintain its structure during sintering.

As the interconnecting struts provide strength and stability, it is undesirable for any microporosity to detrimentallly affect the macrostructure.The integrity of the structure is not affected by the inclusion of a porogen and thus these structures could be suitable for enhancing the biological functionality of the scaffolds, provided the microporosity has the desired effect on cell attachment and proliferation. It was expected that increasing the level of porogen would have decreased the amount of collapse of Ti-6Al-4V samples due to the production of routes for the polymer to be removed from the structure. However, as this is not the case, it is more likely that the greater ease with which the polymer can be removed could exacerbate the problem as the stability previously provided by the foam has been taken away. Thus, provided that there are a sufficient number of coats on the scaffold for it to maintain its integrity, the inclusion of a porogen does not have any adverse effects on the structure.

Finally, the level of porogen had no impact upon the size of the micropores. Instead, increasing the amount of camphene in the slurry caused for an increase in the number and density of the microporosity, to the point where there was coalescence.

Microporosity of Ti sample with no porogenMicroporosity of Ti sample with 5% porogen

Microporosity of Ti sample with 10% porogenMicroporosity of Ti sample with 20% porogen