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Important Findings - Hydroxyapatite

The AFR has been used to produce structures from potential - hydroxyapatite (HA) - and currently used - titanium (Ti) and titanium-aluminium-vanadium (Ti-6Al-4V) - biomaterials.

The crystallography and size of the as recieved biomaterials was analysed prior to scaffold fabrication, as was the pore size of the foam template. The thermal characteristics of both the template and biomaterial slurries was also determined.

Hydroxyapatite Scaffolds

XRD of the raw material indicated that the powder was predominantly HA with some monetite, a calcium phosphate with a Ca:P ratio of 1 also present. The high sintering temperatures required to ensure the structural stability of the fabricated samples led to some phase transformation from HA to whitlockite, and in some cases a slightly biphasic combination of whitlockite and HA.

XRD of raw HA and of powdered scaffolds sintered to various temperatures

Due to the relative versatility of hydroxyapatite and the huge potential it has as a bone replacement biomaterial, constructs were produced using the AFR with varying processing paramaters. Scaffolds were produced from 2 templates: one with 45 pores per inch (ppi) and the other with 90ppi. The structure of the scaffolds closely replicated that of the template, regardless of which was used during manufacture.

SEM micrograph of scaffold produced using 90 ppi templateScaffold produced with 45 ppi template

Increasing the sintering temperature led to an increase in the amount of shrinkage and hence smaller pore and strut sizes, with an increase in the ease of handling of the scaffold. An increase in the number of coats led to an increased strut size and a decreased pore size. There was also an increase in the uniformity of the strut, and pore, sizes as the number of coats increased and an elevated susceptibility to pore occlusion. The increase in pore occlusion with increasing number of coats was more noticable on samples produced using the 90ppi template. Although this was not desired, in no cases has the 3-dimensional interconnectivity been compromised and so this is not expected to have any detrimental effects.

Including a porogen into the slurry to generate microporosity had no discernible effect on the macrostructure. It did however cause the generation of micropores which were between 1-5μm, regardless of the level of porogen included. As the camphene content was increased, the amount and density of these micropores also increased to the point where there was some coalescence, causing larger micropores of 20-30μm at the highest levels of porogen inclusion.

Microstructure of scaffold produced without porogenMicrostructure of scaffold produced with 20% porogen

The porosity of the scaffolds was measured using the Archimedes method and was found to be between 80 and 95% depending on the processing parameters used. These values were used to determine the theoretical yield stress of the scaffolds, with selected samples subjected to compression tests to failure to determine the efficacy of non-destructive analysis on AFR fabricated scaffolds, with good correlation between the two techniques.

Compressive strength of AFR produced HA scaffolds

Cell viability of the scaffolds has been undertaken using live/dead assays, cell number quantification and ALP expression. AFR produced scaffolds contained more metabolically active cells than was seen on a commercially available porous HA, with the generation of microporosity having a varied response depending on the micropore size. The highest level of porogen inclusion

Cell viability on scaffolds from 45 ppi template with 25% porogenCell viability on scaffold from 90 ppi template with 25% porogen