The use of thermal printing to control the properties of calcium phosphate deposits

The objective of this work was to characterize the deposits of calcium phosphate produced by thermal printing in terms of structure, topography and mechanical properties. Hydroxyapatite was molten and directed to (a) a titanium target in relative motion and (b) stationary titanium substrates preheated to 100 °C and 350 °C. Scanning electron microscopy showed round-like deposits, but high resolution profilometry measured the profile. Micro-Raman spectroscopy and X-ray diffraction characterized the surface for structure, while nanoindentation revealed the hardness and elastic modulus. A symmetrical hemispherical deposit was formed on a surface in slow relative motion, but an off-centre shape formed at a higher relative speed. Deposits on preheated surfaces (100 °C and 350 °C) were identified as amorphous calcium phosphate. Nanoindentation revealed no significant difference in hardness between the amorphous deposits (4.0–4.4 ± 0.3 GPa), but the elastic modulus increased from 65 ± 4 GPa (annealed calcium phosphate reference) to 88 ± 3 GPa (100 °C surface) and then to 98 ± 3 GPa (350 °C substrate). The large change in elastic modulus is thought to arise from the dehydroxylation during thermal printing. Production of functional materials through crystallization is discussed to extend the range of possible microstructures. The characterization and testing approach is useful for hemispherical deposits produced by printing, coatings (laser ablation, thermal spraying, simulated body fluid) and melt extrusion elements in scaffolds.