Can Hydroxyapatite Micro-Parts Be Mass-Produced by Vat Photopolymerization? Geometry Is Necessary, But Not Sufficient

Abstract Mass production of discrete, sub-millimetre hydroxyapatite parts — called for as shape-engineered reinforcement in composites — is undocumented for ceramic vat photopolymerization, well established at centimetre scale. We report the fabrication and post-processing of hydroxyapatite cubes (1 mm design) and tetrapods (0.876 × 0.767 × 0.656 mm). Green parts released spontaneously from the build foil after 72 h at 40°C, eliminating mechanical deplatforming with no measurable dimensional change. Co-sintering required no separation medium, with parts in mutual contact remaining free-flowing. Corner rounding of 23–34% was already present in the green state, identifying it as a printing artefact rather than a sintering effect. Most consequentially, two batches from the same feedstock, furnace and thermal schedule reached 94.9%TD (tetrapods) and 79.9%TD (cubes); the tetrapod value sits within the supplier’s own stated range for this material (94–97%TD) while the cube value sits clearly below it. X-ray diffraction excluded phase composition as an explanation, confirming single-phase hydroxyapatite. The tetrapod encloses 77 times less material than the cube; since binder removal is diffusion-limited, the cube requires of order 35 times the debinding time. An independent, geometry-dependent exposure artefact reported here confirms that at sub-millimetre scale, process parameters are geometry-specific rather than size-specific, and that the ratio of enclosed volume to free surface — not nominal part dimension — is the governing quantity. Geometry-matched parameters are therefore necessary for mass production at this scale, but not sufficient alone: throughput economics and the absence of batch-level quality control remain separate barriers.