Acid resistance of hydroxyapatite dental films fabricated through powder jet deposition

This study proposes a novel dental treatment method using powder jet deposition (PJD). PJD can be employed to fabricate hydroxyapatite (HA) films directly on human enamel by blasting fine HA particles. Acid resistance tests were performed on HA films fabricated with two blast angles (α = 60° and 90° with two samples each) and using human enamel (with one sample) to evaluate the usefulness of the HA films in acidic intraoral environments. All samples were dissolved in acid, and their volume gradually decreased over the test time t_acid. Transmission electron microscopy images showed that the grain boundaries near the acid-exposed surface of the HA films were densely modified and prevented acid penetration, whereas the human enamel grains became finer owing to the acid–base reaction. Thus, the HA films lost lesser volume than the human enamel did, indicating higher acid resistance. However, one film fabricated at α = 60° partially peeled off during t_acid = 6–9 h. Analysis of the PJD phenomenon through smoothed particle hydrodynamics indicated that the tensile residual stresses promoting crack propagation and delamination increased as α became more acute, causing HA film peeling. Therefore, particles should be blasted in multiple directions to avoid stress bias.