Rapid regeneration of artificial enamel using laser-assisted biomineralization: With and without mesoscale hierarchy in hydroxyapatite rods

Rapid biomimetic replication of human hard tissues at the nanoscale is an important goal in the field of regenerative medicine. To this end, the repair of human tooth enamel using various techniques has been extensively investigated. In this study, enamel-like hydroxyapatite (HAP) layers were successfully grown on human enamel substrates within 3 min by laser-assisted biomimetic crystallization using indocyanine green (ICG) as a light absorbent. Laser irradiation of ICG-coated substrates in a pseudo-physiological solution enabled enamel regeneration by crystallizing micrometer-thick fluoride-substituted HAP rod layers. The HAP rods formed on the substrates possessed a c-axis-oriented morphology similar to that of natural enamel HAP. The results were reproducible across two substrates derived from two patients; however, the crystallized layers differed in their mesoscale structures. One layer exhibited a mesoscale hierarchical structure, in which approximately 20-nmwide elemental HAP nanorods self-assembled into approximately 230-nm-wide polycrystalline rods with a slight shift in the c-axis direction of each nanorod. The polycrystalline rods were further assembled to form HAP rod groups, generally aligned along the c axis. This layer integrated into the enamel through layers of lownumber-density HAP rods and non-oriented HAP nanoparticles. The layer grown on the other substrate lacked this characteristic mesoscale hierarchy, and a 150-200-nm-wide rod of the HAP single crystals grew without the low-number-density HAP layer. These differences can be attributed to whether or not geometrical selection has occurred at the interface between the enamel substrate and the crystallized layer. The proposed method has the potential to replace conventional resin-based treatments for the repair of early caries lesions.