The interaction of focused ultrasound and dentin: Thermal, structural and chemo-mechanical characterization

This laboratory study aimed to develop an ex vivo model simulating pulpal micro-circulation to characterize temperature variations within the pulpal space following high intensity focused ultrasound (HIFU) exposure. In addition, to assess the interaction and effect of HIFU exposure parameters on human dental pulp stem cells (hDPSCs) response and the variations in dentin surface microstructure and chemo-mechanical characteristics. Thermal camera mapping and intra-pulpal temperature measurements with thermocouples were conducted (continuous measurement of 60 s) at HIFU intensities of 20 and 25 W with varying active-duty ratios. hDPSCs tolerance and viability were assessed via acid-phosphatase assay and live/dead fluorescence imaging. Subsequently the effect of HIFU exposure on the structure, chemical composition and nano-mechanical properties of dentin was characterised by scanning electron microscopy, atomic force microscopy , transmission electron microscopy , Raman spectroscopy and nano-indentation testing. Thermal analyses revealed two potentially thermally favourable HIFU exposure parameters: 20 W and 25 W at active-duty ratio of 1/6 ms. The two identified HIFU parameters were further investigated at 30 and 60 s treatment time on dentin surface. HIFU treatment did not result in statistically detrimental effects on the viability of hDPSCs (p > 0.05). HIFU treatment for 60 s effectively removed the overlying smear layer exposing dentin-tubules orifices and partial revealed collagen fibril's structure. No significant change in roughness or chemical composition (960/1450 cm −1 mineral/organic ratio) was detected after HIFU treatment (p > 0.05). However, HIFU treatment at 25 W for 60 s resulted in a significant decrease (p < 0.05) in surface hardness compared to sound dentin. HIFU treatment at 25 W for 60 s demonstrates effective removal of the dentin smear layer and partial exposure of dentin collagen fibrils without significantly altering the dentin chemical composition without adversely affecting the underlying hDPSCs.