Energy transfer, volumetric expansion, and removal of oral biofilms by non-contact brushing

Non-contact removal of oral biofilms offers advantages beyond the reach of bristles, but it is unknown how energy transfer for removal from brush-to-biofilm occurs. In the present study we evaluated non-contact, oral biofilm removal by oscillating-rotating and sonic toothbrushes, and their acoustic output up to 6 mm distance. Whereas some brushes removed biofilm when used at a distance of up to 6 mm, others lost efficacy at a distance of 2-4 mm from the biofilm. Loss of efficacy was accompanied with high standard deviations and volumetric biofilm expansion. Both sonic and oscillating-rotating brushes caused fluid flows and the inclusion of air-bubbles, while non-contact acoustic energy-transfer was demonstrated to decay with distance for both types of brushes. We put forward the following mechanism for non-contact removal: (i) brush energy is absorbed by biofilm, resulting in the visco-elastic expansion of the biofilm; (ii) if the energy absorbed is sufficient and deformation is beyond the yield point, biofilm removal occurs; and (iii) if deformation is in the plastic range but below the yield point (i.e. at the limiting distance for non-contact removal), biofilm is expanded but not removed.