· Only compatible with Duo Pro and not compatible with Duo· ProFlex Brush Heads - AquaSonic's improved ProFlex Brush heads offer improved plaque removal with precisely angled bristles with alternating hard and soft areas that are gentler on gums and slightly harder on teeth· Improved Plaque Removal - ProFlex brush heads offer 40% improved plaque removal when compared to standard brush heads· Improved Whitening - Whitening is improved 20% with stiffer bristles on the surface enamel while outer bristles remain gentle on gums.