With a specially-designed overlay of concave and convex bristle trim patterns The Gentle Care Refill precisely adapts to the curvature of the tooth and gum line for your deepest and most thorough clean. The cushioning outer ring of bristles has the smallest diameter bristle filaments (4.5 mil.) of any Oral-B Electric Toothbrush Refill – to be extra gentle to protect sensitive teeth and gums. On Oral-B’s signature round brush head Bristles are specially configured to best surround each tooth for the best possible clean. Compatibility Compatible with Oral-B iO only.