Gum Trav-ler Interdental Brushes 0.9mm x 4 are used in addition to brushing to clean very narrow interdental spaces (interproximal brushing). They are ready to use and require no assembly and are hygienic thanks to the resealable cap. Gum Interdental Brushes 0.9mm x 4 have been specially designed for use outside the home (workplace, travel, etc.).