Frictional characteristics of various bracket materials (stainless steel [SS],136 single-crystal sapphire [SCS],124 polycrystalline alumina [PCA],123,124,136 and polycrystalline zirconia [ZrO2]125) in combination with metallic archwires (stainless steel [SS], cobalt-chromium [CoCr], nickel-titanium [NiTi], and beta-titanium [β-Ti]) (A) in the dry state; (B) in the wet state using human saliva