A

Stiffer nylon bristles are produced by increasing filament diameter and/or shortening filament length—stiffness is directly proportional to diameter squared and inversely proportional to length cubed in beam-theory models of toothbrush tufts [Rawls, 1990, PMID 2079171]. Experimental studies also show that bristles of larger diameter yield higher measured stiffness classes, confirming the diameter effect [Yankell, 2001, PMID 11507920].