A flexible security document is provided which contains an authentication device including: a) source of electrical potential (5), the source including a piezoelectric polymeric material including at least one terpolymer of vinylidene fluoride (VDF), trifluoroethylene (TrFE) and a halogenated ethylene based monomer containing at least one non-fluorine halogen atom, the source of electrical potential being activated by mechanical deformation; b) reporter element (3) including a material capable of switching electrically between a first state and a second state, the difference between the first state and the second state being able to be perceived by an unaided human; and c) conducting elements (8) electrically connecting the source of electrical potential and the reporter element to produce an electric circuit. The reporter element (3) may take a number of different forms, such as a light emitting device which lights up or undergoes colour change when activated by the source of electrical potential to provide an indication of authenticity. In a particularly preferred embodiment, the flexible security document is a banknote and the source of electrical potential is applied by printing.