Printing inks containing the photochromic protein bacteriorhodopsin

The biological photo chrome bacteriorhodopsin occurs in nature in the form of a 2D crystalline lattice. In this form, the so-called purple membranes, it is astonishingly stable towards chemical and thermal degradation. Variants of the naturally occurring bacteriorhodopsin can be switched between a purple and a yellowish state with yellow and blue light - no UV light is required. The application of bacteriorhodopsin in the form of a photochromic ink is described. In addition to the optical effect, which can be checked easily without instrumentation, additional optional security elements can be hidden in the material which are very hard to detect and to copy. Among them is the alteration of the aminoacid sequence of bacteriorhodopsin in positions which do not interact with its photochemical properties. By this and related methods even single production batches may be identified. The price of the material to date is too high for a broad commercial application but current efforts to reduce the production costs by several orders of magnitude look promising.