Skin is made up of several layers, including the basal layer, which separates the outer epidermis and the inner dermis. Fingerprint development begins when the foetus is about ten weeks old and the basal layer begins to grow faster than either the epidermis or dermis in the tips of the finger. This leads to an increased stress in the basal layer, which causes it to buckle inwards, creating ridges on the surface of the skin. This explanation was first suggested in the 1920s by the Norwegian scientist Kristine Bonnevie but was subsequently forgotten.

Michael Kücken and Alan Newell at the University of Arizona developed a model in which the basal layer is an elastic sheet confined between the epidermis and dermis, which are modelled as beds of weakly nonlinear springs (figure 1). Due to differential growth -- and the fact that it is constrained -- a compressive stress builds up in the sheet that represents the basal layer. If the compressive stress is high enough, a buckling instability occurs. The scientists analysed this process by minimising the total elastic energy in their system.

Kücken and Newell say that boundary forces -- produced at finger creases and nails -- are the main source of stress in the fingertips of a developing foetus. They also note that stress is produced as the tips of our fingers (known as volar pads) shrink when fingerprints begin to develop. This creates ridges that are perpendicular to the direction of the stress.

Kücken and Newell also observed that the formation of the three basic patterns of fingerprints -- known as arches, loops and whorls -- is related to the geometry of the volar pads as they shrink. Previous research has shown that highly rounded volar pads produce whorls, for instance, and that flatter pads produce arches. The Arizona duo found they could reproduce these patterns in their simulations (figure 2).

They now plan to focus on the defects in the patterns that are important in fingerprint identification, including ridge endings and bifurcations.