Visualizing shed skin cells in fingerprint residue using dark‐field microscopy

This proof‐of‐concept study shows that dark‐field microscopy provides sufficient contrast for cell visualization in fingerprints with high sebum content. Although the application is limited to smooth surfaces that do not scatter light, such as polyethylene terephthalate (PET), it was able to measure the number of cells deposited within a fingerprint residue and the reduction in cell transfer with repeated skin contact. On a PET surface, at roughly 5 N of contact force, a typical finger transfers several hundred cells onto the surface. Over subsequent finger contacts onto a clean PET surface, this number decreased exponentially until a steady state was reached, which is characterized by the transfer of (78 ± 36) cells or (0.46 ± 0.21) cells/mm 2 when normalized for fingerprint area. High uncertainty in cell transfer was due to: the highly variable nature of a human finger (where the number of loose cells varies from person to person and from day to day depending on what they touch) and difficulties in controlling the contact force and finger movement such as twisting during deposition (where twisting of the finger can expose a new patch of skin to the substrate, increasing the number of cell transfer). Plasma etching was also explored as an effective way to validate dark‐field microscopy for cell counting. Although limited to inorganic substrates due to etching effects, exposing the fingerprint for less than 10 min can remove a majority of the sebum while keeping the cells intact for a before‐and‐after comparison using light microscopy.