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Visualizing shed skin cells in fingerprint residue using dark-field microscopy
Shin Muramoto1, William Osborn1, Greg Gillen1
1National Institute of Standards and Technology, Gaithersburg, MD, 20895, USA.
Abstract:
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/mm2 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.
Insights
Dark-field microscopy effectively visualizes cells in fingerprints on smooth surfaces like PET. This method quantics cell transfer, revealing exponential reduction with repeated contact.
Area of Science:
- Forensic science
- Biomolecular imaging
Background:
- Fingerprint analysis often relies on ridge patterns.
- Cellular material within fingerprints offers additional forensic information.
- High sebum content in fingerprints can obscure cellular details.
Purpose of the Study:
- To evaluate dark-field microscopy for visualizing cells in high-sebum fingerprints.
- To quantify cellular deposition and transfer dynamics on smooth surfaces.
- To explore plasma etching as a method for enhancing cell visualization.
Main Methods:
- Dark-field microscopy was used to image cells in fingerprint residue on polyethylene terephthalate (PET).
- Cell transfer was measured across repeated skin contacts under controlled force.
- Plasma etching was investigated for sebum removal to aid microscopy.
Main Results:
- Dark-field microscopy provided sufficient contrast for cell visualization in fingerprints.
- Cell transfer decreased exponentially with repeated contact, reaching a steady state of 78 ± 36 cells.
- Plasma etching effectively reduced sebum while preserving cell integrity for microscopy.
Conclusions:
- Dark-field microscopy is a viable technique for visualizing and quantifying cells in fingerprints on smooth, non-light-scattering surfaces.
- Understanding cell transfer dynamics is crucial for forensic applications.
- Plasma etching offers a complementary method for sample preparation in cell-based fingerprint analysis.
