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Posted: Mar 28, 2014
Thin films yield high-detail latent fingerprints from nonporous surfaces
(Nanowerk News) Latent fingerprints - which cannot be seen without the application of a chemical or physical treatment - are among the most common forms of evidence used to link criminals to a crime scene and provide the proof needed to convict them. However, traditional methods for developing fingerprints (i.e. making them visible), may be inadequate when dealing with latent prints on nonporous surfaces such as knife handles, doorknobs, and garbage bags. A poorly developed print may lead to a wrong identification or no identification at all, putting an innocent person behind bars or missing an opportunity to catch the real perpetrator.
A latent fingerprint contains secretions from both the sebaceous and eccrine glands in the palms. Eccrine secretions are 98 percent water, so over time they will evaporate or become absorbed by other components of the fingerprint. Therefore, a slightly aged print is left primarily with only sebaceous materials (saturated fats, waxes and a hydrocarbon called squalene). To visualize sebaceous secretions, they have to be treated. Standard methods include applying a powder, coating with chemicals (such as ninhydrin and 1,8-diafluorene to react with amino acids or silver nitrate to react with chloride ions) or fuming with cyanoacrylate (the active ingredient in "super glues"). Unfortunately, these methods are not versatile enough.
To remedy the problem, three engineers at Penn State University - Akhlesh Lakhtakia and two of his doctoral students, Steve Swiontek and Drew Pulsifer (now at Intel) - deposited a columnar thin film on top of latent sebaceous fingerprints. "The CTF is an assemblage of nanoscale upright columns that capture the detail of a print's friction ridges [the raised layers of skin that define the print]," Lakhtakia says.
"To understand how it works, imagine putting your hand palm-side up on a table and standing identical pieces of chalk vertically upon the skin until the surface was completely covered. The tops of the chalk pieces would reproduce the shape of the palm, with the resolution of the 'image' increasing as you use smaller and smaller diameter chalk."
With the CTF print development method, the "chalk pieces" are infinitesimally small with the columns measuring approximately 60 nanometers (60 billionths of a meter, about half the size of an influenza virus) in diameter. This, Lakhtakia says, allows them to conform very tightly to the valleys and ridges of a fingerprint. "The coated print can easily be seen by the naked eye and its optical image manipulated by specialized software to yield forensically relevant data," he says.
Using a sophisticated computer analysis system, the Penn State team assessed the quality of fingerprints developed with the CTF method and compared the results to those achieved via traditional methods. CTF-processed prints were as good or better for most nonporous substrates tested. The researchers also found that the CTF method appears to be particularly suitable for developing partial bloody fingerprints, for which standard techniques are ineffective.
Finally, Lakhtakia and his students studied what happened if the columns were slanted instead of straight, and if the CTF was replaced by a dense homogenous layer. In both cases, the visualization quality of the treated fingerprints was significantly reduced.
With the validity of the CTF fingerprint development method established, the researchers will now seek funding for trial programs to see if crime laboratory personnel can be trained to use it. They also plan to investigate how environmental insults - such as prolonged exposure to moisture, dryness, high and low temperatures, etc. - impact the effectiveness of the CTF technique.