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May 19, 2011
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Tim Dees Police Tech & Gear
with Tim Dees

Mat-transfer latent fingerprints

New technique reveals prints in seconds, nondestructively and in daylight

A technology developed at Nanjing University in China allows latent fingerprints to be developed in seconds, using only a special mat and hot air. The method was published on March 17 in the technical journal Angewandte Chemie. Central to the technique is a microfiber mat made from thermoplastic polyurethane (TPU) resin and fluorescein. TPU is an inexpensive commercial resin in common use; fluorescein is an organic compound used as a tracer in medical and industrial applications. In this research, the mats were fabricated using an electrospinning process that produces a pliable fabric of uniform nanofibers about 300 nm in diameter (a human hair varies between 18,000 and 80,000 nm). The finished mat material is straw-colored.

When the mat material is applied to a surface holding a latent fingerprint, the chemical components of the latent print react with a free isocyanate group contained in the TPU. The free isocyanate groups cross-link with one another, releasing the fluorescein. If the mat is left at room temperature, the transferred print(s) will appear on its own within four days. But if the mat is then heated to around 100° C with hot air, the print will develop within 30 seconds. Developed prints appear red on the straw background. Once developed, the transferred print can be photographed or otherwise preserved for comparison and archiving.

The mechanism that causes the TPU fibers to release the fluorescein is called a release-induced response process. When the sweat compounds react with the TPU, a phase change results between the TPU fibers and the fluorescein. The released fluorescein allows visualization of the latent print.

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This technique has several advantages over more traditional methods of developing latent prints, which usually involve application of powder, liquid chemical sprays, or chemical vapor to the surface where the latent print resides. Because the latent print is transferred to the mat, rather than developed on the surface, the process is nondestructive. Once located and identified, the print can be re-processed for trace DNA or drug metabolites. No special light source is required, and the developed print is visible in daylight. There are other techniques that cause latent prints to fluoresce in place, but the fluorescence is usually visible only with the aid of UV light and/or under very low light conditions. The biggest drawback of this new technique is that the mats will have to be sealed until ready for use, and then handled only with uncontaminated gloves, as handling them with bare fingers would transfer sweat from the technician’s fingers onto the mat and render it useless. The only equipment needed will be the special mats and a hardware store heat gun.

This research is very new, and there are as yet no commercial products that incorporate the technology. While it’s technically possible to produce the TPU mats in a lab, most forensic laboratories aren’t equipped with the electrospinning equipment necessary to fabricate suitable mats. Once the technology is adapted for commercial use, the mats should not be terribly expensive. Both TPU and fluorescein are in large-scale commercial use, and neither is especially costly.

 


References
Yang, S., Wang, C.-F. and Chen, S. (2011), A Release-Induced Response for the Rapid Recognition of Latent Fingerprints and Formation of Inkjet-Printed Patterns. Angewandte Chemie International Edition, 50: 3706–3709. doi: 10.1002/anie.201006537

 

About the author

Tim Dees is a writer, editor, trainer, and former law enforcement officer. After 15 years as a police officer with the Reno Police Department and elsewhere in Northern Nevada, Tim taught criminal justice as a full-time professor and instructor at colleges in Wisconsin, West Virginia, Georgia, and Oregon.

He was also a regional training coordinator for the Oregon Dept. of Public Safety Standards & Training, providing in-service training to 65 criminal justice agencies in central and eastern Oregon.

Tim has written more than 300 articles for nearly every national law enforcement publication in the United States, and is the author of The Truth About Cops, published by Hyperink Press. In 2005, Tim became the first editor-in-chief for Officer.com, moving to the same position for LawOfficer.com at the beginning of 2008. He now writes on applications of technology in law enforcement from his home in SE Washington state.

Tim holds a bachelor’s degree in biological science from San José State University, a master’s degree in criminal justice from The University of Alabama, and the Certified Protection Professional credential from ASIS International. He serves on the executive board of the Public Safety Writers Association.

Dees can be reached at tim.dees@policeone.com.

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