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.
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.
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