Touch DNA is one of the richest sources of forensic evidence at a crime scene. It is also little understood, regarding how it gets transferred (especially from a secondary source) and how it can best be found and collected. 

The pressure applied from skin onto a surface is a major driver of how much trace evidence crime scene technicians may find at a scene, according to a recent paper by the University College London in the journal Forensic Science International.

“These results show that pressure increases the transfer of DNA to a surface directly from skin, not just of DNA between surfaces,” write the scientists, from University College London. “Furthermore, these findings show that pressure can significantly impact the amount of DNA deposited, even when DNA deposition significantly varies between individuals.”

The trials were carried out in a controlled laboratory environment.

Polycarbonate boards were first completely sanitized with bleach and then UV radiation, according to their protocols.

Two volunteers then pressed hands down on the boards with low, medium or high amounts of pressure: either 4, 21 or 37 kilopascals. The volunteers also wore surgical masks to prevent DNA transfer by breathing or speaking.

The five fingermark points of contact were sampled with both a wet and dry swab. The DNA was extracted by the QIAamp DNA Investigator Kit, then sequenced by AmpFlSTR NGM Select, and thereafter interpreted GeneMapper IDX v1.3.

While DNA did not significantly vary from hand to hand and day to day for the DNA donors, how hard they pressed on the surface was a crucial factor, they said.

“Increasing the pressure between skin and surface significantly increased the amount of DNA deposited, which resulted in the detection of more alleles, from both the donor and unknown sources,” they conclude. “Pressure is a key variable for crime scene investigators and forensic examiners to consider when prioritizing items/surfaces that are likely to produce successful touch DNA results during a criminal investigation … This suggests that this pressure effect is independent of an individuals’ shedder status, although the pressure used in DNA deposition may impact the detection of shedder status.”

Touch DNA is a huge factor in current criminal investigation. However, increasing sensitivity and better collection methods mean there are still-undefined factors that need to be further studied by forensic scientists. Secondary transfer remains a major factor in criminal investigations. A landmark 2015 study by scientists at the University of Indianapolis determined that a lengthy handshake can transfer DNA from an unwitting donor to a knife they have never even seen. Although touch DNA can be fleeting or difficult to detect, sometimes samples are hardy enough to survive a pipe bomb blast, a team from Sam Houston State University found in a study published last year