Fingering the criminals

Forensic experts in Japan claim to have succeeded in developing a method for obtaining fingerprints left on human skin.

The discovery provides a vital breakthrough in gathering evidence and narrowing down suspects in rape, sexual assault or murder cases.

Fingerprints can currently be obtained from smooth surfaces such as windows, painted surfaces and plastics. Following a crime, these are sprinkled with powder made from substances such as aluminium. This then sticks to natural oils secreted by the skin’s sweat glands, allowing the shape of the print to become clear.

However, identifying patterns from skin oils left on a victim is extremely difficult as these become mixed with skin wrinkles, making powder identification hard.

A team at the forensic science division of the Kagawa Prefecture Police in Takamatsu, Japan, used the fact that the amount of oil deposited on a victim increases with the force of an attacker’s grip.

In order to obtain a print, the team pressed a plastic sheet on to the skin to take an impression of the oil deposits. This was then sprayed with a chemical dye that turned the print blue.

By heating the resulting sample at a low temperature, a colour contrast between oils from the victim and the attacker becomes clear, allowing investigators to achieve a readable print.

This can then be compared to a national criminal database or samples from suspects. Japanese police plan to put the technique into use later this year.

However, there are some limitations to its effectiveness. While collecting prints from a corpse is relatively easy owing to the fact that it no longer sweats, living victims’ sweat may eventually corrupt the print.

Ideally, the image should be taken as soon as possible after the crime has been committed and within four to five hours of this at the most.

The ability to take prints from skin has long been a goal of forensic scientists.’There have been claims that such techniques have been developed before, mostly by US firms,’ said Ronald Cook, director of fingerprint development and analysis laboratory DABS and an assessor for the UK’s Council for Registration of Forensic Practitioners.

‘After closer examination they failed to live up to their promise, though. Skin oils may also be contaminated with blood or substances such as grease, so getting a true image is often hard, even if a print is found on a smooth surface.

‘But if this technique works just once out of 100 times, that would be a success in terms of achieving a prosecution, so if it works we would definitely consider it.’