Separating the male and female components of sexual assault evidence using sound waves could vastly cut the time it takes to identify suspects, researchers in the US and Sweden report.

Typically, crime labs take up to twelve hours to isolate sperm cells from a sexual assault sample. This is because female skin cells greatly outnumber sperm cells, so careful handling and multiple cycles of centrifuging and washing are required. 

This new method promises to greatly simplify the process, and takes only fourteen minutes to extract sperm cells from the mixture. It also leaves a very clean sample that can immediately be used for DNA analysis. 

'We call our process an acoustic tweezer,' said Thomas Laurell, who worked on the project at Lund University in Sweden. 'We can use this to pinch different cells into different locations.'

Laurell explains that the process is similar to a drum skin being hit by a sound wave at a specific frequency - causing grains of sand on the drum's surface to gather at fixed locations. In this case, ultrasound waves are used to vibrate sperm cells out of the mixture.

The technique could enter crime labs in its current form, but the team are developing a simpler method based on a one-time-use chip. The chip comprises a glass slide where the sample is entered and an acoustic unit that vibrates the surface - capable of performing a single separation. 

James Landers, who led the project at the Universityof Virginia, US, is confident that each chip should not cost more than a dollar, and hopes to see similar technology integrated into larger automated DNA analysis suites in the future - bringing together extraction, amplification and detection.

Max Houck, Director of the Forensic Science Initiative at West Virginia University, US, agrees that there is a lot of potential. 'This new process is a tremendous step forwards but the real promise lies in the integrated lab-on-a-chip system that can be sold as a turn-key product to forensic laboratories,' he told Chemistry World. 

'These laboratories are very conservative by nature and it will take what I call the Law of Twice (twice as fast, twice as cheap, or twice as reliable) for this to be accepted,' Houck says. 'Given the times they are talking about, the researchers are certainly beyond twice as fast.'

Lewis Brindley

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