New ‘laser tweezers’ could help solve sexual assault cases much faster

6 Feb 2019

Image: © kasto/

In the delicate circumstance of a sexual assault case, a forensic lab could soon be able to use a ‘laser tweezers’ to find whose DNA belongs to whom.

When genetic material is recovered from a potential crime scene, every precaution has to be taken to make sure that it can be correctly paired with the individuals involved in the alleged crime. However, trying to sort this out can be incredibly difficult and time consuming, with Virginia Commonwealth University’s Tracey Dawson Cruz describing it as “one of the most challenging problems in forensic DNA”.

Now, along with her colleagues, Dawson Cruz is attempting to solve this issue – particularly when it comes to sexual assault cases – by developing technology that can determine which DNA profile came from which person at each genetic locus. This can be achieved using their new ‘optical tweezers’ – a compact, strongly focused laser beam that uses an immersion objective lens on an inverted microscope to create an optical trap.

This makes it extremely effective at separating mixed cells, such as sperm and vaginal cells, within a solution. In testing, the process showed that crime labs could consistently obtain full DNA profiles from just 50 tweezered sperm cells. By comparison, a similar attempt a decade ago needed 500 cells.

Explaining how it works, the researchers said that, as it’s easy to identify sperm cells under a microscope, the laser tweezers is able to pick them out and isolate them from other cells to be extracted for DNA analysis.

Put on a microchip device

“Because [the process] is manual, it is really important that we set the number of individual cells that have to be tweezed out of the mixture,” Dawson Cruz said. “Because, if that is very high, then it could be time prohibitive and it could be too lengthy of a process for a lab to want to take on, so the finding of only needing 50 sperm cells is pretty good.”

The next stage in the research is to create a microchip device that would allow forensic science labs to apply it easily to their day-to-day work analysing DNA evidence.

Dawson Cruz’s colleague, Sarah Seashols-Williams, explained what would be necessary to achieve this.

“If it’s simplified, I think labs would be amenable to implementing it. Our challenges right now are transferring the cells into the DNA workflow,” she said. “That’s why we want this all-in-one device so it’s simplified and so we won’t lose cells in the transfer.”

Colm Gorey was a senior journalist with Silicon Republic