Sorting out Sperm Cells

FORENSIC SCIENCE

A simple microfluidic method could substantially decrease the time required to prepare samples for forensic analysis of sexual assault evidence [Anal. Chem., 77, 742 (2005)]. Faster sample preparation could help eliminate the backlog of such evidence waiting to be analyzed.

Current methods to prepare sexual assault evidence for analysis are based on a process called differential extraction. This technique relies on the ability of sperm cells (from the perpetrator) to survive chemical conditions that rupture the membranes of other cells (primarily epithelial cells from the victim). It is applied to the evidence still on the cotton swab used for collection.

Chemist James P. Landers and his coworkers at the University of Virginia take a different approach: Sort the cells before rupturing any. They have developed a microfluidic device that exploits the physical differences between the two types of cells to separate them.

The simple device consists of two reservoirs connected by a channel. The epithelial cells (cheek cells in the demonstration samples) settle to the bottom of the inlet reservoir, which takes four to five minutes. Then, a pressure-induced flow sweeps the sperm cells into the second reservoir. The separated cells can then go through normal DNA analysis.

Landers must address a number of issues before the method could become practical for real forensic samples. His team is finding that desorption of the cells from the cotton swabs could be an issue. In addition, because cells can rupture while the samples are dry during storage, free DNA in the samples could be another concern, which may have to be addressed by chromatography.

Susan Greenspoon, a forensic molecular biologist with the Virginia Division of Forensic Science, says that "the device is not yet up to the level of performance we can obtain using either semiautomated robotic or manual differential extraction." But she believes that, with further development, it "has real potential to supplant our current methods."