This study examined a technology called holographic optical trapping (HOT) in order to determine its performance in separating sperm cells from epithelial cells in sexual assault samples prior to DNA analysis.
Findings of the study show that HOT offers a powerful alternative approach for separating sperm from epithelial cells, thereby reducing the DNA carry-over problem. Even without the automation of HOT for sperm separation, the time taken for separating sperm from epithelial cells is less than 5 hours, making it amenable for adoption by forensic crime laboratories in their analysis of sexual assault forensic cases. Current practice in most crime laboratories involves a two-step differential extraction process, a chemical process that differentially disrupts sperm and epithelial cells, thus releasing their DNA for downstream STR analysis. This process is time-consuming, labor intensive and commonly results in female DNA carry-over into the male fraction, which interferes with DNA analysis and criminal identification. Optical trapping is a well-established scientific technique that has been widely applied in cell biology to manipulate cells. It is a non-destructive method in which trapped cells can live and reproduce while being held in the traps for extended periods of time. The author's company, Arryx, Inc. has developed a robust commercial research instrument that can simultaneously deploy and move multiple optical traps independently of each other in three dimensions, using the unique and patented Holographic Optical Trapping (HOT) technique. This technology could provide an automated means for separating spermatozoa from epithelial cells in sexual assault samples offering single cell resolution. The research described in this report focuses on various aspects of the technique and its use in handling sexual assault samples. Using mock forensic swabs, this research demonstrated the compatibility of optical trapping with PCR and STR analysis. 23 figures, 1 table and 28 references
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