Efficient separation of sperm and epithelial cells using optical trapping on a microfluidic device

DNA interpretation of biological mixtures is arguably one of the most challenging and time-consuming steps in the forensic DNA analysis workflow, often requiring the use of complex end-point mixture deconvolution procedures. This has led the research community to seek out reliable cell separation methods that are capable of isolating different cell types in an attempt to prevent mixtures entirely. A proposed alternative approach to differential extraction is optical trapping, a non-invasive method that utilizes the properties of light to create an optical trap for cell separation and manipulation within a reconstituted sample with minimal preparation. The optical trap is created by tightly focusing a laser beam through an immersion objective on an inverted microscope, where the force applied by the laser captures individual cells or cell clusters. While previous work demonstrated successful isolation and complete, single-source STR profiles by trapping 15 leukocytes or 40 sperm cells in an open droplet format, this work focused on optimizing optical trapping in a closed microfluidic device system as well as demonstrating success in capture and movement of epithelial cells. Using an improved laser and microfluidic device architecture, optical trapping was consistently successful in isolating and obtaining single-source profiles of both spermatozoa and vaginal epithelial cell contributors on a single microfluidic device, not only from simulated sexual assault mixtures but also from postcoital vaginal swabs. These innovations demonstrate the potential for optical trapping in forensic biological samples, and ultimately could reduce labor and time by eliminating mixtures at the beginning of the DNA analysis workflow.