Optical tweezers as an effective tool for spermatozoa isolation from mixed forensic samples

Area of Science:

• Forensic Science
• Biotechnology
• Optics

Background:

• Optical tweezers utilize a focused laser beam to trap and manipulate microscopic particles, including biological cells.
• Spermatozoa are crucial in forensic investigations, but their isolation for DNA analysis can be challenging.
• Current methods for sperm cell isolation may impact DNA yield and profile completeness.

Purpose of the Study:

• To optimize sperm cell collection using optical tweezers for maximum DNA yield.
• To determine the minimum number of trapped sperm cells required for a complete STR DNA profile.
• To assess the efficacy of optical tweezers in separating mixed cell populations relevant to forensic evidence.

Main Methods:

• Formation of a single-focus optical tweezer using a high numerical aperture immersion objective.
• Isolation and manipulation of individual spermatozoa within an optical trap.
• Collection of trapped sperm cells into a capillary for subsequent DNA analysis.
• Processing of isolated sperm cells using conventional Short Tandem Repeat (STR) analysis.

Main Results:

• Approximately 50 trapped spermatozoa yielded a consistently full DNA profile.
• A complete single-source DNA profile was obtained from a mixture of sperm and vaginal epithelial cells using optical trapping.
• Optical tweezers successfully separated mixed populations of sperm and epithelial cells.

Conclusions:

• Optical tweezers are a viable technology for forensic applications requiring precise cell isolation and manipulation.
• This method offers potential for improved DNA yield and profile quality in forensic casework.
• Optical trapping can facilitate the analysis of complex forensic samples containing mixed cell types.