Orientation- and wavelength-dependent sperm motility in optical trapping

Optical tweezers technology has the characteristics of noncontact manipulation in three dimensions and steerable separation in solutions, and could be applied to obtain a separated sperm with high quality for intracytoplasmic sperm injection (ICSI). However, the effects of optical tweezers on sperm motility are still unclear. To elucidate the effects on sperm motility for optical tweezers, we systematically investigated the correlation between motility parameters and the parameters of optical tweezers (wavelength, power, trapping duration, and trapping orientation). Under three systems of optical tweezers with different laser wavelengths (1064, 850, and 785[Formula: see text]nm), the nine motility parameters of free swimming were mainly affected by trapping orientation (vertical/horizontal) and trapping duration. When 850[Formula: see text]nm laser and 1064[Formula: see text]nm laser are used, vertical trapping significantly reduces sperm free-swimming capability with prolonged exposure time, whereas horizontal trapping exhibits relatively minor interference on sperm motility. Notably, the 785[Formula: see text]nm laser does not induce statistically significant changes in key parameters of sperm motility under any experimental conditions (trapping orientation and duration). For the rolling frequency of trapped sperm, horizontal trapping for three wavelengths has a negligible effect compared with vertical trapping, especially for the 785[Formula: see text]nm laser. In conclusion, horizontal trapping can preserve sperm motility under low power (below 140[Formula: see text]mW at 1064[Formula: see text]nm, below 100[Formula: see text]mW at 850[Formula: see text]nm, below 60[Formula: see text]mW at 785[Formula: see text]nm) and short duration (below 4[Formula: see text]min). This trapping duration is sufficient for the separation procedure of single live sperm in ICSI. This study provides critical parameter optimization guidelines for the safe application of optical tweezers technology in reproductive medicine.