Analysis of sperm motility using optical tweezers.

This study examines the use of optical trapping as a quantitative measure of sperm motility. The effects of laser trap duration and laser trapping power on sperm motility are described between sperm swimming force, swimmimg speed, and speed of progression (SOP) score. Sperm (SOP scores of 2-4) were trapped by a continuous-wave 1064 nm single-point gradient laser trap. Trap duration effects were quantified for 15, 10, and 5 seconds at 420 mW laser power. Laser power effects were quantified at powers of 420 mW, 350 mW, 300 mW, and 250 mW for five seconds. Swimming force, swimming speed, and SOP score relationships were examined at a trap duration and trapping power shown to minimally affect sperm motility. Swimming forces were measured by trapping sperm and subsequently decreasing laser power until the sperm escaped the trap. Swimming trajectories were calculated by custom-built software, and SOP scores were assigned by three qualified sperm scoring experts. A ubiquitous class of sperm were identified that swim with relatively high forces that are uncorrelated to swimming speed. It is concluded that sperm swimming forces measured by optical trapping provide new and valuable quantitative information to assess sperm motility.