The force generated by the radiation pressure of a low power laser beam induces an optical trap which may be used to manipulate sperm. We studied the effect of the optical trap on sperm motility. A Nd:YAG laser beam was coupled to a conventional microscope and focused into the viewing plane by the objective lens. Sperm were caught in the trap and manipulated by a joy stick controlled motorized stage. After different exposure periods, the velocity and patterns were analysed by a computerized image processor. There were minor changes in sperm velocity when exposed to the trap for 30 seconds or less. A gradual decrease in the mean linear velocity was observed after 45 seconds of exposure. This optical micromanipulator may also be useful for studying the force generated by a single spermatozoa and evaluating the influence of drugs on motility.

The development of the single beam gradient force optical trap has made it possible to manipulate cells solely by laser light. A continuous wave Nd:YAG (1.06 microns) laser beam was directed into a conventional microscope and focused onto the viewing plane by the objective lens. The laser beam power at which human sperm were released from the trap was measured and correlated to the sperm's linear velocity before trapping. The mean trapping power readings for slow, medium, and fast motile sperm were 57, 73, and 84 mW, respectively. The analysis of measurements over the total population demonstrated that zig-zag motile sperm had significantly higher mean power readings when compared with straight motile sperm with similar mean linear velocities. In two cases, specimens required significantly less trapping power when the measurements were repeated 24 hours later.