Sperm motility is an important concept in fertility research. To this end, single spot laser tweezers have been used to quantitatively
analyze the motility of individual sperm. However, this method is limited with throughput (single sperm per spot), lacks the
ability of
in-situ sorting based on motility and chemotaxis, requires high laser power (hundreds of milliWatts) and can not be used to dynamically
monitor changes in sperm swimming behavior under the influence of a laser beam. Here, we report a continuous 3-D ring-shaped
laser trap which could be used for multi-level and high-throughput (tens to hundred sperm per ring) sperm sorting based on
their motility and chemotaxis. Under a laser power of only tens of milliWatts, human sperm with low to medium velocity are
slowed down, stopped, or forced to change their trajectories to swim along the ring due to the optical gradient force in the
radial direction. This is the first demonstration of parallel sperm sorting based on motility with optical trapping technology.
In addition, by making the sperm swimming along the circumference of the ring, the effect of laser radiation, optical force
and external obstacles on sperm energetics are investigated in a more gentle and quantitative way. The application of this
method could be extended to motility and bio-tropism studies of other self-propelled cells, such as algae and bacteria.
Keywords Laser trapping - Sperm motility - Cell sorting - Axicon