Tiny device brings out the best in sperm. (Technology).

Sperm cells prepare a lifetime for just one goal: the race to an unfertilized egg. Now there's a microscale apparatus that pits the little wigglers against each other in a preliminary heat. By separating top swimmers from the rest, the itsy-bitsy racecourse may ultimately improve the odds for infertile couples.

In many eases of infertility, the semen contains an insufficient percentage of normal, mobile sperm. Centrifuging and other existing means of separating the healthier sperm cells from the legions of listless ones typically net a low percentage of good swimmers and may even damage them.

The new sperm sorter boosts the percentage of mobile sperm from an average of 44 percent in a semen sample to 98 percent in fluid removed from the device, report Gary D. Smith and Shuichi Takayama of the University of Michigan in Ann Arbor and their colleagues.

As noted in Nature's online news service and described in an upcoming Reproductive BioMedicine Online, two narrow conduits in the device merge into a broader channel a few human hairs wide. That geometry exploits a well-known quirk of miniature fluid flows. When tiny, undisturbed streams of liquids merge, the contents of those streams tend not to mingle. "That's the beauty of this system," Smith says.

In the sorter's main channel, semen running within one conduit meets a spermfree solution from the other conduit. Only active sperm cross the border between the two flows, becoming concentrated in the formerly spermfree solution.

While sorted sperm from a man haven't yet been tested on a woman's egg, mouse eggs have been fertilized by sorted mouse sperm, Smith says.--P.W.