Tiny and fast—even for microorganisms—human sperm are notoriously difficult to study. Nevertheless the team trained their tool on the male reproductive cells rather than easier quarry. Why? Because "sperm is one of the most important microorganisms in life," study leader Aydogan Ozcan said.

(Find out how a man produces 1,500 sperm a second.)

Ozcan and his team began by placing the sperm—obtained from a sperm bank—on a silicon sensor chip.

The researchers shone red and blue LED lights from different directions on the moving sperm—24,000 cells over the course of the study. Each sperm cast two different, and different colored, shadows, which the chip recorded. Later a computer program combined the two sets of data to reconstruct the cells' meandering paths.

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