Poly-deutonic alloy was a metal alloy native to the 29th century. The Doctor's mobile emitter was constructed from this material.

The exoskeleton of One was composed of a poly-deutonic alloy. Seven of Nine theorized that her nanoprobes must have extrapolated that form of technology from the mobile emitter, as it was not native to the 24th century. (VOY: "Drone")

It served to help detect the Doctor aboard USS Voyager, since there was no other poly-deutonic alloy aboard. (VOY: "Renaissance Man")