Poly-deutonic alloy
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. ()

It served to help detect The Doctor aboard , since there was no other poly-deutonic alloy aboard. ()

de: Polydeutronische Legierung
Materials