Bio-damper

A bio-damper is a device, which can be as small as a ring that masks the bio-signature of an individual. The Doctor gave Donna a bio-damper so that she would be hidden from the Racnoss. However, bio dampers are ineffective against Huon particles. (DW: The Runaway Bride).

Martin Trueman had a bio-damper to hide the fact that he had the Ancient Lights in him. (SJA: Secrets of the Stars)