The Zeedex was a device invented by the Vlinx for the purpose of resisting the Toymaker's influence over humanity. According to Kate Stewart, it disrupted the brain, flattened the spike of influence and kept the brain in a "calm" state. They were worn on the left arm.
History
The Zeedex was invented by the Vlinx and used by UNIT in late 2023 to disrupt the influence of a signal that changed the behaviour of humans. At the time of its conception, the exact source of the change in behaviour was unknown, with the Vlinx concluding that it was "generated inside the brain". In actuality, this change in behaviour was generated by the Toymaker through Stooky Bill.
The device was named Zeedex as it was a "good name", according to the Vlinx.
Certain individuals, such as Melanie Bush and Donna Noble, did not require the Zeedex, as they were unaffected by the Toymaker's influence. The Fourteenth Doctor speculated that long-term travel in TARDIS placed them "out of sync". Mel wore a Zeedex nonetheless "just in case".
Some of the wider public were resistant to the Zeedex, with Trinity Wells asserting that the devices were being used to control, monitor, and "microwave [the] brains" of the population on her own show. (TV: The Giggle [+]Loading...["The Giggle (TV story)"])