Nano-recorder

Nanorecorders were personal audio-recording devices used by the Eleventh Doctor. They were small devices implanted in the palms of the users' hands, fusing itself with the cartilage there.

Use
In preparation for fighting the Silence, a race of aliens with the power to erase themselves from the memories of all who encountered them, the Doctor injected nanorecorders into the palms of Amy Pond, Rory Williams, River Song, Canton Delaware, and himself with a pistol-like injector.

When activated by the user pressing on their palm, the device recorded all the user's speech via a telepathic link attuned to the speech centres of the user's brain. In this way, when one of the TARDIS crew encountered the Silence, they could describe the encounter aloud and record it to their nanorecorder. The nanorecorder then glowed with a pulsing red light to indicate it had been used. Afterward, when the user had forgotten the encounter had happened, they would be alerted to it by their glowing palm. They could then activate the nanorecorder again to play back their own message.

When the Silence abducted Amy, they removed her nanorecorder and left it behind. Rory found the device, which had defaulted to live mode, allowing him to hear Amy's speech live via the still-functioning telepathic link.

After the Silence were defeated, Amy caught Rory using the nanorecorder to eavesdrop on her conversation with the Doctor and chided him for it. (DW: Day of the Moon)