Nano-cam
From Tardis Wiki, the free Doctor Who reference
Nano-cams were a miniature observation device. Cuthbert Simpson implanted them into two of Richard Harries' rats. He then used them to observe others in Banquo Manor. Even after Simpson was blinded he was still able to see via the nano-cams and so kept the rats with him. (EDA: The Banquo Legacy)
