Stealth wallpaper could keep LANs secure:

The company has produced panels using the technology to produce a screen that will prevent outsiders from listening in on companies' Wi-Fi traffic but let other radio and mobile phone traffic get through.

The FSS (Frequency Selective Surface) panels are made in the same way as printed circuit boards - layers of copper on Kapton polymer - and used on stealth bombers and fighter jets. They come in two varieties: passive, which is effectively permanent, and active, where various areas can be switched on and off to enlarge or limit the area of the network

(from silicon.com).