Tyres tend to loose air overtime duo to its material and structures. Normally people have to inflate tyres once every several months and it is dirty job that will most likely get grease and dirt on hands.
A company in Japan called Nakano Iron Works Co developed a system named "automatic air replacement device for bicycle tires" is a device that can air your tyres while you pedal.
The innovative product works by converting the kinetic energy of the rotating wheels into the reciprocating motion of an air pump inside a hub at the center of the wheel. The compressed air is fed into the tire tube via a special hose, while appropriate air pressure is maintained by emitting surplus air from an air hole in the hose.
The device is said to maintain appropriate air pressure in the tires as long as the bicycle is ridden at least 3 kilometers a month.
Link to the article: A bicycle Air Hub That Completely Eliminates the Need to Pump Air into Tires
Link to the Company website in Japanese: Nakano Iron Works Co., Ltd.