Fisher Space Pens®

In the 1960's, after much experimentation Paul Fisher perfected a pen refill using thixotropic ink, which stays semisolid until the shearing action of the rolling ball liquefies it, so that only flows when needed. The cartridge was pressurized with nitrogen so that it didn't rely on gravity to make it work, so it was dependable in freezing cold and desert heat. It could also write underwater and upside down. It even works in space! The astronauts on the Apollo 7 mission used the Fisher® AG-7.