In the 1950's there were dozens of ballpoint pen models on the market, and nearly every one took a different cartridge. In 1953 Paul Fisher invented the "Universal Refill" which could be used in most pens.

Not content, Fisher continued working on making a better refill. After much experimentation he perfected a refill using thixotropic ink that would flow only when needed. The cartridge was also pressurised with nitrogen so that it didn't rely on gravity to make it work. The refill was dependable in the freezing cold and desert heat. It could also write underwater and upside down. The trick was to have the ink flow when you wanted it to, and not to flow the rest of the time, a problem Fisher solved.