The space pen is a specially designed pen that can work in zero-gravity, underwater and at lots of temperatures. A regular pen requires gravity in order to work, so in zero-gravity situations such as on the space station, they cannot work. So how does the space pen work without gravity?
The space pen was developed in the 1960s by the Fisher Pen Company, they invested about a million pounds in research for the project.
To allow it to write in zero-gravity, the ink is in a highly pressurised reservoir, with high pressure nitrogen gas separated from the ink. This high pressure also increases the amount of ink stored in the pen so it can last up to 3 times longer than a normal pen.
The ink itself is a thixotropic ink (meaning it will be solid when stationary but liquid in certain conditions), so will only flow when writing. This helps to prevent it from leaking as well as preventing it from evaporating. Usual pens will rely on gravity but this pen uses both pressurized gas and a clever ballpoint design, containing small peaks which break the ink down into liquid form to allow it to flow from the reservoir.
This means that the pen can work in zero-gravity but also at high altitudes and upside down. Because of these properties NASA used them on all their missions from 1968 onwards.
Most fluids (known as Newtonian fluids) will only change viscosity when they are subjected to a change in temperature or pressure, but thixrotopic materials also reduce viscosity when stress increases.
This means that these materials will be in a gel form, or like a solid, when no stress is applied (for example when pen is not being used). If stress is applied it will return to being a liquid which flows easily.