Not vented so much as just not air tight-- no reason to make it air tight, and the inside of the pen is by no means sealed, as you can tell if you've ever taken

Not vented so much as just not air tight-- no reason to make it air tight, and the inside of the pen is by no means sealed, as you can tell if you've ever taken one apart (or had it explode inside your bag). The parts just don't fit that well together. Which I guess is a vent in a sense, although that's probably not the main design consideration. The (obvious) exception being the space pens, which do have some pressure chambers. They solve the vacuum issue by being pressurized, which has the nice benefit of letting them write upside down. I imagine they've made it viscous/non-Newtonian/whatever (not sure on the technical fluid mechanics term) enough such that gravity pushes/pulls it down towards the tip but prevents it from flowing the other way. Edit 1 (just before getting in the shower): Removed references to air pressure, as I don't think that makes a difference. Edit 2 (post shower): Air pressure might a difference after all, it depends on where the seal is. If there's an air tight seal on the tip end (which there might be, if that's how it's kept from leaking) then air pressure might add to the "only goes one direction" effect.