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In some situations it useful to have a material in which air passes but water, and water vapor, does not. Printing cell membranes or cases for something that must be immune to humidity changes (like something that holds a highly oxidative metal) are two examples.

Teflon (PTFE) is a material that has this property. But PTFE is horrendous to 3D print.

Silicone is another material that has this property. 3D printing silicone can be done with a syringe mechanism. But there are also thermoplastic rubbers (e.g. TPU, TPE) that are designed to mimic Silicone.

Are there any filaments on the market which naturally form holes with the porous diameter for this feature? Is there any way to manipulate the diameter of this hole, even starting from pellets? Are some filaments more likely than others to have this feature?

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    $\begingroup$ I doubt you'll find many materials - aside from 3d filaments - which can separate gaseous H20 from the other molecular gases present in our atmosphere. $\endgroup$ – Carl Witthoft Aug 6 '19 at 17:14
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    $\begingroup$ The water vapor molecule is smaller than an Oxygen or Nitrogen molecule. There is no such material in this world. Can you cite a source of that property of PTFE? $\endgroup$ – user77232 Aug 6 '19 at 18:03
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    $\begingroup$ Good point about the size of water, but one thing water is, is polar. So its polarity can be used to filter it out more easily despite its size (like in a cell's phospholipid membrane). $\endgroup$ – K Mmmm Aug 7 '19 at 13:40
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    $\begingroup$ Well, there you go, then-- just use phospholipid-based filament. (Yes this is a stupid joke) $\endgroup$ – Carl Witthoft Aug 7 '19 at 13:56
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It is difficult enough to make 3D-printed objects water-tight (unless you paint them). Making them permeable to air but not water vapour is too big an ask, I think.

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