Card Readers, by definition, are not mechanical because they have no moving parts. There is however one part that does move - the card when it goes in, and when it comes back out.
Since these cards CAN on occasions be removed from the digital camera and then a phone might ring, or you are distracted in some other way, the card CAN end up sitting on a foreign surface. It can, then, pick up minuscule particles of detritus that then find there way into the card reader itself.
Eventually one of these bits of muck will lodge where you would rather it didn't.
The only substrate I have ever found that is both effective and most importantly, trustwothy - is Litmus Paper. It is just the right 'feel', it tends to attract stuff it comes in contact with, and most importantly of all - it is long, thin, and just the right size.
You can get it at any Chemist / Drugstore and it has only a fraction of the destructive capability of a cotton bud! :D