For those worried about leaving a chemical in the fabric, the ink in these pens is acid free and archival quality (though the people at pilot thought I was crazy for asking that- they highly do not recommend using them for archiving, due to the ink disappearing at high temperatures- not good for long term storage, where it might disappear)
The chemicals will not harm fabric. And, it washes out anyway, so it is really only a concern if you aren't washing it.