In the U.S. patent system, if maintenance fees (required at 3.5, 7.5, and 11.5 years for $1,600, $3,600 and $7,400, respectively) are not paid, an issued patent goes into the public domain .
Great article and link to public domain patent database search found here;
(US7989436B2, US20070142307A1, US6660726B2, US6855703B1) I was a lead chemist in identifying novel estrogenic compounds present in hormone replacement therapy products.
That was interesting looking those up again. A couple appear to have changed hands (assignee) from who I thought owned them.
A side effect of getting your name on a patent is that you get lots of junk mail from companies wanting to sell you nice, but expensive, framed / etched patent awards for you to hang on your wall.