Can someone explain what "inter partes review" and "ex parte reexamination" are when challenging patents with prior art?
The question asked for an explanation of inter partes review and ex parte reexamination when considering challenging patent with prior art. This is a big question that cannot be fully answered here.
These are the two primary administrative procedures currently available to a third party to challenge an issued US patent. My answer on this other post discusses a few other options.
In both procedures, a third party can challenge a patent based upon prior art that arguably renders the patent invalid as either anticipated (not novel) or obvious.
The primary difference between the two options involves process.
In ex parte reexamination, the third party is merely a "requester" and has no direct input in the reexamination following the submission of that request. Further, ex parte reexamination is conducted by patent examiners (technical experts) within the USPTO's central reexamination unit (CRU).
Inter partes review is a new process that only became available in September 2012. As such, only a handful of inter partes reviews have been filed and none have been completed (other than some requests rejected as inadequate). Unlike ex parte reexamination, the requester in an inter partes review remains a party to the dispute has an opportunity to continue to file opposition briefs and present evidence throughout the process. Inter partes reviews are conducted by administrative patent judges (legal/technical experts) that serve as members of the Patent Trials and Appeals Board (PTAB).
In one sense, the reexamination is closer to ordinary patent examination while the inter partes review is more like ordinary patent litigation. While both are expensive, hiring a patent attorney to conduct an inter partes review run in the hundreds of thousands of dollars.
There is some fear that ex parte reexamination is ineffective. The idea is that if the patentee already convinced an examiner once, they can probably do it again. That fear used to have some merit when reexaminations were sent back to the same examiner who issued the original patent to check his own work. With the creation of the central reexamination unit, that fear is misplaced. The results from ex parte reexaminations have been impressive with the bulk of reexamined patents being narrowed or outright cancelled.