A number of district courts, as well as the Court of Appeals for the Federal Circuit, have weighed in on whether and to what extent a patent challenger in an inter partes review (IPR) before the Patent Trial and Appeal Board may be estopped from making prior art based challenges in a district court litigation. In Ironburg Inventions Ltd. v. Valve Corporation, the Federal Circuit has clarified that the patent owner, not the patent challenger, bears the burden of proof on estoppel.

In Ironburg, after the conclusion of an inter partes review (IPR), the patent owner, Ironburg, argued that the patent challenger, Valve, was estopped from relying in district court on prior art that was not at issue in the IPR.

In its defense, Valve provided evidence that its own search did not turn up prior art that appeared in another IPR petition, by another challenger, on the same patent. The district court nonetheless held that there was estoppel, and Valve appealed. The Court of Appeals for the Federal Circuit reversed, and placed the burden of proving estoppel on Ironburg, the patent owner, instead of Valve.

The Federal Circuit held that a patent owner asserting estoppel against a patent challenger after an IPR bears the burden of showing that “a skilled searcher exercising reasonable diligence would have identified an invalidity ground” on which the patent challenger wants to rely in district court.

The Federal Circuit’s holding has prompted discussion about how a patent owner might sustain its burden of proof on estoppel. One suggestion is that the patent owner commission its own search, by a skilled searcher. There are concerns with this approach, however. What is a patent litigator to do?

Pitfalls of Patent Owner-Commissioned Search

First, it is normal in patent litigation for an accused infringer challenging validity to request production of any prior art that the patent owner has. The prior art itself, independent of source, is neither privileged nor confidential, and so the patent owner has to produce it. If a patent owner were to commission a prior art search around the time that the accused infringer filed an IPR petition, the patent owner could rely on the search results as being from a “skilled searcher,” to bolster an argument of estoppel. However, because the patent owner would have to produce that prior art, that production could give the accused infringer prior art that is outside of the IPR proceeding and is not barred by any estoppel. The accused infringer then could argue invalidity in in district court based on that new prior art after the IPR proceeding is over.

Second, putting aside the danger of having to produce prior art on which the accused infringer might want to rely, it is possible that the patent owner’s commissioned search results would not include the prior art the patent challenger used in the IPR proceeding. Failure to find that IPR prior art arguably would weigh against an argument that the patent owner’s prior art search was done by a “skilled searcher.”

Third, it is entirely possible, if not likely, that the patent challenger would want to depose the searcher, if only to ask whether the searcher had seen the prior art from the IPR before conducting the search. Such a question should not trigger issues of privilege, as the question does not seek to know how the searcher came to see the IPR prior art. If the searcher had indeed seen the IPR prior art, the patent challenger could argue that the search was not an independent search, and that as a result, the court should give no or little weight to the search results.

Fourth, if the district court sides with the patent challenger against the patent owner, there would be no estoppel, and whatever prior art the patent owner had to produce from the search results would be fair game for the patent challenger to use in court to challenge validity of the patent in suit.

Practice Tips

There are ways other than commissioning a prior art search in which a patent owner can establish that a skilled search would have found the non-IPR prior art on which the patent challenger wants to rely.

First, if the non-IPR prior art comprises patents or patent publications, whether in English or in another language, it will be possible to identify the international classification categories in which the non-IPR patent documents are listed, and compare those categories with the categories in which the IPR patent documents are listed. If there is overlap between the sets of categories, it should be possible to provide a declaration from one or more patent searchers who could attest to their habitual searching within such categories, in addition to keyword searching. That searching practice could lead to a conclusion that a skilled searcher should have found that prior art, such that the challenger is estopped from relying on it.

Second, if the non-IPR prior art comprises articles, whether in English or in another language, it should be possible to provide a declaration from one or more patent searchers who could attest to their identification of relevant search terms, based on the patent in suit, or on customary terminology in the art. If the use of those search terms results in finding the non-IPR prior art, then the patent owner could argue that a skilled search should have found that prior art.


It is important for a patent owner involved in patent infringement litigation to avoid waiver of privilege, and to avoid giving a patent challenger more ammunition with which to challenge the patent in district court. Avoiding commissioning specific searches can address the privilege issue and may also avoid the risks of results that do not support estoppel positions. Reliance on well-known data and approaches to searching, in conjunction with evidence from qualified patent searchers, can aid a patent owner in carrying their burden of proof that a patent challenger is estopped from relying on non-IPR prior art that a skilled searcher reasonably would have found.