In Ironburg Inventions Ltd. v. Valve Corporation, [2024-2088] (Fed. Cir. 2026), the Federal Circuit reversed and remanded the district court’s grant of partial summary judgment in favor of the patent owner on the issue of IPR estoppel, finding that estoppel did not apply because it was not shown that a reasonably diligent search could have been expected to discover the references at issue.
Valve challenged Ironburg’s patent in a first IPR, and again in a second IPR. After the Final Written decision in the first IPR, IPR Ironburg successfully asserted estoppel under 35 USC 315(e)(1), the Board finding that a skilled searcher conducting a reasonably diligent search could have been expected to discover the reference on which the second IPR was based. A third party also filed an IPR against Ironburg’s patent, and Valve tried to amend its invalidity contentions in Ironburg’s infringement suit against it. Ironburg moved for estoppel under 35 USC 315(e)(1), arguing that Valve should be estopped form asserting the grounds raised by the third-party IPR. The District Court found that because the third party found the prior art, a skilled searcher could reasonably be expected to find the same prior art.
In a first appeal, the Federal Circuit held that the district erred in placing the burden on Valve to show that Valve reasonably could not have raised the third-party ground during the first IPR, when the burden was on Ironburg to show that estoppel applied to Valve. The Federal Circuit further held that Ironburg failed to meet its burden by merely relying on the fact that the third party found the prior art, without any evidence about the third party’s diligence in finding the prior art. On remand, the district court again found Valve was estopped as to the prior art in the Valve’ second IPR and as to the prior art from the third party IPR.
Ironburg relied on searches by a private search company to show that the grounds in the prior IPRs would have been discoverable to a skilled searcher exercising reasonable diligence, and the district court determine that the prior art was reasonably discoverable. In the present appeal, Valve argued: (1) that its pre-petition search evidence does not demonstrate that references were discoverable under a proper articulation of the skilled searcher standard, and (2) that Ironburg’s evidence from third-party searchers was irrelevant to the skilled searcher inquiry.
Regarding the skilled searcher inquiry, the Federal Circuit said the statutory language requires a focus on what reasonably, rather than possibly, could have been raised prior to the filing of a relevant IPR petition. The Federal Circuit said when evidence is introduced that sheds light on what a skilled searcher may have discovered only after the time of the relevant IPR petition, such evidence is likely irrelevant to the skilled searcher inquiry.
The Federal Circuit identified two open questions regarding the skilled searcher inquiry: First, whether the skilled searcher inquiry poses a question of law, reviewable de novo, or a question of fact, reviewable for clear error. Second what “discovery” within the skilled searcher inquiry entails. However the Federal Circuit neatly ducked both questions, finding that the reference was unfindable under any conception of what the skilled searcher.
The Federal Circuit said that a reference was not discoverable merely because it is located in classifications that a searcher would believe to be relevant – at least where an unreviewably large number of references are returned by a skilled searcher’s diligent search. This was in line with preponderant evidence of record in this case, which indicated that keyword searches or other narrowing techniques would be used after an initial classification search. Because the district court was confronted with thousands of references in the search results yet still found that no narrowing was required for the reference to be deemed discovered, the Federal Court concluded the district court erred, warranting reversal of the finding of estoppel.
The Federal Circuit then addressed whether a search that was not date restricted to exclude references after the filing of the IPR Petition involved hindsight. Because the searching included forward-and-backward searching the failure to exclude subsequent references which could contain backward citations could misrepresent what a reasonable search could find at the time the IPR was filed. The Federal Circuit agreed, finding that the failure to date limit searches employing forward-and-backward searching taints the results of what a skilled searcher at the time of an IPR would discover.
Judge Stark wrote separately to highlight certain aspects, noting:
- The patentee’s burden to show discoverability of invalidity grounds is not automatically satisfied by proving the findability of the prior art reference(s) on which the invalidity ground is based.
- To prove estoppel, a patent owner must prove: (i) the prior art reference(s) at issue were findable by a skilled searcher conducting a diligent search and (ii) the skilled searcher also reasonably would have been expected to discover, based on the reference(s) found, the invalidity ground the challenger seeks to assert in court.
- in at least some circumstances, “something more” than proving the mere accessibility of a prior art reference is required in order to establish a skilled searcher performing a diligent search would have found the reference.
- “Something more” than accessibility is not demonstrated when the only evidence in the record is that a skilled searcher would have undertaken a classification search that yielded, in addition to the reference now being asserted by the patent challenger, tens of thousands other references.
- “Something more” may be satisfied the reference is expressly cited on the face of the patent being challenged.
