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Federal Circuit Offers Guidance On The “Skilled Searcher” Inquiry Required Under 35 U.S.C. § 315(e)(2)
06/30/2026On June 18, 2026, the Court of Appeals for the Federal Circuit (“CAFC”) issued a precedential opinion reversing and remanding a partial summary judgment from the United States District Court for the Western District of Washington finding that appellant-defendant was estopped under 35 U.S.C. § 315(e)(2) from asserting two invalidity grounds. Ironburg Inventions LTD. v. Valve Corp., __ F.3d __ (Fed. Cir. June 18, 2026). In reaching its decision, the CAFC concluded that the district court relied on insufficient evidence to estop one ground, and failed to adequately account for hindsight bias in estopping the other ground.
Appellee-patentee accused appellant defendant of infringing certain claims of U.S. Patent No. 8,641,525 (“the ’525 patent”). Representative asserted claim 2 of the ’525 patent is directed to a video game controller shaped such that a user’s thumb is positioned over a front control element and a user’s index finger is positioned to operate a top edge control element, and further including two elongated control elements on the back of the controller.
Prior to trial, in April 2016, appellant-defendant filed a petition for inter partes review (“IPR”), which the Patent Trial and Appeal Board (“PTAB”) instituted. During the pendency of this first IPR, appellant-defendant filed a second IPR based on what it alleged to be a newly discovered prior art reference (“Wörn”). After a September 2017, final written decision in the first IPR, patentee appellee moved to terminate this second IPR based on estoppel under 35 U.S.C. § 315(e)(1). In January 2018, the PTAB granted the motion, finding that patentee had proven that “a skilled searcher conducting a diligent search reasonably could have been expected to discover Wörn” by conducting manual searches using patent classifications, as well as using keywords to further narrow the search results to a manageable set for review that would have included Wörn.
A third party also filed an IPR petition challenging the ’525 patent in January 2018, which was instituted and then terminated due to settlement. After the filing of this third-party IPR, appellant defendant amended its invalidity contentions in the district court litigation to raise the invalidity grounds presented in the third-party IPR petition, one based on Kotkin and the other based on a Willner-Koji-Raymond combination. Patentee then filed a motion for IPR estoppel under § 315(e)(2), arguing that appellant defendant should be estopped from asserting the third party IPR invalidity grounds. The district court granted the motion on the basis that Willner was cited on the face of the ’525 patent and that the other references must have been reasonably accessible because they were found by the third-party IPR petitioner.
In an initial appeal by defendant, which followed a jury verdict finding willful infringement and awarding damages, the CAFC agreed with the district court’s conclusion that, under § 315(e)(2), “an IPR petition ‘reasonably could have raised’ any grounds that ‘a skilled searcher conducting a diligent search reasonably could have been expected to discover.’” However, the CAFC held that the district court erred by improperly placing the burden of proof on defendant to show that it could not reasonably have raised the third party IPR grounds in its petition. The CAFC then turned to the evidence considered by the district court and criticized the court’s reliance on the mere fact that certain prior art was included in a third-party IPR petition as proof that the prior art could reasonably have been found by a skilled searcher.
On remand, the district court permitted limited additional discovery. Patentee then filed a renewed motion for IPR estoppel, submitting evidence that it had asked Cardinal IP, a prior art search company, to perform searches after the case was remanded and that these searches identified Kotkin and the Willner-Koji-Raymond references. Appellant-defendant opposed the motion, relying in-part on the searches that its own search agent, Landon IP, had conducted before the filing of its first IPR petition.
On May 3, 2024, the district court again granted patentee’s motion for IPR estoppel, finding in-part that Kotkin and Koji were reasonably discoverable because they were within broad patent classifications included in the Landon IP searches.
Defendant again appealed, primarily arguing: (1) that its pre-petition Landon IP search evidence did not demonstrate that Kotkin and Koji were discoverable under a proper articulation of the skilled searcher standard, and (2) that the ex-post Cardinal IP search evidence offered by patentee is irrelevant to the “skilled searcher” inquiry because it incorporates hindsight information.
In its opinion, the CAFC first affirmed that the statutory language of 35 U.S.C. § 315(e)(2) requires a focus on what reasonably, rather than possibly, could have been raised prior to the filing of a relevant IPR petition. Next, with respect to appellant’s first argument on appeal, the CAFC concluded that the district court erred by finding Kotkin reasonably discoverable based on the mere fact that defendant’s searcher, Landon IP, searched patent classifications that included 26,333 references of which Kotkin was one. The CAFC held that, under these circumstances, further narrowing was required for Kotkin to be deemed discoverable. And, because there was no other evidence of record supporting the discoverability of Kotkin, the CAFC reversed the district court’s decision as to estoppel of the Kotkin invalidity ground.
With respect to appellant’s second argument on appeal, the CAFC agreed that the testimony from two Cardinal IP searchers relied on hindsight and, therefore, did not establish the discoverability of Kotkin, Koji, or Raymond. In particular, the CAFC held that testimony from a first Cardinal IP searcher regarding the discoverability of these references suffered from hindsight bias because one of the search strings employed forward-and-backward citation searching that would have pulled in patents and publications that post-dated appellant’s first IPR petition, and that might cite Kotkin, Koji, and Raymond merely because these three references were raised as part of the third-party IPR that also post-dated appellant’s first IPR petition. The CAFC also agreed with appellant that patentee did not provide sufficient reasoning that excused a second Cardinal IP searcher’s unequivocal admission that a supplemental Cardinal IP search identifying Raymond was only undertaken in the first instance because this reference had not been located in the original search. Again, because there was no evidence of record supporting the discoverability of the Willner-Koji-Raymond references, the CAFC reversed the district court’s decision as to estoppel of the Willner-Koji-Raymond invalidity ground.
In a concurring opinion, Judge Stark expressed his further view that, to invoke estoppel pursuant to § 315(e)(2), a patentee must prove at least the following: (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. Judge Stark noted that the CAFC’s decision had no reason to address the second step of this analysis.
IP Litigation