2025 Summer Associates Zhuoxun Li and Kian Neishaboori contributed to this article.
The estoppel provision of 35 U.S.C. § 315(e)(1) had largely prevented requesters from challenging claims of a patent via ex parte reexamination after an inter partes review (IPR) that resulted in a final written decision (FWD) on those claims. But a series of Federal Circuit decisions might indicate a change is necessary in the almost absolute bar the United States Patent and Trademark Office (USPTO) used for reexamination post-FWD, resulting in novel pathways for reexamination challengers and new strategies needed for patent owners.
The Federal Circuit and district courts have started carving out exceptions to the hard estoppel threshold for district court cases. This article explores whether these new paths to invalidity challenges post-FWD being formed by the Federal Circuit and district courts should extend into the USPTO for ex parte reexaminations, e.g., where patentability challenges are grounded in new legal theories or previously undiscoverable art.
The “Skilled Searcher” Standard and Burden of Proof: Ironburg
In Ironburg Inventions Ltd. v. Valve Corp., 64 F.4th 1274 (Fed. Cir. 2023), the Federal Circuit clarified the objective standard for what “reasonably could have been raised” in an IPR petition pursuant to § 315(e)(1) estoppel provisions. Estoppel applies to any ground that “a skilled searcher conducting a diligent search reasonably could have been expected to discover.” The focus, therefore, is not on what the petitioner actually found, but on what a hypothetical skilled searcher would have found.
Crucially, Ironburg placed the burden of proof squarely on the patent owner asserting the § 315(e)(1) estoppel. The patent owner must prove by a preponderance of the evidence that a skilled searcher would have discovered the prior art in question. This places a higher evidentiary burden on the patent owner, suggesting that mere expert declaration stating a skilled searcher located the prior art after a diligent search may no longer be sufficient. This creates an opening to argue that obscure foreign documents, niche technical disclosures, or art buried in non-traditional sources were not reasonably discoverable, particularly when supported by the timeline and limitations of search tools available at the time.
The “Grounds” vs. “Evidence” Distinction: Ingenico
Another development came in Ingenico Inc. v. IOENGINE, LLC, 136 F.4th 1354 (Fed. Cir. 2025), where the Federal Circuit held that IPR estoppel applies to “grounds,” not the “evidence” used to support them. A “ground” is a specific statutory theory of invalidity (e.g., obviousness based on patents and printed publications), while prior art references are merely the evidence supporting that ground.
The court clarified that since IPRs are limited by statute to grounds based on patents and printed publications, estoppel does not bar a petitioner from later asserting a different ground—such as invalidity based on a prior public use or an on-sale bar—even if the very same publications are used as evidence to support that new ground.
This textualist approach moves away from a holistic “spirit of the law” interpretation and focuses on the specific legal theories available in each forum such that estoppel attaches to the presented legal theory,not the identity of the references. Thus, earlier-cited references in support of different statutory grounds (e.g., § 112 enablement or § 101 ineligibility) or even for policy-laden concepts like obviousness-type double patenting (ODP), which lie beyond IPR’s jurisdiction, can be reused.
Unforeseeable Events and Procedural Safeguards
The concept of unforeseeability has also emerged as a critical exception. In California Institute of Technology v. Broadcom, 25 F.4th 976 (Fed. Cir. 2022) the court signaled that estoppel might not apply to grounds that were dependent on unforeseeable events, such as the Patent Trial and Appeal Board (PTAB) adopting a novel claim construction for the first time in its final written decision. While the ruling in Olaplex, Inc. v. L’Oreal USA, Inc., 855 F. App’x 701 (Fed. Cir. 2021) clarified that the mere possibility of a different claim construction in a different forum is a foreseeable part of our “two-forum system,” it left the door open for arguments based on truly unforeseeable intervening changes in law.
Applying Recent Estoppel Decisions to Reexamination
Although Ingenico primarily interpreted the parallel estoppel provision for district court litigation, § 315(e)(2), its identical statutory language suggests these interpretations should apply with equal force to § 315(e)(1) estoppel at the USPTO. This creates new strategic possibilities for challengers in subsequent ex parte reexaminations, which patent owners have to be ready to rebut:
- A challenger who asserts only an anticipation ground under § 102 in an IPR may be free to later bring an obviousness challenge under § 103 in a reexamination using the same § 102 prior art, particularly if there is a new motivation to combine or new supporting references or evidence through an expert declaration.
- A challenger could argue for a more granular definition of a “ground.” For instance, having unsuccessfully argued in an IPR that combining references A and B was obvious to increase product strength, a challenger could potentially argue in a later reexamination that combining the exact same references A and B is obvious for a different reason, such as to reduce manufacturing costs. While the evidence is the same, this presents a fundamentally different “theory of invalidity,” arguably creating a new “ground.”
The Unforeseeable Intervening Change of Law Theory
This theory posits that a truly unforeseeable, landscape-altering change in the governing legal standard should defeat estoppel. While Olaplex forecloses this argument for case-specific claim constructions, it leaves room for a macro-level legal shift. Examples could include:
- A Supreme Court decision overruling the flexible framework for obviousness from KSR.
The argument is that the grounds in the IPR were evaluated under a legal regime that is now defunct, making it fundamentally unfair to apply estoppel. In Voter Verified v. Election Systems & Software LLC, 887 F.3d 1376 (Fed. Cir. 2018), the Federal Circuit has been reluctant to characterize decisions like Alice as major intervening law. However, even if a legal change is not deemed “major,” a challenger may still argue it was unforeseeable and led to an unforeseeable claim construction or altered the relevance of the prior art.
“Skilled Searcher” Standard in Reexamination
The Ironburg “skilled searcher” standard is objective and evolving. One could argue that certain prior art was not reasonably discoverable at the time of the IPR but has since become so due to the development of advanced AI-powered search tools. The argument would be that what a skilled searcher using conventional tools “could have discovered” in the past is different from what a skilled searcher using today’s cutting-edge AI tools can find. This theory invites the USPTO to treat “reasonably could have raised” as a time-sensitive inquiry, grounded in the technological context of the original petition, not in hindsight availability.
Patent Owner Tools to Continue Pushing for a High Threshold for Estoppel
But patent owners are continuing to fight back against requesters trying to change the high threshold for estoppel. For example, the USPTO vacated a reexamination proceeding because the patent challenger relied on unpatentability grounds that reasonably could have been raised in an earlier inter partes review. Ex parte Reexamination of U.S. Patent No. 6,601,821, 90/014,950, Decision on Petition to Vacate Reexamination Order (USPTO, Nov. 16, 2022). In this case, the patent owner filed a declaration from an “expert prior art searcher” opining that the patent challenger reasonably could find references in the prior IPR proceedings. Under Ironburg, the mere assertion by an expert may not be conclusive, such that the patent owner should also show that the art fell within the skilled searcher’s expected diligence at the time of the IPR.
Will similar arguments work for patent owners against requesters trying to use the other paths being formed in the district court towards post-FWD invalidity challenges using prior art? Only time will tell.
