IPR estoppel
IPR estoppel is the price of admission to inter partes review. Once the PTAB issues a final written decision on a claim, the petitioner — and any real party in interest or privy — may not re-litigate in district court, the USPTO, or the ITC any invalidity ground that was raised or reasonably could have been raised during the IPR. The Federal Circuit's reading of "reasonably could have been raised" expanded sharply in 2022.
The rule
Section 315(e) of Title 35 imposes two parallel estoppels following an IPR final written decision. Subsection (e)(1) bars the petitioner, real party in interest, and privies from requesting or maintaining any subsequent USPTO proceeding — IPR, PGR, ex parte reexamination — on any ground that the petitioner raised or reasonably could have raised during the inter partes review. Subsection (e)(2) imposes an identical bar in any civil action arising under § 1338 of Title 28 and in any section 337 investigation before the International Trade Commission.
The estoppel attaches only when the Board issues a final written decision on the patentability of the challenged claims. Settlement before final written decision, denial of institution, or termination without a decision do not trigger § 315(e). Because the PTAB now institutes on all-or-nothing grounds following SAS Institute Inc. v. Iancu, 584 U.S. 357 (2018), every petition that proceeds to final written decision generates the full statutory estoppel as to all instituted claims and grounds.
Statutory and regulatory source
The statutory text is brief but consequential. Section 315(e)(1) provides:
The petitioner in an inter partes review of a claim in a patent under this chapter that results in a final written decision under section 318(a), or the real party in interest or privy of the petitioner, may not request or maintain a proceeding before the Office with respect to that claim on any ground that the petitioner raised or reasonably could have raised during that inter partes review.
Section 315(e)(2) applies the same standard — "raised or reasonably could have raised" — to civil actions under 28 U.S.C. § 1338 and to ITC investigations. The companion estoppel for post-grant review appears in § 325(e) and reaches the broader range of grounds available in PGR.
No implementing regulation in 37 C.F.R. Part 42 expands or interprets § 315(e); the contour of the estoppel has been worked out exclusively in case law.
The framework
Pre-2018 reading: Shaw and the institution-bound estoppel
The early Federal Circuit decision in Shaw Industries Group, Inc. v. Automated Creel Systems, Inc., 817 F.3d 1293 (Fed. Cir. 2016), construed § 315(e) narrowly. Decided when the Board still routinely engaged in partial institution, Shaw reasoned that grounds the Board had refused to institute were never part of the IPR and therefore could not have been "raised" within it. The court held that estoppel did not attach to non-instituted grounds. District courts widely followed Shaw, though some pushed back on its broader implications, and the case carved a substantial loophole: a petitioner could withhold or stage grounds and, in the event of a partial-institution decision, preserve them for district court.
The SAS turn
In SAS Institute Inc. v. Iancu, the Supreme Court invalidated partial institution and held that the Board must institute on every claim and every ground in the petition — or none. After SAS, Shaw's premise — that some grounds in the petition would not be part of the IPR — collapsed. Once a petition is instituted, every ground in the petition is in the proceeding.
The Caltech expansion
The Federal Circuit in California Institute of Technology v. Broadcom Ltd., 25 F.4th 976 (Fed. Cir. 2022), read SAS and § 315(e) together to overrule Shaw. The court held that estoppel applies not only to grounds actually raised in the petition but also to any ground that reasonably could have been raised — meaning any ground that a skilled searcher conducting a diligent search could have discovered and included in the petition. Caltech rejected the petition-bounded reading of "during" and gave the statute its natural breadth.
Under Caltech, an IPR petitioner who fails to include an available ground in the petition forfeits it: the ground neither was raised (it is missing from the petition) nor was unavailable to a diligent searcher (or it would not have been omitted). The result is a near-categorical estoppel against patents-and-printed-publications challenges in subsequent district court litigation by the same defendant.
Burden, timing, deadlines
The party asserting estoppel — typically the patent owner — bears the burden of showing that each contested ground was raised or that a skilled searcher conducting a diligent search reasonably could have discovered the prior art. The contours of the diligent-searcher standard remain disputed at the district-court level and turn on expert testimony about commercial search tools, foreign-language databases, and the cost-benefit of additional searching.
Estoppel attaches at the moment the PTAB issues its final written decision under § 318(a). Until that point, parallel district court invalidity contentions remain available. Once the decision issues, estoppel applies even while an appeal to the Federal Circuit is pending; reversal on appeal would lift the estoppel.
The estoppel binds the petitioner, any real party in interest, and any privy of the petitioner. The privity inquiry is fact-intensive and looks to control over the IPR, financial relationships, and indemnification arrangements. Co-defendants who declined to file their own IPRs are not automatically bound, but those who funded or controlled a petitioner's IPR may be.
Interaction with district court litigation
IPR estoppel reshapes the validity defenses available in subsequent litigation. After a final written decision, the petitioner-defendant is limited to validity defenses outside the IPR's jurisdiction:
- Section 101 patent eligibility
- Section 112 written description and enablement
- Section 112 indefiniteness
- On-sale bar and public-use bar grounds based on activities (not patents or printed publications)
- Prior physical products or system art that could not have been raised in the IPR (the "prior product art" carve-out — see below)
- Inequitable conduct
Civil action estoppel
Section 315(e)(2) applies to "a civil action arising in whole or in part under section 1338 of title 28." The estoppel runs in any district court patent infringement action — the venue is irrelevant. A defendant in EDTX, WDTX, Delaware, or NDCA who proceeds through IPR to final written decision is estopped equally.
The ITC carve-out
Section 315(e)(2) explicitly extends to ITC section 337 investigations. But the ITC and the courts have read this differently from district court estoppel in several respects. Because the ITC's invalidity determinations have no claim-preclusive effect on subsequent district court litigation, and because the ITC retains jurisdiction over a wider universe of validity issues than the PTAB, ITC respondents commonly raise grounds — including prior physical products and § 112 challenges — that survive § 315(e) on their face.
The product art question
An open and frequently litigated question is whether prior art consisting of a physical product, where corresponding patents or publications describing the product also exist, is foreclosed by IPR estoppel. District court authority is split: some courts hold that the underlying physical product is a separate ground and is not estopped; others reason that, where the publication discloses the same invalidating subject matter, the publication ground reasonably could have been raised in IPR and the product evidence is therefore cumulative and effectively estopped. The Federal Circuit has not yet given a definitive answer.
Practical notes
- Decide whether to file IPR carefully. The strategic value of IPR — claim cancellation before trial, lower cost than district court — must be weighed against the prospect of broad estoppel. A defendant who is confident in non-prior-art defenses (eligibility, indefiniteness, non-infringement) may have less to lose. A defendant whose strongest defenses are prior art has the most to gain and the most to lose.
- Search broadly. Caltech's "reasonably could have been raised" standard rewards thoroughness. Diligent searchers documenting their search are better positioned to argue that newly discovered art could not reasonably have been found earlier.
- Co-defendant coordination raises privity risk. Funding another defendant's IPR, exchanging draft petitions, or entering joint defense agreements concerning IPR strategy may create privity. The case law is fact-intensive and unsettled, and the Board's privity rulings on joinder bear on subsequent estoppel disputes.
- Plan for the § 112 / § 101 toolkit. Counsel routinely preserves written description, enablement, indefiniteness, and eligibility defenses for trial regardless of IPR strategy, since those defenses are categorically outside § 315(e).
- Watch for system-art opportunities. Where a complex system, machine, or apparatus existed publicly before the priority date and was not described in any single publication, that system art may remain available even after IPR — although the analysis turns on the specific district court's view of Caltech.
Open questions
- The product-art issue. Whether a physical product on which a published reference is based is itself an estopped ground remains contested in the district courts. The Federal Circuit has touched on the question without resolving it definitively.
- The reach of "reasonably could have been raised." Caltech tied the standard to a skilled searcher's diligent search, but district courts have varied widely in defining what is reasonable. The amount of foreign-language searching expected, the depth of database coverage, and the role of expert testimony are all unsettled.
- Privity in coordinated defenses. Common defense funds, joint-defense agreements, and indemnification arrangements between defendants and their suppliers continue to generate privity disputes. The Federal Circuit's controlling guidance is sparse.
- Effect of Director review. Following United States v. Arthrex, Inc., 594 U.S. 1 (2021), Director review is part of the final agency action. Whether and when estoppel attaches before the Director acts on a petition for review is an emerging issue.
- The interplay with the AIA estoppel for PGR. Where the same defendant could have filed PGR but instead filed IPR, courts have not consistently addressed whether the broader PGR-eligible grounds — § 101, § 112 — are nonetheless preserved.
See also
- Inter partes reviewThe proceeding that triggers § 315(e) estoppel.
- Post-grant reviewThe companion proceeding with broader § 325(e) estoppel.
- PTAB trial procedureFinal written decision marks the trigger date.
- ITC section 337 investigationsStatutory estoppel reaches ITC, with practical differences.
- Section 101 patent eligibilityOutside § 315(e); preserved for trial.
- Written description and enablementOutside § 315(e); preserved for trial.
- AnticipationA core IPR ground subject to estoppel after FWD.
Last reviewed: 2026