Introduced in the Leahy-Smith America Invents Act, Inter Partes Review (“IPR”) proceedings have enjoyed increasing popularity as an alternative means for challenging patent claims.1 As IPR proceedings present an attractive forum for petitioners, and because IPR petitions must be filed within one year of the service of a complaint alleging infringement, district court litigation and IPR proceedings often run concurrently.2 Thus, the extent of the interplay between IPR proceedings and district court litigation is an important consideration in challenging or protecting the claims of a patent.

In particular, codified at 35 U.S.C. § 315(e)(2), an estoppel provision proscribes a petitioner in an IPR of a patent claim from asserting, in district court litigation, that “the claim is invalid on any ground that the petitioner raised or reasonably could have raised” during the IPR proceeding.3 This statutory IPR estoppel attaches as soon as the PTAB renders a final written decision in an IPR proceeding, which generally occurs no later than twelve months from the formal institution of the IPR proceeding.4

Although the Patent Trial and Appeal Board (“PTAB”) has issued over two hundred final written decisions, each of which potentially triggered estoppel in a parallel district court action, there is scant guidance from the district courts as to what constitutes a ground that reasonably could have been raised.5 Analysis of existing district court dispositions of estoppel issues and legislative history of the estoppel provision may, however, be instructive as to the scope of could-have-raised estoppel, by identifying what arguments are not estopped.

Invalidity Grounds that could not have been Reasonably Raised

Patentable Subject Matter, Written Description, Enablement, and Indefiniteness Grounds

Only challenges grounded in anticipation or obviousness based on prior art—under 35 U.S.C. §§ 102 or 103—fall within the purview of the IPR estoppel provision. This is because, by statute, a petitioner may seek IPR review of patent claims “only on a ground that could be raised under section 102 or 103 and only on the basis of prior art consisting of patents or printed publications.”6Thus, could-have-raised estoppel does not prevent a petitioner from asserting, in district court, that a claim is invalid for failing to comply with the patentable subject matter requirement, or any of the written description, enablement, and definiteness requirements of 35 U.S.C. §§ 101 or 112, as none of these could have been raised in an IPR proceeding.

Reasonably Undiscovered Prior Art

The legislative history of the estoppel provision indicates that the term “reasonably” restricts the scope of could-have-raised estoppel to that which “a skilled searcher conducting a diligent search reasonably could have been expected to discover.”7 In other words, a petitioner need not conduct “a scorched-earth search around the world” in order to uncover prior art.8 While the legislative history suggests that this reasonably undiscovered prior art may avoid the proscription of the IPR estoppel provision, the courts have not yet provided guidance as to what constitutes a reasonably diligent search.

Prior Sales/Uses of Physical Systems

The Central District of California recently carved out an exception to could-have-raised estoppel for a physical machine. In Star Envirotech, Inc. v. Redline Detection, LLC., the plaintiff contended that the defendants should be estopped from relying on a physical machine as part of its invalidity case, because the defendants could have raised the machine’s owner’s manual in a parallel IPR proceeding that had concluded in the Patent Owner’s favor.9 The district court disagreed, however, finding that IPR estoppel extends to “patents and printed publications,” which do not include a physical machine. The court also found that the physical machine was “a superior and separate reference,” as it embodied claimed features that were not described in the instruction manual.10Thus, this opinion supports the conclusion that a prior art device might not be barred by the could-have-raised estoppel provision of § 315(e)(2), as long as at least one claimed feature is embodied in the physical machine and not in the published manual. A similar issue arose in Oil States Energy Services, LLC v. Trojan Wellhead Protection, Inc.11 There, the defendant attempted to amend its invalidity contentions to include as prior art a commercial embodiment of a patent that was already being used as prior art in a parallel IPR proceeding filed by the defendant. The district court denied the motion to amend on the basis that the motion to amend was untimely, but did not reach the question of whether the defendant would be estopped under 35 U.S.C. § 315(e) from relying on the commercial embodiment.12

Prior Art Combinations

In Star Envirotech, the defendants presented the physical machine as part of an invalidity contention that relied on a combination of the machine with six other prior art publications, each of which was raised during the IPR proceeding.13 Notably, the court declined to estop the defendant from relying on these combinations, even though each of the other six publications in the combination would likely be individually estopped under 315(e)(2).14 It may, therefore, be that courts will not estop invalidity grounds based on a combination of references if even a single reference is found to be outside the purview of the IP estoppel provision.


While IPR estoppel can limit the invalidity arguments that a petitioner may raise in a parallel district court proceeding, several exceptions to could-have-raised estoppel have been recognized. Moreover, it may be possible in a district court litigation to rely on references that would be subject to the estoppel provision, when those references are part of an obviousness combination that includes a reference that is not subject to estoppel.