Continued Uncertainty About Estoppel Highlights the Importance of Preparing Carefully

Challengers in post-grant proceedings like IPR may not reassert invalidity arguments in court that they “raised or reasonably could have raised” before the PTAB. Several recent cases illustrate that whether a particular invalidity argument “reasonably could have [been] raised” is a fact-intensive question that the parties must be prepared to address. These cases also show that both patent owners and challengers should develop a factual record relating to whether estoppel applies to a given invalidity theory.

In GREE, Inc. v. Supercell Oy (E.D. Tex.), the patent owner moved to strike the accused infringer’s affirmative defense of invalidity based on estoppel after the patent owner had prevailed at the PTAB.  The court denied the motion because it held that what “reasonably could have been raised” was a question of fact. The court cited Congress’s explanation in the legislative history that “reasonably could have raised” was intended to cover prior art that a “skilled searcher conducting a diligent search” could have located, and concluded this was a factual issue.

Courts appear to agree that the “raised or reasonably could have raised” standard is a factual question, but they vary on the factual threshold required to apply estoppel. For example, in Wi-Lan v. LG (S.D. Cal.), the court held that an accused infringer’s actual discovery of certain references during a post-filing search was sufficient evidence that the art csould have been located through diligent searching before the IPR filing. In contrast, in SiOnyx v. Hamamatsu (D. Mass.), the court denied the patent owner’s motion for summary judgment of estoppel because the patent owner did not present any evidence, such as a search string or expert testimony, showing what a diligent search could have found.


In preparing for post-grant proceedings like IPR, challengers must recognize the risk that their petition may be their only “shot” at certain invalidity arguments. Thus, challengers must carefully evaluate the strength of their arguments and make sure that their petition is as strong as possible.

In litigation, both patent owners and challengers should consider how (if at all) the court has addressed the “reasonably could have raised” question and should try to create a favorable record to support their estoppel arguments. Patent owners, for example, could present expert testimony demonstrating that particular references could have been found with diligent efforts. Challengers, conversely, could try to show unique circumstances that would have prevented them from finding particular references or raising certain arguments at the PTAB.