In a decision addressing an issue of first impression, U.S. District Court for the Northern District of Illinois defined the scope of the estoppel that applies in a district court after the USPTO issues a final written decision in an inter partes review (“IPR”). Clearlamp, LLC v. LKQ Corporation, 1-12-cv-02533 (ILND March 18, 2016). The Court found that the defendant/IPR petitioner was not estopped under 35 U.S.C. § 315(e)(2) from raising in the district court litigation prior art that the court found was not reasonably available during the prior IPR proceeding, even if the art was found cumulative of prior art that had already been used in the IPR.

The parties disagreed on how to interpret the word “reasonably could have raised” used in the statute: “The petitioner in a post-grant review [such as an IPR proceeding] of a claim in a patent under this chapter that results in a final written decision…may not assert…in a civil action…that the claim is invalid on any ground that the petitioner raised or reasonably could have raised during that post-grant review.” (emphasis added).

The Court determined that “any ground that reasonably could have [been] raised” means any ground that could have been raised based on “prior art that could have been found by a skilled searcher’s diligent search.” Acknowledging lack of precedent on the issue, the court found instructive the legislative history of the statute, including Senator Kyl’s Senate floor statement: “The present bill also softens the could-have-raised estoppel that is applied by [IPR] against subsequent civil litigation…Adding the modifier ’reasonably’ ensures that could-have-raised estoppel extends only to that prior art which a skilled searcher conducting a diligent search reasonably could have been expected to discover.” The Court concluded that “[t]he relevant inquiry…is not whether the ground is redundant of a ground that was asserted but, rather, whether the ground reasonably could have been raised.”

Applying this standard, the Court clarified that the proponent of an estoppel argument, the plaintiff/patent owner here, bore the burden of showing that a skilled searcher’s diligent search would have found the prior art in question. One way to make this showing, the Court noted, would have been “(1) to identify the search string and search source that would identify the allegedly unavailable prior art and (2) present evidence, likely expert testimony, why such criterion would be part of a skilled searcher’s diligent search.” Because the plaintiff failed to make this showing for one prior art reference, the court found the defendant was not estopped from raising the reference in the district court the particular prior art. The court found plaintiff’s claims invalid as obvious over the reference.