For an inter partes review of a claim in a patent that results in a final written decision by the Patent Trial and Appeal Board (“PTAB”) under Section 318(a), estoppel applies in a civil action with respect to invalidity of the claim “on any ground that the petitioner raised or reasonably could have raised during that inter partes review.” 35 U.S.C. § 315(e)(2).

As used in Section 315(e)(2), “estoppel applies not just to claims and grounds asserted in the petition and instituted for consideration by the Board, but to all grounds not stated in the petition but which reasonably could have been asserted against the claims included in the petition.” Click-to-Call Techs. LP v. Ingenio, Inc., 45 F.4th 1363, 1370 (Fed. Cir. 2022) (quoting Cal. Inst. of Tech. v. Broadcom Ltd., 25 F.4th 976, 991 (Fed. Cir. 2022)). “[T]he petition, not the institution decision” “defines the scope of the IPR” litigation. Id. at 1369. “[A]ny ground that could have been raised in a petition is a ground that could have been reasonably raised ‘during inter parties review.'” Cal. Inst. of Tech., 25 F.4th at 990.

But who has the burden of proof to show that a ground for patent invalidity could not reasonably have been raised in an IPR?

In Ironburg Inventions Ltd. v. Valve Corp., No. 21-2296, — F.4th — (Fed. Cir. Apr. 3, 2023), the Federal Circuit held that the patent owner had the burden for the affirmative defense of IPR estoppel. Specifically, “that the burden of proving, by a preponderance of the evidence, that a skilled searcher exercising reasonable diligence would have identified an invalidity ground rests on the patent holder, as the party asserting and seeking to benefit from the affirmative defense of IPR estoppel.”

The Federal Circuit noted this is consistent with the general practice that a party asserting an affirmative defense bears the burden to prove it. See, e.g., K-Mart Corp. v. Oriental Plaza, Inc., 875 F.2d 907, 912 (1st Cir. 1989) (“The party asserting the estoppel has the burden of proving it.”); United States v. Asmar, 827 F.2d 907, 912 (3d Cir. 1987) (same); Kennedy v. United States, 965 F.2d 413, 417 (7th Cir. 1992) (same); Van Antwerp v. United States, 92 F.2d 871, 875 (9th Cir. 1937) (same); see also Fuji Photo Film Co. v. Int'l Trade Comm'n, 474 F.3d 1281, 1293 (Fed. Cir. 2007) (placing burden on party asserting affirmative defense of repair); Titan Tire Corp. v. Case New Holland, Inc., 566 F.3d 1372, 1376 (Fed. Cir. 2009) (placing burden on alleged infringer who raises affirmative defense of invalidity).

Professor Dennis Crouch noted that “this result better aligns with both tradition and the reading of the statute.”

In cases where the IPR challenger lacks knowledge of the prior art, estoppel applies to what “a skilled searcher conducting a diligent search reasonably could have been expected to discover.”

Thomas P. Howard, LLC litigates patents nationwide including in Colorado.