Burden on Proponent of Affirmative Defense
Back in 2016 district courts started encountering infringement suits where the subject patents had survived PTAB challenge. In such scenarios, estoppel applied to failed petitioners under 35 U.S.C. § 315(e)(2). To asses whether invalidity defenses asserted in the later court case should be estopped as arguments that “reasonably could have been raised” in the failed IPRs, the courts needed to determine whether the newly asserted art could have been located by a reasonably skilled searcher at IPR filing. But, 325(e)(2) is silent as to which party bears the burden of proof on this issue of estoppel.
That is, does the Patent Challenger have to show that it could not have found the art with reasonable diligence, or, does the Patent Owner have to show that art could have been found with reasonable diligence?
Most courts held that since estoppel was an affirmative defense, the burden fell on the proponent patent owner. Today, in a precedential decision, the Federal Circuit agreed.