Whether, to avoid prosecution history estoppel under Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 'the rationale underlying the amendment' must be the rationale the patentee provided to the public at the time of the amendment

QUESTION PRESENTED The doctrine of equivalents in patent law prevents a would-be infringer from avoiding infringement by making insubstantial changes to a patented invention. But, when a patentee narrows its claims during prosecution to overcome a rejection, the patentee is presumed to be estopped from later invoking the doctrine of equivalents to recapture the territory between the original, broader claim and the narrower, amended one. Prosecution history estoppel ensures that the doctrine of equivalents does not defeat the patent’s public notice function. In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., this Court held that prosecution history estoppel can be rebutted if the patentee demonstrates “that at the time of the amendment one skilled in the art could not reasonably be expected to have drafted a claim that would have literally encompassed the alleged equivalent.” 535 U.S. 722, 741 (2002). As relevant here, a patentee could do so by showing that “the rationale underlying the amendment * * * bear[s] no more than a tangential relation to the equivalent in question.” Id. at 740. Until the decisions in this case and Eli Lilly & Co. v. Hospira, Inc., 933 F.3d 1320 (Fed. Cir. 2019), the Federal Circuit had consistently held that a patentee’s silence at the time of amendment could not satisfy this exception. Here, however, the Federal Circuit held that the rationale can be provided post hoc, in light of the product accused in litigation. The question presented is: Whether, to avoid prosecution history estoppel under Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., “the rationale underlying the amendment” must be the rationale the patentee provided to the public at the time of the amendment. (1)