Prosecution History Estoppel’s Foreseeability Exception Defined By Original Claim Scope

In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd. and SMC Pneumatics, Inc. (“SMC”)(No. 05-1492, July 5, 2007), a divided Federal Circuit panel affirms a district court’s judgment of noninfringement in favor of SMC. At issue was whether Festo’s claim amendment adding “a sleeve made of a magnetizable material” is subject to the doctrine of prosecution history estoppel and, as such, precluded infringement under the doctrine of equivalents by a product having a non-magnetizable aluminum sleeve.

Festo argued that the claimed magnetizable sleeve should be entitled to its full scope of equivalents under the “foreseeability” exception to the doctrine of prosecution history estoppel. That is, the non-magnetizable aluminum sleeve was an “unforeseeable” equivalent and, hence, should not be subject to the doctrine of prosecution history estoppel despite being added during prosecution for reasons related to patentability. In attempting to apply the function/way/result test to this issue of foreseeability, Festo claimed that the aluminum sleeve was not foreseeable at the time of the amendment since it was not known that a non-magnetizable aluminum alloy sleeve would perform the magnetic shielding function of the recited magnetizable sleeve.

Two members of the Federal Circuit panel disagree and find the function/way/result test inapplicable to the determination of foreseeability. The proper test of foreseeability, says the majority, is whether the alternative “is known in the field of the invention as reflected in the claim scope before the amendment” (emphasis added). Here, the aluminum sleeve’s magnetic shielding function was unrelated to the original function of the sleeve, namely, a magnet-enclosing function. The split panel finds that the “the applicant should not be able to recapture [an equivalent] simply by establishing that a property of the equivalent—irrelevant to the broader claim before amendment—was relevant but unknown with respect to the objectives of the narrower amended claim.”