As a Florida Patent Attorney with a sizable docket of patent cases, I constantly worry about the effect my amendments have on the enforceability of the resulting patents. As you know, the doctrine of patent prosecution history estoppel limits the scope of a patentee's claims due to things that happened during patent prosecution. Last week, the Court of Appeals for the Federal Circuit issued an illuminating decision in Funai v. Daewoo on the doctrine of prosecution history estoppel and when it is applied.
At issue in this case was whether the patentee, Funai, lost its range of equivalents with regard to a claim term when Funai cancelled the relevant claim after it was rejected by the Patent Examiner. Does cancelling a claim after a rejection invoke prosecution history estoppel and lead to a loss of a range of equivalents?
Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002) is the landmark case we consider when discussing prosecution history estoppel. Festo tells us that no estoppel exists with respect to a claim term if this aspect is unrelated to or "merely tangential" to the prosecution. In Festo, 535 U.S. at 741, the Court stated: "There are some cases, however, where the amendment cannot reasonably be viewed as surrendering a particular equivalent. . . . [T]he rationale underlying the amendment may bear no more than a tangential relation to the equivalent in question; . . . . In those cases the patentee can overcome the presumption that prosecution history estoppel bars a finding of equivalence."
In the Funai case, the Federal Circuit reviewed the patentee's amendment and decided the following:
"It is apparent that the nature of the [claim term at issue] was not a factor in the allowance of claim 4, for this aspect was not at issue during prosecution. This limitation is in the category that the Court called "merely tangential" to the prosecution, as discussed in Festo. Thus the district court correctly held that the cancellation of claims 1 and 2 did not surrender access to equivalency with respect to the [claim term at issue]."
The moral of the story here is that prosecution history estoppel only applies to aspects of a claim that are directly related to prosecution. Aspects that are merely tangential or secondary to a main issue are not subject to prosecution history estoppel and do not lose their range of equivalents due to an amendment. As a practice tip, if you see this issue come up during patent prosecution, I would explicitly state in the Remarks section of an amendment whether you consider certain aspects of the claim tangential and not at at issue.