The Diminishing Claim Vitiation Limitation to the Doctrine of Equivalents

Under DOE, an accused instrumentality infringes an asserted patent claim if the differences between the missing patent claim limitation and the accused instrumentality are insubstantial. The most common test for determining whether differences are insubstantial (and, thus, the accused instrumentality is an “equivalent”) is the function-way-result test, that is, asking whether the accused instrumentality performs the same function, in the same way, and achieves the same result, as what is disclosed in a patent claim. If the answers are in the affirmative, the accused instrumentality is found to infringe because it includes the equivalent of the missing limitation (as well as literally possessing the remaining limitations of the claim). Because DOE expands the scope of a patent claim, it is a powerful tool for patent holders as well as a source of concern for the courts because the expansive application of DOE may negate the public notice function of patent claims and create more uncertainty for patent defendants. As a result, the courts have placed a number of restrictions on its application in patent cases.

Prosecution history estoppel, or file wrapper estoppel, is one of the most important restrictions that courts have placed on the application of DOE infringement claims. This estoppel generally prohibits the use of DOE on claim limitations that were amended during the prosecution of a patent claim to overcome a rejection by the patent examiner. See generally, Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002). Courts prohibit the use of DOE on the ground that the public has a right to rely on the public statements made by a patent applicant to obtain his or her patent. Id.