Foreseeability as a Bar to the Doctrine of Equivalents

The common-law doctrine of equivalents extends a patent's protection to cover certain similar devices that do not literally infringe on the patent's claims. Broad application of this doctrine can, however, create uncertainty as to how substantial an improvement a competitor must make to avoid non-literal infringement. This uncertainty can chill innovation by exposing the would-be improver to potential liability for infringement under a claim of equivalency. The courts created another rule, prosecution history estoppel, to cabin the expansion of protection provided by the doctrine of equivalents. Prosecution history estoppel prevents a patentee from claiming the equivalents that it surrendered during the prosecution of its patent. Both the U.S. Court of Appeals for the Federal Circuit and the U.S. Supreme Court have grappled with articulating the factual situations in which a patent's specification and prosecution history precludes the added protection afforded by the doctrine of equivalents. Several opinions in these cases suggest that a patentee should not be able to invoke the doctrine of equivalents to obtain protection from equivalents that one skilled in the art would have reasonably foreseen before the patent issued. This paper examines a series of cases related to the applicability of foreseeability to the doctrine of equivalents. It then attempts to synthesize the current state of the law in the area, and analyzes arguments for and against adopting a foreseeability rule