Berkeley Law

In the Festo decision, the Federal Circuit significantly changed the scope of the doctrine of equivalents in patent law. The doctrine of prosecution history estoppel precludes a patent owner from claiming during litigation to own ground given up during patent prosecution. Under the old rule, called the "flexible bar," estoppel was based on a multi-factor test and would apply only if the patentee had no choice but to amend its claims in the way it did. Festo replaced the flexible bar with an "absolute bar," under which virtually any amendment to a patent precludes resort to the doctrine of equivalents for that claim element.

The Supreme Court is now considering whether the flexible or absolute bar is the right rule. We believe there is middle ground in this debate that has been ignored with both parties. Whether prosecution history estoppel applies to any given amendment should depend on the reasonably foreseeable effect of that amendment. Normally, patentees will understand that they are surrendering coverage by amending their patent claims, and so a rule precluding them from reclaiming that ground makes sense. But in some cases the absolute bar will produce unexpected and unintended results. We argue that the application of estoppel should turn on whether the effect of a change would be foreseeable to a reasonable patentee at the time of the amendment. This "foreseeable" bar better balances the competing policies of strong protection for pioneering inventors and notice to improvers who wish to design around a patent.