There are two types of direct infringement: (1) literal infringement; and (2) infringement under the doctrine of equivalents. The former asks whether a product or process meets all of the requirements of a claim, exactly. The latter is a long-standing judicial doctrine that focuses on whether a given requirement of the claim, though not literally met, is nevertheless equivalent to that requirement. Recognizing the potential for the doctrine of equivalents to subvert the public notice function of a patent’s claims, the federal courts have developed a number of legal defenses that constrain its application. Prosecution history estoppel, which generally prevents a patentee from recapturing through the doctrine of equivalents claim scope that it surrendered during prosecution for purposes of patentability (subject to certain exceptions), tends to be the most common. But there are others, and they can be equally important when addressing a doctrine-of-equivalents theory of direct infringement. The ensnarement defense is one of those other defenses, and it holds that a doctrine of equivalents theory cannot be asserted if it will encompass or “ensnare” the prior art.
On September 29, 2017, in Jang v. Boston Scientific Corp., the Federal Circuit issued a relatively rare precedential decision addressing the application of the ensnarement defense. Nos. 2016-1275, 2016-1575, slip op. at 14-23 (Fed. Cir. Sept. 29, 2017). Continue reading