We first posted a reminder that reverse DOE and ensnarement exist back in 2021, and then again in 2023. But here, now, in 2025, we just got some nice guidance from the Court about ensnarement, and it seems like a great time to put out another reminder.
What Are They?
The Reverse Doctrine of Equivalents deals with a situation where the accused product literally reads on the claims, but is not actually doing what the patent is meant to cover. As we noted last time:
The [reverse doctrine of equivalents] rescues from infringement devices that literally satisfy the elements of a claim but perform the same function of the invention in a substantially different way.
Steuben Foods, Inc. v. Shibuya Hoppmann Corp., C.A. No. 19-2181-CFC, 2023 U.S. Dist. LEXIS 42393, at *26 (D. Del. Mar. 14, 2023).
Ensnarement relates to the idea that a patentee should not be able to use the doctrine of equivalents to stretch their patent claims in a way that would cover the prior art.
Why Litigants Don't Often Think About Them
Neither of these things come up very often in practice. The Reverse Doctrine of Equivalents requires the accused infringer to agree, at least in the alternative, that the product meets the literal claim elements (even though it is performs in a substantially different way). That can be a tough pill to swallow.
(The name isn't great either. The Reverse Doctrine of Equivalents only applies when the product literally meets the claim elements.)
Ensnarement, meanwhile, only applies when the patentee asserts infringement by the doctrine of equivalents—an argument that is often inadvertently waived. And even if the patentee sets forth a real DOE analysis, ensnarement is only relevant when a piece of prior art that would not anticipate the literal claim elements would anticipate the claims as expanded by DOE. That's not always the case.
Beyond that, asserting ensnarement requires the awkward argument that the prior art does not anticipate the claims but would if DOE were applied to those claims in the way the patentee is applying it. It doesn't exactly roll off the tongue—and usually litigants want to go full bore on the idea that the prior art meets all of the claim elements regardless of DOE.
Some Guidance From the Court on Asserting Ensnarement
In any event, this renewed reminder was prompted by an order from the Court yesterday.
In Bausch & Lomb Incorporated et al v. SBH Holdings LLC, C.A. No. 20-1463, D.I. 253 (D. Del. Feb. 13, 2025), the Court denied a motion to strike an ensnarement defense. In making its ruling, the Court laid out its thoughts on what is required to assert ensnarement if the parties are using a "hypothetical claim" analysis for DOE:
So far as the Court can tell, the way it is supposed to work (assuming that the parties are relying on a hypothetical claim analysis to assess whether a DOE theory ensnares the prior art) is that: (a) at some point, the patentee has to put the accused infringer on notice that it is asserting infringement under the DOE; (b) then, the accused infringer has to give the patentee some notice that it intends to make an ensnarement defense to the DOE infringement allegation; (c) next, the burden is on the patentee to construct a hypothetical claim that would assertedly cover the accused device; (d) then the accused infringer has the burden to identify prior art that purportedly would render that hypothetical claim invalid; and (e) if the accused infringer meets that burden, then the burden swings back to the patentee to persuade the court that the hypothetical claim is patentable over the prior art identified by the accused infringer.
Id. As you can see, it's not the most straightforward process—but it's interesting to see it spelled out.
The Court went on to deny the motion to strike on the basis that the plaintiff had failed to sufficiently explain why the defendant was required to disclose it:
Plaintiffs argue that their Motion should be granted because although Defendant first raised this theory in opposition to Plaintiffs’ motion for summary judgment of infringement, Defendant “never said anything about any ‘ensnarement’ in any interrogatory response or other pleading” at any time before that. . . . The problem with this argument is that if Plaintiffs are trying to demonstrate that Defendant’s theory should be stricken as untimely, then Plaintiffs need to be specific about how and when they think that Defendant should have first disclosed this theory—so that the Court can evaluate whether it agrees or not. Should the theory have been first disclosed in an “interrogatory response” or in a “pleading”? If Plaintiffs’ argument was the former, then they should also have identified and attached the relevant interrogatory that they argue should have prompted the response (along with Defendant’s answer thereto). . . . And depending on the answer to these questions, Plaintiffs would need to explain why, as of the time the relevant pleading was filed or the relevant interrogatory response was submitted, Plaintiffs had actually informed Defendant of their DOE-related infringement arguments. But Plaintiffs’ Motion did not include these details, and so the Court is not in a position to grant them the relief they seek on this ground.
Id. That's another good reminder—when moving to strike, you must be specific about exactly where the opposing party was required to (and failed to) disclose the material you are moving to strike.
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