Assignor Estoppel: What did the assignor “represent” as his invention?

Hologic v. Minerva, 19-2054 (Fed. Cir. 2022) (en banc petition)

The Supreme Court’s 2021 decision retained assignor estoppel as a cognizable patent law doctrine that can bar a former patent owner from later challenging the patent’s validity.  Although the doctrine was retained, the Court also concluded that the Federal Circuit had applied the doctrine “too expansively.” Like other estoppels, assignor estoppel is equitable in nature and so courts should consider any “unfairness in its assertion” and particularly only apply the doctrine when the assignor has made either “explicit nor implicit representations in conflict with an invalidity defense.” Minerva Surgical, Inc. v. Hologic, Inc., 210 L. Ed. 2d 689 (June 29, 2021).

On remand, the Federal Circuit found Minerva still estopped, despite the narrowed doctrinal scope.  Hologic, Inc. v. Minerva Surgical, Inc., 44 F.4th 1358 (Fed. Cir. 2022); Dennis Crouch, Narrowed Assignor Estoppel Still Bars Minerva Surgical from Challenging Validity, Patently-O (Aug 15, 2022).    Now, Minerva has petitioned the Federal Circuit for an en banc rehearing on the following question:

For purposes of determining whether assignor estoppel applies, how should a court determine an inventor’s representation of patent scope in a never-issued claim in the original patent application?

Minerva Petition.

The basic setup here is that back in the 1990s the inventor (Csaba Truckai) filed his patent application for an endometrial ablation tool that operates as a “moisture transport system.”  The embodiments in the application all use a “moisture permeable” applicator head. The application also the disclosed prior art moisture impermeable approach, disparaged that approach that led to a “problem of steam and liquid buildup at the ablation site.”  The patent document is clearly directed toward the permeable portion as a key aspect of the invention.  But, one claim in the original application (Claim 31) was directed to other aspects of the invention and did not expressly require use of the permeable member.  And, although Truckai argued that the specification’s description should limit the claim scope to inherently include the permeability clause, the Federal Circuit disagreed and instead concluded that “nothing in the intrinsic record supports limiting claim 31” to require a moisture permeable material.  Truckai also prosecuted claim 31 for some time – overcome an anticipation rejection.   Eventually though the claim was dropped by the patentee following a restriction requirement.

The Truckai application was filed in 1998 and that same year Truckai signed an assignment of rights to start-up company Novacept.  Later Novacept was purchased for $300m by Hologic.  In this process, Truckai filed another assignment in 2004 warranting it had “no present knowledge from which it could reasonably conclude” that the assigned IP rights were invalid or unenforceable.”  By that time though, claim 31 had been withdrawn following a restriction requirement.

Now at his next startup (Minerva), Truckai conceived of another device.  The new device relied upon a moisture impermeable head.  Truckai apparently showed the device to Hologic as part of a potential sales agreement.  Rather than buying, Hologic filed a continuation of that 1998 application with new scope expanded out to cover Truckai’s new impermeable membrane.

Once the patent issued, Hologic sued Minerva for patent infringement.  Minerva argued that the new claims were invalid for lack of enablement and written description — that the expanded claim scope improperly covered moisture impermeable heads that were not disclosed in the original specification; Hologic argues that Minerva’s invalidity challenge is barred by assignor estoppel.

The Federal Circuit initially sided with the patentee, but that judgment was vacated by the Supreme Court.  On remand, the Federal Circuit considered the case history and the Supreme Court guidance, and eventually maintained with its original conclusion barring the invalidity defenses under the doctrine of assignor estoppel.  After construing that original Claim 31, the court concluded that it was broad enough to encompass both permeable and impermeable devices; and therefore that Truckai’s actions at least an implicit representation that he was transferring patent rights covering impermeable devices.  Ergo, assignor estoppel prohibits him from now claiming that such expansive claims are invalid.

The new petition for rehearing argues that the Federal Circuit got this all-wrong by applying the principles of claim construction as it did.

The difference between a primary focus on the inventor/assignor’s representations (as the Supreme Court directed) and the mechanics of claim construction (as the Panel purported to apply) is more than semantic.

With claim construction, we know that the use of “comprising” creates an open-ended scope.  Thus, the absence of the permeable clause in claim 31 allows or claim scope that includes impermeable heads.  But, this quirk of claim construction is different from concluding that Truckai had made affirmative statements that his invention covers impermeable heads.  The defendant argues, “silence is not a representation.”  Of course, the Supreme Court made clear that the representations can be “implicit.”

In the lawsuit, Truckai wants to argue that the claims covering impermeable heads are invalid for failure of disclosure.  Assignor estoppel will apply if Truckai made some representation that his assigned patent rights cover impermeable heads.  In its petition, Truckai argues that the court should take special precaution in judging  claims submitted in a patent application that never-issued.  Because those original applied-for claims are “inchoate rights of indefinite scope,” the court should take  extra measures to read meaning from the specification:

But if the task is to  determine what the inventor represented in a never-issued claim, then the fact that the specification declares moisture trapped at the ablation site as the problem the invention solves represents that the device could not work if it included moisture impermeable material. An inventor did not represent that his invention is something his own description of the invention makes clear would not work.


The general rule for en banc petitions is that no response need be filed unless requested by the Court.  However, I expect that such a request will be forthcoming on this interesting issue.

2 thoughts on “Assignor Estoppel: What did the assignor “represent” as his invention?

  1. 1

    Can that question be accurate?

    What exactly is a “never issued claim?”

    Be that as it may (and is actually more a non-sequitor), the actual legal point at hand is that inventors take an oath to the application as filed — as opposed to ANY particular “state of claim.”

    To even consider otherwise would necessitate a full set of oaths with every amendment and/or argument during prosecution.

    The fact of the matter is that a legal mechanism already exists removing that necessity: the requirement that no new matter may be added after the filing date.

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