Inventor Argues Assignor Estoppel Does Not Apply to “Materially Broader” Claims

As we previously reported, the Supreme Court narrowed the doctrine of assignor estoppel. The Supreme Court remanded to the Federal Circuit Minerva Surgical, Inc.’s question of whether Hologic’s claims are “materially broader” than the claims at the time of assignment, which would bar application of the assignor estoppel doctrine under the Supreme Court’s newly delineated limits.1 Put simply, “[i]f Hologic’s new claim is materially broader than the ones [the inventor] assigned, then [the inventor] could not have warranted its validity in making the assignment,”2  and Minerva can challenge the validity of the claims. 

On September 13, 2021, Minerva told the Federal Circuit that assignor estoppel does not bar it from challenging the validity of Hologic’s patent claim, which is “materially broader” than what the inventor (Minerva’s founder and president) assigned.

The asserted patent relates to an endometrial treatment. At Hologic’s urging, the Federal Circuit construed claim 1 of the patent-in-suit3 to cover endometrial treatment devices with moisture impermeable applicator heads. Yet, Minerva asserts that every claim pending at the time of the assignment included an express limitation that the applicator head be moisture permeable, or made of “mesh.” 

Hologic, in a continuation application after the assignment, drafted one of its claims to cover applicator heads, without regard to whether they are moisture permeable. Minerva, however, contends that a claim with a moisture impermeable applicator head would not make sense in the context of the invention and seeks to challenge the claim under 35 U.S.C. § 112(1) because such an embodiment is not adequately described in the specification. Thus, Minerva maintains that the claim is “materially broader” than what the inventor assigned.

In response, Hologic points to a single cancelled claim. The inventor, and applicant, dropped the claim two years before the assignment.4 Moreover, the cancelled claim does not recite an “applicator head.” Instead, Hologic identifies the phrase “electrode array” and attempts to treat the claim as if it recited “applicator head” because the cancelled claim recited an “electrode array” without regard to moisture permeability. 

Minerva urges that this argument fails because, in arguing for its construction of the moisture impermeable applicator heads, counsel for Hologic stated that the “electrode array... is a different part” – and now, Hologic is judicially estopped from arguing that the components are the same. Thus, Hologic’s argument rests on: (1) whether the “electrode array” is effectively the same as the “applicator head,” (2) whether it can rely on a claim cancelled before the assignment, and (3) if so, whether the cancelled claim supports the broader invention of endometrial treatment devices with moisture impermeable applicator heads. Hinging on its contention that assignor estoppel does not apply to the “materially broader” claims, Minerva presses the Federal Circuit to issue a ruling on the merits of its invalidity claims. 

The bounds of the Supreme Court’s decision remain unclear. What it means to be “materially broader” and what evidence will be considered are questions the Federal Circuit will need to address. But the decision stresses the importance and potential implications of assignors making representations that may bar validity challenges or assignees expanding the scope of assigned patent applications. 

We anticipate a decision on remand in Q2/Q3 of 2022. Please contact the authors with any questions, and stay tuned for updates regarding this important case.

Kasey E. Koballa is an associate in the Raleigh, NC, office and April Abele Isaacson is a partner in the San Francisco, CA office of Kilpatrick Townsend & Stockton LLP.


1The Supreme Court held that assignor estoppel does not apply in at least the following scenarios: (1) an inventor assigns rights before she or he can make a representation regarding the validity of any patent claims, such as when an inventor assigns rights in an employment contract to all future inventions developed during her or his employment; (2) a legal development renders the original warranty irrelevant; and (3) the claims of a patent or patent application are materially broadened after the assignment is executed.

2Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440, slip op., at 16 (U.S. June 29, 2021).

3U.S. Patent No. 9,095,348.

4The claim was previously allowed, but later cancelled in response to a restriction requirement.

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