Good practices post-Minerva Surgical, Inc. v. Hologic, Inc.



United States Publication October 2021

In 2001, Csaba Truckai obtained a patent on a medical device for treating abnormal uterine bleeding. He assigned the patent and any continuations to his company Novacept. In 2004, Novacept sold Truckai's patent, which Hologic, Inc. subsequently acquired. Years later, Truckai obtained a patent on a new medical device, which used a moisture-impermeable applicator head, for treating the same condition. Truckai began marketing this device through his new company, Minerva Surgical, Inc. In the meantime, Hologic, Inc. obtained a continuation to Truckai's 2001 patent, which included a claim encompassing applicator heads, generally, without regard to moisture-permeability.

Hologic then sued Minerva, claiming that Truckai's new device infringed the continuation patent claim. Minerva argued that the continuation claim was invalid for lack of written description support and that assignor estoppel should not apply because the continuation claim was materially broader than the 2001 patent claims that Truckai assigned. The district court, however, found that the assignor estoppel doctrine barred Minerva from asserting invalidity, and the Federal Circuit affirmed, holding that whether Hologic expanded the claim scope after Truckai assigned the 2001 patent was irrelevant.

The Supreme Court vacated the Federal Circuit's decision, holding that the appeals court applied assignor estoppel too broadly and "failed to recognize the doctrine's proper limits." Minerva Surgical, Inc. v. Hologic, Inc., 141 S. Ct. 2298, 2304, 2311 (2021). The Supreme Court upheld the assignor estoppel doctrine on the basis that "[i]f one lawfully conveys to another a patented right . . . fair dealing should prevent him from derogating from the title he has assigned." Id. at 2305 (quoting Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U.S. 350 (1924)). However, "[t]he doctrine applies when, but only when, the assignor's claim of invalidity contradicts explicit or implicit representations he made in assigning the patent." Minerva, 141 S. Ct. at 2302. Where the assignor makes no such representations, the assignor does not take an inconsistent position and, thus, assignor estoppel does not apply. See id. at 2310. The Supreme Court remanded Minerva and Hologic's dispute for the Federal Circuit to consider whether Hologic's continuation claims were materially broader. Id. at 2311.


Because the scope of claims in a patent application are less certainly defined than that of granted patent claims, an assignor might not warrant post-assignment claims' validity if such claims are materially broader than assigned claims. See id. at 2310. With this possibility in mind, it is recommend that employers:

  • Include the broadest claims supportable by the specification in an initial provisional or non-provisional patent application;
  • Do not reuse oath, declarations, or assignments from employee-inventors for broadening continuations;
  • Include express waiver of invalidity challenges in employee-inventor assignments; and
  • Obtain employee-inventor assignments closer to issuance of final allowable claims.

The Supreme Court also explained that a present assignment of future inventions, such as within an employment agreement, may not provide sufficient warranties for assignor estoppel to apply. Id. The reason is that such an assignment might relate to inventions that do not yet exist, and thus there can be no representation that any future patent is valid. Id. It is therefore recommended that:

  • Employers consider adding express waivers against challenging the validity of present and future patent assignments to employee agreements;
  • Patent buyers (1) not rely on a seller's employee agreements as proof of asset ownership; (2) seek express assignments from seller's employee-inventors; or (3) negotiate for lower asset prices in the absence of patent-specific assignments; and
  • Patent sellers should (1) seek patent-specific assignments; otherwise (2) the lack of the same may affect the patent's value.1



See Minerva, 141 S. Ct. at 2309 n.4 (explaining that “assignor estoppel furthers some patent policy goals. . . . [by] giv[ing] assignees confidence in the value of what they have purchased [and] rais[ing] the price of patent assignments”)

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