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Supreme Court Upholds, but Limits, Patent Infringement Defense of “Assignor Estoppel”


The U.S. Supreme Court recently decided a case resolving a patent dispute between two medical device companies, Hologic, Inc. and Minerva Surgical.  The opinion was closely watched because it raised the question of whether an inventor who has assigned a patent is legally prevented from later attacking the validity of that same patent — a doctrine historically referred to as “assignor estoppel.”

The Supreme Court’s opinion on June 29, 2021, upheld but limited this doctrine, defining its boundaries and emphasizing it is based on legal principles of equity and fair dealing.

In the case, Csaba Truckai was a listed inventor on a patent application, the rights to which were subsequently acquired by Hologic, Inc. Mr. Truckai then founded Minerva Surgical, Inc. and developed an endometrial ablation system.  Hologic sued Minerva for patent infringement of one of the assigned patents related to endometrial ablation.

In response to the claims of patent infringement, Minerva attacked the patent as allegedly invalid. In response, Hologic argued that, under assignor estoppel, Minerva should be prevented from attacking the patent’s validity because Minerva’s founder, Mr. Truckai, was an inventor on the same patent.

In deciding the case, the Court recognized the fairness principle of assignor estoppel — that an inventor shouldn’t be able to initially tout an invention to the patent office, only to later disclaim its worth after assigning it. However, the Court decided that the lower court had applied assignor estoppel too expansively to muzzle inventors.  Thus, the Court held that the doctrine applies only when an inventor makes statements (explicitly or implicitly) in assigning a patent, and later contradicts those statements in litigating against the owner of the patent. The Court reasoned that an assignment does carry an implied assurance of a patent’s validity, but where the assignor has not made explicit or implicit representations that contradict an invalidity defense, there is no ground for assignor estoppel.

To illustrate the boundaries of assignor estoppel, the Court provided three non-exhaustive examples of when assignor estoppel does not apply:

  • First, when assignment occurs before an inventor can make a warranty of validity (e.g., “when an employee assigns to his employer patent rights in any future inventions he may develop during his employment”);
  • Second, when a later legal development renders the warranty of validity irrelevant (such as a change in the law); and
  • Third, when a change in patent claims occurs for an assigned application (e.g., “the new claims are materially broadened” during patent prosecution after the assignment takes place).

Nevertheless, each assignor’s and each company’s situation is unique, and the application of assignor estoppel depends on the particular situation. Medical device companies and others concerned about patents should seek the guidance of professional legal counsel when making any determination regarding whether assignor estoppel applies.


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