Supreme Court Upholds Narrowed Assignor Estoppel Doctrine


On June 29, 2021, the Supreme Court issued its long-anticipated decision in Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440.  

The case concerned the doctrine of assignor estoppel in patent law.  Prior to this decision, the doctrine generally held that a patent assignor could not later challenge the validity of that patent.  The doctrine dated back to a 1924 Supreme Court case, Westinghouse Elec. & Mfg. v. Formica Insulation Co. 

Justice Kagan delivered the opinion of the Court. She was joined by Chief Justice Roberts and Justices Breyer, Sotomayor, and Kavanaugh.  The majority declined to overrule Westinghouse and held that the doctrine of assignor estoppel remains alive.  But it then narrowed the circumstances in which the doctrine is available.  

Previously, assignor estoppel was ostensibly available to bar almost any invalidity defense an assignor sought to raise. Following today’s Minerva decision, the doctrine “applies only when an inventor says one thing (explicitly or implicitly) in assigning a patent and the opposite in litigating against the patent’s owner.”  Slip op. at 5. 

Justices Barrett, Thomas, and Gorsuch dissented. They would have held that the Westinghouse decision, which interpreted the Patent Act of 1870, was not ratified in the Patent Act of 1952. Nor, in their view, was it part of the “well-settled common-law backdrop” when Congress legislated in 1952. For those reasons, they would have held that the Patent Act of 1952 did not incorporate the doctrine of assignor estoppel, and for that reason, it is no longer good law. 

Justice Alito also dissented. He thought the case could not be decided without considering whether Westinghouse should be overruled, and because the majority and principal dissent refused to consider overruling it, he would dismiss the writ of certiorari as improvidently granted. 

The Minerva decision will likely lead to significant litigation when an issue of assignor estoppel arises.  Parties will certainly dispute, and vigorously contest, whether an inventor “implicitly” said something when assigning a patent or patent application, and whether she is later saying the opposite in litigation against the subsequent owner. The decision is a decidedly middle-ground: it does not foreclose estoppel altogether, but it also doesn’t offer a blanket defense to an invalidity claim by an assignor/ patent challenger.

Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440 (June 29, 2021)

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