U.S. Supreme Court Limits Patent Law Doctrine of Assignor Estoppel

On June 29, 2021, the Supreme Court of the United States issued its ruling in Minerva Surgical, Inc. v. Hologic, Inc. limiting the patent law doctrine of assignor estoppel. The doctrine of assignor estoppel limits a party who has assigned a patent from later challenging the validity of that patent. The doctrine of assignor estoppel is grounded in the principle of fair dealing. When a party assigns its patent rights, the assignor makes an implicit representation to the assignee that the patent at issue is valid. However, in later raising an invalidity defense, the assignor disavows that warranty. In doing so, the assignor seeks to profit doubly—by gaining both the price of assigning the patent and the continued right to use the invention it covers.

In Minerva, Csaba Truckai invented a device for treating abnormal uterine bleeding. Truckai filed a patent application and assigned his rights in the patent to NovaCept, Inc., a company he co-founded, and which was eventually acquired by Hologic, Inc. Truckai later went on to found Minerva Surgical, Inc., where he developed another device for treating abnormal uterine bleeding. Hologic filed a continuation application based on the originally assigned patent with claims covering Minerva’s device. The continuation application issued into a patent.

Hologic sued Minerva for patent infringement. Minerva asserted that Hologic’s patent was invalid and, in response, Hologic invoked the doctrine of assignor estoppel. The District Court agreed that assignor estoppel barred Minerva’s invalidity defense and the Court of Appeals for the Federal Circuit affirmed in relevant part. Minerva appealed to the Supreme Court.

On appeal, the Supreme Court upheld the doctrine of assignor estoppel, but with limits. The Supreme Court held that when the assignor has not made explicit or implicit representations contradicting an invalidity defense, there is no unfairness in allowing the assignor to assert an invalidity defense and assignor estoppel will not apply. Accordingly, a post-assignment change in patent claims removes the rationale for applying assignor estoppel where the new claims are “materially broader” than the claims as assigned. The Supreme Court remanded the case to the Federal Circuit to address whether Hologic’s continuation claims are materially broader than the claims as assigned.

The Supreme Court also identified additional examples where assignor estoppel may not apply. For example, assignor estoppel may not apply when an assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims, which may occur in certain employment arrangements, or when a later legal development renders the warranty given at the time of assignment irrelevant.