Supreme court permits exceptions to sssignor estoppel doctrine

6. July 2021

By Ivan Blomquist. Lawyer.

On June 29, the U.S. Supreme Court held in Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440, 2021 U.S. LEXIS 3563 (June 29, 2021) that inventors may challenge the validity of patents they assigned under specific circumstances.

In this case, the inventor filed a patent application for the device and later assigned the application, along with any future continuation applications, to his company, which was later acquired by another, along with the patent. The inventor left and co-founded his own company, in which he developed a device like the previous one. His previous employer filed a subsequent “continuation” patent application that was broadened to cover the inventor’s new device and was subsequently sued the inventor for infringement, while also invoking the doctrine of assignor estoppel against the inventor’s opposition, arguing that because the inventor had assigned the original patent application, he could not impeach the validity of the patent.

The case reached the Supreme Court where on 5-2 vote decided that assignor estoppel applies only when the assignor’s claim of invalidity contradicts statements he made in assigning the patent.

To such extent, the Supreme Court provided some examples where the doctrine does not apply:

  1. When an inventor signs an invention assignment as part of their employment agreement that pertains to all inventions developed during their employment.
  2. When a later legal development, such as a new rule of claim interpretation, renders irrelevant the warranty given at the time of assignment.
  3. When the scope of the asserted patent is broadened beyond the scope of the assigned prior patent.
  4. When the inventor assigns a patent application, and the assignee later enlarges the coverage of the patent’s claims.


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