The doctrine of assignor estoppel precludes the assignor of a patent from later challenging the validity of that patent, under the premise that doing so is not fair dealing. In Minerva Surgical, Inc. v. Hologic, Inc., the developer of a patent treating abnormal uterine bleeding through a moisture-permeable applicator head gave the patent to his company, Hologic, then left and started another company, Minerva, that developed an improved device with a moisture-impermeable applicator head. Hologic increased its patent’s scope to encompass all applicator heads, and then sued Minerva for patent infringement. When Minerva argued that Hologic’s expanded patent scope was inapplicable, Hologic invoked assignor estoppel. The district court and the Federal Circuit agreed that assignor estoppel precluded Minerva’s defenses. Minerva asked on appeal that the doctrine be jettisoned completely.
The Court, in a 5-4 opinion by Justice Kagan, vacated and remanded. First, it declined to repeal the doctrine as well grounded in ancient common law fairness principles stretching back to 18th-century England that intervening decisions and laws had not contravened. However, it held that the doctrine only applies when an assignor has made representations relied upon by the assignee. When there has been a post-assignment change in the patent, as in this case, the doctrine does not necessarily apply, especially when the patent was made to be materially broader than what was assigned. The case was remanded for the Federal Circuit to re-assess. Justice Barrett, joined by Justices Thomas and Gorsuch, dissented, arguing that the Patent Act’s comprehensive structure practically eliminated assignor estoppel. Justice Alito dissented, arguing that the case should have determined whether Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U.S. 342 (1924), which first recognized assignor estoppel, should be overruled, and chided the others for failing to address it.
A link to the opinion is here: https://www.supremecourt.gov/opinions/20pdf/20-440_9ol1.pdf