Supreme Court Modifies the Bounds of Assignor Estoppel

The Supreme Court in Minerva Surgical, Inc. v. Hologic, Inc., No. 20-440 (S. Ct. June 20, 2021) narrowed the doctrine of assignor estoppel by holding that an inventor that has assigned their patent rights may still challenge their own patent, provided that the challenge does not contradict explicit or implicit representations made by the inventor in the assignment. The doctrine of assignor estoppel is grounded in principles of equity, where the doctrine provides that an inventor (and owner) should not later be able to challenge their own patented invention’s validity when it is owned by another party.

In this case, Hologic, Inc. (“Hologic”) sued Minerva Surgical, Inc. (“Minerva”) for patent infringement over two patents for endometrial ablation.  Both of the patents at issue included Csaba Truckai as an inventor.  Truckai assigned his interest in the patents to a company that he co-founded, where Hologic later came to own the patents through a series of company acquisitions. Meanwhile Truckai started a new company, Minerva, which developed another device for endometrial ablation. After Hologic sued Minerva, Minerva’s defense for non-infringement asserted patent invalidity for lack of enablement and failure to provide an adequate written description. The lower courts barred this defense based on the doctrine of assignor estoppel, granting summary judgement in favor of Hologic. 

On appeal to the Supreme Court, the Supreme Court found error in the application of assignor estoppel by not considering whether Hologic’s patents were warranted by Truckai as to their validity at the time of the assignment. Put another way, if the claims of the issued patents owned by Hologic were of broader protection than the claims at the time of assignment by Truckai, then Hologic could not assert assignor estoppel, and Truckai could assert non-infringement based on patent invalidity.

Overall, this holding narrows the scope of when assignor estoppel is applicable, however, the question of whether assignor estoppel applies is now highly dependent on both the timing of the assignment and scope of claim amendments during patent prosecution. Recipients of patent rights can longer rely alone on the assignment stopping the inventor from challenging the patent on invalidity grounds. Rather, an assessment must be made as to the scope of rights when assigned as compared to the issued patent.