## Facts

- Csaba Truckai invented the NovaSure System, a medical device used to treat abnormal uterine bleeding using a moisture-permeable applicator head.
- Truckai filed a patent application with the U.S. Patent and Trademark Office (PTO) covering the NovaSure invention.
- Truckai assigned his interest in the application to his company, Novacept, Inc., including rights in future continuation applications.
- The PTO issued a patent from the application.
- Hologic, Inc. later acquired Novacept and its assets, including the NovaSure patent rights.
- After leaving, Truckai founded Minerva Surgical, Inc. and developed a competing endometrial ablation device that used a moisture-impermeable applicator head.
- Minerva obtained its own patent for its device, and the product received FDA approval.
- Hologic filed a continuation application tied to the original NovaSure application and sought an additional claim drafted to cover applicator heads generally, without limiting the claim to moisture permeability.
- The PTO issued the continuation patent with the added claim.
- Hologic sued Minerva for infringement. Minerva responded that the added continuation claim was invalid because it did not match the patent’s written description, which described moisture permeability.
- Hologic argued that assignor estoppel barred Truckai (and Minerva as a related party) from asserting invalidity because Truckai had assigned the original application and continuation rights.
- The district court applied assignor estoppel, barred the invalidity defense, and entered judgment for Hologic. The Federal Circuit affirmed the application of assignor estoppel.
- The Supreme Court granted certiorari to address the scope and continued validity of assignor estoppel.

## Issues

1. Should the equitable doctrine of assignor estoppel be abolished in patent infringement cases, or retained?
2. If retained, when does assignor estoppel bar an inventor-assignor (or a party in privity) from raising patent invalidity as a defense?
3. Does assignor estoppel apply when a continuation claim asserted by the assignee is materially broader than the claim scope the assignor could be understood to have warranted at the time of assignment?

## Decision

- The Supreme Court retained assignor estoppel as a judge-made equitable doctrine with deep roots in patent law.
- The Court narrowed the doctrine: assignor estoppel applies only when the assignor’s invalidity defense conflicts with explicit or implicit representations of validity made in connection with the assignment.
- The Court held that the Federal Circuit applied the doctrine too broadly by treating it as a near-automatic bar on invalidity defenses by assignors.
- The Court vacated the judgment and remanded for consideration of whether the continuation claim asserted by Hologic was materially broader than what Truckai assigned; if so, Truckai’s invalidity challenge might not be inconsistent with any representation made at assignment, and estoppel might not apply.

## Legal Principles

- Assignor estoppel is an equitable bar that can prevent an assignor (and parties in privity with the assignor) from contesting a patent’s validity in an infringement suit brought by the assignee.
- The doctrine is limited to situations where the invalidity defense contradicts what the assignor represented—expressly or by implication—when transferring the patent rights for value.
- No estoppel applies when there is no meaningful representation of validity by the assignor, such as when an employee assigns rights in future inventions or future applications before any specific claims exist for the assignor to evaluate.
- No estoppel applies when a later change in law supplies an invalidity argument that was not available (or not true) under the law at the time of assignment, because the assignor is not contradicting a prior representation about validity under a different legal rule.
- No estoppel applies when the assignee later obtains claims that are materially broader than those linked to the assignment such that the assignor could not fairly be said to have warranted the validity of the later-asserted claim.
- The proper focus is consistency: the doctrine targets unfairness from an assignor taking payment for patent rights while later asserting invalidity in a way that conflicts with the assignor’s earlier assurances.

## Conclusion

Minerva Surgical held that assignor estoppel remains part of patent law, but only as a limited equitable bar tied to an assignor’s own representations at the time of assignment; because the lower courts applied the doctrine too broadly without testing whether Hologic’s continuation claim was materially broader than what Truckai could have warranted, the Supreme Court vacated and remanded.


Minerva Surgical, Inc. v. Hologic, Inc., 594 U.S. 559, 141 S. Ct. 2298 (2021)