Assignor estoppel is an equitable doctrine that can amount to an implied warranty of validity to prevent an assignor from assigning the rights to a patent ' such as to an employer or an acquiring company ' for value, and then later assert invalidity of the patent as a defense in a subsequent patent infringement suit. If the assignor were permitted to later assert that an assigned patent or patent application is invalid and unenforceable, the assignment would be ineffective as the assignor would have knowingly transferred little of value. The doctrine is rooted in 'an idea of fair dealing' between the parties. Minerva Surgical Inc. v. Hologic, Inc. 141 S.Ct. 2298, 2302 (2021).
In Minerva Surgical, Inc., the Supreme Court affirmed the ongoing application of the doctrine, but limited its use to 'when an invalidity defense in an infringement suit conflicts with an explicit or implicit representation made in assigning patent rights.' 141 S.Ct at 2311. In so doing, the Supreme Court did not appear to disturb existing precedent related to other proceedings, such as Federal Circuit precedent permitting an assignor to challenge a patent via inter partes review. Hologic, Inc. v. Minerva Surgical, Inc., 957 F.3d 1256, 1266 (Fed. Cir. 2020) (Hologic I). Similarly, an assignor can be permitted to 'argue for a narrow claim construction, or that the accused devices are within the prior art and therefore cannot infringe.' Mentor Graphics Corp. v. Quickturn Design Systems, Inc., 150 F.3d 1374, 1380 (Fed. Cir. 1998).
To add clarity to when the doctrine does and does not apply, the Court discussed scenarios meant to facilitate understanding of the boundaries of the doctrine. A common theme among these scenarios is that the assignor would not have been able to make representations about the validity of the patent claims that later became the subject of a dispute. For example, assignor estoppel may not apply where the assignment occurred before...
