What is Assignor Estoppel and What Implications Remain Post Minerva?August 23, 2021

Particularly within the patent industry, it is common to see that an employment agreement contains a provision where an employee agrees to assign the rights in any future inventions developed during the course of employment to the employer. Provided the prevalence of assignments filed in patent applications, inventors and patent owners should be aware as to how assignor estoppel applies and what implications remain after the United States Supreme Court issued their opinion in Minerva Surgical, Inc. v. Hologic, Inc.

Assignor estoppel is an equitable doctrine which bars the assignor of a patent from later attacking the patent’s validity. Essentially, the doctrine operates to prevent an inventor who assigns a patent to another for value from later claiming that the patent assigned is invalid or worthless during litigation. Such a doctrine is well grounded in centuries-old fairness principles, including fair dealings when selling patents. While the doctrine of assignor estoppel has not been frequently (or consistently) applied, the Supreme Court confirmed in Minerva that the doctrine continues to be an available defense in patent infringement suits to challenge the validity of an asserted patent.

Assignor estoppel may serve as a limit to assignors, however, it is important to note that the assignor estoppel doctrine is not a blanket ban on invalidity challenges by assignors. As further clarified by the Supreme Court in Minerva, the primary purpose of the doctrine is to prevent an inventor from making inconsistent representations of the validity of a patent. However, the doctrine may not apply in instances where a later challenge would pose little risk of unfairness or inconsistencies. Fortunately, the Supreme Court provided a few examples where assignor estoppel would not be available as a defense to an inventor’s challenge of invalidity:

  • When an assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims – This situation arises when an employee assigns to their employer patent rights in any future inventions provided within employment agreements.
  • Later legal developments render irrelevant the warranty given at the time of the assignment.
  • A post-assignment change in patent claims can remove the rationale for applying assignor estoppel – This situation arises when an inventor assigns a patent application, rather than an issued patent, where an assignee may return to the United States Patent and Trademark Office to broaden the patent’s claims (such as through a continuation or divisional). If the new claims are materially broader than claims at the time of assigning, the assignor did not warrant to the new claims’ validity.

While the boundaries and limitations of the doctrine may not be explicitly clear, Minerva confirms that the doctrine is still available as a defense and provides some insight into how patent owners may be able to utilize such doctrine as an effective defense, and provides a reminder to inventors to fully consider any implications associated when signing agreements regarding their patent rights. If you have any questions regarding the assignor doctrine, assignments, or other matters related to your patent rights, it is important that you seek advice and counsel of an attorney with experience and knowledge in this area. The attorneys at MVS are always available should you have any questions.

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