Takeaway: Assignor Estoppel is applicable in limited circumstances for patent inventors or owners who assign their patents, and then later claim the patents are invalid.
The U.S. Supreme Court narrowed the circumstances in which inventors can challenge their own patents. For multiple reasons, inventors may wish to challenge their own patents, including instances where they have assigned their own patents. This principle of challenging your own patent has its limits, however, known as “assignor estoppel.”
Justice Kagan wrote that this “doctrine applies when an inventor says one thing (explicitly or implicitly) in assigning a patent and the opposite in litigating against the patent’s owner.” The Supreme Court held that the doctrine is needed to prevent “unfair dealing” where someone receives money for a patent and then claims it is invalid.
Assignor estoppel, however, is not applicable in situations such as common employment agreements where workers agree to assign to their employer all future inventions developed on the job and where a patent changes after assignment.