Assignor estoppel is a judicially created doctrine that bars the assignor of a patent (commonly the inventor) from attacking the validity of an assigned patent.  The U.S. Supreme Court in Minerva Surgical, Inc. v. Hologic, Inc., Cytyc Surgical Products, LLC limited the scope of assignor estoppel, holding that it “should apply only when its underlying principle of fair dealing comes into play” and only when “an invalidity defense … conflicts with an explicit or implicit representation made in assigning patent rights.”

The Court provided three examples of when the doctrine would not be applied:

1. The assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims (g., an employment agreement requiring assignment of inventions not yet conceived);
2. A change in the law impacts the warranty of validity; and
3. A change in the scope of the invention's patent claims after the warranty of validity is made.

The common thread in the examples provided is that something must have changed subsequent to the assignor's “representations.”  The extent of the change in these circumstances is not unlimited such that it can be any change however minimal that allows an assignor to contest validity. Instead, the Court made clear that the change must be “material.” The Court appears to have left considerable discretion in light of the particular facts of the case with Minerva being remanded to the U.S. Court of Appeals for the Federal Circuit to determine, akin to the third example above, “whether Hologic's new claim is materially broader” than the claims that were assigned. Although not addressed by the Court, it is assumed that the first and second examples would be subject to a similar “materiality” analysis requiring something more than inconsequential changes.

The principal dissent, which was written by Justice Barrett, examined the doctrine of assignor estoppel from a foundational perspective. Justice Barrett questioned whether the Supreme Court's decision in Westinghouse, which was the birthplace of assignor estoppel, is still the law given that it preceded the codification of the Patent Act of 1952.  Justice Barrett concludes it is not. Several arguments were presented but the most compelling argument of the principal dissent is that Westinghouse premised the doctrine of assignor estoppel on a similar right under real property law after the Court drew an equivalence between real property and “patent property.” The Patent Act of 1952, however, explicitly provided that “patents shall have the attributes of personal property.” Justice Barrett concluded that “[b]ecause this language is inconsistent with the premise on which Westinghouse rested, it undercuts the argument that Congress ratified Westinghouse in the new assignment provision.”

Given the subjective nature of the material change test set forth by the majority and the unsteady ground on which the doctrine stands as set forth in the principal dissent, assignees would be prudent to secure contractual obligations from an assignee in order to not only prohibit all invalidity attacks on assigned intellectual property, but also to specifically provide for injunctive relief if that prohibition is breached.  Such provisions are commonly used in patent licensing agreements and not only address and are consistent with the equitable concerns of the doctrine of assignor estoppel, but also provide the added benefit of creating a separate cause of action for breach of contract should the assignee later challenge the validity of an assigned patent.