Assignor Estoppel

Assignor estoppel is an equitable doctrine in patent law that prevents a party who assigned a patent (or the original inventor who assigned the patent application) from later challenging the validity of that patent in a patent infringement lawsuit. The doctrine is based on fairness: when you represent to a buyer that you have valuable patent rights worth assigning (and receive compensation for them), you cannot later turn around and tell a court that those rights were worthless because the patent is invalid. Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co. (S.Ct. 1924) established the doctrine; Diamond Scientific Co. v. Ambico (Fed. Cir. 1990) extended it to inventors who assign before the patent issues.

Minerva Surgical, Inc. v. Hologic, Inc. (S.Ct. 2021) preserved assignor estoppel but defined its scope more precisely in a 5-4 opinion (Kagan, J.). The Court held: the doctrine applies when the assignor's invalidity defense contradicts the explicit or implicit representations the assignor made in the assignment. It does NOT automatically bar all invalidity challenges. The key distinction: (1) If a claim was UNCHANGED from what the assignor assigned — the assignor implicitly represented it was valid when assigning, and is estopped from saying it isn't; (2) If a claim was BROADENED after assignment by the patent prosecution — the assignor never made any representation about the broader claims and cannot be estopped from challenging that specific broadened scope. The dissents (Thomas, Alito, Gorsuch, Barrett) would have eliminated the doctrine entirely as contrary to 35 U.S.C. § 282 which allows any party to challenge validity.

Assignor estoppel is not limited to the original assignor — it extends to parties in privity with the assignor. Privies typically include: (1) a company founded by the assignor after the assignment, especially when the company's business is built around technology related to the assigned patent; (2) employees of the assignor's new company who work on the infringing technology and were closely involved with the original inventor; (3) parties who worked closely with the assignor on the relevant technology after the assignment. The privy analysis is fact-specific — courts look at whether the privy's interests and participation were so closely aligned with the original assignor that it would be inequitable to allow them to challenge validity when the assignor could not.

Historically contested, the Federal Circuit held in Hologic v. Minerva Surgical (prior to the S.Ct. ruling) that assignor estoppel DOES apply in IPR proceedings — the assignor cannot petition for IPR to challenge its own former patent. The Supreme Court's Minerva decision did not directly resolve the IPR question. The PTAB has applied assignor estoppel to bar IPR petitions, but the scope of this application (given Minerva's narrowing) is still being developed. The practical implication: companies founded by former inventors who want to challenge a competitor's patent (their former employer's patent) may find both IPR and district court invalidity defenses unavailable to them.

ASSIGNOR ESTOPPEL: Prevents an assignor from challenging the validity of a patent they assigned. Based on representations implied in the assignment transaction. Preserved (albeit narrowed) by Minerva Surgical (2021). LICENSEE ESTOPPEL: Would prevent a licensee from challenging the validity of a licensed patent while benefitting from the license. This doctrine was ABOLISHED by Lear, Inc. v. Adkins (S.Ct. 1969), which held that the public interest in eliminating invalid patents outweighs the licensor's private interest in being free from validity challenges by its licensee. Key asymmetry: licensees CAN challenge patent validity even while paying royalties; assignors generally CANNOT, because the assignor's position (having been paid to transfer rights) is fundamentally different from a licensee's.