Assignor estoppel

• For example, if Tom sold his U.S. patent rights to Jerry, and was sued by Jerry over infringement of that patent later, Tom is not allowed to challenge the patent's validity because he was the inventor. : "Doctrine of assignor estoppel prevents unfairness and injustice of permitting party to sell patent rights and later assert that what he sold is worthless." • This doctrine extends to those in "privity" with the assignor. If Tom becomes another company's largest shareholder, and that company is sued by Jerry over patent infringement, that company could very likely be barred from raising patent validity as a defense even if Tom was not personally involved in the infringing process. == Case law ==

• Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U.S. 342, 45 S.Ct. 117 (1924). • Diamond Scientific Co. v. Ambico, Inc., 848 F.2d 1220 (Fed. Cir. 1988), in some circumstances, equity may outweigh the estoppel doctrine. • Mentor Graphics Corp. v. Quickturn Design Sys., Inc., 150 F.3d 1374, 1377-79 (Fed. Cir. 1998) ("Without exceptional circumstances...one who assigns a patent surrenders with that assignment the right to later challenge the validity of the assigned patent"). • Shamrock Technologies, Inc. v. Med. Sterilization, Inc., 903 F.2d 789, 793 (Fed. Cir. 1990) (Holding that assignor estoppel extends to parties in privity with the assignor). • Minerva Surgical, Inc. v. Hologic, Inc. 594. U.S. ___ (2021) re-affirms the principle of assignor estoppel as long as the assignor contradicts "explicit or implicit representations he made in assigning the patent." == See also ==