The popularity of IPR proceedings will likely increase as assignor estoppel does not apply to IPRs.

Inter Partes Review (IPR) proceedings have become a very popular method of challenging patent validity for actual and potential defendants in infringement actions. Of late, however, another advantage of IPR proceedings to petitioners has emerged from a consistent series of decisions by the PTAB holding that the doctrine of assignor estoppel does not apply to IPRs. Ever since the Federal Circuit’s decision in Diamond Scientific Co. v. Ambico, Inc., 848 F.2d 1220 (Fed. Cir. 1988), the doctrine has barred patent assignors, most notably assignor inventors, from challenging the validity of their patented inventions when defending a claim of infringement. The doctrine not only bars the assignor but anyone else judged to be in privity with the assignor. As articulated by the Federal Circuit,

[A]ssignor estoppel is an equitable doctrine that prohibits an assignor of a patent or patent application, or one in privity with him, from attacking the validity of that patent when he is sued for infringement by the assignee....Assignor estoppel is thus a defense to certain claims of patent infringement.

Semiconductor Energy Laboratory Co., Ltd. v. Nagata, 706 F.3d 1365, 1369 (Fed.Cir. 2013) (emphasis added) (citations omitted).