Patent assignor estoppel applies when, but only when, the assignor’s claim of invalidity contradicts explicit or implicit representations he made in assigning the patent, and whether an assignee has materially expanded the assigned patent application or claim is relevant to determining whether assignor estoppel is applicable, abrogating Diamond Scientific Co. v. Ambico, Inc., 848 F.2d 1220, 6 USPQ2d 2028 (Fed. Cir. 1998).