After patent issuance, the patentee has enforceable rights in that he/she can sue another entity for damages and/or an injunction to stop that entity from making, using, selling, offering for sale, or importing the patented invention into the United States. The ability to sue for past damages goes back six (6) years from the date of the suit with the earliest date that damages can be calculated typically the date of patent grant (unless the patentee is enforcing provisional patent rights based on the claims in a corresponding published patent application).
However, the ability to even seek a remedy against an alleged infringer can be entirely cut off based on prior communications of the patentee and alleged infringer under the Doctrine of Equitable Estoppel. The Equitable Estoppel Doctrine could bar the patentee from seeking past damages, or an injunction, and even bars the lawsuit.
In Aspex Eyewear Inc. v Clariti Eyewear, the patentee was estopped (i.e., prevented) from suing the alleged infringer. About three years prior to the initiation of the lawsuit, the patentee sent a letter to the alleged infringer notifying the alleged infringer that the patentee held a patent and that the products of the alleged infringer may infringe the patent. For three years, the patentee remained silent regarding the patent until the filing of the lawsuit. In light of the three-year period of silence along with other circumstances between the parties, the Court held that (1) the patentee misled the alleged infringer to believe that the patentee would not enforce the patent, (2) the alleged infringer relied on the misleading conduct of the patentee and (3) due to its reliance, the alleged infringer has been materially prejudiced if the patentee were permitted to proceed with its charge of infringement.
Based on this case, a patentee should consider the ramifications of sending a notification or cease and desist letter to potential infringers of a patent. The alleged infringer during litigation may argue that the patentee is equitably estopped from claiming patent infringement based on the cease and desist letter and subsequent silence. In sending the letter, the patentee should also consider the possibility of a declaratory judgment action by the alleged infringer. The declaratory judgment action is a lawsuit filed by the alleged infringer typically in the home court of the alleged infringers which forces the patentee to litigate the issue immediately and in a location more convenient to the alleged infringer.
Should you have any questions, please do not hesitate to contact me at (949) 433-0900.