My Oct. 6, 2009, blog entry identified five common equitable affirmative defenses that can be used by a party accused of patent infringement. These defenses include laches, equitable estoppel, patent misuse, inequitable conduct and unclean hands. My next three blog entries will address these defenses, as well as the defense of prosecution laches.

Laches and Equitable Estoppel

Laches
The affirmative defense of laches requires a showing that the patent owner has unreasonably and inexcusably delayed filing suit of infringement to the material prejudice of the infringer. See e.g.,Gasser Chair Co., Inc. v. Infiniti Chair Manufacturing Corp., 60 F.3d 770, 773(Fed. Cir. 1995). This means that a patent owner that discovers that a potential defendant may be infringing its patent must file a patent infringement suit within a reasonable period of time, so that the defendant’s rights are not prejudiced in an unfair manner.

The period of time that must pass before a claim for patent infringement may be barred by laches is unclear. Normally, laches is presumed if a patent owner waits more than six years after discovery of infringing activity to file a patent infringement action. However, some accused infringers have succeeded in barring patent infringement claims based upon shorter periods of delay.

Similarly, patent owners may defeat a laches defense if they can show that the delay was excusable, even if the period of delay exceeds six years. Possible excuses for delaying the filing of a lawsuit include other litigation, negotiations and even wartime conditions.

The prejudice element of a laches defense may be satisfied by showing that an accused infringer’s defense to the patent infringement claim has been impaired. This may occur through the loss of key evidence, such as through the loss of key documents or through the death of a key witness.

Equitable Estoppel
Equitable estoppel arises when “(1)[a] patentee, through misleading conduct, led the alleged infringer to reasonably believe that the patentee did not intend to enforce its patent against the infringer; (2) the alleged infringer relied on that conduct; and (3) due to its reliance, the alleged infringer would be materially prejudiced if the patentee were permitted to proceed with its charge of infringement.”Aspex Eyewear, Inc. v. Clariti Eyewear, Inc.,605 F.3d 1305, 1310(Fed. Cir. May 24, 2010)(citingA.C. Aukerman Co. v. R.L. Chaides Construction Co.,960 F.2d 1020, 1043 (Fed. Cir. 1992)).

Equitable estoppel is similar to laches. However, an accused infringer must establish that the patent owner engaged in some sort of conduct that led the accused infringer to believe that the patent owner would not enforce its patent. Silence alone will not result in equitable estoppel.

Also, the accused infringer must demonstrate that it relied upon that conduct to detriment to succeed in asserting an equitable estoppel defense. Such reliance must be reasonable.