Unclean Hands
The assertion of a separate equitable defense that is based upon “unclean hands” is also a common litigation tactic. The principle (or maxim) behind the doctrine of unclean hands is that “one who comes into equity must come with clean hands.” In other words, a party must be acting equitably in order to receive equitable relief.

The concept of “unclean hands” is very broad and amorphous. Other equitable defenses, such as inequitable conduct and patent misuse, fall within the scope of the doctrine of unclean hands so that the scope of acts that can be the basis for an unclean hands defense is broader than the scope of acts that may constitute inequitable conduct or patent misuse.

An unclean hands defense may be based upon acts that occurred during the prosecution of the patent at issue, acts that occurred during the sale of products covered by the patent at issue or acts that occurred during the licensing of the patent at issue.

Also, the doctrine of unclean hands can be used by a patent owner to preclude the application of a laches defense. See Serdarevic v. Advanced Med. Optics, Inc., 532 F.3d 1352, 1361 (Fed. Cir. 2008).

Prosecution Laches
Prosecution laches is a very rare equitable defense that “may render a patent unenforceable when it has issued only after an unreasonable and unexplained delay in prosecution.” Symbol Tech., Inc. v. Lemelson Med. Educ. & Research Found., 422 F.3d 1378, 1384-85 (Fed. Cir. 2005). The Symbol court affirmed a finding that an 18- to 39-year delay in filing and prosecuting a patent claim rendered that claim unenforceable. See id.

The defense of prosecution laches limits the enforceability of certain patents that are kept alive for years through various patent prosecution machinations by their owners. Such patents are called “submarine patents” because they tend to “surface” upon issuance to “torpedo” unsuspecting infringers.