35 U.S.C. Section 112 paragraph 6 provides a statutory basis for drafting patent claims in what is known as means-plus-function language. The language allows a patentee to write a claim based on the function of a device rather than its actual structure. This language is, in the words of Gene Quinn, "dangerously seductive." Means claims are easy to write, and there is a belief that the means language adds some "fuzzy math" to the court’s calculation of infringement.
Over the past several years, the Federal Circuit has continually limited the scope of means limitations. Such a limitation is construed by the court to cover the corresponding structure as described in the specification and equivalents thereof. Thus, if you use means claim limitations, you should make absolutely sure that the structure is completely (and broadly?) described in the specification.
In an admittedly unscientific survey of case law, attorney Hal Wegner searched through the last fifteen CAFC opinions that dealt with the interpretation of means-plus-function language. Results:
Patentee’s construction of "means" term accepted: 0%
Patentee’s argument that term was not a "means" term accepted: 14
Summary judgment of non-infringement reversed: 7%
Losses for Patentee: 80%.
Thus, the means language was a loser for the patent holder in 80% of cases. (12 of 15 cases). (Hal, thanks for conducting the study).
Bottom Line: MPF claims are risky. They are fine to add as insurance after you have drafted more descriptive claims, but they should not be trusted.
If you are still writing MPF claims, you are not alone — thousands of patents using such language are issued annually.