by Dennis Crouch
I have been thinking about the potential rise in the use of means-plus-function (MPF) claims as a reaction to indefiniteness cases such as Williamson v. Citrix as well as pressure from other doctrines, including enablement, written description, and eligibility. But, my preliminary data show that the foretold rise has not yet come. As the chart above illustrates, MPF claims continue their descent into obscurity, with no sign from the preliminary data of rising from the grave any time soon.
The chart above shows a visualization of the percentage of issued patents that include means-plus-function (MPF) claims over time, with two separate estimates based on the wording used in the claims. MPF claims are a particular type of claim in patent law that allows an inventor to claim an invention based on the function that it performs, rather than the specific structure or materials used. 35 U.S.C. 112(f). Although MPF claims may seem broad on their face, the statute limits their scope to cover only the corresponding structures disclosed in the patent document (the specification) and their equivalents.
The chart shows a clear trend: a decline in the use of MPF claims from 1980 through to the mid-2020s. The high estimate in the chart indicates the broader inclusion of MPF claims that use the word “Means,” shows a more substantial presence and a steeper decline than the low estimate, which tracks claims specifically using the phrase “Means For” or “Means To.”
The decline in MPF claims can be linked to several legal decisions and changes in patent doctrine. Historically, MPF claims were indeed a powerful tool, as they allowed patent holders to claim a range of equivalent structures that performed the same function, without having to list them all out. However, this breadth of protection has been significantly curtailed by precedential decisions that have limited the scope of the claims (as required by the statute) and invalidated many MPF claims as indefinite (something that I believe is not required by the statute). These decisions from the 1980s and 1990s began the downward drive of MPF usage.
Functional claiming is a constant lure for patent drafters. The an invention’s function (what result it obtains) is typically tied more directly to the market and business goals than the particular structure or technological details used to reach that end. A functional claim limitation offers the tantalizing ideal of broad coverage and technological flexibility. By defining an invention by what it does rather than the nuts and bolts of how it is accomplished, functional claims can appear to encompass any means for achieving the stated function. This breadth can deter competitors by making it more difficult for them to design around the patent. Additionally, defining an invention functionally may allow the claim scope to adapt to future technological advances that arise after the patent filing date, as long as the new technologies perform the claimed function. The functional format also simplifies claim drafting for complex inventions and supports filing patents amidst technological uncertainty before landing on a final design.
As MPF claims were declining, patent applicants still felt the draw for functional limitations and began to use alternative claiming strategies. Rather than a “means for” performing an algorithm, applicants began claiming a “processor configured to” perform the algorithm, or other alternatives that avoided the magical words “means.” Under the (old) precedent, the use of alternative words such as “processor for” were ordinarily not treated as means-plus-function form and therefore did not have the narrow construction and invalidity risk mentioned above.
In its important 2015 decision in Williamson v. Citrix Online, LLC, 792 F.3d 1339 (Fed. Cir. 2015) (en banc), the Federal Circuit changed the law again — going after these non-means nonce words. Williamson established the absence of the term “means” no longer creates a strong presumption against applying MPF construction. Thus, words such as “mechanism for,” “module for,” “device for,” and “member for” are all likely to be treated as invoking 112(f) means-plus-function construction absent some indication that the limitation directly imparts a structural limitation.
Moreover, enablement and written description requirements have clamped down on non-MPF-style functional claims. For example, in Amgen v. Sanofi, the 2023 Supreme Court invalidated Amgen’s broad functional genus claim covering for monoclonal antibodies that bind to PCSK9 to lower cholesterol. The case suggests that some broad functional limitations might never be enabled even with extensive disclosure. Eligibility cases raise similar issues with functional limitations undergoing significant scrutiny. In Electric Power Group, for instance, the Federal Circuit explained that the “result-focused, functional character of claim language has been a frequent feature of claims held ineligible under § 101.”
One seeming solution to all of these arising issues is for patent applicants to more directly assert MPF claim limitations and include a number of embodiments. But, for now at least, the miracle resurgence of MPF remains elusive.
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Just to confirm that there not a lagging rise in MPF usage, I did the parallel analysis on claims found in published applications and found similar results with continued decline of the use of “means” and “means for” language up through applications published in 2023.