MPEP 2173.05(g)
Functional Limitations

Ninth Edition of the MPEP, Revision 07.2022, Last Revised in February 2023

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2173.05(g)    Functional Limitations [R-07.2022]

A claim term is functional when it recites a feature "by what it does rather than by what it is" (e.g., as evidenced by its specific structure or specific ingredients). In re Swinehart, 439 F.2d 210, 212, 169 USPQ 226, 229 (CCPA 1971). There is nothing inherently wrong with defining some part of an invention in functional terms. Functional language does not, in and of itself, render a claim improper. Id. In fact, 35 U.S.C. 112(f) and pre-AIA 35 U.S.C. 112, sixth paragraph, expressly authorize a form of functional claiming (means- (or step-) plus- function claim limitations discussed in MPEP § 2181 et seq.). Functional language may also be employed to limit the claims without using the means-plus-function format. See, e.g., K-2 Corp. v. Salomon S.A., 191 F.3d 1356, 1363, 52 USPQ2d 1001, 1005 (Fed. Cir. 1999). Unlike means-plus-function claim language that applies only to purely functional limitations, Phillips v. AWH Corp., 415 F.3d 1303, 1311, 75 USPQ2d 1321, 1324 (Fed. Cir. 2005) (en banc) ("Means-plus-function claiming applies only to purely functional limitations that do not provide the structure that performs the recited function."), functional claiming often involves the recitation of some structure followed by its function. For example, in In re Schreiber, the claims were directed to a conical spout (the structure) that "allow[ed] several kernels of popped popcorn to pass through at the same time" (the function). In re Schreiber, 128 F.3d 1473, 1478, 44 USPQ2d 1429, 1431 (Fed. Cir. 1997). As noted by the court in Schreiber, "[a] patent applicant is free to recite features of an apparatus either structurally or functionally." Id.

A functional limitation must be evaluated and considered, just like any other limitation of the claim, for what it fairly conveys to a person of ordinary skill in the pertinent art in the context in which it is used. A functional limitation is often used in association with an element, ingredient, or step of a process to define a particular capability or purpose that is served by the recited element, ingredient or step. In Innova/Pure Water Inc. v. Safari Water Filtration Sys. Inc., 381 F.3d 1111, 1117-20, 72 USPQ2d 1001, 1006-08 (Fed. Cir. 2004), the court noted that the claim term "operatively connected" is "a general descriptive claim term frequently used in patent drafting to reflect a functional relationship between claimed components," that is, the term "means the claimed components must be connected in a way to perform a designated function." "In the absence of modifiers, general descriptive terms are typically construed as having their full meaning." Id. at 1118, 72 USPQ2d at 1006. In the patent claim at issue, "subject to any clear and unmistakable disavowal of claim scope, the term ‘operatively connected’ takes the full breath of its ordinary meaning, i.e., ‘said tube [is] operatively connected to said cap’ when the tube and cap are arranged in a manner capable of performing the function of filtering." Id. at 1120, 72 USPQ2d at 1008.

Other examples of permissible function language include the following.

It was held that the limitation used to define a radical on a chemical compound as "incapable of forming a dye with said oxidizing developing agent" although functional, was perfectly acceptable because it set definite boundaries on the patent protection sought. In re Barr, 444 F.2d 588, 170 USPQ 330 (CCPA 1971).

In a claim that was directed to a kit of component parts capable of being assembled, the court held that limitations such as "members adapted to be positioned" and "portions... being resiliently dilatable whereby said housing may be slidably positioned" serve to precisely define present structural attributes of interrelated component parts of the claimed assembly. In re Venezia, 530 F.2d 956, 189 USPQ 149 (CCPA 1976).

Notwithstanding the permissible instances, the use of functional language in a claim may fail "to provide a clear-cut indication of the scope of the subject matter embraced by the claim" and thus be indefinite. In re Swinehart, 439 F.2d 210, 213 (CCPA 1971). For example, when claims merely recite a description of a problem to be solved or a function or result achieved by the invention, the boundaries of the claim scope may be unclear. Halliburton Energy Servs., Inc. v. M-I LLC, 514 F.3d 1244, 1255, 85 USPQ2d 1654, 1663 (Fed. Cir. 2008) (noting that the Supreme Court explained that a vice of functional claiming occurs "when the inventor is painstaking when he recites what has already been seen, and then uses conveniently functional language at the exact point of novelty") (quoting General Elec. Co. v. Wabash Appliance Corp., 304 U.S. 364, 371 (1938)); see also United Carbon Co. v. Binney & Smith Co., 317 U.S. 228, 234 (1942) (holding indefinite claims that recited substantially pure carbon black "in the form of commercially uniform, comparatively small, rounded smooth aggregates having a spongy or porous exterior"). Further, without reciting the particular structure, materials or steps that accomplish the function or achieve the result, all means or methods of resolving the problem may be encompassed by the claim. Ariad Pharmaceuticals., Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1353, 94 USPQ2d 1161, 1173 (Fed. Cir. 2010) (en banc). See also Datamize LLC v. Plumtree Software Inc., 417 F.3d 1342, 75 USPQ2d 1801, (Fed. Cir. 2005) where a claim directed to a software based system for creating a customized computer interface screen recited that the screen be "aesthetically pleasing," which is an intended result and does not provide a clear cut indication of scope because it imposed no structural limits on the screen. Unlimited functional claim limitations that extend to all means or methods of resolving a problem may not be adequately supported by the written description or may not be commensurate in scope with the enabling disclosure, both of which are required by 35 U.S.C. 112(a) and pre-AIA 35 U.S.C. 112, first paragraph. In re Hyatt, 708 F.2d 712, 714, 218 USPQ 195, 197 (Fed. Cir. 1983); Ariad, 598 F.3d at 1340, 94 USPQ2d at 1167. For instance, a single means claim covering every conceivable means for achieving the stated result was held to be invalid under 35 U.S.C. 112, first paragraph because the court recognized that the specification, which disclosed only those means known to the inventor, was not commensurate in scope with the claim. Hyatt, 708 F.2d at 714-715, 218 USPQ at 197. For more information regarding the written description requirement and enablement requirement under 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph, see MPEP §§ 2161-2164.08(c). Examiners should keep in mind that whether or not the functional limitation complies with 35 U.S.C. 112(b) or pre-AIA 35 U.S.C. 112, second paragraph, is a different issue from whether the limitation is properly supported under 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph, or is distinguished over the prior art.

When a claim limitation employs functional language, the examiner’s determination of whether the limitation is sufficiently definite will be highly dependent on context (e.g., the disclosure in the specification and the knowledge of a person of ordinary skill in the art). Halliburton Energy Servs., 514 F.3d at 1255, 85 USPQ2d at 1663. For example, a claim that included the term "fragile gel" was found to be indefinite because the definition of the term in the specification was functional, i.e., the fluid is defined by what it does rather than what it is ("ability of the fluid to transition quickly from gel to liquid, and the ability of the fluid to suspend drill cuttings at rest"), and it was ambiguous as to the requisite degree of the fragileness of the gel, the ability of the gel to suspend drill cuttings (i.e., gel strength), and/or some combination of the two. Halliburton Energy Servs., 514 F.3d at 1255-56, 85 USPQ2d at 1663. In another example, the claims directed to a tungsten filament for electric incandescent lamps were held invalid for including a limitation that recited "comparatively large grains of such size and contour as to prevent substantial sagging or offsetting during a normal or commercially useful life for such a lamp or other device." General Elec. Co., 304 U.S. at 370-71, 375. The Court observed that the prior art filaments also "consisted of comparatively large crystals" but they were "subject to offsetting" or shifting, and the Court further found that the phrase "of such size and contour as to prevent substantial sagging and offsetting during a normal or commercially useful life for a lamp or other device" did not adequately define the structural characteristics of the grains (e.g., the size and contour) to distinguish the claimed invention from the prior art. Id. at 370. Similarly, a claim was held invalid because it recited "sustantially (sic) pure carbon black in the form of commercially uniform, comparatively small, rounded smooth aggregates having a spongy or porous exterior." United Carbon Co., 317 U.S. at 234. In the latter example, the Court observed various problems with the limitation: "commercially uniform" meant only the degree of uniformity buyers desired; "comparatively small" did not add anything because no standard for comparison was given; and "spongy" and "porous" are synonyms that the Court found unhelpful in distinguishing the claimed invention from the prior art. Id. at 233.

In comparison, a claim limitation reciting "transparent to infrared rays" was held to be definite because the specification showed that a substantial amount of infrared radiation was always transmitted even though the degree of transparency varied depending on certain factors. Swinehart, 439 F.2d at 214, 169 USPQ at 230. Likewise, the claims in another case were held definite because applicant provided "a general guideline and examples sufficient to enable a person of ordinary skill in the art to determine whether a process uses a silicon dioxide source ‘essentially free of alkali metal’ to make a reaction mixture ‘essentially free of alkali metal’ to produce a zeolitic compound ‘essentially free of alkali metal.’" In re Marosi, 710 F.2d 799, 803, 218 USPQ 289, 293 (Fed. Cir. 1983).

Examiners should consider the following factors when examining claims that contain functional language to determine whether the language is ambiguous: (1) whether there is a clear cut indication of the scope of the subject matter covered by the claim; (2) whether the language sets forth well-defined boundaries of the invention or only states a problem solved or a result obtained; and (3) whether one of ordinary skill in the art would know from the claim terms what structure or steps are encompassed by the claim. These factors are examples of points to be considered when determining whether language is ambiguous and are not intended to be all inclusive or limiting. Other factors may be more relevant for particular arts. The primary inquiry is whether the language leaves room for ambiguity or whether the boundaries are clear and precise.

During prosecution, applicant may resolve the ambiguities of a functional limitation in a number of ways. For example: (1) "the ambiguity might be resolved by using a quantitative metric (e.g., numeric limitation as to a physical property) rather than a qualitative functional feature" (see Halliburton Energy Servs., 514 F.3d at 1255-56, 85 USPQ2d at 1663); (2) applicant could demonstrate that the "specification provide[s] a formula for calculating a property along with examples that meet the claim limitation and examples that do not" (see id. at 1256, 85 USPQ2d at 1663 (citing Oakley, Inc. v. Sunglass Hut Int’l, 316 F.3d 1331, 1341, 65 USPQ2d 1321, 1326 (Fed. Cir. 2003))); (3) applicant could demonstrate that the specification provides a general guideline and examples sufficient to teach a person skilled in the art when the claim limitation was satisfied (see Marosi, 710 F.2d at 803, 218 USPQ at 292); or (4) applicant could amend the claims to recite the particular structure that accomplishes the function.