In patent prosecution, we've often encountered situations where claim language, either explicit or implicit, seems to describe a limitation negatively, but the phrasing is not immediately obvious. The distinction between a positive limitation and a negative limitation may seem straightforward based on the words used, but things can get complicated. And the proper interpretation, whether viewed as positive or negative, can in turn affect the claim scope, especially under the USPTO’s Broadest Reasonable Interpretation (BRI) standard.

We’ve discussed negative limitations in previous blog posts—those that describe what is not included or what cannot be present in the claimed invention. These types of limitations are often tricky, but can be valuable during prosecution as they can explicitly exclude an interpretation used by an examiner to maintain a rejection.

For example, consider the following two limitations:

    Explicit Negative Limitation: "A bottle without an opening."
    Implicit Negative Limitation: "A bottle that is fully sealed."

On the surface, the first limitation is explicitly negative, stating that the bottle cannot have an opening. Consider prior art applied by the examiner where there is a bottle with an opening. The second limitation, however, uses more positive language by stating that the bottle must be "fully sealed." An examiner might maintain a rejection based on the same prior art, arguing that the bottle with an opening is fully sealed with the lid is in place. So, while at first glance one may consider the limitations equivalent, in practice there can be ways in which the second limitation differs from the first depending on the particular language and facts of the case.

This brings us to an important question in patent prosecution: Should the semantic structure of the limitation—the choice of words—determine how the limitation is interpreted, or should the actual concept behind the language be the guiding factor?

To illustrate this issue further, consider a recent case that touched on the distinction between explicit and implicit negative limitations. Appeal 2024-002425, Application 17/487,575. The invention relates to a vaporizing hydrogen fuel, and the applicant is Rolls Royce. Claim 1 is reproduced below, with emphasis on the limitation in question.

1. A hydrogen fuel vaporiser for vaporising a cryogenically-stored hydrogen fuel prior to injection into a gas turbine engine,
comprising:
a fuel offtake configured and arranged to divert a portion of the cryogenically-stored hydrogen fuel from a main fuel conduit;
a burner configured and arranged to bum the portion of the cryogenically-stored hydrogen fuel diverted from the main fuel conduit; and
a heat exchanger configured and arranged to transfer a heat produced by the burner to a remaining cryogenically-stored hydrogen fuel in the main fuel conduit after the portion of the cryogenically-stored hydrogen fuel is diverted from the main fuel conduit,
wherein the burner comprises the heat exchanger and the burner is concentric around an entire circumference of the main fuel conduit that contains all of the remaining cryogenically-stored hydrogen fuel such that the all of the remaining cryogenically-stored hydrogen fuel always flows past the burner via the main fuel conduit when the cryogenically-stored hydrogen fuel flows through the hydrogen fuel vaporiser.

The applicant was attempting to use the positively claimed feature to essentially operate as a negative limitation to avoid prior art. The examiner had relied on an argument that took a selected portion of the prior art fuel lines in order to argue that fuel never reaches the evaporator and that the fuel always flows to the burner.

However, the PTAB did not agree that the feature was truly a negative limitation that circumscribed the cite art. From the opinion:

Third, a “negative limitation” is a claim limitation that excludes a particular structural element. Omega Engineering, Inc. v. Raytek Corp., 334 F.3d 1314, 1322–23 (Fed. Cir. 2003). To the extent Appellant might intend the limitation “always flows past the burner via the main fuel conduit when the cryogenically-stored hydrogen fuel flows through the hydrogen fuel vaporizer” to be construed as a “negative limitation,” Appellant does not identify a particular structural element excluded in claim 1 by this limitation.

The PTAB relied on a failure mode in the prior art to state find a way to apply to the cited limitation, noting that there was no structural exclusion.

As patent prosecution professionals, we must be careful not to overlook subtle distinctions in claim language that may lead to unnecessary complications or misunderstandings during examination.

So, be careful to recognize the true bounds of claim language, whether positive or negative.