Meaning of silence: negative patent claim limitations

“Silence is of different kinds, and breathes different meanings.”

— “Villette” by Charlotte Brontë

Silence is not disclosure, except when it is. This year, the Federal Circuit decided multiple patent cases addressing negative claim limitations and the meaning of silence.

The decisions provide different standards for determining how much disclosure is required to support a negative claim limitation in a patent specification versus in the prior art.

As a result, it is easier to invalidate a negative claim limitation with prior art than to support it in a patent specification.

Negative claim limitations are limitations in patent claims that speak to the absence of a feature, as opposed to positive claim limitations which claim the presence of a feature.

Negative claim limitations are entirely proper. “There is nothing inherently ambiguous or uncertain about a negative limitation.” (See The Manual of Patent Examining Procedure §2173.05(i).)

Indeed, negative claim limitations can be useful in some circumstances, such as when the defining feature of an invention is the absence of a function, or when amending claims to distinguish prior art.

Negative claim limitations can raise thorny issues in court. This year alone, the Federal Circuit has addressed three issues.

First, the Federal Circuit considered whether a patent specification that is silent on the absence of an element supports a negative claim limitation. Second, the court addressed the mirror question of whether silence in a prior art reference supports a rejection of claim with a negative limitation. Finally, the Federal Circuit opined on negative claim limitations introduced through claim construction.

A critical issue with negative claim limitations is how much disclosure is required in the patent specification to support such limitations. The Federal Circuit addressed that issue in Novartis Pharms. Corp. v. Accord Healthcare, Inc., 38 F.4th 1013 (Fed. Cir. 2022).

At issue was a patent covering a particular medical treatment that included a dosage regimen for a pill. The claims required “a daily dosage of 0.5 mg, absent an immediately preceding loading dose regimen.”

The negative claim limitation was the absence of a preceding loading dose. A loading dose is a higher-than-daily dose usually given as the first dose in a treatment regimen.

The patent did not mention loading doses anywhere in the specification. That is not necessarily surprising because the negative limitation was added during prosecution to distinguish over the prior art.

The Federal Circuit held the claim invalid for lack of written description under Section 112. There was no support in the specification for the absence of a loading dose; the patent never mentioned loading doses or the significance of not having a loading dose.

Silence is not disclosure, and to hold otherwise would mean that any negative limitation would be supported by mere silence.

The Federal Circuit reiterated that a negative claim limitation can be supported implicitly or explicitly by: (1) stating a feature is not necessary, (2) listing alternatives to a feature, or (3) providing a reason not to include a feature.

As a guiding principle, the specification must contain some language that would convey to a person of ordinary skill in the art that the inventor intended the exclusion.

This guidance is helpful, except when examining prior art. The Federal Circuit applies a different standard of analysis when determining whether a prior art reference discloses a negative limitation.

For example, a prior art reference may be silent on the absence of a feature and yet still disclose the feature’s absence. A negative limitation may be disclosed in the prior art if a person of ordinary skill in the art would recognize the negative limitation based on the disclosure.

In Almirall, LLC v. Amneal Pharms. LLC, the Federal Circuit held that “a reference need not state a feature’s absence in order to disclose a negative limitation.” (28 F.4th 265 (Fed. Cir. 2022).)

The patent at issue included a limitation explicitly excluding adapalene from the claimed chemical formulation. The prior art disclosed a chemical compound used to treat acne but made no mention of adapalene. The Federal Circuit reasoned that a person of ordinary skill in the art would realize that the prior art compound was a complete compound that did not require adapalene, and thus the absence of adapalene was disclosed.

The Almirall decision is in tension with Novartis. It is noteworthy that the prior art in Almirall did not include any language that would support a negative claim limitation based on the Federal Circuit’s three grounds given in Novartis. The prior art did not disclose reasons to exclude adapalene, did not provide alternatives including adapalene, and did not explicitly exclude adapalene.

Instead, the court reasoned that the lack of adapalene would be obvious to a person of ordinary skill in the art based on the entire disclosure.

The Federal Circuit’s reasoning in Almirall would likely result in a different outcome in Novartis. In Novartis, the specification did disclose daily dose regimens where each dose was the same value. The specification made no mention of a loading dose, and at least two expert witnesses testified that a person of ordinary skill in the art would understand a loading dose was not required.

Nevertheless, the Federal Circuit rejected the expert testimony because the issue was whether silence necessarily excluded a loading dose. The dissent challenged the “necessarily excluded” standard as inconsistent with precedent and the written description requirement of Section 112.

Accordingly, it is easier to invalidate a negative claim limitation than to support it.

Negative claim limitations can also be created by courts as a result of claim construction.

In Sound View Innovations LLC v. Hulu LLC, the Federal Circuit affirmed a negative claim construction but rejected a grant of summary judgment based on that limitation. (33 F.4th 1326 (Fed. Cir. 2022).)

The claim limitation at issue required downloading a first portion of a file to a buffer, and then transmitting the first file portion to a client while simultaneously downloading a next portion of the file to the buffer.

The District Court held that the buffer could not transmit the first file portion to another buffer on the same server. Hulu moved for summary judgment, arguing that no buffer on its edge servers received and transmitted file portions as required. Sound View countered that caches in Hulu’s edge server could meet the buffer requirement.

The District Court held that the accused caches could not be buffers and granted summary judgment. The Federal Circuit reversed the summary judgment decision.

The Federal Circuit held that the District Court correctly created a negative claim construction by defining “buffer” to exclude “cache,” but erred in finding that the cache in the accused product could not be the claimed buffer. “What was needed [from the District Court] was an affirmative construction of buffer — which could then be compared to the accused-component caches based on more than a mere name.”

The court further observed that “cache” is not a term of such uniform meaning to a person of skill in the art as to rule out the possibility that buffers could encompass caches.

Tips for practitioners

The Federal Circuit’s focus on negative claim limitations has practical implications for patent practitioners.

First, patent attorneys should draft applications to implicitly support negative claim limitations. Many attorneys do not draft patent applications with an intent to use negative claim limitations. Indeed, some practitioners are taught not to draft claims with negative claim limitations. Nevertheless, careful drafting of patent applications can create opportunities to rely on negative claim limitations, if necessary.

For example, a patent application can identify alternatives for key features or elements, and provide figures showing some, but not all, of the features in each embodiment. These alternatives and figures can be used later to support a negative claim limitation, even if such limitation was not explicitly considered.

In addition, practitioners should consider which features are unnecessary in each embodiment of the invention, and whether to provide reasons that support (implicitly or explicitly) excluding the feature.

Second, practitioners seeking to challenge the validity of negative claim limitations should consider prior art that is silent on the negative claim element. Such prior art may still be relevant if a person of ordinary skill in the art would understand the disclosure is complete and therefore the missing feature is not required.

Third, during prosecution, applicants should carefully review claim amendments and arguments that either require or imply the absence of a feature. Patent attorneys should only rely on negative claim limitations that are supported by the patent specification. Without proper support, such limitations can be held invalid for lack of written description.

The Federal Circuit’s guidance on negative claim limitations creates opportunities for careful patent drafters and pitfalls for the unwary.

Craig R. Smith is a partner at intellectual property law firm Lando & Anastasi in Boston. He can be contacted at [email protected]. Kevin M. Eckert is an associate at the firm and can be contacted at [email protected].