Understanding Negative Limitations in Patent Claims: A Shift in Perspective

[Contributor - Devashish Agarwal]

In the world of patent law, negative limitations have historically been viewed with caution. These limitations, which define an invention by stating what it is not rather than what it is, were once considered improper except in exceptional circumstances. However, recent developments and court rulings have brought about a significant shift in how negative limitations are perceived and utilized.

The Evolution of Negative Limitations

The Manual of Patent Examining Procedure (MPEP) 2173.05(i) reflects the evolving stance on negative limitations. The current judicial view is that there is nothing inherently ambiguous or uncertain about a negative limitation. Consequently, negative limitations are now generally accepted, provided they are clear and precise.

Examples and Acceptability

Under this updated practice, descriptions like "halogen other than fluorine" are considered acceptable. Similarly, terms such as "noncircular," "nonmagnetic," or "colourless" have long been deemed acceptable because they concisely convey the intended limitation. These expressions often represent the most practical way to define specific characteristics of an invention.

MPEP 2173.05 (i) further illustrates acceptable negative limitations with examples such as "not in excess of 10% structure" and "incapable of forming a dye with...". These examples demonstrate how negative limitations can effectively convey precise constraints without ambiguity.

Strategic Use of Negative Limitations

Negative limitations can be strategically employed to avoid restrictive language such as "consisting" or "consisting essentially." For instance, if a patent claim covers a combination comprising elements A, B, and C, and the examiner cites a combination of A, B, C, and a large amount of D, it might be unobvious to eliminate D. In such cases, the claim could be amended to read "comprising A, B, C, and absent sufficient D to... [hurt the combination]," as seen in the Duva case. This approach allows the claim to cover combinations where D is minimized without excluding additional elements that do not affect the overall invention.

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Case Laws: The Use of Negative Limitations

Case Law 1: Ex parte Grasselli

In this case, the inventors were working on a catalyst for the oxidation of olefins. They used a negative limitation to exclude the presence of a specific metal that would interfere with the catalytic process. The claim read:

"a catalyst comprising... absent sufficient iron to substantially affect the oxidation process."

Outcome: The use of the negative limitation was upheld by the Board of Patent Appeals and Interferences. The decision was based on the clarity provided by the negative limitation, which precisely defined the invention's scope by excluding the unwanted metal.

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Case Law 2: Allergan, Inc. v. Sandoz Inc.

Allergan's patent for the glaucoma treatment Combigan included negative limitations. The claims described the combination of specific active ingredients and excluded certain preservatives that could destabilize the formulation. The claim read:

"a pharmaceutical composition comprising... and absent sufficient benzalkonium chloride."

Outcome: The court upheld the patent, recognizing the clear and specific nature of the negative limitations. This case illustrates how combining both types of limitations can provide a robust definition of an invention's scope.

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Case Law 3: In re Duva

The landmark case of In re Duva solidified the legitimacy of negative limitations in patent claims. In this case, the court upheld a negative limitation that stated:

"absent sufficient CN [cyanide] ions to prevent deposition...,"

even though a positive expression could have been used. This ruling established that negative limitations could be valid, even when they define a point of novelty.

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Conclusion

While the historical opposition towards negative limitations has softened, it remains advisable to use them sparingly and judiciously. Negative limitations should be employed when they represent the clearest or only way to define a limitation. As a general rule, it's preferable to describe what an element is, rather than what it is not.

In summary, the modern view of negative limitations opens new avenues for patent claims, offering flexibility and precision. Understanding when and how to use these limitations can enhance the clarity and enforceability of your patent applications.