© 2003, Dawsey Co., LPA
October 2003

Most inventors are well aware of the use of the open-ended term “comprising” in claims. With such open ended language, an invention that includes the elements recited will potentially infringe the patent, even if additional features are present. Some inventors are also aware of the use of closed-ended term “consisting of,” wherein only inventions having the same elements can infringe. Generally, inventors prefer open-ended language as it allows a much greater breadth of protection, since a potential infringer cannot escape liability by adding additional elements to get around an existing patent. But in crowded or old areas of art, it may be difficult to achieve allowable claims using open-ended terminology. In such cases, the phrase “consisting essentially of” in a patent claim represents a middle ground between the open-ended term “comprising” and the closed-ended phrase “consisting of.” “Consisting essentially of” has long been understood to permit inclusion of components not listed in the claim, provided that they do not materially affect the basic and novel properties of the invention. This is a critical proviso that was recently reviewed by the U.S. Court of Appeals for the Federal Circuit in AK Steel Corp. v. Sollac.

Background

AK Steel owns two patents directed to hot-dip aluminum-coated stainless steel. Hot-dip aluminum-coated steel is produced by passing heated steel strips through molten aluminum; however, it is challenging to get the aluminum to adhere or wet the steel. The inventors solved the wetting problem by maintaining the steel strip in a hydrogen atmosphere prior to entry into the aluminum coating bath. The inventors also discovered that their invention did not work well unless the aluminum is substantially pure.

The appeals court considered the following representative claim language: “the strip including at least about 6% by weight chromium, the coating metal consisting essentially of aluminum.…”

Examination of the Specification

The court found that the specification itself answered the question as to what constituted “consisting essentially of.” The specification drew a precise line demarking the exact percentage of silicon that the inventors considered to be too much silicon, when it states, “[s]ilicon contents in the coating metal should not exceed about 0.5% by weight.” Silicon in excess of 0.5% by weight would materially alter the basic and novel properties of the invention, and that the claims of the patent must therefore be interpreted to permit no more than 0.5% silicon by weight in the aluminum coating.

Test Remains “Materially Change …Characteristics”

The court thereby affirmed the test that a claim expressed in the language of “consisting essentially of” is narrower than that using “comprising,” but not as narrow as that expressed as “consisting of.” The test will remain that extra elements not described in the claim can be disregarded only if they do not materially change the novel characteristics of the invention, and that the specification itself will be one of the primary authorities consulted as to that essential character.