Nautilus v. Biosig — Patent Claims Must Inform Skilled Artisans With Reasonable Certainty; Insolubly Ambiguous Standard Rejected

Background

Biosig Instruments held a patent on a heart rate monitor designed for use with exercise equipment. The monitor used two sets of electrodes — one for the left hand and one for the right — positioned in a “spaced relationship” to each other in order to filter out electrical noise from the muscles (EMG signals) while accurately detecting the heart’s electrical signals (ECG signals).

Nautilus, Inc. sold exercise equipment and was sued by Biosig for infringement. Nautilus argued that the patent claim term “in spaced relationship with each other” was fatally indefinite because a person skilled in the art could not determine from the specification how far apart the electrodes needed to be. The Federal Circuit disagreed, applying its standard that a patent claim is indefinite only if it is “insolubly ambiguous” — meaning courts cannot even begin to construe it. The Supreme Court granted certiorari to address whether this lenient standard was correct.

The Court’s Holding

Justice Ginsburg, writing for a unanimous Court, held that the Federal Circuit’s “insolubly ambiguous” standard was too permissive and failed to satisfy the definiteness requirement of 35 U.S.C. §112, ¶2. The correct standard is that a patent claim is invalid for indefiniteness if it fails to inform a person skilled in the relevant art, with “reasonable certainty,” about the scope of the invention.

The Court remanded the case to the Federal Circuit to apply the correct standard, noting that the “insolubly ambiguous” test tolerates too much uncertainty — patent claims must give the public fair notice of what is and is not protected, and that notice must be reasonably certain, not merely resolvable through heroic interpretive effort. The Court balanced this against the recognized reality that patents are written in advance of full development and some claim language will inherently have some ambiguity.

Key Takeaways

• Patent claims are indefinite — and thus invalid — if they fail to inform a person skilled in the art with “reasonable certainty” about the invention’s scope.
• The Federal Circuit’s former “insolubly ambiguous” standard was too lenient and is no longer good law.
• The reasonable certainty standard gives the public clearer notice of patent rights and creates a higher bar for claim clarity during prosecution and litigation.
• Patent drafters must ensure claim language provides meaningful guidance on scope, not merely language that is theoretically construable.

Why It Matters

Patent claims define the metes and bounds of intellectual property — they tell competitors what they must design around and tell courts what to enforce. A lenient definiteness standard allows patent owners to maintain broad, vague claims that chill innovation without providing clear notice of what is protected. Nautilus tightened this standard significantly, giving defendants a stronger tool to challenge imprecise patent claims.

For patent prosecutors and litigants, the decision raised the bar on claim drafting and increased the frequency of indefiniteness challenges at the Patent Office and in district courts. Companies defending against vague patent claims — particularly from non-practicing entities (patent trolls) who sometimes rely on ambiguous language to cover a broad range of products — gained new leverage. The “reasonable certainty” standard is now a central feature of patent validity analysis.