## Facts

- Biosig owned U.S. Patent No. 5,337,753 covering a heart-rate monitor used with exercise equipment to detect ECG signals while reducing EMG noise.
- The patented monitor used an elongate bar with, on each side, a “live” electrode and a “common” electrode connected to circuitry including a difference amplifier.
- A representative claim required the electrodes on each side to be “mounted … in spaced relationship with each other.”
- Biosig sued Nautilus for infringement in the Southern District of New York.
- After claim construction, the district court granted Nautilus summary judgment that the asserted claims were invalid as indefinite under 35 U.S.C. § 112 ¶ 2 because “spaced relationship” lacked a precise boundary.
- The Federal Circuit previously reversed under its prior definiteness standard; the Supreme Court vacated and remanded, directing application of the “reasonable certainty” test.
- On remand, the Federal Circuit reconsidered indefiniteness under the Supreme Court’s standard.

## Issues

1. Whether the claim phrase “mounted … in spaced relationship with each other,” read in light of the specification and prosecution history, informs a person of ordinary skill in the art with reasonable certainty about the scope of the claimed electrode configuration under 35 U.S.C. § 112 ¶ 2.

## Decision

- The Federal Circuit reversed the district court’s summary judgment of indefiniteness and remanded.
- Applying the “reasonable certainty” standard, the court held “spaced relationship” was not indefinite.
- The court relied on the intrinsic record to conclude that a skilled artisan would understand the spacing as constrained by the claimed differential measurement and noise-cancellation function.
- The court treated the prosecution history as confirming a narrowing, technically meaningful electrode arrangement rather than creating ambiguity.

## Legal Principles

- A patent claim is indefinite under 35 U.S.C. § 112 ¶ 2 if, read in light of the specification and prosecution history, it fails to inform those skilled in the art, with reasonable certainty, about the scope of the invention.
- Absolute numerical precision is not required if the intrinsic record provides sufficient technical guidance for skilled artisans to determine what falls within the claim.
- Functional context in the specification, including how claimed structures must operate within the invention, can supply objective boundaries for otherwise general terms.
- Prosecution history can clarify claim scope by showing how the patentee distinguished prior art based on specific technical features tied to the claim language.

## Conclusion

The Federal Circuit held that the ’753 patent’s “spaced relationship” limitation, interpreted through the specification’s functional description and the prosecution history, provided reasonably certain notice to skilled artisans and therefore did not render the claims indefinite.


Biosig Instruments, Inc. v. Nautilus, Inc., 783 F.3d 1374 (Fed. Cir. 2015)