The US Court of Appeals for the Federal Circuit vacated a district court’s claim constructions concerning generic independent claims that were amended after a species restriction requirement, because the district court disregarded the doctrine of claim differentiation after incorrectly concluding that the examiner had mistakenly rejoined withdrawn claims. Littelfuse, Inc. v. Mersen USA EP Corp., Case No. 21-2013 (Fed. Cir. Apr. 4, 2022) (Prost, Bryson, Stoll, JJ.)

Littelfuse owns a patent directed to a fuse end cap for providing an electrical connection between a fuse and an electrical conductor. The specification teaches three embodiments of the invention:

1. A single-piece machined end cap comprising a mounting cuff and a terminal

2. A single-piece stamped end cap comprising a mounting cuff and a terminal

3. A two-piece assembled end cap comprising a mounting cuff, a terminal and a fastening stem attaching the mounting cuff to the terminal.

The originally filed claims included independent claims covering an end cap with a mounting cuff and a terminal, and dependent claims directed to the three embodiments. The claims directed to the two-piece assembled end cap embodiment contained the limitation that the terminal is press-fit onto the fastening stem.

During prosecution, the examiner issued a restriction requirement, asserting that the independent claims were generic to the three species in the dependent claims. Littelfuse elected to prosecute the assembled end cap species and the examiner withdrew the claims directed to the other embodiments. In response to a novelty rejection, Littelfuse amended the independent claims by adding the fastening stem element without specifying that the terminal is press-fit onto the stem. After allowing the amended independent claims, the examiner concluded that the previously withdrawn claims “require all the limitations of the . . . allowable claims,” and thus rejoined them.

Littelfuse sued Mersen for selling allegedly infringing fuses. The parties asked the district court to determine whether the fastening stem element in the independent claims limited Littelfuse’s patent to multi-piece end caps, despite the rejoined dependent claims being directed to one-piece embodiments. The district court found that the claim language, the specification and the prosecution history required the invention to have a multi-piece construction. First, the district court determined that the plain meaning of “fastening stem” was “a stem that attached or joins the other two components of the apparatus.” The district court then noted that the fastening stem was only mentioned in the specification in relation to the multi-piece embodiment in which the terminal is joined to the mounting cuff by the fastening stem. While Littelfuse argued that the US Patent & Trademark Office’s rejoining of the withdrawn claims meant that the independent claims covered unitary and multi-piece embodiments, the district court reasoned that the claims were rejoined based on a “misunderstanding” because they referred to the original independent claim, which did not include a fastening stem. In light of the district court’s finding that the independent claims covered only a multi-piece apparatus, the parties stipulated to non-infringement. Littelfuse appealed.

Applying the doctrine of claim differentiation, the Federal Circuit concluded that the district court erred in finding that the independent claims covered only multi-piece devices. The Court noted that the rejoined claims limited the independent claims by requiring that the end cap be formed from a single piece of material. Because an independent claim is broader than a depending claim, the court determined that the independent claims must cover the single-piece embodiments—otherwise, the dependent claims would be meaningless.

The Federal Circuit also found that the presumption of claim differentiation was not overcome by the specification or prosecution history. The Court observed that nothing in the specification stated that a fastening stem cannot be present in a single-piece apparatus, and cautioned against limiting the claimed invention to preferred embodiments in the specification. The Court also found that the examiner’s determination that the withdrawn claims “require all the limitations of the . . . allowable claims” was logical, because “the independent claims are not limited to a multi-piece construction and the dependent claims form a coherent invention.” The Federal Circuit thus concluded that the fastening stem was not required to attach the mounting cuff to the terminal, and that the independent claims were not limited to multi-piece embodiments. The Court vacated the district court’s judgment and claim constructions and remanded for further proceedings.