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the doctrine of claim differentiation after incorrectly concluding that the examiner had mistakenly rejoined withdrawn claims.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 After allowing the amended independent claims, the examiner concluded that the previously withdrawn claims “require all the limitations of the .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 [...]

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