Background

Patent applicants often employ the strategy of filing a relatively narrow parent application with the aim of quick allowance, followed by multiple divisional applications to secure protection for claim sets of varying scope. Recently, the Supreme Court provided guidance regarding the interpretation of functional claim language in divisional applications, and specifically how arguments asserted during the prosecution of the parent application may limit the scope of later-filed divisional applications that may be broader than the parent application. 

In Supreme Court Decision 2023Hu11340 (July 17, 2025), the Court held that a divisional patent claim based on a functional term must be narrowly interpreted if the claim does not specify concrete structural elements of the term and the specification only discloses narrow embodiments. The Court further emphasized that arguments made during the prosecution of the parent application—such as excluding certain technologies—must be considered in interpreting the scope of a divisional patent. 

This decision is significant as it exposes risks in the common strategy of quickly obtaining narrow parent claims followed by filing broader divisional claims, particularly when functional claim language is involved.

Procedural History

The patent at issue related to a second generation divisional application (a divisional of a divisional) claiming a self-cleaning feature of a water purifier's storage tank. The claim included the functional term "cleaning means," which was a broad term without giving any specifics of the actual claimed structure. 

During prosecution of the parent application, the examiner rejected the claim based on prior art disclosing an electrolysis device that generated hypochlorous acid (a sterilizing substance) by electrolyzing a chloride solution to clean a water storage tank, which the examiner determined corresponded to the cleaning means of the parent application.

To overcome this prior art rejection, the patentee amended the claim to specify that "the cleaning means is configured to comprise a filter or a cleaning/sterilizing agent dosing system, which contains cleaning or sterilizing materials internally." The patentee expressly argued that unlike the prior art's electrolysis approach, the cleaning means of the parent application contained internal cleaning or sterilizing materials, and thus did not require electrolysis, saving time and energy for cleaning the water storage tank. The parent application was subsequently granted with these limitations.

After filing two successive divisional applications, the patentee obtained the grant of the patent at issue ("Divisional Patent") with claims broadly reciting "cleaning means," without the limitations that had been added to the parent patent. The granted claims of the Divisional Patent only recited limitations regarding the position of the cleaning means, not its structure. When a competitor sought a declaration of non-infringement for their electrolysis-based cleaning system, the main issue was whether the broadly claimed "cleaning means" in the Divisional Patent encompassed the electrolysis-based cleaning systems that the patentee had disclaimed during prosecution of the parent application. The lower court interpreted the functional term broadly, finding that the electrolysis-based cleaning constituted an equivalent mechanism. 

However, the Supreme Court rejected this approach, holding that this broad interpretation was "clearly unreasonable" in light of the specification and prosecution history, and that the term must be limited to the embodiments expressly supported by the disclosure. The Court further noted that this narrow interpretation would be correct regardless of whether the narrowing resulted in the independent claim becoming substantially similar to certain dependent claims.

Supreme Court's Reasoning

The Supreme Court applied its established doctrine regarding interpretation of functional claim language, which permits a narrowing interpretation when literal reading would be "manifestly unreasonable" in view of the technical content of the invention, the specification as a whole, the patentee's intent, and legal certainty for third parties. Such situations include cases where: (i) some interpretations within the literal scope lack support in the specification, or (ii) the patentee has consciously excluded certain embodiments from the claim scope, etc.

Here, the Court narrowly interpreted the "cleaning means" recited in the Divisional Patent based on the following: (i) the specification disclosed only a "cleaning means" containing cleaning or sterilizing materials internally, with no mention whatsoever of electrolysis-based cleaning devices; and, more significantly, (ii) the patentee had expressly disclaimed electrolysis-based cleaning systems from the scope of the claims during prosecution of the parent application in order to distinguish the prior art. The Court held that the prosecution history of the parent application must be considered when interpreting claims of subsequent divisional applications. Consequently, the "cleaning means" in the Divisional Patent was limited to means that contained cleaning or sterilizing materials internally, and excluded electrolysis-based systems, regardless of the broad literal claim language.

Implications

This decision carries significant implications for Korean patent prosecution strategy. While it has been well established that prosecution history estoppel and narrow interpretation of functional claim language for a specific application operate to limit the scope of that application, it was previously unclear whether statements made during a parent application's prosecution would be binding on divisional applications, as the Court has now affirmed.

The decision particularly impacts the common practice of using divisional applications to pursue broader protection after securing narrow parent claims, since where functional claim language is involved, arguments made to distinguish prior art in the parent application will now limit all subsequent divisional applications as well. This risk is especially acute in complex prosecution scenarios where multiple divisional applications may be contemplated over extended periods.

For practitioners, a more holistic approach to prosecution strategy will be required in response to this decision. When responding to rejections in parent applications, careful consideration must be given to potential future divisional filings, since overly narrow distinctions or unnecessary claim limitations made to expedite allowance of a parent application may foreclose broader protection in later divisional applications. This is particularly critical when dealing with functional claim language, where the specification's disclosure and prosecution statements can dramatically impact claim scope.