The South Korean Supreme Court has rendered a ruling clarifying when arguments or amendments made during prosecution of a parent application may be used to narrow the interpretation of its divisional application claims (Supreme Court Decision 2023Hu11340, July 17 2025). This suggests that patent practitioners should be more careful when employing the common prosecution strategy of first pursuing narrow claims in a parent application and seeking broader claims – particularly when using functional terms – in its divisional applications.
Case history
The subject patent, directed to a self-cleanable water purifier, was pursued as a second-generation divisional application; i.e., a second divisional application divided from the first divisional application. Claim 1 of the subject patent recited “a cleaning part which is connected to said storage tank so as to wash said storage tank by using the water filtered by said filtration part, and comprises a cleaning means”, without defining the specific technical elements of the functional term “cleaning means”.
Meanwhile, during prosecution of the parent application, in order to resolve a rejection from the examiner based on prior art, the patentee amended the claims by limiting the “cleaning means” to “the cleaning means configured to comprise a filter or a cleaning/sterilising agent dosing system, which contains cleaning or sterilising substances therein”. Furthermore, the patentee expressly argued that, unlike the prior art reference – which disclosed the use of electrolysis for generating a sterilising substance to wash a storage tank – the “cleaning means” of the parent application contains cleaning or sterilising substances therein. The patentee emphasised that this difference removes the need to generate such substances by electrolysis and results in the technical effects of saving time and energy for cleaning or sterilisation.
The parent application was subsequently allowed with these narrowed claims, after which the subject divisional application was filed to pursue broader claims as described above.
Following the grant of the divisional application, a competitor filed an action seeking to confirm that its product fell outside the subject patent’s scope. The competitor’s product included an electrode steriliser for electrolysing filtered water to generate electrolysed water for self-washing the water purification tank. The electrode steriliser did not contain cleaning or sterilising substances.
The key issue was whether limitations arising from the prosecution history of the parent application should be taken into account when interpreting the functional claim term “cleaning means” in the subject divisional application.
The Intellectual Property High Court sided with the patentee, but the Supreme Court overturned the lower court’s decision, finding that the latter erred when interpreting the functional expressions included in claims. Additionally, the court applied the principle of prosecution history estoppel in claim construction.
The Supreme Court’s ruling
Under established Korean precedents, claims drafted in functional terms – rather than by reference to specific structural features – may be subject to a narrow interpretation where a literal reading would be “evidently unreasonable”. This assessment is made in light of:
The technical substance of the invention;
The specification as a whole;
The patentee’s intent; and
The need for legal certainty for third parties.
Such circumstances arise, for example, where certain interpretations falling within the literal scope of the claim lack support in the specification, or where the applicant has clearly and deliberately excluded particular embodiments from the scope of the claims.
The Supreme Court, when applying the principle established in these precedents, determined that the “cleaning means” in the subject patent should be narrowly interpreted. The reason was that a broad, literal interpretation of the term would be obviously unreasonable in light of the specification and the prosecution history of the parent application. Accordingly – despite the use of the broad functional claim language – the court interpreted “cleaning means” as limited to a means for cleaning the storage tank, comprising cleaning or sterilising substances therein, wherein the cleaning or sterilising substances are diluted in the filtered water from the filtration part and supplied to the storage tank to perform the cleaning function.
In reaching this conclusion, the Supreme Court relied on a few key considerations:
First, the court confirmed that the term “cleaning means” is a purely functional expression – referring to “a means for performing a cleaning function” – that does not, on its own, convey any specific technical feature.
Second, the specification disclosed only embodiments in which the “cleaning means” contains cleaning or sterilising substances therein. Thus, it did not provide any support for a cleaning means that generates sterilising substances through electrolysis.
Third, and perhaps of most significance, given that the patentee expressly had made an argument to exclude electrolysis-based cleaning means from the claims due to the prior art rejection during prosecution of the parent application, the court highlighted that the prosecution history of the parent application could be taken into account when interpreting the claims of the subsequent divisional application.
Implications
This ruling makes clear that statements made during prosecution of a parent application may subsequently influence the interpretation of the claims of a divisional application.
Since arguments or amendments made to distinguish the claimed invention from prior art in the parent application may come to limit the scope of divisional claims – particularly in cases involving functional claim language – patent practitioners would be wise to adopt a more thoughtful and holistic prosecution strategy when responding to rejections in the parent application.
