Both the doctrine of equivalents and prosecution history estoppel play important roles in the determination of patent infringement. The prosecution history estoppel has a priority when there is a conflict between the applications of these two principles, because prosecution history estoppel is one of the factors that limit the application of the doctrine of equivalents. But before that, it is necessary to estimate whether the prosecution history estoppel can be applied in the determination of equivalent infringement.
During the patent prosecution or invalidity procedures, applicants or patentees usually make amendments on claims or make statements that limits and narrows the scopes of claims in order to accommodate the patent law. Adding new technical features into the independent claim ——picking technical features from the specification and adding them into the independent claim or using a subordinate claim as a new independent claim——is a common manner of amendments in practice, and therefore leads to the change of the scope of the independent claim. It was once believed that if such a change produced an essential effect on the patent right, for example overcoming the lack of novelty or inventive step, then all of the other technical features other than the new added one should be excluded from the scope of the independent claim (i.e., the prosecution history estoppel should be applied). While the court of a patent infringement case holds a different view on this matter, and we will introduce this typical case focusing on whether the prosecution history estoppel is applicable or not in facing of equivalent infringement.
In December 2018, the Supreme People’s Court of China made a judgment on a patent infringement case of Tianjin Lianli Chemical Co., Ltd.(“Lianli” for short) v. Zhejiang Friend Chemical Co., Ltd.(“Friend” for short). The court reversed the judgment of the first and second instance courts that Friend infringed the patent right of Lianli, and decided that the accused products did not fall into the scope of the claims involved.
One dispute of the case is whether “the material level switch” in claim 1 should apply the prosecution history estoppel or not.
The accused product lacks of “the material level switch” in comparison with claim 1. According to the specification, the function of the material level switch is detecting whether the aluminum powder in a feed tank has been completely transferred. During the first trial, Friend had ever clearly expressed that the accused product used pressure measurement to check whether the aluminum powder was emptied.
The patentee held that the pressure measurement of the accused product was basically the same as what was recorded in the specification, i.e. the material level switch in the form of “tuning forks, capacitors or others like them”. With the same method, the same function and the same effect, they constituted the equivalent features.
Friend argued that the patent was authorized based on a new claim 1 which was amended from the original one by Lianli during the procedure of prosecution, i.e. the technical feature “material level switch” was added to the original claim. In addition, during the patent invalidation procedure, the Patent Reexamination Board held that the difference between the patent and the prior art was “material level switch” which was non-obvious for the ordinary skilled person in the field. Therefore, the prosecution history estoppel should be applied to prevent “material level switch” expanding to its equivalent technical features, i.e., the technical feature of pressure measurement should not be regarded as an equivalent technical feature of material level switch.
The Court of Second Instance held that the prosecution history estoppel should not be applicable in this case, on the grounds that Friend failed to prove that the relevant statements made by the patentee during the invalidation procedure had a substantial impact on the patentability.
The Supreme Court rectified the applicable law of the court of second instance and held that: in the situation of no other evidences having proved that the patentee has abandoned a certain technical solution through amendment or statement, it cannot be deemed that patentee has completely abandoned all other technical features which are equivalent to the additional technical feature and the prosecution history estoppel cannot be applied, simply because the scope of the claim is narrowed down by adding a technical feature. Otherwise, the scope of the claim will be overly limited after being added a technical feature, and it’ll be too easy for the accused infringer to avoid infringement by modifying and replacing the added technical feature, resulting in an imbalance of interests between the patentee and the public.
If asserting applying the prosecution history estoppel, the accused infringer should provide the specific proofs of the patentee’s “narrowly amendment or statement” and whether the patentee has abandoned any ” technical solution “. While if asserting the prosecution history estoppel is not applicable due to “no abandon on any technical solution”, the patentee should bear the burden of proof for “the narrowly amendment or statement is explicitly negated”. It should be ascertained whether the patentee has abandoned a specific technical solution through “narrowly amendment or statement”, rather than merely considering whether the “narrowly amendment or statement” has a substantial impact on the patentability.
Ultimately, the supreme court held that: 1) the judgment of the court of second instance was correct, but the applicable law was inappropriate; 2)no evidence supported the patentee had abandoned the technical solution of pressure measurement by the “narrowing amendment or statement” and therefore the prosecution history estoppel was not applicable.
According to the view of the supreme court, the key point of whether the prosecution history estoppel is applicable depends on if any technical solution is abandoned or not. The prosecution history estoppel is a necessary restriction on the doctrine of equivalence. Its purpose is to propel the parties to be honest and trustworthy in litigation activities and avoid inconsistent interpretations during administrative authorization procedures and infringement litigations.
For sure,the added technical feature mentioned above has narrowed the scope of the patent claim, but it doesn’t mean that the patentee had abandoned the equivalent technical features. The prosecution history estoppel cannot be expansively applied arbitrarily.
Under which situations would it be recognized as “abandoning a technical solution ”? With referring to other representative decisions (e.g. Darts-ip reference: darts-889-929-A-zh, darts-653-218-F-zh, darts-037-482-G-zh), the situations are summarized as following:
Assumming that a patentee amended the dependent claim N to a new independent claim during the procedure of prosecution or invalidation.
>If the scope of the original independent claim is A+B ,and the scope of the dependent claim N is A+B’, wherein B is the superordinate concept of B’, then it can be inferred that all the equivalent technical features other than the technical feature B’ have been abandoned, as the technical feature B’ in the dependent claim N is covered by the original independent claim and an original reference for the technical features B’ had ever existed.
>If the scope of the original independent claim is A+B, and the scope of the dependent claim N is A+B+C, wherein C is a new added technical feature, then it cannot be inferred that all the equivalent technical features other than the technical feature C have been abandoned, as the technical feature C in the dependent claim N is not covered by the original independent claim and no original reference for the technical features C had ever existed.
Assumming that the scope of the independent claim is A + B, and the technical solution of A + B’ is disclosed by prior art where B’ is equivalent to B.
If the patentee added a new technical feature or stated that there was an obvious difference between B and B ‘ during the procedure of prosecution, and therefore caused patentability of the invention, then it can be inferred that all the equivalent technical features other than the technical feature B have been abandoned, as the patentee’s action shows that the patentability is essentially based on the technical feature B which makes the patent different from the prior art.
 Means the scope of a patent right not only includes the essential technical features, but also the technical features that equivalent to the essential technical features in substance.
 Means a court won’t determine in favor of an applicant or patentee, if he surrenders a subject matter during prosecution and uses the doctrine of equivalents to recover for infringement based on that same subject matter .