United States: Plain Talk About Infringement

The U.S. Supreme Court Decision in Festo Corp. v. Shoketsu Kogyo Kabushiki Co., Ltd.

On May 28, 2002, the United States Supreme Court issued a ruling in the case of Festo Corp. v. Shoketsu Kogyo Kabushiki Co., Ltd. ("Festo") (535 U.S. ___ (2002)). In a unanimous decision, the Court held that prosecution history estoppel may apply to any claim amendment made to satisfy the Patent Act's requirements and not just to amendments made to avoid the prior art. The Supreme Court additionally held that estoppel need not bar suit against every equivalent to the amended claim element. The Court remanded the case for further proceedings consistent with its opinion.

Part I of this paper provides the background of the Festo case, particularly focusing on the proceedings before the U.S. Court of Appeals for the Federal Court, the ruling of which shocked inventors and patent practitioners worldwide. Part II discusses the much-anticipated ruling by the U.S. Supreme Court.

PART I: A SURPRISING DECISION BY THE U.S. COURT OF APPEALS FOR THE FEDERAL CIRCUIT

Background
The Technology

Petitioner Festo Corporation (hereinafter "Festo") owns two patents, referred to herein as the "Stoll patent" and the "Carroll patent". The two patents are directed to magnetic rodless cylinders, i.e., piston-driven devices that rely on magnets to move objects in a conveying system. Copies of the front pages of the Stoll and Carroll patents are provided in the Appendix hereto. The front pages of the patents contain illustrations of the invention device. Each device contains a piston, a cylinder and a sleeve. The piston is located inside the cylinder and is moved by fluid under pressure. The sleeve is located on the outside of the cylinder and is magnetically coupled to the piston. Magnetic attraction between the sleeve and the piston causes the sleeve to follow the piston when it moves along the inside of the cylinder. The sleeve is used to move objects on a conveying system.

In the magnetic rodless cylinder disclosed in the Stoll patent, the piston includes magnets, two elastomeric sealing rings and a pair of guide rings. The sealing rings prevent any impurities from getting on the piston. The guide rings, which project beyond the piston's surface, pass along the internal cylinder surface in a sliding fit and help prevent impurities from dirtying the piston. The sleeve includes magnets and an outer body made of a magnetizable material.

In the Carroll patent, the sleeve of the magnetic rodless cylinder is a permanent magnet attached to a gripping device and surrounding part of the exterior of the cylinder. Each end of the piston has a sealing ring set in an annular groove. The sealing rings engage the inner wall of the cylinder and form a fluid tight seal that allows compressed air or other pressurized fluid to move the piston in either direction. The polarization of the magnets on the piston and the sleeve causes the sleeve to follow the movement of the piston.

Prosecution History of the Stoll Patent

Originally, the Stoll patent application contained twelve claims. Claim 1, set forth below, initially read:

1. A linear motor for use in a conveying system,

said motor being operable by a pressure medium and comprising a tubular part connectible to a source of the pressure medium,

a piston which is slidable in said tubular part and which has sealing means at each end for [w]iping engagement with an internal surface of the tubular part and so as to form a seal for the pressure medium,

and a driven assembly which is slidable on the tubular part and which has means at each end for [w]iping engagement with an external surface of the tubular part,

the piston and the driven assembly each carrying a drive magnet arrangement in the form of a hollow cylindrical assembly,

each magnet arrangement having radial play relative to the adjacent surface of the tubular part,

and surfaces of the magnet arrangements which face the tubular part being closely adjacent to the respective surfaces of the tubular part [Emphasis Added].

Dependent claims 4 and 8, set forth below, of the Stoll patent application are relevant:

4. A linear motor according to any of claims 1 to 3, wherein the sealing means of the piston comprise sealing rings and the piston is provided with sliding guide rings near the sealing rings [Emphasis Added].

8. A linear motor according to any of the preceding claims wherein the driven assembly is provided with a sleeve made of a magnetisable material, which encircles the hollow cylindrical assembly of the magnet arrangement [Emphasis Added].

All twelve claims of the patent application were rejected in the first Office Action.

Claims 1-12 were rejected under 35 U.S.C. ,§ 112, first paragraph, because the "exact method of operation is unclear." The patent examiner asked whether the device was a "true motor or magnetic clutch". Claims 4-12 were further rejected under 35 U.S.C. ,§ 112, second paragraph, because they were "improperly multiply dependent."

In response to the Office Action, Stoll amended some claims, including claim 1, and cancelled others, including claims 4 and 8. Claim 1 was amended to recite the following:

"plural guide ring means . . . and first sealing rings located axially outside said guide rings" on the piston

and

"a cylindrical sleeve made of a magnetizable material".

In the remarks accompanying the amendments, Stoll stated that

[e]ach of the claims now present in this application has been reviewed for compliance with the provisions of Title 35 USC 112. Accordingly, further consideration of these claims, particularly with respect to the provisions of Title 35 USC 112, is respectfully solicited.

In the response to the Office Action, Stoll also made two German patents of record in the application. In the remarks section, Stoll stated that the "claims now present in th[e] application" are distinguishable over these references and that "i[t] is clear that neither of these two references discloses the use of structure preventing the interference by impurities located inside the tube and on the outside of the tube while the arrangement is moved along the tube."

The examiner allowed the amended claims and requested that all references to "linear motors" be deleted from the specification because this phrase "connotes a different device having different operational characteristics."

Prosecution History of the Carroll Patent

The relevant portion of the Carroll Patent's prosecution history is its reexamination. Before reexamination, claim 1 of the Carroll patent application read as follows:

1. A device for moving articles comprising

a cylinder of non-ferrous material,

a piston including a permanent magnet having a pole-piece on each axial side thereof,

a body disposed outside and adjacent to said cylinder, said body including a permanent magnet which substantially surrounds the cylinder, there being a pole piece on each axial side of the permanent magnet included in said body, and

means for controlling the admission of pressure fluid into the cylinder and exhaust of fluid from the cylinder for moving the piston in the cylinder,

the attractive forces between the permanent magnets being such that movement of the piston causes corresponding movement of the body below a predetermined load on the body and such that above said predetermined load movement of the piston does not cause corresponding movement of the body.

Claim 1 did not recite the sealing rings disclosed in the specification.

In his request for reexamination, Carroll cited German Patent No. 1,982,379, which was not of record in the Carroll Patent's prosecution history.

During reexamination, Carroll cancelled claim 1 and added claim 9, which explicitly recited " a pair of resilient sealing rings situated near opposite axial ends of the central mounting member and engaging the cylinder to effect a fluid-tight seal therewith." In the remarks accompanying the amendment, Carroll stated that the now-amended claims "more clearly and more specifically" define the "features of the patentee's invention that distinguish over the art of record, including" the German patent cited in the request for reexamination.

Carroll also noted that the structure now described in claim 9 was not disclosed in the art of record. He further pointed out that "the particular structure of the inner piston and outer body now specifically set forth in new claim 9 is not taught or suggested by the German patent." Carroll particularly noted the recitation of the placement and plurality of magnets for both the piston and outer body and the recitation of resilient materials and cushion materials on the ends of the piston.

The examiner allowed the amended claims, stating that "the prior art does not teach or render obvious the claimed combination which includes the plurality of magnets, end members, and cushion members in the claimed relationship."

The Alleged Infringement

After Festo began selling its rodless cylinder, respondents Shoketsu Kinzoku Kogyo Kabushiki Co. and SMC Pneumatics Inc. (collectively "SMC") entered the market with a device similar, but not identical, to the ones disclosed by Festo's patents. Rather than using two one-way sealing rings, SMC's cylinder used a single sealing ring with a two-way lip. Furthermore, SMC's sleeve was made of a nonmagnetizable alloy.

Festo brought action against SMC in the U.S. District Court for the District of Massachusetts for infringement of both the Stoll and Carroll Patents.

The District Court Proceedings

The district court held that SMC could not literally infringe the Stoll patent because SMC's devices did not have magnetizable sleeves.

SMC asserted that prosecution history estoppel barred the application of the doctrine of equivalents to the Stoll patent because the magnetizable sleeve element initially was not recited in claim 1 but was added to the claim after the first Office Action.

The district court stated that the reason for the magnetizable sleeve amendment was "a mystery" because it did not appear to be related to any of the examiner's ,§ 112 rejections and did not appear to distinguish the invention over the prior art. Therefore, the district court declined to hold that prosecution history estoppel barred a finding that the Stoll patent was infringed under the doctrine of equivalents.

The issue was taken to a jury, which concluded that claim 1 of the Stoll patent was infringed under the doctrine of equivalents. The jury found that Festo had proven by a preponderance of the evidence that SMC's non-magnetizable sleeve and single sealing ring performed substantially the same function in substantially the same way to obtain substantially the same result as the claimed magnetizable sleeve and pair of sealing rings.

SMC argued that its device did not infringe the Carroll patent because the single sealing ring in the SMC piston was not equivalent to the pair of sealing rings recited in claim 9 of the Carroll patent. However, Festo presented expert testimony that SMC's single seal was equivalent to the two seals recited in the claims. In rebuttal, SMC referred to a statement made by Stoll during prosecution of the patent to the effect that two sealing rings were necessary to prevent dirt on the piston's magnet. The district court found that this statement did not bear on the "meaning and function of the sealing rings as described" in the Carroll patent. Thus, the court granted Festo's motion for summary judgment of infringement of claims 5, 6 and 9 of the Carroll patent under the doctrine of equivalents.

SMC appealed the judgments of infringement to the U.S. Court of Appeals to the Federal Circuit (CAFC), a panel of which affirmed the district court's holdings. Granting a writ of certiorari filed by SMC, the U.S. Supreme Court vacated the CAFC's holding and remanded the case back to the CAFC. The Supreme Court instructed the CAFC to reconsider its earlier decision in view of the intervening Supreme Court decision in Warner-Jenkinson v. Hilton Davis Chemical Co.

Proceedings in the CAFC

The CAFC took the Festo case en banc "to resolve certain issues relating to the doctrine of equivalents that remained in the wake of the Supreme Court's decision in" Warner-Jenkinson." The CAFC's opinion was divided into four sections, with the first section providing a brief overview of the doctrine of equivalents and prosecution history estoppel, the second section discussing the Supreme Court's decision in Warner-Jenkinson, the third section dealing with the CAFC's answers to a series of en banc questions the court had asked the parties to brief, and the fourth section setting forth the CAFC's application of the foregoing answers to the facts of the case.

I. The Doctrine of Equivalents and Prosecution History Estoppel

The doctrine of equivalents prevents an accused infringer from avoiding liability for infringement by changing only minor or insubstantial details of a claimed invention while retaining the invention's essential identity. As stated by the CAFC, "the doctrine attempts to strike a balance between ensuring that the patentee enjoys the full benefit of his patent and ensuring that the claims give 'fair notice' of the patent's scope. However, quoting Warner-Jenkinson, the CAFC warned that "[t]his balance can be easily upset â,¦ because 'the doctrine of equivalents, when applied broadly, conflicts with the definitional and public-notice functions of the statutory claiming requirement."

The CAFC then pointed out that prosecution history estoppel "is one tool that prevents the doctrine of equivalents from vitiating the notice function of claims." Citing a previous CAFC decision , the court stated that "[p]rosecution history estoppel precludes a patentee from obtaining under the doctrine of equivalents coverage of subject matter that has been relinquished during the prosecution of its patent application." According to the CAFC:

[t]he logic of prosecution history estoppel is that the patentee, during prosecution, has created a record that fairly notifies the public that the patentee has surrendered the right to claim particular matter as within the reach of the patent.

II. Warner-Jenkinson

The CAFC then discussed the effect of Warner-Jenkinson on the case law relating to the doctrine of equivalents and prosecution history estoppel.

In Warner-Jenkinson, the patent alleged to be infringed disclosed an improved process for purifying dyes which used a method called "ultrafiltration". During prosecution, the patentee amended the claims to recite that the process is carried out "at a pH from approximately 6.0 to 9.0." The allegedly infringing process was carried out at a pH of 5.0. In view of these facts, the Supreme Court sought "to clarify the proper scope of the doctrine" of equivalents.

Warner-Jenkinson (the alleged infringer) argued that the doctrine of equivalents, as established in the 1950 U.S. Supreme Court decision Graver Tank & Mfg. Co. v. Linde Air Prods. Co., did not survive the 1952 revision of the Patent Act. The Supreme Court dismissed this argument but noted that "the doctrine of equivalents, as it has come to be applied since Graver Tank, has taken on a life its own." The Supreme Court agreed with Warner-Jenkinson "that Graver Tank did not dispose of prosecution history estoppel as a legal limitation on the doctrine of equivalents."

Warner-Jenkinson further argued that "the reason for an amendment during patent prosecution is irrelevant to any subsequent estoppel." The Supreme Court disagreed and noted that "[i]n each of our cases cited by petitioner and by the dissent below, prosecution history estoppel was tied to amendments made to avoid the prior art, or otherwise to address a specific concern such as obviousness that arguably would have rendered the claimed subject matter unpatentable." The Court therefore saw "no substantial cause for requiring a more rigid rule invoking an estoppel regardless of the reasons for a change."

In Warner-Jenkinson, the Supreme Court noted that the parties did not dispute that the upper pH limit of 9.0 was added to avoid the prior art. However, the Court found that "the reason for adding the lower limit of 6.0 is unclear." Because the record did not reveal the reason for including the lower pH limit of 6.0, the Supreme Court placed "the burden on the patent holder to establish the reason for an amendment during patent prosecution." Courts then would have to "decide whether the [proffered] reason is sufficient to overcome prosecution history estoppel as a bar to application of the doctrine of equivalents to the element added by that amendment." The Supreme Court further stated that:

"[w]here no explanation is established, â,¦ the court should presume that the patent applicant had a substantial reason related to patentability for including the limiting element added by the amendment.

Therefore, according to the Court, "prosecution history estoppel would bar the application of the doctrine of equivalents as to that element."

Warner-Jenkinson proposed a number of restrictions on the doctrine of equivalents, which the Supreme Court rejected. The Court also refused to limit the doctrine of equivalents to "equivalents that are disclosed within the patent itself." According to the Court, the "proper time for evaluating equivalency . . . is at the time of infringement, not at the time the patent was issued."

The Supreme Court stated that the doctrine of equivalents "should be applied as an objective inquiry on an element-by-element basis." Although it noted that "[p]rosecution history estoppel continues to be available as a defense to infringement", the Court also stated that "if the patent holder demonstrates that an amendment required during prosecution had a purpose unrelated to patentability, a court must consider that purpose in order to decide whether an estoppel is precluded." However, if "the patent holder is unable to establish such a purpose, a court should presume that the purpose behind the required amendment is such that prosecution history estoppel would apply."

Because the respondent did not proffer a reason for the addition of the lower pH limit, the Supreme Court in Warner-Jenkinson could not determine whether the reason for the addition could properly avoid an estoppel. The Court thus remanded this issue to the CAFC to consider whether reasons for the addition of the lower pH limit were offered or not and whether further opportunity to establish such reasons would be proper.

III. Answers to En Banc Questions

The CAFC in Festo then proceeded to discuss its answers to questions which the court had asked the parties to brief for rehearing en banc. Although five questions had been asked, the CAFC only answered the first four, stating that the court did not need to reach the fifth question (which related to the "all elements" rule). The four "answers" are set forth below:

(1) For the purposes of determining whether an amendment gives rise to prosecution history estoppel, a "substantial reason related to patentability" is not limited to overcoming or avoiding prior art, but instead includes any reason which relates to the statutory requirements for a patent. Therefore, a narrowing amendment made for any reason related to the statutory requirements for a patent will give rise to prosecution history estoppel with respect to the amended claim element.

(2) Voluntary claim amendments are treated the same as other amendments. Therefore, a voluntary amendment that narrows the scope of a claim for a reason related to the statutory requirements for a patent will give rise to prosecution history estoppel as to the amended claim element.

(3) When a claim amendment creates prosecution history estoppel with regard to a claim element, there is no range of equivalents available for the amended claim element. Application of the doctrine of equivalents to the claim element is completely barred (a "complete bar").

(4) When no explanation for a claim amendment is established, no range of equivalents is available for the claim element so amended.

With respect to the first answer, i.e., a narrowing amendment made for any reason related to the statutory requirements for a patent will give rise to prosecution history estoppel with respect to the amended claim element, the CAFC, while acknowledging that Warner-Jenkinson focused on claim amendments made to overcome or avoid prior art, stated that:

[h]owever, there are a number of statutory requirements that must be satisfied before a valid patent can issue and that thus relate to patentability. In addition to satisfying the novelty and nonobviousness requirements of 35 U.S.C. ,§ ,§ 102 and 103 â,¦, the claims must be directed to patentable subject matter and the claimed invention must be useful, as set forth in 35 U.S.C. ,§ 101 (1994). Additionally, the first paragraph of 35 U.S.C. ,§ 112 requires that the patent specification describe, enable, and set forth the best mode of carrying out the invention â,¦, while the second paragraph of section 112 requires that the claims set forth the subject matter that the applicant regards as his invention and that the claims particularly point out and distinctly define the invention â,¦. The Patent Office will reject a patent application that fails to satisfy any one of these statutory requirements.

Thus, the CAFC found that an "amendment related to any of these statutory requirements is an amendment made for 'a substantial reason related to patentability.'"

The CAFC further stated that:

[i]n view of the functions of prosecution history estoppel preserving the notice function of the claims and preventing patent holders from recapturing under the doctrine of equivalents subject matter that was surrendered before the Patent Office we see no reason why prosecution history estoppel should not also arise from amendments made for other reasons related to patentability. . . . Indeed, the functions of prosecution history estoppel cannot be fully satisfied if substantial reasons related to patentability are limited to a narrow subset of patentability issues. Rather, substantial reasons related to patentability include 35 U.S.C. ,§ ,§ 101 and 112 issues, as well as 35 U.S.C. ,§ ,§ 102 and 103 issues.

With respect to the second answer, i.e., voluntary amendments are treated the same as other amendments, the CAFC stated that:

[b]oth voluntary amendments and amendments required by the Patent Office signal to the public that subject matter has been surrendered. There is no reason why prosecution history estoppel should arise if the Patent Office rejects a claim because it believes the claim to be unpatentable, but not arise if the applicant amends a claim because he believes the claim to be unpatentable.

Citing previous CAFC cases holding that arguments made voluntarily during prosecution may give rise to prosecution history estoppel if they evidence a surrender of subject matter, the CAFC concluded that "[t]here is no reason why an amendment-based surrender of subject matter should be given less force than an argument-based surrender of subject matter."

With respect to the third answer, i.e., when a claim amendment creates prosecution history estoppel with regard to a claim element, there is no range of equivalents available for the amended claim element, the CAFC stated that "it is fair to say that the question of the scope of equivalents available when prosecution history estoppel applies to a claim element has not been directly addressed or answered by the Supreme Court, at least in circumstances where the claim was amended for a known patentability reason." According to the CAFC, "[i]n Warner-Jenkinson, the Court focused its attention more on the circumstances under which prosecution history estoppel arises than on the range of equivalents that might generally be available despite the existence of prosecution history estoppel." The CAFC further stated that "[t]he only statements in Warner-Jenkinson as to the range of equivalents that is available when prosecution history estoppel applies are found in the Court's discussion of unexplained amendments. Id. At 33-34. For those amendments, the Court held that 'prosecution history estoppel would bar the application of the doctrine [of] equivalents as to that element.'"

The CAFC held that "prosecution history estoppel acts as a complete bar to the application of the doctrine of equivalents when an amendment has narrowed the scope of a claim for a reason related to patentability." According to the CAFC:

[A] problem with the flexible bar approach is that it is virtually impossible to predict before the decision on appeal where the line of surrender is drawn. The patentee would draw the line just at or slightly short of the prior art, leaving a wide range of equivalents untouched by prosecution history estoppel. The accused infringer, however, would draw the line close to the literal terms of the claims, leaving little or no range of equivalents. These considerations, we think, contribute to the difficulty under the flexible bar approach in predicting with any degree of certainty the scope of surrender that will be found when prosecution history estoppel applies.

The CAFC also believed that the flexible bar approach posed "a direct obstacle to the realization of important objectives." These objectives include: "giving effect, when prosecution history estoppel arises, to a narrowing amendment's operation as a disclaimer of subject matter"; "preserving the notice function of patent claims"; and "promoting certainty in patent law". According to the court:

[a]llowing some range of equivalents gives the patentee some benefit of the doubt as to what was disclaimed, a benefit that comes at the public's expense. A complete bar therefore best serves the notice and definitional function of patent claims.

Under the flexible bar approach, however, the exact range of equivalents when prosecution history estoppel applies is virtually unascertainable, with only the prior art marking the outer limits of the claim's scope. There is no precise metric to determine what subject matter was given up between the original claim and the amended claim.

If prosecution history estoppel acts as a complete bar to application of the doctrine of equivalents, both the patentee and the public are on notice as to the scope of protection provided by a claim element narrowed for a reason related to patentability. The patentee and the public can look to the prosecution history, a public record, to determine if any prosecution history estoppel arises as to any claim element. If so, that element's scope of protection is clearly defined by its literal terms.

The CAFC concluded that:

we see no overriding benefit to the flexible bar approach. Although a flexible bar affords the patentee more protection under the doctrine of equivalents, we do not believe that the benefit outweighs the costs of uncertainty.

With respect to the fourth answer, i.e., when no explanation for a claim amendment is established, no range of equivalents is available for the claim element so amended, the CAFC quoted the following passage from Warner-Jenkinson:

Where no explanation is established, . . . prosecution history estoppel would bar the application of the doctrine [of] equivalents as to that element. (emphasis added)

IV. The CAFC's Application of the En Banc Answers to the Facts Before It

The CAFC then applied the en banc answers to the determination of whether the Stoll and Carroll patents were infringed by SMC's device under the doctrine of equivalents.

Stoll Patent

The jury in the district court proceedings found that SMC's device infringed claim 1 of the Stoll patent under the doctrine of equivalents. The two elements found to be infringed by equivalents were the "cylindrical sleeve made of a magnetizable material" and the "first sealing rings located axially outside . . . [the] guide rings."

The CAFC concluded that prosecution history estoppel barred the application of the doctrine of equivalents to these claim elements.

The Magnetizable Sleeve Element

SMC argued that the magnetizable sleeve element was not entitled to any range of equivalents.

According to SMC, Festo had not demonstrated that the amendment was made for a reason unrelated to patentability and that, therefore, the presumption of Warner-Jenkinson applied and the doctrine of equivalents was barred. SMC asserted that the voluntary nature of the amendment was irrelevant to the prosecution history estoppel inquiry because Warner-Jenkinson placed the burden on a patent holder to establish the reason for an amendment, regardless of whether the amendment was required or voluntary. SMC argued that Festo disclaimed non-magnetizable sleeves when it amended the claim to recite a magnetizable sleeve. In addition, SMC argued that the public, including competitors like itself, would reasonably understand from the prosecution history of the patent that Festo had surrendered any device with a non-magnetizable sleeve.

Festo responded that the Warner-Jenkinson presumption did not apply to voluntary amendments. Festo emphasized that the magnetizable sleeve amendment was not made in response to any prior art rejection and was not needed to overcome the 35 U.S.C. ,§ 112 rejections of the original claims. Thus, Festo argued that prosecution history estoppel did not bar the application of the doctrine of equivalents.

According to the CAFC, "[t]o determine whether a claim amendment gives rise to prosecution history estoppel, we first must determine whether the amendment narrowed the literal scope of the claim."

Noting that the added claim element was introduced through a new claim instead of through an amendment to an original claim, the CAFC stated that:

Nevertheless, the addition of the magnetizable sleeve claim element can be said to have narrowed the scope of the original claim because the new claim replaced the original claim. Specifically, the only original independent claim, which did not recite a magnetizable sleeve, was replaced with an independent claim which does recite a magnetizable sleeve.

The court then stated that "[b]ecause the amendment narrowed the literal scope of the claim, we must determine whether Festo has established that it was made for a reason unrelated to patentability."

The court agreed with SMC that the reason for the amendment adding the magnetizable sleeve element was not evident from the prosecution history. Original claim 1 had not recited a magnetizable sleeve, although this feature was recited in original dependent claim 8. In response to the first Office Action, Festo replaced original claim 1 with a claim reciting a magnetizable sleeve and cancelled claim 8. The amendment was not responsive to any of the rejections set forth in the Office Action, which had rejected all of the claims under 35 U.S.C. ,§ 112, first paragraph, because it was not clear to the examiner whether the claimed device was a true motor or a magnetic clutch. In addition, claims 4-12 had been rejected under 35 U.S.C. ,§ 112, second paragraph, for being improperly multiply dependent. The amendment adding the magnetizable sleeve element did not address either of these rejections. Moreover, the court noted that there was no statement in the prosecution history that explained why this element was included in the independent claim.

In its Supplemental Brief on Remand from the Supreme Court, Festo argued that the amendment was made to clarify the claim. According to Festo, the "hollow cylindrical assembly" recited in original claim 1 was "rewritten more clearly as "a cylindrical sleeve made of a magnetizable material." The CAFC found this assertion "inadequate to escape the Warner-Jenkinson presumption" because "nothing in the prosecution history of the Stoll patent indicates that the magnetizable sleeve element was merely added for purposes of clarification unrelated to patentability concerns."

On remand, Festo also argued that the voluntary nature of the amendment adding the magnetizable sleeve claim element prevented the amendment from giving rise to prosecution history estoppel. The CAFC rejected this argument and stated that "voluntary amendments are treated the same as other amendments."

The court then concluded that:

Festo has thus failed to meet its burden under Warner-Jenkinson of establishing a reason unrelated to patentability for the amendment that added the magnetizable sleeve element. The amendment therefore gave rise to prosecution history estoppel. See Warner-Jenkinson, 520 U.S. at 40-41. Because prosecution history estoppel acts as a complete bar to the doctrine of equivalents, application of the doctrine of equivalents is barred as to this claim element.

The Sealing Ring Element

SMC argued that the sealing ring claim element was added to distinguish the prior art and, therefore, was not entitled to any range of equivalents. SMC asserted that arguments accompanying the amendment made clear that the amendment was made to distinguish the prior art.

Festo argued that there was no substantial difference between original claim 1 and the amended claim with respect to the sealing ring element. Specifically, Festo contended that the original claim recited the sealing ring element in means-plus-function language, whereas the amended claim recited the structure described in the specification as performing the recited function ("the corresponding structure"). Festo also argued that the claim amendment did not give rise to prosecution history estoppel because it was made to respond to the 35 U.S.C. ,§ 112 rejection, not to avoid the prior art. Festo asserted that the statements accompanying the amendment did not evidence a clear and unmistakable surrender of subject matter, and, therefore, did not give rise to prosecution history estoppel.

The sealing ring element had been added to claim 1 when the original independent claim 1 was replaced with the independent claim that issued as claim 1. The CAFC found that "[t]his amendment narrowed the literal scope of the claim because it substituted an independent claim that recited a sealing ring element for an independent claim that did not recite such an element." The court pointed out that:

Even if the amendment that added the sealing ring merely replaced the means-plus-function language with a recitation of the corresponding structure, the amendment had the effect of narrowing the scope of the claim. A claim element recited in means-plus-function language literally encompasses the corresponding structure and its equivalents (citations omitted). In contrast, a claim element that recites the corresponding structure does not literally encompass equivalents of that structure. Thus, a claim amendment that replaces means-plus-function language with language reciting the corresponding structure narrows the literal scope of the claim.

Thus, the CAFC concluded that "Festo has not established that the amendment that added the sealing ring element was made for a reason unrelated to patentability."

In response to Festo's argument that the amendment was made to respond to the 35 U.S.C. ,§ 112 rejection, the CAFC stated that "[b]ecause a claim will not issue unless it satisfies the requirements of section 112, an amendment made to satisfy the statute is an amendment made for a reason related to patentability."

The court also found that the amendment appeared "to have been made to distinguish the prior art." The amendment had been accompanied by a statement that the German patents submitted with the amendment "are obviously clearly distinguishable over the subject matter of the claims now present in th[e] application," i.e., the amended claims. The amendment also was accompanied by an assertion that "[i]t is clear that neither of these two references discloses the use of structure preventing interference by impurities located inside the tube and on the outside of the tube while the arrangement is moved along the tube." In view of these statements, the CAFC concluded "that the amendment adding the sealing ring element was made to distinguish the German patents and, therefore, was made for a reason related to patentability.

Thus, the court concluded that "Festo cannot establish that the amendment was made for a reason unrelated to patentability. The amendment therefore gave rise to prosecution history estoppel and â,¦ no range of equivalents is available for the sealing ring element."

Pointing out that the "jury's finding of infringement was based on an application of the doctrine of equivalents to the magnetizable sleeve and sealing ring claim elements," the CAFC held that it "must reverse the judgment that claim 1 of the Stoll patent was infringed under the doctrine of equivalents."

Carroll Patent

The district court had granted Festo's motion for summary judgment of infringement under the doctrine of equivalents with respect to independent claim 9 and dependent claims 5 and 6 of the Carroll patent. The element in all three claims found to be infringed by equivalents was "a pair of resilient sealing rings situated near opposite axial ends of the central mounting member" (a "pair of sealing rings"). This element was added to claim 9 during reexamination of the Carroll patent.

The CAFC held that "prosecution history estoppel bars the application of the doctrine of equivalents to this element of the claims of the Carroll patent."

SMC argued that Festo's purpose for this amendment was not clear. SMC stated that because Festo specifically canceled original claim 1, which did not recite a sealing ring at each end of the piston, and added claim 9, which did recite a sealing ring at each end of the piston, the only reasonable conclusion was that the amendment was made for a purpose related to patentability. SMC also argued that if the purpose for the amendment was unclear, the Warner-Jenkinson presumption applied and application of the doctrine of equivalents was barred.

In response, Festo argued that the amendment adding the pair of sealing rings element was not required and thus was voluntary. According to Festo, because the amendment was voluntary, it could not give rise to prosecution history estoppel under Warner-Jenkinson. Festo also stated that the amendment could not have been required to distinguish the German patent that prompted the reexamination, because the German patent disclosed a piston with sealing rings. Festo argued that because the amendment was not made for a purpose related to patentability, the amendment did not create prosecution history estoppel.

To "determine whether this claim amendment gave rise to prosecution history estoppel," the CAFC first determined "whether the amendment narrowed the literal scope of the claim."

The claim element at issue in the Carroll patent had been introduced through a new claim instead of through the amendment of a pending claim. Specifically, during reexamination, independent claim 1, which did not recite a pair of sealing rings, was replaced by independent claim 9, which did recite a pair of sealing rings. The CAFC found that this "amendment narrowed the literal scope of the claims of the Carroll patent."

The court then considered the reasons for the amendment.

Festo admitted that there had been no specific mention of the sealing rings in the prosecution history record. The CAFC stated that the voluntary nature of the amendment was "irrelevant to the inquiry." The CAFC found that the prosecution history of the Carroll patent revealed that the amendment which added the pair of sealing rings claim element was motivated by a "desire to avoid the prior art." In the remarks accompanying the amendment that introduced claim 9, which recited the pair of sealing rings, Carroll had stated that the amendment defined the "features of the patentee's invention that distinguish over the art of record, including" the German patent cited in the request for reexamination. Although the German patent disclosed a piston with sealing rings, Carroll argued that the combination of features recited in the claims, which included the pair of sealing rings, distinguished the claims over the German patent. In addition, when the examiner allowed the reexamined claims, he stated that "the prior art does not teach or render obvious the claimed combination which includes the plurality of magnets, end members, and cushion members in the claimed relationship." The CAFC noted that, although the examiner did not specifically reference the pair of sealing rings in his statement of reasons for allowance, his statement emphasized that it was the claimed combination of elements which was found to be patentable.

The CAFC concluded that:

[i]n view of this prosecution history, Festo cannot establish that the amendment that added the pair of sealing rings element was made for a reason unrelated to patentability. Indeed, the prosecution history indicates that the amendment was made for a reason related to patentability.

The court then held that "prosecution history estoppel bars application of the doctrine of equivalents to the pair of sealing rings element."

The court then reversed the judgment that claims 5, 6 and 9 of the Carroll patent were infringed under the doctrine of equivalents.

Festo petitioned the U.S. Supreme Court for a writ of certiorari, which the Supreme Court granted.

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