Opinion ID: 771385
Heading Depth: 2
Heading Rank: 4

Heading: Infringement of Festo's Patents

Text: 77 Festo is the owner by assignment of the Stoll patent and the Carroll patent, both of which are directed to magnetically coupled rodless cylinders. Festo sued SMC in the United States District Court for the District of Massachusetts, alleging infringement of the patents. The jury found that SMC infringed the Stoll patent under the doctrine of equivalents and assessed damages accordingly. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., No. 88-1814-PBS, slip op. at 2-3 (D. Mass. Oct. 27, 1994) (Judgment) (Festo I (Judgment)). The district court previously had granted Festo's motion for summary judgment that certain models of SMC's rodless cylinders infringed the Carroll patent, also under the doctrine of equivalents. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., No. 88-1814-PBS, slip op. at 14-15 (D. Mass. July 11, 1994) (Order) (Festo I (Order)). 3
78 Both the Stoll patent and the Carroll patent disclose magnetic rodless cylinders. The claimed devices are composed of three basic parts: a piston, a cylinder, and a sleeve. In basic terms, the piston is on the inside of the cylinder, and is moved by fluid under pressure. The sleeve is on the outside of the cylinder, and is magnetically coupled to the piston. The 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. 79 1. The application for the Stoll patent was filed on May 28, 1980; the patent issued on October 12, 1982. The following figure, the only drawing in the patent, shows the sleeve (18), the cylinder (10), and the piston (16) of the Stoll device: 80 [Tabular or Graphical Material Omitted] 81 Inside the cylinder, the piston is driven by a pressurized fluid. Stoll patent, col. 3, ll. 13-19. As the piston travels through the cylinder, the magnetically coupled sleeve follows the piston along the outside of the cylinder. Id. 82 The piston includes magnets (20) and two elastomeric sealing rings (26). Id. at col. 3, ll. 20-32. The sealing rings prevent any impurities from getting on the piston. Id. at col. 3, ll. 48-55. There also are a pair of guide rings (24) on the piston. Id. at col. 3, ll. 26-30. As the patent explains, the guide rings, which project beyond the piston's surface, pass along the internal [cylinder] surface in a sliding fit, id. at col. 3, ll. 29-30, and help prevent impurities from dirtying the piston, id. at col. 3, ll. 51-55. 83 The sleeve of the Stoll patent device is made of multiple parts that include magnets (32) and an outer body made of a magnetizable material (30). Id. at col. 3, ll. 60-65. According to the patent, the magnetizable material on the sleeve that encircles the sleeve's magnets allows magnetic leakage fields in the vicinity of the driven assembly to be kept to a minimum, preventing any unwanted braking forces. Id. at col. 2, ll. 24-35. 84 Claim 1 of the Stoll patent is representative of the claims asserted by Festo, and is the only claim of the Stoll patent at issue on appeal: 85 1. In an arrangement having a hollow cylindrical tube and driving and driven members movable thereon for conveying articles, the improvement comprising 86 wherein said tube is made of a nonmagnetic material, 87 wherein said driving member is a piston movably mounted on the inside of said tube, said piston having a piston body and plural axially spaced, first permanent annular magnets encircling said piston body, 88 said piston further including first means spacing said first permanent magnets in said axial spaced relation, the radially peripheral surface of said magnets being oriented close to the internal wall surface of said tube, 89 said piston further including plural guide ring means encircling said piston body and slidingly engaging said internal wall and 90 first sealing rings located axially outside said guide rings for wiping said internal wall as said piston moves along said tube to thereby cause any impurities that may be present in said tube to be pushed along said tube so that said first annular magnets will be free of interference from said impurities, 91 wherein said driven member includes a cylindrical sleeve made of a magnetizable material and encircles said tube, 92 said sleeve having plural axially spaced second permanent annular magnets affixed thereto and in magnetically attracting relation to said first permanent annular magnets, 93 and second means spacing said second permanent annular magnets in said axially spaced relation, the radially inner surface of said magnets being oriented close to the external surface of said tube, 94 said sleeve having end face means with second sealing rings located axially outside said second permanent annular magnets for wiping the external wall surface of said tube as said driven member is moved along said tube in response to a driving movement of said piston to thereby cause any impurities that may be present on said tube to be pushed along said tube so that said second permanent annular magnets will be free of interference from said impurities. 95 Stoll patent, col. 5, l. 23 - col. 6, l. 18 (paragraphing added). 96 2. The application for the Carroll patent was filed on February 17, 1972; the patent issued on December 18, 1973. A reexamination certificate was issued on October 25, 1988, with amended claims. The Carroll patent is directed to the same technology as the Stoll patent. An exterior view of the Carroll patent device is shown in Figure 1 of the Carroll patent, reproduced below: [Tabular or Graphical Material Omitted] 97 In the disclosed embodiment, the sleeve (28) is described as a permanent magnet that is attached to a gripping device (30) and that surrounds part of the exterior of the cylinder (10). Carroll patent, col. 2, ll. 17-26. As in the device of the Stoll patent, the sleeve of the Carroll patent moves along the cylinder in response to a magnetic piston which moves inside the cylinder. Id. Each end of the piston has a sealing ring set in an annular groove. Id. at col. 2, ll. 1-16. According to the patent, the sealing rings engage the inner wall of the cylinder and form a fluid tight seal that allows compressed air, or any other pressurized fluid, injected into port (12) or (14) on the outside of the cylinder to move the piston in either direction. Id. at col. 2, ll. 10-16, 42-59. The polarization of the magnets on the piston and the sleeve causes the sleeve, which is located on the outside of the cylinder, to follow the movement of the piston, which is located on the inside of the cylinder. Id. at col. 2, ll. 17-24. 98 Claim 9 of the reexamined Carroll patent is representative of the claims asserted by Festo: 99 9. A device for moving articles, which comprises: 100 a hollow cylinder formed of non-ferrous material and having opposite axial ends; 101 a piston mounted in the interior of the hollow cylinder and reciprocatingly slidable therein, the piston including a central mounting member disposed axially in the cylinder, 102 a plurality of cylindrically-shaped permanent magnets mounted on the central mounting member and spaced apart axially from each other, each magnet having a bore formed axially there-through for receiving the central mounting member, 103 at least one pair of end members mounted on the central mounting member and disposed on opposite axial sides of the plurality of magnets,a pair of cushion members formed of resilient material, the cushion members being situated near opposite axial ends of the central mounting member to help prevent damage to the piston when the piston contacts an axial end of the cylinder, 104 and 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; 105 a body mounted on the exterior of the hollow cylinder and reciprocatingly slidable thereon, 106 the body including a plurality of annularly shaped permanent magnets surrounding the cylinder and spaced apart from each other, the permanent magnets of the piston and body being polarized so as to magnetically couple the body to the piston whereby movement of the piston inside the cylinder causes a corresponding movement of the body outside the cylinder, 107 the body further including means provided thereon for holding on the body an article to be moved; and 108 means for controlling the admission of pressure fluid into the cylinder and exhaust fluid from the cylinder for moving the piston in the cylinder, 109 the attractive forces between the permanent magnets of the piston and the body 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. 110 Reexamined Carroll patent, col.1, l. 34 - col. 2, l. 37 (additional paragraphing added). 111 3. The SMC devices that were found to infringe the Stoll and Carroll patents under the doctrine of equivalents have two notable differences from the structures claimed in the patents. First, the SMC devices, although having pistons with two hard plastic guide rings, have only a single resilient two-way sealing ring, located on one end of the pistons. Thus, while the patents disclose and claim devices with a pair of sealing rings, the SMC devices have only single two-way sealing rings. 4 Second, the outer portion of the sleeves of SMC's devices is made of an aluminum alloy, a material that the parties agree is not a magnetizable material. Thus, while the Stoll patent discloses and claims a sleeve made of a magnetizable material, the SMC devices have sleeves that are not made of a magnetizable material. 112
113 1. The Stoll patent application was filed in the United States as the U.S. counterpart of a German patent application. As filed, claim 1 of the Stoll patent initially read: 114
115 said motor being operable by a pressure medium and comprising a tubular part connectible to a source of the pressure medium, 116 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, 117 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, 118 the piston and the driven assembly each carrying a drive magnet arrangement in the form of a hollow cylindrical assembly, 119 each magnet arrangement having radial play relative to the adjacent surface of the tubular part, 120 and surfaces of the magnet arrangements which face the tubular part being closely adjacent to the respective surfaces of the tubular part. 121 The original application also included two dependent claims of relevance, claims 4 and 8: 122 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. 123 8. A linear motor according to any of the preceding claims wherein the driven assembly is provided with a sleeve made of a magentisable material, which encircles the hollow cylindrical assembly of the magnet arrangement. 124 (Emphasis added.) 125 In the first Office Action, the patent examiner rejected all twelve original claims, and cited three patents as references believed pertinent. Claims 1-12 were rejected under 35 U.S.C. § 112, ¶ 1, because the exact method of operation is unclear. Is [the] device a true motor or magnetic clutch? In addition, claims 4-12 were rejected under 35 U.S.C. § 112, ¶ 2, because they were improperly multiply dependent. 126 In response, Stoll amended some claims, including claim 1, and canceled others, including claims 4 and 8. Claim 1 was amended to recite plural guide ring means . . . and first sealing rings located axially outside said guide rings on the piston and to recite 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. 127 When Stoll submitted this amendment, he also made two German patents of record in the application, No. 27,37,924 and No. 19,82,379. Stoll had received these patents in the first office action in the corresponding German application. Stoll argued in the remarks accompanying the amendment that the claims now present in th[e] application are distinguishable over these references. Stoll stated 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. 128 After considering this response, the examiner allowed the amended claims, requesting that all references to linear motors be deleted from the specification, because this phrase connotes a different device having different operational characteristics. 129 2. The relevant portion of the Carroll patent's prosecution history is its reexamination. Before reexamination, claim 1 of the Carroll patent read as follows: 1. A device for moving articles comprising 130 a cylinder of non-ferrous material, 131 a piston including a permanent magnet having a pole-piece on each axial side thereof, 132 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 133 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, 134 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. 135 Original Carroll patent, col. 4, ll. 4-19 (paragraphing added). This claim did not recite the sealing rings disclosed in the specification. 136 Carroll requested reexamination on March 18, 1988, citing German Patent No. 1,982,379, which was not of record in the Carroll patent's prosecution history. In his request for reexamination, Carroll asserted that the German patent presented a substantial new question of patentability because the Patent Office may find the German patent, in combination with the other references which were cited during prosecution of the Carroll patent[,] . . . to disclose several of the primary structural features of the device defined by Claim 1. 137 The German patent described rodless cylinders having several of the features of the device described in the Carroll patent, including a pair of sealing rings. The Patent Office granted Carroll's request for reexamination, finding that the German patent discloses an article transport device which is movable in response to a hydraulically operated magnetic piston, which is a feature that was not found by the Examiner during the prosecution of the Carroll patent. 138 During reexamination, Carroll canceled claim 1 and added claim 9, which explicitly recites 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 argued that the now-amended claims 5 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. Carroll further stated 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, particularly noting 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. 139 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.
140 Festo sued SMC in the district court for infringement of both the Stoll and Carroll patents. Festo's claims of infringement and damages and SMC's counterclaims of invalidity were referred to a special master for consideration. The special master determined that both the Stoll and Carroll patents were not invalid. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., No. 88-1814-MA, slip op. at 42, 19 (D. Mass. Spec. Master Apr. 27, 1993) (Report). The special master also determined that the SMC devices at issue in this appeal did not infringe the Stoll patent, id. at 42, 47, but did infringe claims 5, 6, and 9 of the Carroll patent under the doctrine of equivalents, id. at 25. 141 In due course, the district court entertained summary judgment motions from both parties on the issues of infringement and validity. Festo I (Order), slip op. at 1-3. The district court denied all summary judgment motions except Festo's motion for summary judgment of infringement of the Carroll patent. Id. at 2. In ruling on the motions, the district court determined that SMC could not literally infringe the Stoll patent because SMC's devices did not have magnetizable sleeves. Id. at 6. The court also determined that there was a genuine issue of material fact regarding infringement under the doctrine of equivalents. Id. at 11. The court addressed SMC's assertion that prosecution history estoppel barred the application of the doctrine of equivalents to the Stoll patent because the magnetizable sleeve element was not initially recited in claim 1, but was added to the claim after the first Office Action. Id. at 9-10. The court concluded 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 35 U.S.C. § 112 rejections, and it did not appear to distinguish the invention over the prior art. Id. at 10. The district court therefore declined to hold that prosecution history estoppel barred a finding that the Stoll patent was infringed under the doctrine of equivalents. Id. at 11. 142 Turning to the Carroll patent, the court noted that the only argument of noninfringement that SMC made was that the single sealing ring in its piston was not equivalent to the pair of sealing rings recited in claim 9 of the Carroll patent. Id. at 14. However, Festo had presented expert testimony that SMC's single seal was equivalent to the two seals recited in the claims. Id. at 14. To rebut this testimony, SMC cited a statement Stoll had made during prosecution of the Stoll patent to the effect that two sealing rings are necessary to prevent dirt on the piston's magnet. Id. at 14-15. The district court found that the statement made during prosecution of the Stoll patent did not bear on the meaning and function of the sealing rings as described in the Carroll patent. Id. at 14-15. The court therefore granted Festo's motion for summary judgment of infringement of claims 5, 6, and 9 of the Carroll patent under the doctrine of equivalents. Id. at 16. 143 The remaining issues, infringement under the doctrine of equivalents of the Stoll patent, and validity of the Carroll and Stoll patents, were tried to a jury. Festo I (Judgment), slip op. at 1. The jury rendered a verdict on July 14, 1994, concluding that both patents were not invalid and finding that claim 1 of the Stoll patent was infringed under the doctrine of equivalents. Id. at 2-3. The special verdict form indicates that 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. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., No. 88-1814-PBS (D. Mass. July 14, 1994) (Special Verdict Form) (Festo I (Special Verdict Form)).
144 SMC appeals the judgment of infringement of the Stoll patent, which was entered pursuant to the jury's verdict that the patent was infringed under the doctrine of equivalents. Infringement under the doctrine of equivalents is a question of fact. Warner-Jenkinson, 520 U.S. at 38. We must overturn the jury's finding on a factual issue if it is not supported by substantial evidence or if it is based on an erroneous legal determination. Kearns v. Chrysler Corp., 32 F.3d 1541, 1547-48, 31 USPQ2d 1746, 1751 (Fed. Cir. 1994). Prosecution history estoppel is a legal question that is subject tode novo review by this court. LaBounty, 867 F.2d at 1576, 9 USPQ2d at 1998. Thus, when reviewing the jury verdict, we will independently decide the legal question of the application of prosecution history estoppel to the Stoll patent. 145 SMC also appeals the district court's judgment of infringement of the Carroll patent, which was entered in accordance with the court's grant of Festo's motion for summary judgment that claims 5, 6 and 9 were infringed under the doctrine of equivalents. Summary judgment shall be rendered forthwith if the pleadings, depositions, answers to interrogatories, and admissions on file, together with the affidavits, if any, show that there is no genuine issue as to any material fact and that the moving party is entitled to a judgment as a matter of law. Fed. R. Civ. P. 56(c); Vivid Techs., Inc. v. Am. Sci. & Eng'g, Inc., 200 F.3d 795, 806-07, 53 USPQ2d 1289, 1297 (Fed. Cir. 1999). We review the grant of a motion for summary judgment without deference, Conroy v. Reebok, Int'l, Ltd., 14 F.3d 1570, 1575, 29 USPQ2d 1373, 1377 (Fed. Cir. 1994), drawing all reasonable factual inferences in favor of the non-moving party, Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 255 (1986); Semiconductor Energy Lab. Co. v. Samsung Elecs. Co., 204 F.3d 1368, 1378, 54 USPQ2d 1001, 1008 (Fed. Cir. 2000). 146 1. When infringement is alleged to occur under the doctrine of equivalents, two primary legal limitations on the doctrine are to be determined by the court, either on a dispositive pretrial motion or on a motion for judgment as a matter of law at the close of evidence and after the jury verdict. Warner-Jenkinson, 520 U.S. at 39 n.8. Those legal limitations are prosecution history estoppel and the all elements rule. Id. 147 The first legal limitation a court should consider is prosecution history estoppel, because prosecution history estoppel may completely bar the application of the doctrine of equivalents to a given claim element. The first step in a prosecution history estoppel analysis is to determine which claim elements are alleged to be met by equivalents. Then, the court must determine whether the elements at issue were amended during prosecution of the patent. If they were not, amendment-based estoppel will not bar the application of the doctrine of equivalents. However, the court still may need to consider whether statements made during prosecution give rise to argument-based estoppel. See e.g., Pharmacia & Upjohn, 170 F.3d at 1377, 50 USPQ2d at 1036. 148 If the claim elements at issue were amended, the court first must determine whether the amendment narrowed the literal scope of the claim. If so, prosecution history estoppel will apply unless the patent holder establishes that the amendment was made for a purpose unrelated to patentability. Warner-Jenkinson, 520 U.S. at 40-41. If the patent holder fails to do so, prosecution history estoppel will bar the application of the doctrine of equivalents to that claim element. 149 In Warner-Jenkinson, the Supreme Court explained the purpose of placing on the patent holder the burden of establishing the reason for an amendment: allocating the burden in this manner gives proper deference to the role of claims in defining an invention and providing public notice. Id. at 33. Public notice considerations also have been fundamental to our decisions regarding the scope of prosecution history estoppel. E.g., Pharmacia & Upjohn, 170 F.3d at 1377, 50 USPQ2d at 1036 (To determine what subject matter has been relinquished, an objective test is applied, inquiring whether a competitor would reasonably believe that the applicant had surrendered the relevant subject matter. (citation and quotations omitted)). In order to give due deference to public notice considerations under the Warner-Jenkinson framework, a patent holder seeking to establish the reason for an amendment must base his arguments solely upon the public record of the patent's prosecution, i.e., the patent's prosecution history. To hold otherwise--that is, to allow a patent holder to rely on evidence not in the public record to establish a reason for an amendment--would undermine the public notice function of the patent record. If the reasons for the amendment do not appear in the public record of the patent's prosecution, the reasons in most cases will be known only to the patent holder. We therefore hold that a narrowing amendment will give rise to prosecution history estoppel unless the prosecution history of the patent reveals that the amendment was made for a purpose unrelated to patentability concerns. 6 150 If prosecution history estoppel does not bar the application of the doctrine of equivalents, the court should consider the second legal limitation on the doctrine, the all elements rule, see, e.g., Pennwalt Corp. v. Durand-Wayland, Inc., 833 F.2d 931, 4 USPQ2d 1737 (Fed. Cir. 1987) (en banc) (holding that there can be no infringement under the doctrine of equivalents if even one element of a claim or its equivalent is not present in the accused device). If the court determines that a finding of infringement under the doctrine of equivalents would entirely vitiate a particular claim element, then the court should rule that there is no infringement under the doctrine of equivalents. Warner-Jenkinson, 520 U.S. at 39 n.8. 151 2. The jury found that claim 1 of the Stoll patent was infringed under the doctrine of equivalents. Festo I (Special Verdict Form). 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. Id. Both of these elements were added during prosecution of the patent. Following the methodology outlined above, we conclude that prosecution history estoppel bars the application of the doctrine of equivalents to these claim elements. In view of this determination, we do not reach the all elements rule. 152 We begin our analysis with the magnetizable sleeve element. SMC argues that this claim element is not entitled to any range of equivalents. SMC contends that Festo has not demonstrated that the amendment was made for a reason unrelated to patentability and that, therefore, the presumption of Warner-Jenkinson applies and the doctrine of equivalents is barred. SMC asserts that the voluntary nature of the amendment is irrelevant to the prosecution history estoppel inquiry because Warner-Jenkinson places the burden on a patent holder to establish the reason for an amendment, regardless of whether the amendment was required or voluntary. SMC argues that Festo disclaimed non-magnetizable sleeves when it amended the claim to recite a magnetizable sleeve. SMC also argues 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. 153 Festo responds that the Warner-Jenkinson presumption does not apply to voluntary amendments. Festo emphasizes 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. Accordingly, Festo argues, prosecution history does not bar the application of the doctrine of equivalents to this claim element. 154 To 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. Here we are presented with the situation where the added claim element was introduced through a new claim, instead of through an amendment to an original claim. 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. Because 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. 155 We agree with SMC that the reason for the amendment adding the magnetizable sleeve element is not evident from the prosecution history. Original claim 1 did not recite a magnetizable sleeve, although this feature of the invention 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 canceled claim 8. Although the amendment was submitted in the response to the first Office Action, the amendment itself was not responsive to any of the rejections set forth in the Office Action. As discussed above, the Office Action rejected all of the claims under 35 U.S.C. § 112, ¶ 1 because it was not clear to the examiner whether the claimed device was a true motor or a magnetic clutch; in addition, the Office Action rejected claims 4-12 under 35 U.S.C. § 112, ¶ 2 for being improperly multiply dependent. The amendment adding the magnetizable sleeve element did not address either of these rejections. Moreover, there is no statement in the prosecution history that explains why this element was included in the independent claim. 156 In its Supplemental Brief On Remand from the Supreme Court, Festo argued that the amendment was made to clarify the claim. Specifically, Festo asserted that the 'hollow cylindrical assembly' recited in original claim 1 was rewritten more clearly as 'a cylindrical sleeve made of a magnetizable material.' Appellee's Supplemental Brief On Remand from the United States Supreme Court, at 7. This assertion is inadequate to escape the Warner-Jenkinson presumption, however, 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. 157 On remand, Festo also argued that the voluntary nature of the amendment that added the magnetizable sleeve claim element prevents the amendment from giving rise to prosecution history estoppel. Our answer to En Banc Question 2, which holds that voluntary amendments are treated the same as other amendments, compels us to reject this argument. 158 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. 159 We turn now to the sealing ring element. SMC argues that the sealing ring claim element was added to distinguish the prior art and, therefore, is not entitled to any range of equivalents. SMC asserts that arguments accompanying the amendment make clear that the amendment was made to distinguish the prior art. SMC contends that a competitor such as itself reasonably would conclude from the prosecution history that Festo surrendered the difference between the originally claimed sealing means and the sealing rings recited in the amended claims. By the same token, SMC argues, Festo disclaimed the difference between the original and amended claims. 160 Festo's principal argument is that there is no substantial difference between original claim 1 and the amended claim with respect to the sealing ring element. Specifically, Festo argues that the original claim recited the sealing ring element in means-plus-function language, whereas the amended claim recites the structure described in the specification as performing the recited function (the corresponding structure). Festo also argues 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 contends that the statements accompanying the amendment do not evidence a clear and unmistakable surrender of subject matter and, therefore, did not give rise to prosecution history estoppel. 161 The sealing ring element was added to claim 1 when the original independent claim 1 was replaced with the independent claim that issued as claim 1. This 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. Even if the amendment that added the sealing ring element 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. Laitram Corp. v. Rexnord Inc., 939 F.2d 1533, 1536, 19 USPQ2d 1367, 1370 (Fed. Cir. 1991). In contrast, a claim element that recites the corresponding structure does not literally encompass equivalents of that structure. Id. Thus, a claim amendment that replaces means-plus-function language with language reciting the corresponding structure narrows the literal scope of the claim. 162 We conclude that Festo has not established that the amendment that added the sealing ring element was made for a reason unrelated to patentability. Festo argues that the amendment was made to respond to the 35 U.S.C. § 112 rejection. Because 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. See supra Answer to En Banc Question 1. The amendment also appears to have been made to distinguish the prior art. Submitted with the amendment was a statement to the effect that German Patent No. 27,37,924 and German Patent No. 19,82,379 are obviously clearly distinguishable over the subject matter of the claims now present in th[e] application, i.e., the amended claims. Also submitted with the amendment was an assertion 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. In view of these statements, we conclude 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. See Warner-Jenkinson, 520 U.S. at 30-31 (noting that amendments made to avoid the prior art have been held to give rise to prosecution history estoppel). Thus, Festo cannot establish that the amendment was made for a reason unrelated to patentability. The amendment therefore gave rise to prosecution history estoppel and, in accordance with our Answer to En Banc Question 3, no range of equivalents is available for the sealing ring element. 163 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; accordingly, we must reverse the judgment that claim 1 of the Stoll patent was infringed under the doctrine of equivalents. 164 3. The district court 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. Festo I (Order), slip op. at 15. The element in all three claims found to be infringed by equivalents is a pair of resilient sealing rings situated near opposite axial ends of the central mounting member (a pair of sealing rings). 7 Id. at 14. This element was added to claim 9 during reexamination of the Carroll patent. Following the methodology outlined above, we conclude that prosecution history estoppel bars the application of the doctrine of equivalents to this element of the claims of the Carroll patent. In view of this determination, we do not reach the all elements rule. 165 SMC argues that Festo's purpose for this amendment is not clear. 8 SMC states 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 does recite a sealing ring at each end of the piston, the only reasonable conclusion is that the amendment was made for a purpose related to patentability. SMC further asserts that because claim 9 is a combination claim, its patentability hinges on the novelty of the recited combination, including the recited pair of sealing rings. SMC also argues that if the purpose for the amendment is unclear, the Warner-Jenkinson presumption applies, and application of the doctrine of equivalents is barred. 166 Festo responds by arguing that the amendment adding the pair of sealing rings element was not required, and thus was voluntary. Festo states that because the amendment was voluntary, it cannot give rise to prosecution history estoppel underWarner-Jenkinson. Festo also states that the amendment could not have been required to distinguish the German patent that prompted the reexamination, because the German patent discloses a piston with sealing rings. Festo argues that because the amendment was not made for a purpose related to patentability, the amendment did not create prosecution history estoppel. 167 To determine whether this claim amendment gave rise to prosecution history estoppel, we first must determine whether the amendment narrowed the literal scope of the claim. As with the elements of the Stoll patent discussed above, the claim element at issue in the Carroll patent was 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 does recite a pair of sealing rings. This amendment narrowed the literal scope of the claims of the Carroll patent. 9 Accordingly, we must consider the reasons for the amendment. 168 As discussed above, under Warner-Jenkinson, Festo bears the burden of establishing that the amendment was made for a reason unrelated to patentability. Warner-Jenkinson, 520 U.S. at 40-41. It has failed to do so. Festo admits that there is [n]o specific mention of the sealing rings in the prosecution history record. En Banc Responsive Brief of Plaintiff-Appellee Festo Corp., at 49. Moreover, in view of our answer to En Banc Question 2, the voluntary nature of the amendment is irrelevant to the inquiry. 169 The prosecution history of the Carroll patent reveals that the amendment that added the pair of sealing rings claim element was motivated by at least one reason related to patentability: a desire to avoid the prior art. In the remarks accompanying the amendment that introduced claim 9, which recites the pair of sealing rings, Carroll 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. Thus, although the German patent disclosed a piston with sealing rings, Carroll did argue that the combination of features recited in the claims, which includes the pair of sealing rings, distinguished the claims over the German patent. Moreover, 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. (Emphasis added.) Although the examiner did not specifically reference the pair of sealing rings in his statement of reasons for allowance, his statement emphasizes that it is the claimed combination of elements that was found to be patentable. In 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. In accordance with our answer to En Banc Question 3, prosecution history estoppel bars application of the doctrine of equivalents to the pair of sealing rings element. Accordingly, we must reverse the judgment that claims 5, 6, and 9 of the Carroll patent were infringed under the doctrine of equivalents.