274-1 Federal Circuit Revisits American Axle & Manufacturing; Case Remanded to Determine if One of the “Hooke’s Law” Claims is Ineligible under Other Theories of Eligibility
The Federal Circuit recently issued a modified opinion regarding patent eligibility of “natural phenomena.” In the original case involving propeller shaft dampening and Hooke’s law, the court affirmed the district court’s finding that all claims were directed to patent-ineligible subject matter. In the modified opinion, one claim survives appeal and is remanded to determine if the claim also runs afoul of being an “abstract idea.” See Am. Axle & Mfg. v. Neapco Holdings LLC, No. 2018-1763, 2020 U.S. App. LEXIS 24216, at *11 (Fed. Cir. July 31, 2020) (Before Dyk, Moore, and Taranto, Circuit Judges) (Opinion for the Court, Dyk, Circuit Judge) (Dissenting opinion, Moore, Circuit Judge). For original opinion, see Am. Axle & Mfg. v. Neapco Holdings LLC, No. 2018-1763, 2019 U.S. App. LEXIS 29655 (Fed. Cir. Oct. 3, 2019) (Before Dyk, Moore, and Taranto, Circuit Judges) (Opinion for the court, Dyk, Circuit Judge) (Dissenting opinion, Moore, Circuit Judge).
U.S. Patent No. 7,774,911 (the ’911 patent) owned by American Axle & Manufacturing, Inc. (AAM) is generally related to a method for manufacturing driveline propeller shafts, which are employed in automotive vehicles, with liners that would “attenuate . . . vibrations transmitted through a shaft assembly.” Prior art methods of dampening the vibrations of the propeller shafts included weights, dampers, and liners, the latter of which the patent described as fibrous material like carboard. The ordinary vibration modes of the propeller shafts include bending, shell, and torsion vibrations, and the’911 patent asserted that the designs presented therein were able to dampen two modes simultaneously, unlike prior art designs.
Two independent claims at issue in this case, claims 1 and 22, describe methods of manufacturing a shaft assembly. Of particular interest in the case are the following limitations of those claims:
1. A method for manufacturing a shaft assembly of a driveline system...
tuning at least one liner to attenuate at least two types of vibration transmitted through the shaft member; and
positioning the at least one liner within the shaft member such that the at least one liner is configured to damp shell mode vibrations in the shaft member....
22. A method for manufacturing a shaft assembly of a driveline system...
tuning a mass and a stiffness of at least one liner....
AAM sued Neapco Holdings LLC for infringement of the ’911 patent in 2015, and the parties filed cross-motions for summary judgment as to patent ineligibility under Section 101. The district court concluded that the asserted claims were directed to laws of nature, specifically Hooke’s law and friction damping, and that the claimed additional steps regarding tuning the liners merely consisted of well-understood, routine, and conventional activity that “add[ed] nothing significant beyond the sum of their parts taken separately.” The district court therefore found the claims patent ineligible, and AAM appealed.
In the first opinion by the Federal Circuit on October 3, 2019, the same panel as here held that both independent claims were patent ineligible under section 101. At step one of the Alice/Mayo test, the court determined that the claims were merely directed to an application of Hooke’s law, a natural law that relates the mass and stiffness of an object to the vibration frequency of the object. The court found that, while the claims stated that the “liner” is tuned to dampen vibrations, the claims did not describe a specific application of Hooke’s law in order to achieve the dampening.
At step two of the test, the court found that both independent claims did not amount to significantly more than Hooke’s law. “This direction to engage in a conventional, unbounded trial-and-error process does not make a patent eligible invention, even if the desired result to which that process is directed would be new and unconventional.” Accordingly, in the prior opinion, the court affirmed the district court’s determination that both claims were patent ineligible.
AAM petitioned the court for rehearing en banc, accompanied by several amici curiae briefs. The original panel issued the present modified opinion in response to the petition.
The court first addressed claim 22, which described “tuning a mass and a stiffness of at least one liner.” The court’s opinion on this claim remained largely unchanged from its original October opinion. Turning again to the Alice/Mayo test, the courts “must determine whether the claimed processes have transformed these unpatentable natural laws into patent-eligible applications of those laws.” (Quoting Mayo Collaborative Services v. Prometheus Laboratories, Inc., 566 U.S. 66, 72 (2012)). The “tuning a mass and a stiffness of at least one liner” in claim 22 was construed to mean “controlling the mass and stiffness of at least one liner to configure the liner to match the relevant frequency or frequencies.” In other words, the claim merely “confers patent coverage if the attenuation goal is achieved by one skilled in the art using any method, including any method implemented by computer modeling and trial and error,” to achieve vibration attenuation. Expert testimony similarly supported that adjusting stiffness and mass are “variable directly implicated by Hooke’s law.”
The court agreed that “sophisticated” models were used to fine tune the deigns, but those models and processes were not claimed. Like it did in the original opinion, the majority cited Parker v. Flook, 437 U.S. 584 (1978), which states, “[A] claim to a natural law concept without specifying the means of how to implement the concept is ineligible under section 101.” The court concluded step one by holding the claimed method merely instructs the user how to achieve an end result, but did not describe particular ways to do so.
At step two, the court found that nothing in claim 22 contributed to an “inventive concept” to transform the instructions to achieve a result into eligible subject matter. The court held that the inventive concept is how to tune the liner to achieve the results, and those concepts were not in the claim. The court then concluded that Claim 22 and all claims depending therefrom were ineligible and affirmed the district court.
The court then turned to claim 1, and changed its original opinion. Claim 1 requires “tuning at least one liner to attenuate at least two types of vibration transmitted through the shaft member,” which the court found to be different than claim 22’s tuning “the mass or stiffness” of the liner. The district court construed this limitation to mean “controlling characteristics of at least one liner to configure the liner to match a relevant frequency or frequencies to reduce at least two types of vibration transmitted through the shaft member.” (Emphasis in opinion). That said, “characteristics” may be different than stiffness and mass. The claim also included the limitation “positioning the at least one liner within the shaft member.” Although the district court may have suggested the broader concept of “tuning” in this claim may also be an abstract idea (and not a natural phenomenon), the abstract idea concept was not adequately litigated at the district court. Accordingly, the Federal Circuit vacated and remanded to address the theory of abstractness.
The majority opinion then addressed Judge Moore’s dissent. The majority stressed that, although the claims did not “recite” Hooke’s law, eligibility is also in question for claims that recite the “result” or “effect” of the natural law. The majority also replied to the dissent’s assertion that the opinion is merging enablement and eligibility. According to the majority, patent law includes two “how” requirements. The first “how” is for the claims, and requires that the functional result of the claim is “specified at some level of concreteness” in the claims. This assessment, dating back to O’Reilly v. Morse, is related to eligibility. The other “how” applies to the specification, and requires the specification to lay out how to practice the claimed invention.
The court concluded by affirming the district court’s finding that claim 22 was ineligible, but vacated and remanded with respect to claim 1.
Judge Moore revisited her dissent from the original opinion. She again asserted that, “[t]he majority’s decision expands § 101 well beyond its statutory gate-keeping function and collapses the Alice/Mayo two-part test to a single step—claims are now ineligible if their performance would involve application of a natural law.” She believed the determination that “nothing more” than Hooke’s law is recited in the claim should be remanded to the district court, where it can be addressed by experts and not judges. Judge Moore also noted that the fusion of enablement and eligibility creates a new “superpower” for § 101 invalidation, which can cause confusion to litigants and the courts alike.
The Federal Circuit remains vastly split on the law of patent ineligibility under § 101. Practitioners should recall that, even if a claim is not ineligible as directed to a natural phenomenon or a law of nature, that same claim may still be assessed and/or invalidated under standards for abstract ideas.
274-2 Federal Circuit Denies En Banc Rehearing of Driveshaft Patent Eligibility Under §101 in an Equally-Divided Vote
After releasing a modified version of its original controversial opinion, the Federal Circuit recently decided, in an even 6-6 split, not to institute a full-court review of the decision that invalidated American Axle’s driveshaft patent for claiming a natural law. See Am. Axle & Mfg. v. Neapco Holdings LLC, No. 2018-1763, 2020 U.S. App. LEXIS 24216 (Fed. Cir. July 31, 2020) (Before Prost, Chief Judge, Newman, Lourie, Dyk, Moore, O’Malley, Reyna, Wallach, Taranto, Chen, Hughes, and Stoll, Circuit Judges) (Denying en banc rehearing).
The case arises out of a patent infringement dispute in which AAM sued Neapco for infringement of certain claims of the ’911 patent. Neapco challenged the claims under § 101, and the district court granted Neapco’s motion and held that the asserted claims were ineligible under § 101. Representative independent claims 1 and 22 are reproduced below:
1. A method for manufacturing a shaft assembly of a driveline system, the driveline system further including a first driveline component and a second driveline component, the shaft assembly being adapted to transmit torque between the first driveline component and the second driveline component, the method comprising:
providing a hollow shaft member;
tuning at least one line to attenuate at least two types of vibration transmitted through the shaft member; and
positioning the at least one liner within the shaft member such that the at least one liner is configured to damp shell mode vibrations in the shaft member by an amount that is greater or equal to about 2%, and the at least one liner is also configured to damp bending mode vibrations in the shaft member, the at least one liner being tuned to within about ±20% of a bending mode natural frequency of the shaft assembly as installed in the driveline system.
* * *
22. A method for manufacturing a shaft assembly of a driveline system, the driveline system further including a first driveline component and a second driveline component, the shaft assembly being adapted to transmit torque between the first driveline component and the second driveline component, the method comprising:
providing a hollow shaft member;
tuning a mass and a stiffness of at least one liner; and
inserting the at least one liner into a shaft member;
The updated opinion invalidated claim 22 for claiming a natural law, with the majority holding that “tuning a mass and stiffness of at least one liner” invokes Hooke’s law in all but name, but remanded claim 1 for further proceedings, holding that claim 22 may be more general than an application of Hooke’s law for the inclusion of, inter alia, “tuning at least one liner to attenuate at least two types of vibration transmitted through the shaft member.”
While the majority held that the holdings of the case follow from a simple application of § 101 precedent, the dissenting opinion alleges that the Federal Circuit has created a new and confusing § 101 test – the “nothing more test,” introducing even more uncertainty in the realm of patent eligibility. The dissent notes that the establishment of such precedent may have far reaching consequences, leading to eligibility challenges to many other patents directed to mechanical inventions and beyond. Because a petition for rehearing en banc requires a majority vote to institute, the motion failed in a split 6-6 vote.
The Federal Circuit produced six opinions in support of the split vote. Judge Dyk, joined by Judges Wallach and Taranto, concurred in the denial of the petition for rehearing en banc. Judge Dyk posited that the instant claims in no way “recite the process and machinery necessary to produce the desired effect of reducing vibrations in a shaft assembly,” as claimed by Judge Stoll in his dissent. Judge Dyk argued that the majority opinion does not suggest that claims that properly describe how an objective is to be achieved are patent ineligible. Rather, the claims at issue did not describe how the objective of dampening two modes of vibrations was to be accomplished. Finally, Judge Dyk reiterated that the court had not created a new “nothing more” test for § 101.
Judge Chen, joined by Judge Wallach, concurred in the denial of the petition for rehearing en banc. Judge Chen reiterated that the allegedly new “nothing more” test has been in use since at least 1853 and cited to O’Reilly v. Morse for support. 56 U.S. (15 How.) 62 (1853). Chen continued by pointing out that the scope of the majority’s holding was illustrated by the difference in outcome between claim 1 and claim 22. Chen argued that had claim 22 omitted reference to mass and stiffness, there would be no basis to say that claim 22 invokes Hooke’s law. For these reasons, asserted Chen, claim 1 was remanded for further findings while claim 22 was invalidated for being directed to a natural law.
Judge Newman, joined by Judges Moore, O’Malley, Reyna, and Stoll, dissented from the denial of the petition for rehearing en banc. Judge Newman held that the holding in the instant case reflects a trend of courts narrowing the range of inventions and discoveries that are eligible for patent protection under the Mayo/Alice test. Judge Newman pointed out that other patentability factors such as written description and enablement were not considered, and that many inventions of practically applied processes may be reduced to a mathematical abstraction or an algorithm, and provided the example of an internal combustion engine being merely an application of PV=nRT and other laws of nature.
Judge Stoll, joined by Judges Newman, Moore, O’Malley, and Reyna, dissented from the denial of the petition for rehearing en banc. Judge Stoll expressed concern that the majority holding extended § 101 to put in doubt the eligibility of mechanical inventions which presents a “question of exceptional importance” that warrants consideration in an en banc rehearing. Judge Stoll opined that the majority had created a new “nothing more” test for determining whether a claim is ineligible for being directed to a natural law. Judge Stoll criticized the majority’s comparison of the instant claims to those in O’Reilly, in which a claim directed to the application of electro-magnetism to transmit signals at a distance was held ineligible under § 101. Judge Stoll pointed out that despite the majority position that the “nothing more” test was a simple application of O’Reilly, the district court opinion never mentioned the case nor how the “nothing more test” is a simple application of O’Reilly.
Judge O’Malley, joined by Judges Newman, Moore, and Stoll, dissented from the denial of the petition for rehearing en banc. Judge O’Malley argued that the majority holding included at least three major issues: (i) it announced a new test for patentable subject matter – the “nothing more test”; (ii) rather than remand to the district court to apply the newly created test, the majority applied the test in the first instance; and (iii) the holding sua sponte construed claim terms in an effort to distinguish claims and render them patent ineligible. More particularly, Judge O’Malley took issue with the majority’s construing claims 1 and 22 to have a patentably distinct difference without sufficient record from the trial court to support the distinction.
Judge Lourie dissented without opinion from the denial of the petition for rehearing en banc.
The Federal Circuit decided against the rehearing of a controversial opinion finding a mechanical invention directed to a natural law. Whether the court’s jurisprudence applied long established § 101 precedent or created a new test for patent eligibility remains to be seen.
274-3 Federal Circuit Deemed Thyroid Pharmaceutical Patent Indefinite under 35 U.S.C. § 112
The Federal Circuit recently issued a ruling affirming the District Court for the District of Delaware’s holding that IBSA’s pharmaceutical patent is invalid as indefinite under 35 U.S.C. § 112. See IBSA Institut Biochimique v. Teva Pharms. USA, Inc., No. 2019-2400, 2020 U.S. App. LEXIS 24150 (Fed. Cir. July 31, 2020) (Before Prost, Chief Judge, Reyna and Hughes Circuit Judges) (Opinion for the Court, Prost, Chief Judge).
ISBA Institut Biochimique SA (IBSA) is the assignee of U.S. Patent No. 7,723,390 (“the ’390 patent”) directed to “pharmaceutical formulations based on thyroid hormones enabling a safe and stable oral administration in the framework of the strict therapeutic index prescribed in case of thyroid disorders.” The formulation patented in the ’390 patent is listed as the FDA approved Tirosint® product in the Orange Book. The ’390 patent claims priority to an Italian patent application originally filed in Italian and incorporated into the file history of the ’390 patent. To market a generic version of Tirosint®, Teva Pharmaceuticals USA, Inc. (Teva) filed an Abbreviated New Drug Application (ANDA) asserting the ’390 patent is “invalid, unenforceable, or will not be infringed by Teva’s generic product.” Subsequently, IBSA filed a suit asserting patent infringement of the ’390 patent by Teva.
During the proceedings, the central issue disputed by the parties was the construction of the term “half-liquid” as used in claim 1 of the ’390 patent. Claim 1 recites a pharmaceutical composition comprising thyroid hormones or their sodium salts in the form of “a soft elastic capsule consisting of a shell of gelatin material containing a liquid or half liquid inner phase”. IBSA asserted the term “half-liquid” should be interpreted to mean “semi-liquid, i.e., having a thick consistency between solid and liquid.” In contrast, Teva argued the term “half-liquid” is indefinite or should be interpreted as “a non-solid, non-paste, non-gel, non-slurry, non-gas substance.” Ultimately, the district court found the asserted claims indefinite because the term “half-liquid” does not have a definite meaning in the specification of the ’390 patent, the Italian priority patent application, or scientific literature.
The district court first examined the evidence on the record to determine if the term “half-liquid” was defined. The district court noted that the parties agreed the intrinsic evidence does not define “half-liquid.” Further, IBSA asserted that the Italian priority patent application uses the term “semiliquido” in instances where the ’390 patent uses the term “half-liquid.” IBSA argued that a person having ordinary skill in the art would understand that “half-liquid” and “semi-liquid” are interchangeable. However, the district court disagreed, explaining that numerous differences existed in the ’390 patent and the Italian priority patent application, and that these differences were intentional. The district court also noted that during prosecution of the ’390 patent, the applicant included a claim specifically using the term “semi-liquid” that was dependent on claim 1 including the term “half-liquid,” further suggesting the terms are not synonymous. Additionally, the district court concluded that the extrinsic evidence of dictionary definitions presented by IBSA were not sufficient to support IBSA’s construction of the term “half-liquid,” because the definitions were not in the same context as the claimed invention.
Second, the district court determined that a skilled artisan would not be able to ascertain a reasonably certain definition of “half-liquid.” The district court noted that claim 1 and the specification do not provide clear evidence of what the term “half-liquid” includes or means. Further, during prosecution of the ’390 patent, the applicant distinguished the claimed invention from prior art by specifically arguing that the claimed invention is not a “macromolecular gel-lattice matrix” and is not a “high concentration slurry.” Lastly, the district court explained the extrinsic evidence of testimony provided by Teva’s expert further indicated that it is not well known in the art what the term “half-liquid” means.
Accordingly, the district court found the asserted claims of the ’390 patent invalid as indefinite. Subsequently, IBSA appealed to the Federal Circuit.
Under 35 U.S.C. § 112, a “claim is invalid for indefiniteness if its language, read in light of the specification and prosecution history, ‘fail[s] to inform, with reasonably certainty, those skilled in the art about the scope of the invention.’” HZNP Meds. LLC v. Actavis Labs. UT, Inc., 940 F.3d 680, 688 (Fed. Cir. 2019) (quoting Nautilus, Inc. v. Biosig Instruments, Inc., 572 U.S. 898, 901 (2014)). The first step to determining whether the meaning of a term in a claim is definite, is to analyze the language of the claim. See Berkheimer v. HP Inc., 881 F.3d 1360, 1363 (Fed. Cir. 2018). Accordingly, the Federal Circuit noted that the parties agree the language of the claim does not define the term “half-liquid.”
The next step in determining whether the meaning of the term is definite is to examine the specification. The Federal Circuit agreed with the district court that the specification does not provide clear meaning. In particular, the Federal Circuit explained that passages in the ’390 patent disclose that the drug formulation can have an inner phase that consists of “a liquid, a half-liquid, a paste, a gel, an emulsion or a suspension comprising the liquid (or half-liquid) vehicle and the thyroid hormones together with possible excipients in suspension or solution.” Accordingly, the use of the disjunctive ‘or’ indicates “half-liquid” is not a gel or paste. However, gels and pastes have a thick consistency which have a thickness between a liquid and a solid and thus would be included in IBSA’s proposed interpretation of “half-liquid.” If gels and pastes are included in the proposed interpretation, this would create uncertainty as to the boundaries of a “half-liquid.”
The Federal Circuit then examined the prosecution history, including the Italian priority patent application. In addition to the Italian priority patent application including the term “semiliquido” in instances where “half-liquid” is used in the ’390 patent, the Federal Circuit noted that claim 1 of the ’390 patent and claim 1 of the Italian priority patent application are different, as claim 1 of the ’390 patent includes an additional embodiment and that Italian priority patent application does not include the term “gel.” Accordingly, a person having ordinary skill in the art would understand there is a difference between “half-liquid” and “semiliquido.” The Federal Circuit also explained that, contrary to IBSA’s assertion, the weight of the evidence of given to the foreign priority application “does not amount to a refusal to consider a foreign priority document;” instead, “when discrepancies between a foreign priority document and the U.S. filing exist, it may be proper to view the discrepancies as intentional.” Abbott Labs. v. Sandoz, Inc., 566 F.3d 1282, 1290 (Fed. Cir. 2009). Accordingly, the Federal Circuit agreed with the district court’s conclusion that the intrinsic evidence, including the language of the claim, the specification, and prosecution history, does not sufficiently establish the definition and boundaries of “half-liquid.”
After considering the intrinsic evidence, the Federal Circuit also agreed with the district court’s conclusion that the extrinsic evidence, including dictionary definitions, other patents, and expert testimony, does not sufficiently establish the definition and boundaries of “half-liquid.” The Federal Circuit noted that IBSA provided no scientific definitions of the “half-liquid.” Further, other patents including “half-liquid” use the term in a different context than the claimed invention and thus do not help define “half-liquid” as used in the ’390 patent. Lastly, the Federal Circuit explained the expert testimony affirmed a person having ordinary skill in the art would not be able to determine the boundaries of a “half-liquid.”
Accordingly, the Federal Circuit affirmed the district court’s holding that the asserted claims of the ’390 patent are indefinite because the term “half-liquid” is not reasonably ascertainable when considering both intrinsic and extrinsic evidence.
A term used in a claim can be deemed indefinite when it cannot be given a construction that is sufficiently supported by the claim language, the specification, prosecution history, or extrinsic evidence.
274-4 Federal Circuit Affirms PTAB’s Obviousness Decision Involving Remote Gambling Patent
The Federal Circuit recently affirmed a final written decision of the Patent Trial and Appeal Board (PTAB) finding that the PTAB, in an inter partes review, did not fail to comply with the Administrative Procedure Act (APA) and that the Board’s obviousness findings were supported by substantial evidence. See FanDuel, Inc. v. Interactive Games LLC, No. 2019-1393, 2020 U.S. App. LEXIS 23892 (Fed. Cir. July 29, 2020) (Before Dyk, Moore, and Hughes, Circuit Judges) (Opinion for the Court, Hughes, Circuit Judge) (Opinion concurring in part and dissenting in part, Dyk, Circuit Judge).
During interpartes review, the PTAB found all claims of U.S. Patent No. 8,771,058 (the ’058 patent) unpatentable except for claim 6. In reaching this conclusion, the PTAB found that FanDuel, Inc. (FanDuel), the petitioner, had failed to prove that claim 6 was obvious in view of the asserted prior art.
The ’058 patent, owned by Interactive Games LLC (Interactive Games), is directed to a system for remote gambling. Claim 6, in particular, is directed to a system that “access[es] a look-up table which contains an ordered list of locations and associated game configurations.” See ’058 patent, claim 6. In its inter partes review petition, FanDuel challenged claim 6 as obvious over three prior art references. In rejecting FanDuel’s obviousness challenge to claim 6, the PTAB disagreed with FanDuel that the prior art disclosed the use of look-up tables to carry out the function of determining the gaming rules in a specific jurisdiction.
On appeal, FanDuel argued that the PTAB violated the APA by adopting a new theory that the combination of the prior art failed to disclose jurisdictional profiles stored in a database employing look-up tables including an ordered list, which the patent owner never raised during the proceeding. FanDuel asserted on appeal that, under the APA, it was entitled to notice and an opportunity to respond before the PTAB rejected its obviousness challenge based on insufficient disclosure in the asserted references. The Federal Circuit, however, saw it differently, holding that “there was nothing inconsistent about the PTAB instituting an IPR on obviousness grounds and then ultimately finding that the petitioner did not provide preponderant evidence that the challenged claim was obvious.” FanDuel, Inc. at 11. The court further reasoned that the burden of proving invalidity in an IPR remains on the petitioner throughout the proceeding and that FanDuel failed to prove such invalidity as to claim 6.
FanDuel further challenged the substance of the PTAB’s determination arguing that the substantial evidence did not support the decision to sustain the patentability of claim 6. The PTAB, however, was unpersuaded finding that FanDuel had not met its burden in proving obviousness. The court agreed with the PTAB that a logical gap existed between the concept of a look-up table being well-known in the art and the beneficial application of that concept to the referenced prior art. The court concluded that the PTAB’s decision was supported by substantial evidence.
Having concluded that the PTAB did not violate the APA and that its decision was supported by substantial evidence, the court affirmed the final written decision.
Offering an opinion concurring in part and dissenting in part, Judge Dyk concurred that the PTAB had not violated the APA but argued that it erred when it determined that FanDuel failed to show that claim 6 would have been obvious. Judge Dyk argued that the PTAB used an incorrect standard for obviousness that was an impermissibly rigid view of the prior art, because it required a specific motivation to combine look-up tables and ordered lists. Judge Dyk argued that, because the use of a look-up table and an ordered list was only one of a number of finite, predictable solutions, it would have been obvious to use the well-known technique. Judge Dyk opined that the PTAB erred by requiring FanDuel to provide a specific motivation to use a look-up table and an ordered list when that choice would have been a simple design choice for the claimed technology.
The PTAB does not violate the Administrative Procedure Act when it holds that a petitioner in an inter partes review fails to establish challenged claims are unpatentable based on arguments not advanced by the patent owner. This is because the petitioner bears the burden of persuasion to prove unpatentability by a preponderance of the evidence in an inter-partes review, and that burden never shifts to the patentee.