Nickel-free corrosion-resistant steels have been developed for several decades, but nickel-free ferritic or martensitic stainless steels are ferromagnetic and have poor ductility. Nickel-free austenitic stainless steel is non-ferromagnetic, and the nickel in steels can be replaced by either nitrogen or manganese or both. From the modified Schziffier diagram, the direction for development of the biocompatible nitrogen-containing austenitic stainless steels is presented schematically in figure [21]. Owing to the strong effect of nitrogen to increase the austenite stability, mechanical properties and corrosion resistance of steels, as well as to prevent the formation of ferritic phase, high-nitrogen nickel-free stainless steel can solve the ‘nickel problem’ in medical stainless steels.
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It should be noted that cryogenic toughness of high-nitrogen austenitic stainless steels becomes a problem at high nitrogen content. High-nitrogen austenitic stainless steels are very peculiar as far as the toughness is concerned, so that a number of recent studies were devoted to this issue. Metals possessing fcc lattice usually exhibit good ductility without brittle fracture. However, austenitic stainless steels having high nitrogen and/or manganese contents are exceptions [ 63 ]. There is a ductile-to-brittle transition temperature (DBTT), below which these austenitic stainless steels could be fractured in a brittle manner [ 63 – 65 ]. Particularly, as the nitrogen level increases in the steels, their DBTT or brittleness will increase too [ 66 ]. Therefore, it was suggested to keep the nitrogen content in nickel-free stainless steel below 0.9% for medical application, in order to avoid the ductile-to-brittle transition at body temperature [ 22 ].
For single-phased austenitic stainless steels, several hypotheses have been put forth to explain the beneficial effects of nitrogen on the localized corrosions. These hypotheses include: (i) the formation of NH 4 + (or NH 3 ) by nitrogen in pits/crevices can consume the protons, thereby leading to smaller reduction in the pH value [ 57 , 58 ] and promoting repassivation; here the rate of generation for NH 4 + increases proportionately with nitrogen content [ 59 ]; (ii) the formation of corrosion-inhibiting nitrates (NO 2 – ,NO 3 – ) can prevent the anion attack because of the nitrogen enrichment at the film/substrate interface during the passivation [ 60 ]. Some researchers suggested that the formation of a stable protective layer of Cr, Mo and N-rich nitride (Cr 2 Mo 3 N) is responsible for the protection [ 61 , 62 ]. However, for the corrosion fatigue in a NaCl solution, for example, nitrogen alloying may improve the corrosion fatigue behavior of austenitic stainless steels, because nitrogen strongly enhances the pitting corrosion resistance.
With increasing use of high-nitrogen stainless steels, the effect of nitrogen on the corrosion behavior of steels has been intensively studied. The beneficial role of nitrogen is clearly identified on the localized corrosion phenomena, e.g. pitting corrosion and crevice corrosion. This net effect of nitrogen, in conjunction with chromium and molybdenum, can exceed 20 to 30 times that of chromium alone, as represented by formula PREN (pitting resistance equivalent number) =%Cr+3.3%Mo+20%N [ 56 ]. Clearly nitrogen significantly increases corrosion resistance for steels, which has been well confirmed and should result in a reduction of metallic ions and corrosion products in clinical applications.
The metallic implants which replace the failed hard tissues, such as artificial joints, bone plates and dental implants, are conventionally used under severe cyclic loading conditions. Therefore, the high-nitrogen nickel-free stainless steels, which typically exhibit high strength, ductility and toughness, are the perfect candidates for the structural material of these implants. Maruyama et al compared the fatigue behavior in air and in the simulated body fluid for two high-nitrogen nickel-free stainless steels made, respectively, by nitrogen absorption (Fe-23Cr-1N) and pressure electroslag remelting (P-ESR) processes (Fe-24Cr-2Mo-1N) [ 55 ]. The result revealed no difference between the S-N (stress-number of cycles to failure) curves in air and in simulated body fluid for both steels. The fatigue strength at 10 7 cycles for Fe-24Cr-2Mo-1N was 320 MPa, higher than 245 MPa for Fe-23Cr-1N.
With the increasing popularity of research on high-nitrogen stainless steels, many recent studies focused on the effects of nitrogen on the fatigue properties of steels. Addition of nitrogen to austenitic stainless steels changes the stacking fault energy and hence affects the dislocations structure and fatigue behavior. Planar dislocation arrangement due to nitrogen alloying is thought to be one of the reasons for the enhanced fatigue properties [ 50 , 51 ]. Maeng and Kim [ 52 ] recently compared the dislocation structure at the fatigue crack tips of 316L and 316LN stainless steels. They concluded that the nitrogen would prevent dislocations from crossing slips in the plastic zone around crack tips, and thus fatigue crack growth would be retarded in 316LN. Therefore, nitrogen alloying is believed to be beneficial to the fatigue resistance of stainless steels [ 51 , 53 , 54 ], reducing the tendency for dislocation cross slips and favoring the planar dislocation slips, thereby promoting the slip reversibility and giving rise to the earlier cyclic softening [ 51 , 54 ].
High-nitrogen stainless steels, in addition to high strength, also show high work-hardening rates [ 45 , 46 ] and good high-temperature mechanical properties as compared with conventional steels. The strengthening mechanisms of nitrogen alloying are attributed to: (i) interaction/pinning between dislocations and interstitial nitrogen atoms because of the electrostatic attraction [ 47 ] and (ii) formation of dislocation-nitrogen complexes that may drag the dislocations [ 48 ]. Large lattice distortion associated with interstitial nitrogen also strengthens steels [ 47 ]. Short-range ordering involving substitutional and interstitial elements (like Cr-N) may contribute to the strengthening of N-containing stainless steels [ 49 ].
Corrosion-resistant nickel-free austenitic stainless steel can be produced if high amounts of chromium, molybdenum and nitrogen are alloyed in the steel. Both nitrogen and carbon are potent and effective stabilizers of austenite structure; however, small addition of carbon can decrease the corrosion resistance of steel and also enhance its tendency to form precipitates. Therefore, the carbon content in medical stainless steel is restricted to less than 0.03%. The best and only acceptable element to stabilize the austenitic phase in nickel-free steels should be nitrogen. Thus, nickel-free austenitic stainless steels have been developed in which nickel is completely replaced by nitrogen. Additionally, high nitrogen content is also responsible for the increased strength and improved corrosion resistance.
As nitrogen is an austenitic phase forming element, it is used to develop low-nickel or nickel-free austenitic stainless steels. Nitrogen has been taken as an alloying element in many industrial stainless steels to replace and save the expensive nickel. This type of nitrogen-containing austenitic stainless steels possesses excellent combination of strength, toughness, good corrosion and wear resistance, and it is expected to eliminate nickel allergy in medical applications.
The currently used stainless steels for medical and surgical purposes, such as 316L, still contain 13–16 wt% Ni [25–27]. Because of the potential hazards of Ni, some nitrogen-containing low-Ni or Ni-free austenitic stainless steels have been gradually developed over the last years, and this progress for surgical stainless steel can be seen in table [67–71]. With the development of new surgical stainless steels and the modification of ASTM medical standard, the nickel content is decreasing and nitrogen content is increasing, up to 1.1% in F2229 stainless steel, i.e. Biodur 108 alloy developed by Carpenter Technology Corporation of USA. Biodur 108 has been widely used in processing of medical devices or instruments [72, 73].
Generally, the steels with nitrogen content that can be achieved with the conventional melting technique is defined as the high-nitrogen steels, including the ferritic steels with nitrogen over 0.08% and the austenitic steels with over 0.4% of nitrogen. Currently, high-nitrogen steels are usually produced by pressured electroslag remelting, counter-pressure casting, plasma arc melting and powder metallurgy [74–76]. In the development and production of high-nitrogen nickel-free stainless steels, Mn is also added to further enhance the nitrogen content; Mn is also a nickel substituting alloy element. Therefore the properties and related mechanisms of Fe–Cr–Mn–N and Fe–Cr–Mn–Mo–N types of nickel-free stainless steels are studied and applied in different industrial fields.
In 1995, at the fourth international conference on high-nitrogen steels (HNS95), Uggowitzer et al from Switzerland introduced the development and the properties of a new austenitic stainless steel [22], which contained 15–18% chromium, 3–6% molybdenum, 10–12% manganese and about 0.9% nitrogen. Besides being nickel-free, the steel was further characterized by excellent corrosion resistance, absence of ferromagnetism and outstanding mechanical properties. These properties are beneficial for medical applications. At the same conference, Menzel et al from Germany proposed using high-nitrogen Ni-free austenitic stainless steels for medical applications [21]. They analyzed the feasibility and application of a new biocompatible high-nitrogen nickel-free stainless steel, Fe-18Cr-18Mn-2Mo-1N, and suggested to develop a Fe-15Cr-(10-15) Mn-4Mo-0.9N stainless steel by reducing the Mn and Cr contents and increasing the Mo content. Then, numerous studies have focused on the properties, especially corrosion and wear in body fluid, and on in vitro and in vivo biocompatibility of different Fe–Cr–Mn–Mo–N nickel-free stainless steels.
In 1999, Uggowitzer and Thomann studied wear-corrosion behavior of biocompatible austenitic stainless steels in Hank's solution, distilled water and NaCl solution [77], and compared the newly developed P558 alloy (Fe-17Cr-10Mn-3Mo-0.49N-0.2C) with 316L and Rex734 (Fe-21Cr-9Ni-3Mn-2Mo-0.41N) stainless steels. P558 alloy showed an outstanding mechanical properties and excellent corrosion resistance; it was more resistant against the dry wear, wear–corrosion and crevice corrosion than Rex734, and its pitting corrosion resistance was equal to that of Rex734.
Also in 1999, Carpenter Technology Corporation reported a new high-nitrogen nickel-free austenitic stainless steel (108 alloy), Fe-23Mn-23Cr-1Mo-0.9N [78–80], which can be considered as an alternative to the two common austenitic stainless steels, BioDur Type 316L alloy (ASTM F138) and BioDur 734 alloy (ASTM F1586); Biodur 108 was listed in ASTM standard in 2002 (ASTM F2229). This alloy exhibits significantly higher strength, in both annealed and cold worked conditions, than any of the conventional nickel-containing stainless steels used in medical fields.
A typical yield strength of BioDur 108 alloy is approximately 606 MPa (88 ksi) in the annealed condition. In comparison, typical yield strength of BioDur 316L is approximately 241 MPa (35 ksi). The nitrogen-strengthened BioDur 734 and Fe-22Cr-13Ni-5Mn steels, with more nitrogen than BioDur 316L but less than BioDur 108 alloy, typically exhibit approximately 448 MPa (65 ksi) of yield strength in the annealed condition. The high nitrogen content in BioDur 108 alloy enhances the effect of cold working and further increases the strength level [72, 73]. Figure shows the yield strength of BioDur alloys achieved with different amount of cold working.
It was demonstrated that the corrosion resistance of BioDur 108 alloy is essentially equivalent to that of BioDur 734 alloy and BioDur 22Cr-13Ni-5Mn alloy, and is significantly greater than that of the widely used BioDur 316L alloy [72]. Finally, biocompatibility of BioDur 108 alloy is favorable in every respect, qualifying the alloy as a candidate material for biomedical implant and instrument applications [72].
Kraft et al from Germany studied the Biodur 108 alloy implant in vivo in striated muscle microcirculation [81]. They associated reduction of the nickel content in stainless steel with a considerably lower inflammatory response to the skeletal muscle microvascular system, compared with the regular 316L steel. Preliminary biological and mechanical studies of the novel nickel-free stainless steel approved it as a feasible alternative to the conventional stainless steels.
Mölder and Fischer et al from Germany studied the mechanical, chemical and tribological properties of a nickel-free austenitic steel, X13CrMnMoN18-14-3 (brand name P2000) [82], and assayed its biocompatibility by osteoblastic MC3T3-E1 cells [83]. This nickel-free austenitic steel showed extremely high strength, high ductility and superior corrosion resistance, and the cells growing directly on this steel were indistinguishable from the control cell cultured plastic material with respect to morphology and growth parameters during the cells test. However, regarding the biocompatibility, further studies are needed to understand the influence of cellular functions.
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Montanaro et al from Italy have investigated in vitro the mutagenicity and genotoxicity of a new nickel-free stainless steel (Fe-17Cr-10Mn-3Mo-N), namely P558, in comparison to the AISI 316L [84, 85]. The result of the cytogenetic effect and Ames test proved that P558 alloy is devoid of genotoxicity and mutagenicity, and suggested that this nickel-free stainless steel represents a better alternative to other conventional medical stainless steels.
Fini et al studied effects of P558 in vitro on primary osteoblasts and in vivo after bone implantation into the sheep tibia, with comparison to ISO 5832-9 stainless steel and Ti6Al4 V [86, 87]. The in vitro results demonstrated that the effect of P558 on osteoblast viability, pro-collagen I, transforming growth factor β-1 and tumor necrosis factor did not significantly differ from those exerted by Ti6Al4 V and controls. Furthermore, P558 enhanced osteoblast differentiation, as confirmed by alkaline phosphatase activity and osteocalcin levels, and reduced interleukin-6 production. At 26 weeks, the bone-to-implant contact was higher for P558 than for stainless steel (28%, P<0.005) and Ti6Al4 V (4%, P<0.05), and was higher for Ti6Al4 V than for stainless steel (22%, P<0.005). The results demonstrated that P558 has good biocompatibility and is a promising implant material. The biocompatibility of P558 was prbably due to the absence of Ni-related negative effects on cells and tissues. Fini et al also evaluated comparably the soft tissue response to P558, ISO 5832-9 stainless steel and Ti6Al4 V [88]. Four and 12 weeks after surgery, the histomorphometric measurements of implants with surrounding tissue revealed a stronger inflammatory response, in terms of capsule thickness, surrounding ISO 5832-9 stainless steel implants (with 9.8% Ni content) both in rat subcutis and in rabbit muscle, independent of shape and site of implantation. However, a progressive decrease in capsule thickness could be seen around P558 (with <0.02% Ni content) and Ti6Al4 V implants, respectively. Fini et al concluded that this nickel-free stainless steel would be a good substitute biomaterial for conventional 316L and Ti6Al4 V in orthopedic field.
Most of coronary stents are made of a Cr–Ni–Mo stainless steel (AISI 316L) due to its excellent combined properties, but this steel is a potential allergen. Most other materials, like cobalt-based L605 or tantalum alloys, are relatively expensive and are rarely used. The newly developed high-nitrogen nickel-free austenitic stainless steel (Fe–Cr–Mn–Mo–N) may offer an alternative material for such specific application. Weiss et al [89] investigated comparably the simulation for fatigue deformation and microstructure characterization of stents with equal design, produced from 316L and from high-nitrogen nickel-free stainless steel (DIN EN 1.4452, similar to ASTM F2229-02). They concluded that this high-nitrogen nickel-free stainless steel stents can be suitable for clinical use, but further study is still needed. Their results gave a more comprehensive understanding of the influence of the stents material on the structure–property relationship under monotonic and cyclic deformations, as a basis for ongoing development of new materials for stents optimization.
Although manganese is widely used as a substitute for nickel in high-nitrogen nickel-free stainless steel, its addition may hinder production of fine steel foils and wires. To get around this problem, Niinomi et al [90] and Kuroda et al [91] have devised a new, ingenious way to produce the nickel-free austenitic stainless steel wires. They used a Fe-24Cr-2Mo ferritic stainless steel as the starting material. After shaping the raw material into wires, the wires were heated in nitrogen atmosphere, thereby realizing austenitization. Nickel-free austenitic stainless steel products and small precision devices can be easily obtained through this process. Since wires and thin plates are commonly produced in steel-making industry, the effect of nitrogen adsorption on austenitic stainless steel wires and plates has been recently studied by Tsuchiyama et al [92] and Kuroda et al [93]. Yamamoto et al conducted the cytotoxicity tests on Fe–Cr–Mo, Fe–Cr–Mo–N and 316L stainless steels in both static and dynamic conditions to evaluate the biocompatibility of Fe–Cr–Mo–N, a high-nitrogen nickel-free austenitic stainless steel manufactured by nitrogen adsorption [94, 95]. Fe–Cr–Mo–N steel showed higher cell growth than 316L in static and dynamic conditions, as shown in figure , i.e. Fe–Cr–Mo–N steel has better cytocompatibility than 316L—a clear advantage for its application in medical fields.
High-nitrogen nickel-free stainless steels with lower toxicity to human body constitute the next generation of stainless steels for surgical implant applications. The currently used metallic implants still suffer from the problem that their elastic modulus is different from that of the bones [96]. However, mechanical properties of the porous metals can be adjusted to match those of replaced bones by changing the porosity and pore sizes. The pores on metals can also permit good attachment of tissues to the surface of biocompatible metals, allowing the tissue to enter the metals [97]. Alvarez et al from Japan studied the lotus-type porous Fe-25Cr, Fe-23Cr-2Mo and AISI 446 stainless steels that were fabricated by continuous zone melting technique in pressurized hydrogen and helium gas [98–101]. The porosity of the samples varied in the range 44–48% and the mean pore sizes (145–374 μm) were in the desired range for medical applications. The fabricated lotus-type porous nickel-free stainless steel was nitridized at high temperature, up to a nitrogen concentration of 1.0%, which was sufficient for the steel to maintain almost single-phase austenitic structure at room temperature. The combination of very low magnetic susceptibility, light weight, mechanical properties close to the human cortical bones, together with good enough corrosion resistance of high-nitrogen nickel-free stainless steel, makes this lotus-type porous Fe–Cr–N alloys very attractive for bone implant applications.
Alvarez et al also investigated the bone response to the lotus-type porous nickel-free stainless steel implants using Sprague–Dawley rats [102]. The histological examination showed that the bone grew into the pores (sizes between 70–650 μm) with apparent direct contact to the implant surface. At 12 weeks, new bone covered almost the entire available pore space and there was a scarce thin fibrous band on the surface of the implant. Maximum compressive shear strength of 24 MPa was obtained at 12 weeks, which was substantially higher than the typical shear strength achieved by porous coated materials. These results clearly indicate that the lotus-type porous structure could allow bone cells and tissues to penetrate the implant throughout superficial porous spaces, which could result in an efficient biological fixation responsible for the mechanical stability at the implantation site. Therefore the lotus-type porous nickel-free stainless steel may be a potential biomaterial in some special clinical applications.
In China, studies on melting, microstructure and properties of high nitrogen nickel-free stainless steels have been extensively conducted at the Institute of Metal Research CAS, Northeastern University, Yanshan University, China Iron and Steel Research Institute and other institutions [103–107]. Yang and co-workers from Institute of Metal Research, CAS, developed a new high-nitrogen nickel-free austenitic stainless steel (BIOSSN4) for medical application [108–112], with nominal composition of Fe-18Cr-15Mn-2Mo-(0.45–0.7)N. BIOSSN4 steel has excellent combination of strength and toughness, sufficient corrosion fatigue strength, good wear resistance, better corrosion resistance, favorable biocompatibility and good processability, compared with the conventional 316L stainless steel. Table lists the mechanical properties of BIOSSN4 steel in comparison to 316L. It can be seen that both yield strength and ultimate strength of BIOSSN4 steel are much higher than those of 316L steel, 2–3 times higher in strength and close in plasticity, and the strength of BIOSSN4 steel was increased with increase of the cold deformation. Beyond 30% cold deformation, the ultimate strength of BIOSSN4 was increased about 50% and its yield strength was more than doubled.
The fatigue behavior and erosion resistance of BIOSSN4 steel, compared with 316L stainless steel, at ambient conditions and in 37 °C 0.9% NaCl solution were also studied [110]. Compared to 316L, the high-nitrogen BIOSSN4 steel showed higher fatigue strength, as demonstrated in figure , and better wear resistance, as summarized in table .
The newly developed high-nitrogen nickel-free BIOSSN4 stainless steel has passed the standard biocompatibility evaluation by the Medical and Biological Products Inspection Institute of China, including cytotoxicity, hemolytic, acute toxicity, sensitization, genotoxicity and other necessary tests for the implants [109]. Additionally, Yang and co-workers also studied in vitro the blood platelets adhesion on surfaces of BIOSSN4 in comparison to 316L stainless steel [108, 109]. BIOSSN4 steel showed better platelets adhesion resistance compared with 316L stainless steel after dipping in fresh human blood plasma for 25 min and 3 h, as shown in figures and , respectively [109]. Fewer human blood platelets clung to the BIOSSN4 samples than to the 316L samples, and the platelets on BIOSSN4 showed weaker agglomeration and distortion, indicating that BIOSSN4 should possess better anti-platelet adhesion performance than the widely used 316L stainless steel. The above result further suggests that the high-nitrogen nickel-free stainless steels possess better blood compatibility, representing a prospective application potential for coronary stents.
Yang and co-workers also studied the clotting kinetics of blood on BIOSSN4 steel in comparison to 316L stainless steel (see figure ) [111]. The initial clotting time of BIOSSN4 was about 44.6 min, and that of 316L steel was obviously shorter (about 38.6 min), indicating that the high-nitrogen nickel-free stainless steel should have better thrombin resistance than 316L stainless steel.
In the past decade, several high-nitrogen nickel-free austenitic stainless steels have been developed for medical application, as can be seen in table , mainly focusing on Fe–Cr–N, Fe–Cr–Mo–N and Fe–Cr–Mn–Mo–N systems. Nickel-free stainless steels generally show excellent mechanical properties (see table ), and even in the annealed condition (soft state), their strength is much higher than that of the conventional stainless steels used for implants, e.g., 316L, whereas their elongation is comparable. As the strength of stainless steels can be increased by cold deformation, nickel-free stainless steels also exhibit a stronger potential for work hardening than the conventional stainless steels (figure ). This will open up new possibilities for higher strength implants or for reduction of implant sizes where limited anatomical space is often an issue, for instance, microvascular stents with finer meshes. Although the evaluation of nickel-free stainless steels with regard to surgical implants is yet incomplete, these steels demonstrate satisfactory biocompatibility and might be used in medical fields in the near future. Recently, BioDur 108 stainless steel has become available on the market.
The austenitic structure provides stainless steels with good ductility and formability. The common 18% chromium/ 8% nickel Type 304 in particular shows good stretch-forming characteristics. A slightly higher nickel content further increases the stability of the austenite and reduces the work-hardening tendency, increasing suitability for deep drawing. Unlike low-nickel, high-manganese alloys, these alloys are not prone to delayed cold cracking. Their excellent formability has led to 300-series austenitic alloys being widely used for items such as kitchen sinks and cooking pots.
Many pieces of stainless steel equipment are fabricated by welding. In general, nickel austenitic alloys are better for welding than other alloys, with Types 304 and 316 being the most widely-fabricated stainless steels in the world. Unlike ferritic alloys, they are not prone to brittleness as a result of high-temperature grain growth and the welds have excellent bend and impact properties. They are readily weldable in both thick and thin sections.
Toughness - the ability of a material to absorb energy without breaking - is essential in many engineering applications. Most stainless steels have good toughness at room temperature, however, as temperature decreases the ferritic structure becomes progressively more brittle, making ferritic stainless steels unsuitable for use at cryogenic temperatures. In contrast, the common austenitic stainless steels retain good toughness even at liquid helium temperatures (-270oC), which is why grades such as Type 304 are widely used for cryogenic applications.
Adding nickel gives the austenitic alloys of stainless steel significantly greater high-temperature strength than other alloys, particularly the ability to resist the tendency to move slowly or deform permanently under mechanical stresses, known as creep. These alloys are also much less prone to forming damaging brittle phases when exposed to temperatures in excess of 300oC. Nickel also stabilises the protective oxide film and reduces spalling during thermal cycling. This is why austenitic alloys are preferred for high-temperature applications and where fire resistance is needed.
Most nickel-containing materials are fully recyclable at the end of the product’s useful life; indeed their high value encourages recycling. This, in turn, lessens the environmental impact of nickel-containing stainless steels by reducing both the need for virgin materials and the energy that their production uses. For example, the amount of stainless steel scrap currently being used reduces the energy required for stainless steel manufacture by around one-third over using 100% virgin materials.
The durability of stainless steels can be seen in buildings. The restorations of St Paul’s Cathedral and the Savoy Hotel canopy in London, U.K. (1925 and 1929 respectively), the Chrysler Building in New York City and the Gateway Arch in St Louis in the U.S.A (1930 and 1965), the Progreso Pier in Mexico’s Yucatan state (c. 1940) and the Thyssen Building in Düsseldorf, Germany (1960) all testify to the longevity that can be expected from nickel-containing stainless steel.
Ease of production is not something that is immediately apparent to the end user. However, long experience of manufacturing the common austenitic alloys, their widespread use, their versatility and the scale of their production have allowed them to become widely and economically available in all shapes and quantities and in all parts of the world.
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