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Platinum Metals Rev., 2012, 56, (3), 213

doi:10.1595/147106712x651748

Final Analysis: Palladium in Temporary and Permanently Implantable Medical Devices

    • By Brian Woodward
    • Johnson Matthey Medical Components,
    • 12205 World Trade Drive, San Diego, California 92128, USA

Email: woodwbk@jmusa.com

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For more than forty years platinum alloys have been employed extensively in a range of medical devices and components (1). What is less well known is that palladium, another of the platinum group metals, has recently emerged as a viable alternative to platinum in certain medical device applications. The relative cost of palladium has been much lower than that of platinum (2) and this has led some medical device designers and developers to consider palladium as a replacement for platinum in temporary and permanently implantable electronic devices. Palladium is already widely used in dental applications, where its biocompatibility has proven to be satisfactory; and palladium shares many of the properties and performance characteristics which make platinum so suitable for medical use, such as strength, corrosion resistance and radiopacity. Work has been undertaken at Johnson Matthey Inc, USA, to compare the mechanical properties of platinum and palladium alloys and to develop suitable palladium alloys for a range of biomedical applications.

Palladium in Dentistry

Palladium-based alloys have been used as dental restorative materials for more than two decades with a good clinical history. Palladium alloys have long been tested and used in dental implant applications as dental casting alloys and have been shown to be reliable and relatively risk free (3). Palladium has a good range of solubility with several metals (helpful for alloying) and an ability to impart good mechanical properties including strength, stiffness and durability to the resulting alloys. It has excellent tarnish/corrosion resistance and biocompatibility in the oral environment. These properties make it ideally suited for use in dental crown and bridge alloys (those fitted in the as-polished state) and generally such palladium-based alloys are ‘white’, however many gold-based alloys also contain small amounts of palladium (typically 1–5%) to improve resistance to tarnishing and corrosion without significant loss of colour (4). Palladium is usually mixed with gold or silver as well as copper and zinc in varying ratios to produce alloys suitable for dental inlays, bridges and crowns where the alloy forms the core onto which porcelain is bonded to build up an artificial tooth.

Mechanical Properties

Palladium shares many of platinum's mechanical properties despite its lower density and melting point. Table I summarises the mechanical properties and currently available product forms for a variety of these alloys. The density of pure palladium is 12.02 g cm−3, approximately 40% lower than that of platinum at 21.45 g cm−3, making its relative consumption rate significantly lower for a component of the same dimensions. This, combined with the lower weight-for-weight cost of palladium compared to platinum (2), makes it an attractive substitution choice, as long as other requirements in terms of its properties can be met.

Table I

Mechanical Properties and Product Forms of Platinum and Palladium Alloys for Biomedical Applications

MaterialDensity, g cm−3Melting point, °CUltimate tensile strength, kpsiElongation, %Young's modulus, kpsiResistivity, μΩ cmProduct form






As-drawnStress relievedAnnealedAs-drawnStress relievedAnnealedSheet Tube Wire
Pure platinum 21.45 1769 70 55 23 <2 >5 >20 168 10.6 Yes Yes Yes
Platinum 5% iridium 21.51 1775 130 80 40 <2 >2 >10 171 19 Yes Yes Yes
Platinum 10% iridium 21.56 1790 155 140 55 <2 >2 >10 172 25 Yes Yes Yes
Platinum 15% iridium 21.62 1820 200 160 75 <2 >2 >10 187 29 Yes Yes Yes
Platinum 20% iridium 21.68 1830 210 175 105 <2 >2 >10 199 31 Yes Yes Yes
Platinum 25% iridium 21.74 1860 230 195 120 <2 >2 >10 218 33 Yes Yes
Platinum 4% tungsten 21.34 1780 180 140 75 <2 >2 >10 195 36 Yes
Platinum 8% tungsten 21.23 1870 220 170 130 <2 >3 >20 210 62 Yes
Platinum 10% nickel 18.8 1650 240 190 120 <2 >2 >20 215a 29.8 Yes
Platinum 30% nickel 15.07 1460 300 200 120 <2 >5 >20 218a 22.7 Yes
Platinum 49% nickel 12.69 1430 300 190 110 <2 >2 >20 222a 19.1 Yes
Pure palladium 12.02 1554 110 65 25 <2 >5 >10 118 9.98 Yes Yes Yes
Palladium 5% rhenium 11.71 1560 115 200 55 <2 >2 >10 125a 21.2 Yes Yes Yes
Palladium 10% rhenium 12.29 1620 230 200 80 <2 >2 >10 195a 32.5 Yes
Palladium 14% rhenium 12.79 1650 250 200 100 <2 >2 >10 200a 40a Yes
Palladium 5% iridium 12.31 1590 100 70 40 <2 >2 >10 225 11.6a Yes Yes Yes
Palladium 10% iridium 12.61 1625 110 75 50 <2 >2 >10 232 16.3 Yes Yes Yes
Palladium 15% iridium 12.93 1675 160 85 60 >2 >10 270 20.3 Yes
Palladium 20% iridium 13.27 1740 200 100 80 <2 >2 >10 292 25a Yes
Palladium 20% platinum 13.18 1600 110 55 35 <2 >5 >10 155a 12.3 Yes Yes Yes

[i] a Estimated values

To date, most of the technical development has been focused on replacing platinum alloy wires with palladium alloy wires on feedthrough filter housings which make up parts of cardiac pacemaker, defibrillator and neurostimulator device terminals (Figure 1). Such filter housings serve to shield electronic components from electromagnetic interference, with the terminal pins transmitting and receiving electrical signals to and from a patient's heart while hermetically sealing the inside of the medical instrument against body fluids that could otherwise disrupt the instrument's operation (5). The replacement of platinum and platinum alloys by palladium and its alloys can currently offer lower cost, without loss of mechanical properties. After high temperature brazing, there was found to be no significant degradation in the mechanical properties of palladium, such as in strength and elongation. Palladium also has comparable soldering and welding characteristics and good radiopacity. It has been found to be biocompatible under both soft tissue and bone studies (6) and is regarded as chemically inactive within the body environment.

Fig. 1.

(a) A perspective view of an internally grounded feedthrough capacitor assembly with palladium terminal pins; (b) an enlarged sectional view (5)

(a) A perspective view of an internally grounded feedthrough capacitor assembly with palladium terminal pins; (b) an enlarged sectional view (5)

The replacement of platinum-based alloys with palladium-based alloys can be carried out using the same manufacturing processes and generally without adding a secondary manufacturing stage. Palladium therefore provides a good alternative to conventional platinum or platinum-iridium alloys as a corrosion resistant material for terminal pins in feedthrough filter housings.

Radiopacity of Palladium Alloys

Palladium alloys have also been tested as catheter guidewire and electrode components on temporary implants used to treat cardiovascular and peripheral vascular disease. Palladium and platinum, when alloyed with superelastic metals such as nickel-titanium (Nitinol), have also been shown to improve the radiopacity in tubular stents compared with those made from stainless steel (7). Johnson Matthey manufactures a wide range of palladium-based alloys for these applications ranging from pure palladium to palladium-20% platinum (Table I). Figure 2 shows the results of an investigation into the relative radiopacities of two palladium alloys compared to two traditional platinum alloys used in guiding catheter applications, plus Nitinol, demonstrating a good level of equivalency in the radiopacity of the precious metal alloys.

Fig. 2.

Comparative radiopacities of two platinum alloys, two palladium alloys and Nitinol as coiled fine wire

Comparative radiopacities of two platinum alloys, two palladium alloys and Nitinol as coiled fine wire

Conclusions

The most recently developed platinum substitution materials for certain temporary and permanently implantable medical devices have been palladium alloys. Palladium's physical, mechanical and chemical properties have been found to be comparable to those of platinum in a variety of biomedical device applications. Palladium has a long history of reliability for use in dental restoration applications. However, while palladium has been demonstrated to be a good replacement for platinum in certain medical device applications, platinum and platinum alloys continue to be the first choice for device companies seeking a proven and reliable implantable metal that is biocompatible, radiopaque and electrically conductive.

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References

  1.  A. Cowley and B. Woodward, Platinum Metals Rev., 2011, 55, (2), 98 LINK https://www.technology.matthey.com/article/55/2/98-107/
  2.   Platinum Today, Price Charts: http://www.platinum.matthey.com/pgm-prices/price-charts/ (Accessed on 24th May 2012)
  3.  J. C. Wataha and C. T. Hanks, J. Oral Rehabil., 1996, 23, (5), 309 LINK http://dx.doi.org/10.1111/j.1365-2842.1996.tb00858.x
  4.  R. Rushforth, Platinum Metals Rev., 2004, 48, (1), 30 LINK https://www.technology.matthey.com/article/48/1/30-31/
  5.  C. A. Frysz and S. Winn, Greatbatch Ltd, ‘Feedthrough Filter Capacitor Assemblies Having Low Cost Terminal Pins’, US Patent 7,564,674; 2009
  6.  “Microfabrica Materials Dossier: Biocompatibility”, Microfabrica Inc, Van Nuys, CA, USA, 2010: http://www.microfabrica.com/pdf/Biocompatibility_092310.pdf (Accessed on 28th May 2012) LINK http://www.microfabrica.com/downloads/Microfabrica-Biocompatibility.pdf
  7.  J. F. Boylan and D. L. Cox, Advanced Cardiovascular Systems, Inc, ‘Radiopaque Nitinol Alloys for Medical Devices’, US Patent 6,855,161; 2005

The Author

Brian Woodward has been involved in the electronic materials and platinum fabrication business for more than 25 years and is currently the General Manager of Johnson Matthey's Medical Components business based in San Diego, CA, USA. He holds BS and MBA degrees in Business and Management and has been focused on value-added component supply to the global medical device industry.

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