Medical Device Link.

MATERIALS

Defining precious

For a material scientist, precious metals are those elements that, independent of their aggregate state, are stable in air and do not corrode. These are ruthenium (Ru), gold (Au), platinum (Pt), palladium (Pd), iridium (Ir) and rhodium (Rh). Osmium is also defined as a precious metal, but it is not extensively used because it is toxic at certain temperatures. Silver (Ag) is considered to be a semi-precious metal, because it is affected by sulphur. Pt has the highest electrical potential in the electrochemical series, which makes it the standard for measuring other precious metals.

Pure precious metals are soft and can be easily deformed. To achieve higher mechanical strength they are alloyed. Au, Pt, Pd and Ag are combined with small amounts of non-precious metals such as copper, zinc, tin and indium to create a number of alloys with varying properties. It is this variety that makes precious metals interesting materials. Better mechanical properties also require a fine “crystal” structure and this is achieved during casting. To gain this effect, Ir, Ru and/or rhenium, which have high melting points (above 2300 °C), are added to common precious metals with lower melting points (1000–2000 °C ) such as Au, Pt and Pd. Those with a higher melting point act as nuclei onto which metal crystals grow before the liquid as a whole solidifies.

Use and value

Figure 1: Long-term corrosion of precious metal alloys. (click image to enlarge)

Applications of precious metals include dentistry (dental attachments), permanent implantable devices such as cochlea implants/hearing aids, connectors and marking pins when taking X-rays. Production costs represent a small amount of the value of precious metals. The desirability associated with these materials together with speculation on future price increases their worth. Prices can rise dramatically and increase the cost of medical devices. For example, dental systems are fabricated from Pd-based alloys; between 2001 and 2004 the price of Pd rose from US$ 5000–60000 per kilogram. Today, the cost of precious metals is slightly higher than production costs. The high material costs are compensated for by the properties they offer medical device designers.

Properties

Figure 2: Stress–strain plot of Au 750Pd 210 at different cold-worked rates. (click image to enlarge)

The range of properties achievable with precious metals stems from their bonding characteristics. The electrons on the outer shell of the atoms (the conductive band) form a free electron gas, the so-called Fermi lake. This free electron gas builds the background bonding force of the atoms in a metal. The atoms are relatively mobile, yet highly ordered in crystal structures. This gives the materials their ability to undergo strain similar to that achieved with plastic materials. Precious metals are good conductors because they have many electrons in the conductive band. The electronic configuration of precious metals and their alloys prevents the exchange of electrons with atoms such as oxygen, which is the reason for their low corrosion rates. Their resistance to oxidisation means they offer high biological compatibility. This makes them suitable for use in implantable electronics, which is a growing area of application.

Figure 3: Stress–strain plot of cold-worked precious metal alloys. (click image to enlarge)

Corrosion resistance. Figure 1 shows corrosion measurements on three precious metal alloys; even after 80 days the corrosion rate is below 1 µg/cm2. If a dental restoration has a surface of 1 cm2 and a weight of 1 g, it would take 200000 years to completely corrode away this restoration.

Plastic deformation. Figure 2 shows stress-strain curves for one alloy in different conditions: annealed and cold worked at 25%, 50% and 75%. Precious metals have a great capacity to be cold worked, particularly in the annealed condition (here it is nearly 35%). With repeated cold work and annealing, an homogenous structure can be obtained. This is the basic requirement for obtaining parts of high precision and small dimensions. Precious metals and their alloys offer high mechanical strength at the same time as high deformability. There are alloys with strengths of more than 1000 MPa that show strain values of 10% (Figure 3).

Colour and conductivity. Precious metal alloys offer a diversity of colour, from golden to white, and electrical conductivity and X-ray opacity.

Dr Niklaus Baltzeris Head of Development of Materials Cendres+Métaux, Rue de Boujean 122, Biel-Bienne, Switzerland, tel. +41 32 344 22 11, e-mail: niklaus.baltzer@csma.ch, www.cmsa.ch.