Platinum/Iridium Alloy Wire: The Material Behind Pacemakers, STM Tips, and Spark Plugs

The 90% platinum, 10% iridium alloy has an unusual distinction: it was the material chosen to make the International Prototype of the Kilogram, the physical cylinder that defined one kilogram for the entire world from 1889 to 2019.

Scientists at the Bureau International des Poids et Mesures selected it because no other material offered the same combination of chemical inertness, dimensional stability, and resistance to wear. Today, those same properties — refined and applied across a far wider range of technologies — make platinum/iridium alloy wire one of the most specified precious metal alloys in precision research and industry.

Science Made Simple

Pure platinum is an excellent material in many ways: it doesn't corrode, it conducts electricity reliably, and it is safe in contact with the human body. But it is also quite soft, which means fine platinum wire bends and deforms easily under mechanical stress. Pure iridium is extremely hard, but it's so brittle and difficult to process that drawing it into a fine wire is not practical.

Alloying the two solves both problems at once. Adding 10% iridium to platinum more than doubles the hardness of the resulting wire, while the material keeps virtually all of platinum's corrosion resistance and biocompatibility. The result is a wire that can be drawn to very fine diameters, holds its shape under stress, survives harsh chemical environments, and works safely inside the human body.

What Iridium Actually Does to Platinum

The Vickers hardness of pure platinum sits at around 56 HV. A 90/10 Pt/Ir alloy reaches approximately 130 HV in the annealed condition — and higher-iridium compositions climb further still. The alloy also has a melting point above 1,800°C, a density of around 21.5 g/cm³, and electrical resistivity of approximately 25 μΩ·cm for the 90/10 composition. Critically for applications involving fine wire, the alloy's increased Young's modulus means it resists bending — a limitation that makes pure platinum wire impractical for probe and electrode applications where dimensional precision matters.

Medical Implants and Neural Electrodes

Platinum/iridium alloy is the standard material for implantable stimulation electrodes across several medical device categories: pacemakers, cochlear implants, deep brain stimulation (DBS) systems, and spinal cord stimulators. The requirement in each case is the same — an electrode that remains stable and electrochemically active inside the body for years, without corroding or triggering an immune response.

Researchers at the University of Applied Sciences Mannheim and Precisis GmbH in Heidelberg published work in 2024 quantifying the safe operating conditions for large-area Pt/Ir macroelectrodes used in neurostimulation, characterising the reversible electrochemical boundaries within which these electrodes can be driven without triggering degradation (DOI: 10.1371/journal.pone.0315779). Understanding those boundaries is directly relevant to device safety in long-term implants. The study used standard electrochemical methods — cyclic voltammetry, impedance spectroscopy, and biphasic pulse testing — on Pt/Ir electrodes to establish the limits that device designers need to work within.

Scanning Tunnelling Microscopy and Probe Applications

STM tips made from Pt/Ir alloy wire are a standard consumable in surface science laboratories. The tip needs to be mechanically cut or electrochemically etched to an atomically sharp point, and it needs to hold that geometry under the forces involved in scanning across a surface at nanometre resolution. Platinum alone is too soft to maintain tip geometry reliably; tungsten is harder but oxidises in air. Pt/Ir alloy offers a practical combination: hard enough to form a stable, sharp tip by mechanical cutting, corrosion-resistant enough to operate in ambient conditions without an oxide layer disrupting the tunnelling signal. The flat electronic density of states near the Fermi level is an additional advantage for reliable spectroscopic measurements.

Spark Plug Electrodes and High-Temperature Contacts

In high-performance automotive and aviation spark plugs, the electrode tip operates under repeated thermal shock and arc erosion every time the plug fires. Platinum/iridium electrodes survive this environment significantly longer than copper or nickel alternatives: the wear resistance of the alloy at elevated temperatures means the electrode geometry — and therefore ignition timing and efficiency — remains consistent over a longer service life. The same high melting point and oxidation resistance that made Pt/Ir suitable for precision metrology standards makes it effective wherever electrical contacts face extreme temperature cycling.

Specifying Pt/Ir Alloy Wire for Your Application

Pt/Ir alloy wire is available in a range of compositions — typically from PtIr5 through to PtIr30 — with different balances of hardness, ductility, and electrical resistivity depending on the iridium content. For medical electrode and neural probe applications, 90/10 is the most widely specified. For probe and STM tip applications, 80/20 is also common. Diameter tolerance and surface finish matter significantly for electrode fabrication, where the consistency of the wire directly affects the reproducibility of the electrode.

Advent Research Materials supplies platinum/iridium alloy wire across the standard composition range, to precise diameter tolerances, for research and industrial applications.