Corrosion Testing of Small Implantable Medical Devices

- ASTM F2129 Procedure - 		Downloadable information on ASTM F2129 as a pdf file.
FDA regulations specify ASTM F2129 (Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices) as the standard to follow in determining the corrosion resistance properties of an implant. In order to determine corrosion resistance in a given environment, the material must undergo some testing in that environment. These tests can take place “in vitro” (out of the body) or “in vivo” (inside the body.) When animal or human studies are not possible or practical, that testing environment is usually a buffered saline (or bile) solution at 37 C (98.6 F)

Procedure
In the ASTM F2129 test procedure, the implant material and/or medical device (i.e., Nitinol or Type 316L stainless steel) is exposed to an electrolytic test solution (buffered saline, simulated bile, etc.) The testing is facilitated by what we call a three electrode system. The first electrode is the implant itself. A wire is attached to the implant to allow connection to a potentiostat along with a reference electrode (saturated calomel) and a counter electrode (usually graphite or platinum.) The three electrodes are connected to the potentiostat which controls the experiment. The device is allowed to equilibrate with the test environment for 60 minutes after which the sample is polarized. Using Ohm’s law (E=IR) where E is voltage, I is current, and R is resistance in the circuit, the voltage is steadily increased while the current produced is recorded by a computer. The resistance in the system is primarily due to the passive (oxide) film at the surface of the metal. The ultimate goal of the F2129 procedure is to break down the passive film which is indicated by a notable increase in current observed during the test.

Results
The data is plotted and interpreted for several key data points. Each metal sample has different results; however, the similarity of the results within a given set of samples is essential in determining if the medical device is fit to be used in the human body.