Medical Implant Wear, Fatigue & Corrosion Testing

Exponent provides consulting services and performs corrosion testing for many medical device companies to assist them with product development, quality assurance, and Food and Drug Administration (FDA) submissions. Implants, such as stents, clips, valves, and coated implants, must be resistant to in-vivo corrosion and release a minimal amount of metal ions during their service life.

Exponent performs in-vitro corrosion tests on medical devices to assess the potential pitting, crevice, galvanic, or fretting effects caused by the device design or expected under in-vivo conditions. With immersion durations ranging from one hour to many months, Exponent is able to evaluate the short- and long-term corrosion potential, metal ion release, and breakdown and repassivation potentials for pitting, of implants in a wide range of shapes and sizes.

  

Figure 1. This cardiovascular stent is an example of a typical medical device tested at Exponent.

Corrosion tests are performed in accordance with ASTM Standards F2129 and G71, ISO Standard 16429, and many customized tests. We evaluate the electrochemical characteristics, including resistance to different types of corrosion for various alloys (NiTi, CoCr alloys, titanium alloys, and stainless steels) that are used in medical devices. 

Figure 2. Pitting corrosion tests involve the use of cyclic potentiodynamic polarization. Once pitting corrosion has initiated, it is visually confirmed prior to removal of the device from the test cell.

  

     

Figure 3. Scanning electron microscope (SEM) capabilities allow Exponent’s corrosion scientists to perform detailed examinations of the nature and location of pitting.

    

Figure 4. State-of-the art facilities are available for corrosion testing and SEM.

 Metal ion release rate, or leaching rate, is an important parameter to determine for medical devices as it can have long-term in-vivo implications. Exponent has measured the metal ion release rate for medical devices as a function of time in various physiological solutions at body temperature, 37°C, for a variety of implantable medical devices.

Figure 5. Typical metal ion release curve for nitinol, a superelastic alloy of nickel and titanium when exposed to body temperature physiological solutions.

Exponent uses testing in various types of synthetic body fluids (blood, bile, saliva, sweat, etc.) to evaluate the open-circuit potential (OCP), also known as the rest potential (Er) or corrosion potential (Ecorr), and breakdown potential (Eb) for a wide variety of long-term implantable medical devices. This work is done to help companies estimate the in-vivo corrosion performance to comply with FDA submissions. The figure below is an example of how the pitting susceptibility (Eb-Er) varies with immersion time in deaerated phosphate buffered saline (PBS), a commonly used simulated physiological solution.

Exponent provides many additional services for medical device companies such as litigation support, examination of explanted devices, finite element analysis (FEA), failure analysis, and material verification. Material verification includes fatigue and tensile testing, Af verification (bend and free recovery as well as DSC), and inclusion analysis.

Inclusion type and size distributions can affect corrosion and fatigue performance and Exponent has helped several medical device manufacturers characterize the amount, size distribution, and chemical composition of inclusions in their alloys. These inclusions (see images below), and associated voids and cracking around the particles, can intersect the wire surface and reduce the corrosion and fatigue resistance of the material.

    

    

Figure 6. Inclusions from metallurgical cross section of NiTi wire. Longitudinal (upper left) and transverse (upper right) of wire from vendor A. The corresponding longitudinal and transverse metallographic sections from vendor B wire are shown at the bottom left and right.

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