Implants in the human body are exposed not only to mechanical stresses caused by the patient’s movements but also to the corrosive attack by bodily fluids. For the correct dimensioning of implants, it is therefore not sufficient to know only the static strength of the implant materials. The fatigue and corrosion properties of the different materials must also be investigated – both separately and in interaction with each other.
A new testing station at the Institute of Materials Science (IW) of Leibniz Universität Hannover now enables application-oriented fatigue testing taking into account additional corrosion effects. In the future, this test setup can be used to investigate new types of materials. These include, for example, resorbable iron-based implant materials, which are being developed by the IW.
Resorbable implants as customizable bone substitutes
Biocompatible resorbable materials – i.e. materials that dissolve inside the body – are used for implants where they are only needed temporarily. Thus, patients can avoid the additional stress caused by the surgical procedure for explantation – the subsequent removal of the implant.
The materials used are adjusted so that they degrade slowly inside the human body and the resulting reaction products are metabolized or excreted. This allows the use of implants for the fixation of bone fragments, while the slow degradation of the implant material enables the successive regeneration of the bone.
The use of additively manufactured implants that can be individually adapted to the patient is particularly promising in this context. Additive manufacturing processes (colloquially called 3D printing) often create defects such as pores, which make it necessary to investigate the fatigue behavior of the generated parts.
Application-oriented fatigue testing
IW scientists characterize material behavior under cyclic mechanical loading by fatigue testing, for example, using the MTS Landmark® 100 kN servo-hydraulic universal testing machine (see Figure 1). With the previous setup, however, it was only possible to investigate fatigue behavior in air.
To better understand the material behavior under cyclic loading inside the human body, it is necessary to test the fatigue strength in a corrosive medium. This can rule out the possibility of implants failing prematurely as a result of the combined corrosive and cyclic mechanical loading inside the human body. Within the test setup, different corrosive media can be used to simulate the environmental conditions at the intended location of the implant inside the human body.
For this purpose, IW has developed a test setup that enables fatigue testing in which the gauge length of the test specimen is immersed in a corrosive medium for the entire duration of the test. The test setup has a transparent container that holds the corrosion medium. A pump can replace it at freely configurable intervals. In addition, the medium can be tempered from 20 °C to 80 °C in order to determine the temperature influence on the corrosion rate.
For automated monitoring of the fatigue tests, the test stand has a leakage sensor that stops the test in the event of uncontrolled leakage of medium and pumps the medium out of the test stand. This allows the test to be performed unattended, which is especially helpful since the test of a single specimen may take several days.
Fatigue behavior under corrosive conditions
In initial tests with the new testing station, the fatigue behavior of pure iron in simulated body fluid was investigated (see Figure 2). From the investigations, it was possible to extend an existing service life prediction model to include the additional influence of corrosion. This model forms the starting point for the development of novel alloys for the additive manufacturing of resorbable implants.
At IW, the specimens can be prepared in such a way that light and electron microscopic examination of the crack paths becomes possible. The use of scanning electron microscopy, for example, enables scientists at IW to draw conclusions about the main failure mechanism acting during fatigue. Figure 3 shows an exemplary image of the crack progression of a cyclically loaded pure iron sample with the kernel misorientation.
New possibilities for material development
Currently, the development of iron-based alloys with precisely adjusted corrosion rates is supported by tests with the new testing station. In addition to implant development, however, IW scientists can also use the new setup to gain insights into the fatigue behavior of a wide range of materials under corrosive conditions. Thanks to the widely adjustable temperature range and the possibility of precise dosing of the electrolyte, a wide range of experiments is possible. In the future, the test setup can also be used to conduct application-oriented tests for other fields of application such as aviation or the offshore industry.