Fatigue Testing Of Polyimide-Based Micro Implants

Due to its excellent chemical and mechanical properties, polyimide is frequently used as substrate material for electrodes in biomedical applications. In addition, highly flexible polyimide ribbon cables are integrated in hybrid micro devices such as silicon-based neural probes to connect these recording and/or stimulation units with external instrumentation. Typically a metallization layer, e.g. gold or platinum, is sandwiched between two polyimide layers with an implant thickness in the range of 10 mu m. This paper describes a bending setup constructed to perform fatigue tests with dedicated polyimide specimens in physiological environment. A pivot mounted rod bends the specimens around defined radii of 1, 1.5, 2 or 6 mm at a maximum frequency of 2 Hz. The system concept allows to freely choose angels between 0 degrees and 90 degrees in steps of 0.9 degrees. The applied test method is adapted to the European standard DIN EN 45502-2-1 for pacemaker leads, stipulating the application of mechanical loads by repetitive bending of the specimens. The machine allows the simultaneous testing of up to five samples in a physiological environment. Additionally, online four-wire resistance measurements are performed to detect functional failures during the test. Control specimens are stored at equal conditions without mechanical load. The resistance of all tested samples remained stable longer then the 47,000 cycles demanded in the standard.