Investigation of Mechanical Reliability of Flexible/Stretchable Electronic Materials Using Multi-Axial Stretch Techniques

Research in the field of flexible and stretchable electronics has grown rapidly in recent years due to their applications in the fields of biomedical engineering, wearable electronics, consumer electronics, and the defense industry. The materials used in flexible/stretchable electronics have different mechanical test requirements for their development compared to rigid electronics and therefore require additional mechanical testing methods such as biaxial stretch test methods. This article presents the development of the Radial Stretch Test (RST), which is a modified version of an existing radial stretch system used to characterize stretchable electronics. The test is also compared with the Bladder Inflation Stretch (BIS) test. The RST creates radial stretching using a mechanism similar to the iris of an eye or the aperture control of a camera. This mechanism of the RST converts rotational motion of a stepper motor to an inward/outward movement to radially stretch materials. The RST improves upon previous designs by utilizing readily available off-the-shelf components for the fastners, bearings, gears, and stepper motor. It also modifies the design of the custom cut laser components to allow smooth actuation of the components during operation. In this article the accuracy and repeatability of the RST is characterized with respect to positional accuracy with varied parameters of diametrical displacement, speed, and applied force during monotonic and cyclic operation. The error in all of these characterizations of the RST was less than 1% and negligible for the larger displacements that are common during mechanical testing of stretchable electronics. The equibiaxial strain state created by stretching materials with the RST was confirmed using digital image correlation (DIC). Additionally, the RST was used in monotonic stretching mode to determine the electrical failure strain of a silver nanoflake electrically conductive ink (2 mm wide, 10 μm thick) patterned on a thermoset polymer film (100 μm thick) to be 8.6%. This measurement was verified using the BIS test on the same sample type with a measured electrical failure strain of 7.9%. Additionally, the RST's ability to perform cyclic fatigue tests for stretchable electronic materials was also demonstrated for the same test sample type. Test samples were cycled for 100 cycles and showed a gradual cyclic increase in electrical resistance due to the accumulation of damage in conductive ink. Overall, this initial demonstration of the RST shows its positional accuracy and its potential use as an alternative to other biaxial stretch tests for characterization of flexible electronics.