Bladder Inflation Stretch Test Method for Reliability Characterization of Wearable Electronics

The recent development of electronic materials that can maintain electrical performance while undergoing large applied strains have demonstrated potential for use in a new breed of electronic systems. The rapid development of these electronic systems that are flexible, stretchable, and/or wearable necessitates the concurrent development of robust mechanical and electrical test methods to improve their design and reliability. In this paper, one such mechanical test method is discussed in which a stretchable electronic test coupon is mounted onto an inflatable bladder of known geometry to induce multiaxial strains, while in-situ 4-point resistance measurement is employed to assess the device's performance and electromechanical integrity. The material combination of a stretchable screen-printed silver ink cured onto a thermoplastic polyurethane (TPU) substrate is studied given the proclivity for the use of TPU in wearable devices. A dome-shaped bladder configuration is employed in this work to study the performance of printed conductors under biaxial stretching. Various monotonic and cyclic loading regimes are employed to characterize the fatigue behavior and maximum use conditions of the samples. Volume of water displaced into the bladder during inflation is measured and correlated to the induced multiaxial strains on the mounted devices using 3D digital image correlation. Relationships between resistance and applied multiaxial strains are presented. Experimental results are compared with literature, and plausible extensions of the test method including direct printing on the bladder material are discussed.