Stretchability of Serpentine Interconnect on Polymer Substrate for Flexible Electronics: A Geometry and Material Sensitivity Analysis

Stretchable electronics are enabling revolutionary solutions for wearable textiles, bendable displays and skin-attached monitoring sensors. However, high strain concentration in the brittle metallic interconnects at extensive mechanical stretching required by bio-integrated electronics has strongly constrained the stretchability of flexible electronic systems. Therefore, special geometry design, such as the serpentine shape, has been introduced to facilitate high stretchability ratio without creating high local strains. However, when the serpentine interconnect is printed on a non-conductive polymer substrate, the effectiveness of the serpentine design will be affected by the stiffness ratio of the polymer substrate to the serpentine interconnect. In this work, the stretchability of silver nano-ink printed serpentine interconnect on a polyethylene terephthalate (PET) substrate has been investigated. A finite-element model has been developed to determine the strain distribution in the serpentine interconnect under mechanical stretching. Both numerical and analytical formulations for in-plane stiffness calculation of serpentine interconnect and the PET substrate have been proposed. Different material properties of commonly used substrates and various geometric parameters of the serpentine interconnects have been studied, and a relationship between the stiffness ratio of the polymer substrate and the serpentine interconnect has been obtained to reduce the maximum strains in the serpentine interconnect under stretching. It is found that serpentine structures do not provide any strain-reduction benefits, when printed on stiff substrates, especially when the stiffness of the substrate is four orders of magnitude greater than the stiffness of the conductor.