Crack prevention of highly bent metal thin films in woven electronic textiles

Abstract

Recent smart textile fabrication methods that are aimed at increasing the integration of electronics with textiles have involved fabricating micro-electronic components directly at the yarn level. Our approach to creating smart textiles is to fabricate thin-film devices and interconnects on plastic strips to create ‘e-fibers’ and weave them into a textile using a commercial weaving machine. e-Fibers are exposed to bending radii as small as 165 μ m during weaving. If patterned interconnect lines and device layers on the surface of the e-fiber are not designed correctly, they will crack due to the high strain and lose their electronic functionality. Brittle sensor and transistor device layers may be protected locally using rigid encapsulation materials, but cracking remains an issue for long metal interconnect lines which require flexibility. We investigated two strain-control methods to prevent the thin-film interconnect lines from cracking during weaving: (1) patterning the metal interconnect lines with a geometric design to slow propagation and merging of cracks and (2) encapsulation of interconnect lines to shift the deposited films to the neutral plain of the substrate. The mechanical behavior of interconnect lines exposed to tensile bending was studied by measuring the change in interconnect resistance versus bending radii ranging from 5 mm to 50 μ m. The critical bending radius, X C , defined as the radius at which the normalized interconnect resistance changes to 1.1 (indicating the onset of film rupturing) was 150 μ m for standard interconnect lines. Patterned interconnect lines had a radius X C of 115 μ m while encapsulated interconnect lines never reached this critical bending radius and showed a maximum resistance change of 1.02 at 100 μ m. These results show that it is possible to design interconnect lines with reduced cracking behavior when exposed to high strain during commercial weaving.