Flexible electronics offer tantalizing potential in health and environmental monitoring. A crucial component of these devices is the electrical connection provided by conductive inks consisting of metallic flakes embedded in a flexible elastomer. Such composite connectors must remain conductive even after repetitive loading, such as stretching. Presently, there is limited knowledge of how the resistance to current flow is affected by cyclic loading. There is also little understanding of the causes of catastrophic failure of the conductor, which is essential for life-critical applications where current flow may be interrupted. This award seeks to establish an understanding of the mechanical and electrical response under cyclic loading for conductive composite inks through experiments and models. This effort can guide design of stretchable electronics and ensure reliability of such devices in a range of applications including health monitoring. The award also supports development of new class-based projects in undergraduate and graduate courses for both on campus and remote learning students. Additionally, wearable electronics learning modules will be developed for middle schoolers in the local counties with large underrepresented minority populations.
The objective of this work is to obtain a fundamental understanding of the relationship between mechanical and electrical behavior of metal-polymer composite inks for wearable and stretchable applications when subjected to monotonic and cyclic loading, with a specific focus on strain localization. It is intuitive to assume that localized deformation dictates the evolution of the conductive percolation network and affects the resulting resistance. In-situ scanning electron microscopy and optical confocal microscopy, as well as focused ion beam sectioning will be used to understand the mechanistic origins of strain localization and quantify its evolution under various loading conditions. These experiments will also produce datasets on the statistical distribution of metallic flakes and voids, which will be used to understand the evolution of the percolating network. Several flexible and stretchable inks will be investigated based on material properties, defect distribution, and flake volume fraction. Lastly, an existing percolation model will be enhanced with spatiotemporal descriptions of these distributions to produce complementary data, predict electrical resistance evolution, and provide composite ink design guidelines.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
