Computational modeling of polymer-based stretchable electronic systems

Mechanical flexibility and stretchablity are the next frontier in consumer electronics. New technologies in material development and fabrication techniques allow researchers and companies to create stretchable electronic devices that can become part of human clothes and can be attached on human skin as wearables. These types of devices have numerous applications in health monitoring, fashion, and sports. The newly developed stretchable electronic materials possess unique and new properties that make them suitable for wearable applications. Even though a number of experimental results are available in the literature reporting mechanical and electronic responses of these materials, computational models for evaluating and designing stretchable electronic circuits are not widely available. Therefore, in this thesis, we present computational models and mathematical frameworks for describing various behaviors of stretchable electronic materials. Mechanical and electronic simulations are great tools to understand, evaluate, compare, and design stretchable electronic devices. Developing these simulations helps us better understand functionalities and limitations of flexible devices. This study of simulation tools and material models is presented in several steps. First, we introduce a number of areas where purely mechanical simulations provide valuable insight into designing and analyzing stretchable electronic components. These mechanical finite element simulations focus on perfectly reproducing the geometry of the domain, the boundary conditions of stretchability tests, and relatively accurate mechanical material models. The next focus of the thesis is on developing multi-physics and multi-scale material behaviors. As an example, the detailed micro-mechanical properties of polymer chains in natural rubber, which can be used as a stretchable substrate, is studied here with emphasis on the strain-induced crystallization of rubbery materials. The initiation and evolution of crystallization are calculated at representative polymer chains and the result is homogenized from the micro-scale to the macro-scale space of the material body. This multi-scale material formulation is the first time a time-dependent rubber material is used with the novel homogenization approach called the Maximal Advance Path Constraint (MAPC). In order to better understand the electrical conductivity of stretchable conductors, a new formulation for electro-mechanical simulation of carbon nano-tube thin films is presented. It is observed that simple micro-mechanical mechanisms such as sliding and buckling can provide insights into irreversible change of electrical conductivity as a function of the applied strain. The simulation methods and mathematical formulations that are presented here provide a consistent set of metrics to evaluate the performance of stretchable electronic devices. Concepts such as circuit component density, total interconnect length, strain energy distribution, and electronic stability can be used to evaluate and compare different designs of a stretchable device. By using these comparison metrics, we present an optimization framework to compute the best layout configuration which provides excellent stretchability. The optimization framework presented here is the first prototype of a design framework for intrinsically stretchable electronic devices. It furthermore signifies the importance of accurate material models, better meshing techniques, and efficient simulation methods as fundamental pillars of a great design framework

Advisor: Linder Christian (Engineering professor) 
Borja Ronaldo I. (Ronaldo Israel)
Law K. H. (Kincho H.)