Graphene e-tattoos : design, fabrication, characterization, and applications as wearable sensors
The remarkable mechanical robustness and unique electrical/optical properties make atomically thin graphene a promising candidate for future flexible, stretchable, and bio-integrated electronics. Our invention of sub-micron-thin graphene e-tattoos (GET), designed as filamentary serpentine ribbons of a graphene-polymer bilayer, has demonstrated superior skin-conformability, imperceptibility, and low contact impedance for monitoring various physiological signals, such as ECG, EMG, EEG, skin hydration, and temperature. However, there are unanswered questions on the failure mechanisms of GET and unsolved challenges to make GET applicable in ambulatory sensing. This dissertation attempts to address those critical issues. First, to reveal the failure mechanism of GET and its electrical contacts, I conducted uniaxial tensile tests with in situ microstructure and Raman investigations. I discovered four deformation/fracture stages of GET: pre-cracking elastic deformation, limited micro-cracking in graphene, extensive micro-cracking in graphene, and macro-cracking in the supporting polymer layer. Various conductive overlayers need to directly laminate on graphene to make electrical contacts. I placed gold/polyethylene terephthalate (Au/PET) as well as GET over GET. I found that the Au/PET - GET interface is very vulnerable due to the large stiffness mismatch between them but the GET - GET interface behaves very similar to intact GET electromechanically. Second, to reliably connect GET to rigid back-end-circuits with orders of magnitude mismatch in mechanical stiffness, I proposed the idea of heterogeneous serpentine ribbons (HSPR), which refer to serpentine GET partially overlapping with a serpentine gold-polyimide (Au/PI) ribbon. When the Au/PI step edge is located at the arm of the GET serpentine, 50 folds of strain reduction in GET using HSPR vs. heterogeneous straight ribbons (HSTR) have been confirmed. This simple method offers a generic remedy for the long-standing interconnect challenges between ultrathin sensors and rigid electronics. Finally, based on the electromechanical understanding of GET and the novel design of HSPR, I successfully created an unobstructive and robust GET sensor that is capable of continuous and ambulatory monitoring of electrodermal activity (EDA) on the palm.