Optimization of Inkjet-Printed Seed Layer Based Flexible, Transparent Metal Electrode for Bio-Signal Sensing

A micro-electrode array (MEA) is essential in the bio-medical field to measure various bio-signals in vitro and in vivo environments. The transparent MEA allows imaging of cell surfaces and organs inside the body. Also, when we perform light-based modulation, such as optogenetics, higher efficiency in response to light can be obtained with a transparent MEA. Here, instead of well-known direct electrode material printing, we print a polymer seed layer that can induce the formation of transparent ultrathin $(< 10\ \text{nm})$ metal electrodes with the merits of fabrication simplicity, low processing temperature, and design customizability. We optimized Au deposition thickness and metal film morphology to form conductive and transparent electrodes on selectively printed polymer seed layer regions. These electrodes show improved impedance at low frequencies compared to well-known thick Au-based electrodes. Finally, we successfully recorded brain signals in vivo by placing the flexible electrode array on the surface of the mouse brain.