A Comparative Study of Silver Microflakes in Digitally Printable Liquid Metal Embedded Elastomer Inks for Stretchable Electronics

Printed soft conductive materials for stretchable electronics should have low electrical resistivity, high strain limit, and stable electrical properties when stretched. Previously, it has been shown that a bi-phasic ink composed of silver (Ag) microflakes, eutectic gallium-indium (EGaIn) alloy, and styrene isoprene (SIS) block copolymer is a promising formulation for printed soft electronics and has the potential to satisfy the necessary criteria. In this study, further improvements to the ink formulation are explored, with a focus on how the choice of Ag microflakes affects the electrical and electromechanical properties of the composite. By using specific Ag microflakes, AgInGa-SIS inks that have conductivity as high as 6.38 x 10(5) S m(-1) and a strain limit of over 1000%, with low electromechanical coupling can be synthesized. More broadly, when comparing the composite with different silver flakes, there is a 176% relative difference in conductivity, >600% difference in strain limit, and 277% relative difference in electromechanical coupling. To demonstrate the applicability of these inks for various use cases such as wearable bioelectronics, interconnects are printed for connecting electronic breakout boards with microcontrollers that provide a stable electrical connection when stretched, and the interconnects and electrodes of a wearable electrocardiography system that monitors the heart pulses in real-time.