Fast Coating of Polydopamine Enables Scratch/Moisture Resistant Flexible Electronics

High biocompatibility and stability against water and scratch are crucial for liquid metal-based flexible electronics. Although previous studies have reported the chemical modification of liquid metal nanoparticles which can promote their water-stability and solution-processability, the modification process is complicated and hard to scale up. In particular, polydopamine (PD)-coated liquid metal nanoparticles (LMNPs) have not been used in flexible devices. We report the synthesis of the PD on LMNPs by thermal processing, which is controllable, fast, straightforward, and scalable. Because of the adhesiveness of PD, the PD@LM ink enables high-resolution printing on many substrates. The circuit printed by PD@LM shows high stability against repeated stretch in water (sustaining cardiomyocyte beating for 1 month─around 3 million times) and scratch. This conductive ink is highly biocompatible, conductive (4000 s/cm), and stretchable (up to 800% elongation). We cultured cardiomyocytes onto the PD@LM electrode and recorded the membrane potential change under electrical stimulation. For in vivo use, we fabricated a stable electrode to detect the electrocardiogram signal of a beating heart.