3D Printed Dry Electrodes for Electrophysiological Signal Monitoring: A Review

3D printed on-skin electrodes are of notable interest because, unlike traditional wet silver/silver chloride (Ag/AgCl) on-skin electrodes, they can be personalized and 3D printed using a variety of materials with distinct properties such as stretchability, conformal interfaces with skin, biocompatibility, wearable comfort, and, finally, low-cost manufacturing. Dry on-skin electrodes, in particular, have the additional advantage of replacing electrolyte gel, which dehydrates and coagulates with prolonged use. However, issues arise in performance optimization with the recently discovered dry materials. These challenges become even more critical when the on-skin electrodes are scaled down to a miniaturized size, making the detection of various biosignals while keeping mechanical resilience under several conditions crucial. This review paper aims to provide researchers interested in the 3D printing and manufacturing field for healthcare applications, specifically dry electrophysiological (EP) on-skin electrodes and biosignal sensing methodologies, with a reference guide about the various state-of-the-art 3D printing techniques and materials that have been developed with a focus on the main applications of EP electrodes, such as an electrocardiogram, electrooculogram, electromyogram, and electroencephalogram.