This work demonstrates that engineering of the connection channels in gold thin films is an effective way to alter its resistivity for improved sensitivity in strain sensors. We investigated the formation of channel cracks and explored the corresponding piezoresistive behavior. The developed strain sensor possessed GFs as high as 200 (ε < 0.5%), 1000 (0.5% < ε < 0.7%), and even greater than 5000 (0.7% < ε < 1%), which are among the highest values reported thus far at such small deformation, and are promising in the applications of electronic skin, wearable sensors and health monitoring platforms.

Highlights: Polyimide-based flexible microneedle array (PI-MNA) electrodes realize high electrical/mechanical performance and are compatible with wearable wireless recording systems. The normalized electrode–skin interface impedance (EII) of the PI-MNA electrodes reaches 0.98 kΩ cm² at 1 kHz and 1.50 kΩ cm² at 10 Hz, approximately 1/250 of clinical standard electrodes. This is the first report on the clinical study of microneedle electrodes. The PI-MNA electrodes are applied to clinical long-term continuous monitoring for polysomnography. Microneedle array (MNA) electrodes are an effective solution to achieve high-quality surface biopotential recording without the coordination of conductive gel and are thus very suitable for long-term wearable applications. Existing schemes are limited by flexibility, biosafety, and manufacturing costs, which create large barriers for wider applications. Here, we present a novel flexible MNA electrode that can simultaneously achieve flexibility of the substrate to fit a curved body surface, robustness of microneedles to penetrate the skin without fracture, and a simplified process to allow mass production. The compatibility with wearable wireless systems and the short preparation time of the electrodes significantly improves the comfort and convenience of electrophysiological recording. The normalized electrode–skin contact impedance reaches 0.98 kΩ cm² at 1 kHz and 1.50 kΩ cm² at 10 Hz, a record low value compared to previous reports and approximately 1/250 of the standard electrodes. The morphology, biosafety, and electrical/mechanical properties are fully characterized, and wearable recordings with a high signal-to-noise ratio and low motion artifacts are realized. The first reported clinical study of microneedle electrodes for surface electrophysiological monitoring was conducted in tens of healthy and sleep-disordered subjects with 44 nights of recording (over 8 h per night), providing substantial evidence that the electrodes can be leveraged to substitute for clinical standard electrodes.

Rationale: Delivery of nucleic acid molecules into skin remains a main obstacle for various types of gene therapy or vaccine applications. Here we propose a novel electroporation approach via combined use of a microneedle roller and a flexible interdigitated electroporation array (FIEA) for efficient delivery of DNA and siRNA into mouse skin.