As the demand for soft and flexible devices steadily increases, the ionic applications demonstrated with gel materials have come under the spotlight. Here, stretchable and wearable ionic diodes (SIDs) made from polyelectrolyte hydrogels are introduced. Polyelectrolyte hydrogels are mechanically modified using methacrylated polysaccharides while preserving the ion‐permselectivity of poly(sulfopropyl acrylate) potassium salt (PSPA) and poly([acrylamidopropyl]trimethylammonium chloride) (PDMAPAA‐Q), forming ionic copolymers. Then, SIDs composed of polyelectrolyte copolymer hydrogels are fabricated in VHB substrates as a stretchable and transparent insulating layer which is engraved by a laser. The SIDs show rectifying behaviors beyond the stretch of 3 with the aid of perfect adhesion between hydrogels and elastomeric substrates, and preserve their rectifications over hundreds of cycles. The operation of the SID is visualized by a wearable ionic circuit which rectifies ionic currents and lightens the LED under the forward bias during finger movements.

been applied more closely to the human body. Owing to the soft and curved nature of human skin, the systems need to be flexible, robust, and transparent to ensure operational reliabilities and to provide comforts. [3,4] However, it has been challenging to ensure these desirable properties due to the complicated structures of conventional touch-sensing systems, which contain a network of individual electrodes and stacked multilayers. [4,5,8,10,12] More importantly, sensing systems inevitably rely on external power sources, which potentially sacrifice flexibility, add weight, and decrease the lifetime of the sensing systems. [4,5,8,[12][13][14] To address the issues originated from the use of external power sources, systems that rely on energy-harvesting technologies have been highlighted as an alternative to conventional touch-sensing systems. [3,[15][16][17] Recently, triboelectric nanogenerators (TENGs), which convert mechanical touch to electrical energy, have been developed to provide a platform for touch-sensing capability that does not rely on additional power sources. [16][17][18][19][20] The self-powered touch-sensing capability of TENGs can be realized with a couple of materials, i.e., a dielectric layer attached to a conducting layer, based on the combined effects of contact electrification and electrostatic induction. [3,17,18,21] To ensure that triboelectric touch sensors are skin-mountable, researchers have worked to make the components stretchable and transparent. Soft materials such as poly(dimethylsiloxane) (PDMS) and ionically conductive gels are suitable for use in skin-mountable triboelectric touch sensors thanks to their high transparency, stretchability, resilience, and easily tunable mechanical properties. [17,[22][23][24][25] Nonetheless, the development of triboelectric touch position sensors with reliable stretchability and transparency remains challenging due to the complicated structures of the sensors, which contain multiple stacked layers and arrays of individual electrodes. [16,17,19,21,26,27] Unfortunately, the stacking process deteriorates the outstanding mechanical/optical properties of the soft materials in TENGs since the sensors require an additional conducting layer of a material such as metal, ceramic, carbon materials, etc. [16,[28][29][30] The inclusion of transparent and stretchable materials like gels also results in easy delamination and blurring at the bonding interfaces. [17,19,31] Relying on a complex array of individual electrodes and corresponding wires Recent growing pursuit of skin-mountable devices has been impeded by the complicated structures of most sensing systems, containing electrode grids, stacked multilayers, and even external power sources. Here, a type of touch sensing, termed "triboresistive touch sensing", is introduced for gridless touch recognition based on monolayered ionic power generators. A homogeneous monolayer, i.e., ionic poly(dimethylsiloxane) (PDMS), generates electricity based on the electric field generated by touch. Voltages generated at ...

By classifying stretchable ionic materials into three types of components (ionic conductors, ionic semiconductors, and ionic insulators), we summarized materials development in stretchable iontronics in terms of molecular interactions.