High-Precision Stretchable Ionic Temperature Sensor Passivated with a Liquid Metal/Block Copolymer Multilayer Film

Advancements in atomic-scale interface engineering for flexible electronics: enhancing flexibility and durability

Flexible electronics, such as wearable displays, implantable electronics, soft robots, and smart skin, have garnered increasing attention. Despite notable advancements in research, a bottleneck remains at the product level due to the prevalent use of polymer-based materials, requiring encapsulation films for lifespan extension and reliable performance. Multilayer composites, incorporating thin inorganic layers to maintain low permeability towards moisture, oxygen, ions, etc, exhibit potential in achieving highly flexible barriers but encounter challenges stemming from interface instability between layers. This perspective offers a succinct review of strategies and provides atomic-scale interface modulation strategy utilizing atomic layer integration technology focused on enhancing the flexibility of high-barrier films. It delves into bendable multilayers with atomic-scale interface modulation strategies, encompassing internal stress and applied stress modulation, as well as stretchable composite structural designs such as gradient/hybrid, wavy, and island. These strategies showcase significant improvements in flexibility from bendable to stretchable while maintaining high barrier properties. Besides, optimized manufacturing methods, materials, and complex structure design based on atomic-scale interface engineering are provided, better aligning with the future development of flexible electronics. By laying the groundwork for these atomic-scale strategies, this perspective contributes to the evolution of flexible electronics, enhancing their flexibility, durability, and functionality.

Phase-separated stretchable conductive nanocomposite to reduce contact resistance of skin electronics

Skin electronics, facilitating a high-quality interface between external devices and human skin for recording physiological and/or electrophysiological signals as well as delivering external electrical and/or mechanical energy into the human body, has shown significant progress. However, achieving mechanically conformal contact and electrically low contact resistance at the device-skin interface remains challenging. Here, we propose a material strategy to potentially address such an issue by using phase separation of silver nanowires and silver nanoparticles (Ag NWs and Ag NPs) within a stretchable conductive nanocomposite (NC). This phase-separated NC ensures low contact resistance and high conductivity, which are key requirements in skin electronics, while maintaining excellent mechanical contact with the skin. To achieve phase separation, we hydrophobically treated the surfaces of Ag NWs and Ag NPs. Then, as the NC solidified, the solvent contained in the NC was slowly evaporated to sufficiently precipitate Ag NPs within the NC. As a result, the phase-separated NC exhibited high conductivity (~ 18,535 S cm-1), excellent stretchability (~ 80%), and low contact resistance on both the top and bottom NC surfaces (average ~ 0.132 Ω). The phase-separated NC has enabled implementation of high performance skin-mounted devices, including strain sensors, electrophysiological sensors, and a wearable heater.

Purposive Design of Stretchable Composite Electrodes for Strain-Negative, Strain-Neutral, and Strain-Positive Ionic Sensors

Soft ionic sensors have emerged as a promising device form to accommodate various future electronic applications. One of the hurdles in ionic sensors is that the sensing signals by mechanical deformation and other stimuli are mixed up. Although the performance of the ionic sensors is highly dependent on the structure of electrodes, systematic investigation of purposive electrode design has been rarely explored. This study proposes a simple strategy for designing stretchable composite electrodes which make the ionic sensor strain-negative, strain-neutral, and strain-positive. This study reveals that such strain-responses can be obtained by adjusting the surface coverage of the electrically-effective conductive fillers. On the basis of the concept, deposition of a Au film on an elastomer composite and crack formation of the Au film are presented for the practical fabrication of a highly reproducible strain-neutral ionic sensor. A completely strain-independent temperature sensor is demonstrated by using the Au crack-based ionic sensor. In addition, this study demonstrates a two-terminal shear sensor capable of recognizing shear directions by combining the strain-positive and strain-negative electrodes.

Shear-Pressure Decoupling and Accurate Perception of Shear Directions in Ionic Sensors by Analyzing the Frequency-Dependent Ionic Behavior

In artificial tactile sensing, to emulate the human sense of touch, independent perception of shear force and pressure is important. Decoupling the pressure and shear force is a challenging task for ensuring stable grasping manipulation for both soft and brittle objects. This study introduces a deformable ion gel-based tactile sensor that is capable of distinguishing pressure from shear force when pressurized shear force is applied in any direction. Recognition of the decoupled forces and precise shear directions is enabled by acquiring tactile data at only two frequencies (20 Hz and 10 kHz) based on the frequency-dependent ion dynamics. This study demonstrates monitoring the changes in pressure, shear force, and shear directions while performing practical robotic actions, such as pouring a water bottle, opening a water bottle cap, and picking up a book and placing it on a shelf.