Deposition of particular layers of solid materials on a swelling polymer leads to the formation of functional wrinkles after the release of polymer strain. Unlike traditional mechanical stretching, polymer swelling could introduce uniform wrinkle structures on complex substrates as a result of isotropic polymer elongation. In this work, conductive silver wrinkles are grown on an elastomer by combining polymer swelling with electroless deposition. By adjusting the crosslinking ratio of polymer substrate or deposition time, the amplitude and wavelength of wrinkles can be tuned to meet demands for ultrasensitive pressure sensors. The detectable pressure limit is successfully reached below 1.0 Pa. C omplex wrinkle patterns exist in many thin-film systems typically with layered structures, such as natural wrinkles in aging skin, drying fruits, and fingerprints. 1 In the past decades, fabrication of artificial microwrinkles with tunable periodical structures has become a steadily developing field. 2 Some particular wrinkle structures offered fascinating promise for serving as optical devices, 3 self-cleaning coatings, 4 selfassembly templates, 5 or microchannels. 6 To expand their more attractive and challenging potentials, great concerns have been paid to the wrinkles with predictable topologies grown on soft substrates. 7 Such an integration leads to a tremendous increase of possibilities for flexible and stretchable devices. 8 Among them, a striking possibility is a skin-like piezoresistive pressure sensor, 9 which is routinely based on the lamination of two deformable conductive layers. The strain-gauge sensitivity relies on the degree of conformal contact between two conductive layers upon an external pressure. 10 In this regard, conductive layers with nonplanar morphologies become extremely more sensitive compared with planar films. However, practical examples of ultrasensitive pressure sensors assembled by nonplanar conductive wrinkles are sparse due to the lack of "wave"-like layout with remarkable surface buckling and satisfactory adhesion on soft substrates.The formation of surface wrinkles is a process of mechanical instability whereby a flat film develops out-of-plane undulations on a substrate with different Young's modulus. 11 Amplitude referring to the degree of surface buckling is limited in a narrow range because of its correlation with film thickness and Young's modulus. Wrinkles grown on nonplanar or patterned substrates become more deformable which may meet the demands for ultrasensitive pressure sensors. 12 Mechanical stretching has been exploited as one effective way to prepare two-dimensional surface wrinkles. 13 Periodical wrinkles as a result of compression of a stiff thin film are typically generated along the relaxation of a prestressed soft substrate. 14 However, this method is invalid for the preparation of periodical wrinkles on the nonplanar and patterned substrates. The tensile stress from edges is only allowed to be delivered in the direction parallel to the applied force. Wrinkles are ...
Electronic skins, as the integration of multiple distinct sensors, have aroused broad interests owing to their great potential in sensing applications. However, problems including the interference between sensing components and the difficulty in synchronous monitoring are practically encountered when they are applied to mixed signals. In this work, efforts are devoted to trouble‐free technical strategies for laminating three sensors with different sensing abilities into a skin‐like electronic device. The use of ionic liquid, combined with particular circuit topologies, ensures the reliable stability against mechanical disturbance during the real‐time sensing tests. The intrinsic layered structure and three independent sensing functions of natural skins are successfully presented by this particular device in which three sensors with the ease of preparation are spatially integrated. The changes of temperature, pressure, and infrared light can be recorded simultaneously yet without mutual signal interference. The perfect integration of multiple functional sensors into a single skin‐like device without any signal interference makes an important progress for pursuing the goal of future electronic skins that can practically be used as skin.
Early fire alarming is of vital importance to lower the damages led by forest fires. Thus far, methods to monitor the forest fires at their early stage are mainly focused on artificial ground patrol, unmanned aerial vehicle cruise monitoring, observation by watchtower, or satellite inspection, whereas these methods are practically encountered with the problems of untimely feedback before the forest fires are out of control. This work proposes a particular kind of self-powered, low-cost, and green thermoelectric paper chips based on the principle of self-assembly and disassembly of ionic liquids on the surface of gold electrodes. By adjustment of the species of ionic liquids, both "nand p-type" thermoelectric behaviors have been exploited that correspond to the opposite open-circuit voltages. Owing to the fluidic nature of ionic liquids, those "nand p-type" thermoelectric units can be readily connected in series on one paper chip, leading to remarkable voltage signals in the presence of the temperature difference of 35 K. Followed by signal acquisition and transmission, such a thermoelectric paper chip successfully affords immediate electrical alarming at the early stage of an afire circumstance.
Thermoelectric materials represent a new paradigm for harvesting low-grade heat, which would otherwise be dissipated to the environment uselessly. Relative to conventional thermoelectric materials generally composed of semiconductors or semi-metals, ionic thermoelectric materials are rising as an alternative choice which exhibit higher Seebeck coefficient and lower thermal conductivity. The ionic thermoelectric materials own a completely different thermoelectric conversion mechanism, in which the ions do not enter the electrode but rearrange on the electrode surface to generate a voltage difference between the hot and cold electrodes. This unique character has inspired worldwide interests on the design of ionic-type thermoelectric converters with attractive advantages of high flexibility, low cost, limited environmental pollution, and self-healing capability. Referring to the categories of ionic thermoelectric conversion, some representative ionic thermoelectric materials with their respective characteristics are summarized in this minireview. In addition, examples of applying ionic thermoelectric materials in supercapacitors, wearable devices, and fire warning system are also discussed. Insight into the challenges for the further development of ionic thermoelectric materials is finally provided.
Liquid sensors composed of ionic liquids are rising as alternatives to solid semiconductors for flexible and self-healing electronics. However, the fluidic nature may give rise to leakage problems in cases of accidental damages. Here, we proposed a liquid sensor based on a binary ionic liquid system, in which a flowing ionic liquid [OMIm]PF6 is confined by another azobenzene-containing ionic liquid crystalline [OMIm]AzoO. Those crystal components provide sufficient pinning capillary force to immobilize fluidic components, leading to a freestanding liquid-like product without the possibility of leakage. In addition to owning ultra-high temperature sensitivity, crystal-confined ionic liquids also combine the performances of both liquid and solid so that it can be stretched, bent, self-healed, and remolded. With respect to the reconfigurable property, this particular class of ionic liquids is exploited as dynamic circuits which can be spatially reorganized or automatically repaired.
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