Color‐tuning of electrochemiluminescence (ECL) emission by frequency modulation of the alternating‐current (AC) applied to a single ECL cell containing two different luminescent molecules is demonstrated. Yellow light emission is observed for 1000 Hz AC, while white light is emitted when the frequency is switched to 300 Hz (figure). The ECL cells are simply fabricated by sandwiching an electrolyte solution between transparent electrodes.

An AC voltage-driven electrochemiluminescent (ECL) device containing a ruthenium(II) complex as luminescent species was fabricated, and advantages of the AC-driven method such as electrochemical and emission response were studied. The cell can be easily fabricated by simply placing the emitting solution between electrodes. The emission turn-on response time and emission intensity were dramatically improved by introducing the AC method. The turn-on response time was speeded up by increasing applied frequency. 4 ms was achieved at 200 Hz, which was much faster than the DC method (1.5 s). The current efficiency for the emission was estimated to be 0.59 cd A À1 . The mechanism of the improved ECL properties was discussed from a viewpoint of electrochemical reaction.

An AC-driven light-emitting cell based on the electrochemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(II) [Ru(bpy)3 2+] was fabricated by simply placing the electrolyte solution between transparent electrodes, and this AC-driven ECL cell was demonstrated for comparison to a DC-ECL cell. The properties of the ECL cell were dramatically improved by using the AC method. The AC-ECL cell showed the luminance of 56.4 cd/m2, the current efficiency of 0.78 cd/A and the turn-on response time of ca. 15 ms under application of 4 V, 50 Hz AC. We also elucidated the detailed mechanisms of the AC-ECL reaction to monitor the faradaic current. These improved properties and the frequency dependence of the AC-ECL cell were discussed in the relation to the revealed mechanisms.

A poly-DL-lactide (PLA) fiber film was prepared using the electrospinning method. This film consisted of randomly oriented PLA nanofibers. Consequently, it had sponge-like structure and was quite soft compared to PLA films prepared by spin coating. The average diameter of the fibers and the density of the film were 730 nm and 20%, respectively. By applying a voltage, the PLA film was subjected to electric-field-induced strain: expansion and compression in the thickness direction. When a voltage of -200 V was applied to the film, its thickness shrank from 13.5 µm to 10.0 µm (a 26% reduction). Electric-field-induced strain can occur via two different mechanisms: The first is electrostrictive behavior. That. is, in a highly electric field region, a change of film thickness occurs (compression only) from the electrostatic force between electrodes. The second mechanism is piezoelectric-like behavior that occurs in racemic PLA, wherein a PLA nanofiber is expanded and compressed by applying positive and negative voltage. Such piezoelectric-like behavior was not observed in spin-coated PLA films.

Electromechanical micrometer/submicrometer polymer fibers are promising components for wearable pressure sensors because the fibers are mechanically flexible, lightweight, and breathable. Although piezoelectric polymers are normally used as their materials, electrospun fibers of some nonpiezoelectric polymers have exhibited high electrical actuation, similar to the inverse-piezoelectric effect of piezoelectric materials. However, the pressure sensing properties and the origin of the behavior of the fibers have been unclear. This study demonstrates the electromechanical properties of an as-electrospun fiber mat composed of a nonpiezoelectric polymer, atactic polystyrene, and analyzes the origin of the properties. The fiber mat demonstrates a high apparent piezoelectric constant (d ) of 950-1400 pC/N with an applied load of 0.05-0.28 N, and a Young's modulus of 6.40 kPa indicates a soft nature. The surface potential of the stacked fiber mats with forward and opposite stacking confirms that the mat was charged with bipolar and uneven electric charges. This unique charge distribution is the likely origin of the fiber mat's electromechanical properties. The generated electric charge increased linearly with increasing indentation depth, which can be explained using a theoretical model of bipolarly charged polymer layers and air gaps, similar to the theoretical model of cellular ferroelectrets. This result also supports the bipolarly charged model, and the theoretical model implies that the high d results from the soft and modestly charged nature of the fiber mat. This finding will pave the way for the development of soft, lightweight, and breathable wearable pressure sensors from a variety of materials with high electromechanical performance.

Flexible and lightweight pressure sensors have attracted tremendous attention as a promising component of wearable biological motion sensors and artificial electronic skins. Here, the electromechanical response of as‐electrospun fiber mats composed of a commodity polymer, atactic polystyrene, which can be applied in low‐cost/large‐area, flexible, and lightweight pressure sensors is demonstrated. The fiber mat demonstrates a significantly high apparent converse piezoelectric constant of >30 000 pm V−1 under static measurement and ≈13 000 pm V−1 even at a high frequency of 1 kHz. The first theoretical model to explain the unique electromechanical response is constructed, which reveals that the softness and moderate charge of the fiber mat are the reasons for the significantly high electromechanical response. Further, apparent piezoelectric constants obtained by direct measurement are lower than those obtained by the converse measurement, which is attributed to the densification and hardening of the fiber mat due to prepressure applied in direct measurement. These findings are likely to serve as a milestone for the development of large‐area, flexible, and lightweight pressure sensors at low cost, as well as highly movable actuators like optical modulators without a substantial mechanical load.

A piezoelectric effect, such as actuation behavior with voltage application, could be observed from a poly(methyl methacrylate) (PMMA) fibrous mat fabricated by electrospinning. This fibrous mat increased or decreased its thickness in accordance with the polarity of the applied voltage, which appears to be an inverse piezoelectric effect. The appearance d T constant was as large as 8.5 nm/V owing to the softness of the fibrous structure, and the coupling constant K T = 0.31 indicated its efficient piezoelectric property. This piezoelectric behavior was repeatedly observed to be stable at room temperature. In addition, the polarization components of the fibrous mat, which are considered to be the origin of its piezoelectric effect, and its relaxation behavior were confirmed from the results of thermally stimulated current measurements.

We developed the stretchable wiring made from oriented short conductive fibers and applied it to a stretchable pressure sensor. The short fibers were mechanically oriented on an elastic substrate/film across the long wiring axis of wiring. The stretchable wiring with 5 mm in width had 17.0 X in initial resistance per 1 cm in length and the change in its resistance was only 214 % at 100 % elongation. Even if the wiring is stretched in the long axis direction, the short fibers form junctions with other fibers, thereby retaining their electric conductivity. A stretchable pressure sensor sheet, which is sensitive to pressure in the vertical direction and insensitive to elongation in the horizontal direction, was also successfully developed using the stretchable wiring and elastomer films. These sensor cells sensitively measured the pressure up to ca. 64 kPa. This sensor sheet was demonstrated as a shoe insole pressure sensing system and the sensor system could detect a pressure distribution from soles to adapt to its deformation.