Protonated Organic Semiconductors: Origin of Water-Induced Charge-Trap Generation.

Despite dramatic improvements in the electronic characteristics of organic semiconductors, the low operational stability of organic field-effect transistors (OFETs) hinders their direct use in practical applications. Although the literature contains numerous reports on the effects of water on the operational stability of OFETs, the underlying mechanisms of trap generation induced by water remain unclear. Here, a protonation-induced trap generation of organic semiconductors is proposed as a possible origin of the operational instability in organic field-effect transistors. Spectroscopic and electronic investigation techniques combined with simulations reveal that the direct protonation of organic semiconductors by water during operation may be responsible for the trap generation induced by bias stress; this phenomenon is independent of the trap generation at an insulator surface. In addition, the same feature occurred in small-bandgap polymers with fused thiophene rings irrespective of their crystalline ordering, implying the generality of protonation induced trap generation in various polymer semiconductors with a small bandgap. The finding of the trap-generation process provides new perspectives for achieving greater operational stability of organic field-effect transistors.

Visco-Poroelastic Electrochemiluminescence Skin with Piezo-Ionic Effect.

Following early research efforts devoted to achieving excellent sensitivity of electronic skins, recent design schemes for these devices have focused on strategies for transduction of spatially resolved sensing data into straightforward user-adaptive visual signals. Here, a material platform capable of transducing mechanical stimuli into visual readout is presented. The material layer comprises a mixture of an ionic transition metal complex luminophore and an ionic liquid (capable of producing electrochemiluminescence (ECL)) within a thermoplastic polyurethane matrix. The proposed material platform shows visco-poroelastic response to mechanical stress, which induces a change in the distribution of the ionic luminophore in the film, which is referred to as the piezo-ionic effect. This piezo-ionic effect is exploited to develop a simple device containing the composite layer sandwiched between two electrodes, which is termed "ECL skin". Emission from the ECL skin is examined, which increases with the applied normal/tensile stress. Additionally, locally applied stress to the ECL skin is spatially resolved and visualized without the use of spatially distributed arrays of pressure sensors. The simple fabrication and unique operation of the demonstrated ECL skin are expected to provide new insights into the design of materials for human-machine interactive electronic skins.

Scalable Superior Chemical Sensing Performance of Stretchable Ionotronic Skin via a π-Hole Receptor Effect.

Skin-attachable gas sensors provide a next-generation wearable platform for real-time protection of human health by monitoring environmental and physiological chemicals. However, the creation of skin-like wearable gas sensors, possessing high sensitivity, selectivity, stability, and scalability (4S) simultaneously, has been a big challenge. Here, an ionotronic gas-sensing sticker (IGS) is demonstrated, implemented with free-standing polymer electrolyte (ionic thermoplastic polyurethane, i-TPU) as a sensing channel and inkjet-printed stretchable carbon nanotube electrodes, which enables the IGS to exhibit high sensitivity, selectivity, stability (against mechanical stress, humidity, and temperature), and scalable fabrication, simultaneously. The IGS demonstrates reliable sensing capability against nitrogen dioxide molecules under not only harsh mechanical stress (cyclic bending with the radius of curvature of 1 mm and cyclic straining at 50%), but also environmental conditions (thermal aging from -45 to 125 °C for 1000 cycles and humidity aging for 24 h at 85% relative humidity). Further, through systematic experiments and theoretical calculations, a π-hole receptor mechanism is proposed, which can effectively elucidate the origin of the high sensitivity (up to parts per billion level) and selectivity of the ionotronic sensing system. Consequently, this work provides a guideline for the design of ionotronic materials for the achievement of high-performance and skin-attachable gas-sensor platforms.

Deformable Ionic Polymer Artificial Mechanotransducer with an Interpenetrating Nanofibrillar Network.

We demonstrate an ionic polymer artificial mechanotransducer (i-PAM) capable of simultaneously yielding an efficient wide bandwidth and a blocking force to maximize human tactile recognition in soft tactile feedback. The unique methodology in the i-PAM relies on an ionic interpenetrating nanofibrillar network that is formed at the interface of (i) an ionic thermoplastic polyurethane nanofibrillar matrix with an ionic liquid of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]+[TFSI]-) and (ii) ionic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) conducting polymer electrodes with dimethyl sulfoxide and [EMIM]+[TFSI]- as additives. The i-PAM-based actuator with the ionic PEDOT:PSS exhibits a stable operation up to 200 Hz at low voltage as well as a blocking force of 0.4 mN, which can be potentially adapted to soft tactile feedback. Furthermore, on the basis of this fast i-PAM, we realized alphabet tactile rendering by using a 3 × 3 i-PAM array stimulated by a dc input of 2 V. We believe that our proposed approach can provide a rational guide to the human-machine soft haptic interface.

An ultrathin conformable vibration-responsive electronic skin for quantitative vocal recognition.

Flexible and skin-attachable vibration sensors have been studied for use as wearable voice-recognition electronics. However, the development of vibration sensors to recognize the human voice accurately with a flat frequency response, a high sensitivity, and a flexible/conformable form factor has proved a major challenge. Here, we present an ultrathin, conformable, and vibration-responsive electronic skin that detects skin acceleration, which is highly and linearly correlated with voice pressure. This device consists of a crosslinked ultrathin polymer film and a hole-patterned diaphragm structure, and senses voices quantitatively with an outstanding sensitivity of 5.5 V Pa-1 over the voice frequency range. Moreover, this ultrathin device (<5 µm) exhibits superior skin conformity, which enables exact voice recognition because it eliminates vibrational distortion on rough and curved skin surfaces. Our device is suitable for several promising voice-recognition applications, such as security authentication, remote control systems and vocal healthcare.

Pressure/Temperature Sensing Bimodal Electronic Skin with Stimulus Discriminability and Linear Sensitivity.

Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E-skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus-discriminating and linearly sensitive bimodal E-skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature-independent material, near-perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface-wrinkled microstructure and the optimally reduced graphene oxide in the E-skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E-skin exhibits a linear and high pressure sensitivity of 0.7 kPa-1 up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E-skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K-1 in the temperature range 22-70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.

An Ultrastable Ionic Chemiresistor Skin with an Intrinsically Stretchable Polymer Electrolyte.

Ultrastable sensing characteristics of the ionic chemiresistor skin (ICS) that is designed by using an intrinsically stretchable thermoplastic polyurethane electrolyte as a volatile organic compound (VOC) sensing channel are described. The hierarchically assembled polymer electrolyte film is observed to be very uniform, transparent, and intrinsically stretchable. Systematic experimental and theoretical studies also reveal that artificial ions are evenly distributed in polyurethane matrix without microscale phase separation, which is essential for implementing high reliability of the ICS devices. The ICS displays highly sensitive and stable sensing of representative VOCs (including toluene, hexane, propanal, ethanol, and acetone) that are found in the exhaled breath of lung cancer patients. In particular, the sensor is found to be fully operational even after being subjected to long-term storage or harsh environmental conditions (relative humidity of 85% or temperature of 100 °C) or severe mechanical deformation (bending to a radius of curvature of 1 mm, or stretching strain of 100%), which can be an effective method to realize a human-adaptive and skin-attachable biosensor platform for daily use and early diagnosis.

Highly Sensitive Flexible Pressure Sensors Based on Printed Organic Transistors with Centro-Apically Self-Organized Organic Semiconductor Microstructures.

A highly sensitive pressure sensor based on printed organic transistors with three-dimensionally self-organized organic semiconductor microstructures (3D OSCs) was demonstrated. A unique organic transistor with semiconductor channels positioned at the highest summit of printed cylindrical microstructures was achieved simply by printing an organic semiconductor and polymer blend on the plastic substrate without the use of additional etching or replication processes. A combination of the printed organic semiconductor microstructure and an elastomeric top-gate dielectric resulted in a highly sensitive organic field-effect transistor (FET) pressure sensor with a high pressure sensitivity of 1.07 kPa-1 and a rapid response time of <20 ms with a high reliability over 1000 cycles. The flexibility and high performance of the 3D OSC FET pressure sensor were exploited in the successful application of our sensors to real-time monitoring of the radial artery pulse, which is useful for healthcare monitoring, and to touch sensing in the e-skin of a realistic prosthetic hand.

An Ultrasensitive, Visco-Poroelastic Artificial Mechanotransducer Skin Inspired by Piezo2 Protein in Mammalian Merkel Cells.

An artificial ionic mechanotransducer skin with an unprecedented sensitivity over a wide spectrum of pressure by fabricating visco-poroelastic nanochannels and microstructured features, directly mimicking the physiological tactile sensing mechanism of Piezo2 protein is demonstrated. This capability enables voice identification, health monitoring, daily pressure measurements, and even measurements of a heavy weight beyond capabilities of human skin.