The triboelectric nanogenerator (TENG) is a new type of energy generator first demonstrated in 2012. TENGs have shown potential as power sources for electronic devices and as sensors for detecting ...mechanical and chemical stimuli. To date, studies on TENGs have focused primarily on optimizing the systems and circuit designs or exploring possible applications. Even though triboelectricity is highly related to the material properties, studies on materials and material designs have been relatively less investigated. This review article introduces recent progress in TENGs, by focusing on materials and material designs to improve the electrical output and sensing performance. This article discusses the current technological issues and the future challenges in materials for TENG.Nanotechnology: Materials for harvesting energy from motionThe development of materials for a technology that uses the movement of the human body to provide power has been reviewed by scientists in South Korea. A triboelectric nanogenerator converts mechanical energy into electricity by harnessing the fact that two surfaces rubbing against one another can become electrically charged. This is known as the triboelectric effect. One exciting use for these nanogenerators is in wearable electronics, where the motion of the body provides the power. Unyong Jeong and colleagues from Pohang University of Science and Technology have reviewed recent progress in material advances in the four main elements of a triboelectric nanogenerator: the charge-generating layer, the charge-trapping layer, the charge-collecting layer, and the charge-storage layer. These improvements all aim to increase the electrical output of such devices.
As industrial needs for healthcare sensors, electronic skin, and flexible/stretchable displays increase, interest in stretchable materials is increasing as well. In recent years, the studies on ...stretchable materials have spread to various pivot components, such as electrodes, circuits, substrates, semiconductors, dielectric layers, membranes, and active nanocomposite films. The block copolymer (BC) elastomers have been playing considerable role in the development of stretchable materials. Since BCs are soft elastomers based on physical cross-links, they show differences in physical properties from normal elastomers formed with chemical cross-linking. BC elastomers does not require additional chemical cross-linking procedure, so they can be easily processed after dissolved in various solvents. Their viscoelasticity and thermoplasticity enable the BCs to become moldable and sticky. Although their unique physical properties may serve as disadvantages in some cases, they have been actively applied to create various stretchable electronic materials and their uses are expected to be enlarged more than ever. In this Account, we summarize recent successful applications of BCs for the stretchable electronic devices and discuss the possibility of further uses and the challenges to be addressed for practical uses. Studies on BC-based stretchable materials have focused initially on the fabrication process of stretchable conductors; mixing conductive fillers physically with BCs, infiltrating BCs in a conductive filler layer, and converting metal precursors into metal nanoparticles inside BCs. When conductive fillers with high aspect ratios, such as nanowires or nanosheets are used, the fillers can be infiltrated by the BCs after deposited. Since the contacts between the fillers are maintained during the infiltration process, even thin composite films possess high conductivity and stretchability. The metal precursor solution printing is suggested as a promising approach because it is compatible with traditional printing techniques without clogging the nozzles and allows high filler loading efficiency. When using a BC as a substrate, it is advisable to use a BC/PDMS double layer because of viscoelastic and thermoplastic properties of BCs. If BC/PDMS double layer is used with much thicker PDMS layer instead of viscoelastic BC alone, the double layer substrate can show a perfect elastomeric behavior, and the advantages of the BC substrate are preserved. Additionally, the use of conventional manufacturing techniques is important for commercialization of the stretchable devices. BC substrates having preformed microfibril network on their surfaces facilitate the fabrication of high-resolution circuitry by directly depositing metals through a mask on the substrate. Recent successes of fabricating stretchable organic transistors were obtained based on in situ phase separation of polymer semiconductors to form nanofibril bundles on the surface of a BC substrate. They have led to the achievement of high resolution transistor array printed in large area. BCs are expected to expand their applicability, including stretchable batteries, since they make it feasible to fabricate various hybrid nanocomposites, pore size-controlled membranes, and microstructured surfaces. However, it is necessary to secure long-term stability under heat, solvent, and UV; in addition, there is a need for the synthesis of functional BCs for use in stretchable implanted biomedical devices.
•Design of conductive elastomers.•Elastic composites designed with five distinct methods.•A variety of applications: electronic skins, stretchable transistors, antennas, displays and energy-related ...devices.•Technological issues that require further improvement along with possible solutions.
A growing interest in deformable electronics has fueled remarkable advancement in the area of conductive, stretchable composites suitable for use in electric circuits, interconnects and electrodes. This review provides a brief introduction as to the effect of conductive filler geometry on percolation and summarizes the design of the elastic conductive composites on the basis of five different methods: implanting conductive fillers in elastomers, filling microchannels with liquid metals, infiltrating elastomers in conductive-filler networks, blending conductive fillers with elastic polymers, and synthesizing metal fillers within elastomers. For each method, remarkable advances are described and the technological issues that require further improvement are presented, along with possible solutions. This review provides perspectives as to the application of conductive composite elastomers for deformable electronics and energy-related devices.
Human skin has different types of tactile receptors that can distinguish various mechanical stimuli from temperature. We present a deformable artificial multimodal ionic receptor that can ...differentiate thermal and mechanical information without signal interference. Two variables are derived from the analysis of the ion relaxation dynamics: the charge relaxation time as a strain-insensitive intrinsic variable to measure absolute temperature and the normalized capacitance as a temperature-insensitive extrinsic variable to measure strain. The artificial receptor with a simple electrode-electrolyte-electrode structure simultaneously detects temperature and strain by measuring the variables at only two measurement frequencies. The human skin-like multimodal receptor array, called multimodal ion-electronic skin (IE
-skin), provides real-time force directions and strain profiles in various tactile motions (shear, pinch, spread, torsion, and so on).
For the past two decades, the researches on stretchable physical sensors have made great technological advances, and have been applied to various applications such as electronic skin for robots, ...haptic devices, bionics, and wearable/implantable healthcare sensors, etc. The deformable physical sensors have been investigated in two approaches: the electronic sensors with the well-developed fabrication technologies and the iontronic sensors as a new technological alternative. There has been a wide spectrum of researches branching from the two approaches. They have evolved from simple-structured sensors with a single function into high-resolution array sensors with multiple functions. A variety of technological methods and principles have been explored depending on the target applications and the materials in use. The deformable sensors can be differentiated according to the specific methodology and principle; i) type of electrical signal (resistance or impedance, capacitance, induction, voltage or current, frequency), ii) power consumption (use of external power (passive) or self-signaling (active)), iii) data acquisition method (time-division multiple access or event-driven parallel access), and iv) multiple functionality (combination of different sensing units or multimodality). It is not clear at this stage which approach and method are suitable for which applications and materials. This review begins with a comparison of the technological methodologies and summarizes the evolution of deformable physical sensors. Focusing on key conceptual achievements according to the technological methodologies, this review introduces the advances of the electronic sensors in section 2 and the iontronic sensors in section 3. Section 4 discusses the challenges and directions for future researches, along with some possible technological solutions.
The nebulous term phase‐change material (PCM) simply refers to any substance that has a large heat of fusion and a sharp melting point. PCMs have been used for many years in commercial applications, ...mainly for heat management purposes. However, these fascinating materials have recently been rediscovered and applied to a broad range of technologies, such as smart drug delivery, information storage, barcoding, and detection. With the hope of kindling interest in this incredibly versatile range of materials, this Review presents an array of aspects related to the compositions, preparations, and emerging applications of PCMs.
PCMs on the rise: As a result of their sharp melting points and large heats of fusion during phase transition, phase‐change materials (PCMs) have already found commercial use in thermal management. The vast potential of this class of fascinating materials has recently been tapped in a diverse array of high‐tech applications such as controlled release, information storage, sensing/detection, and barcoding.
A simple strategy for changing a brittle conducting polymer (PEDOT:PSS) into a solution‐processed highly deformable viscoelastic polymer is presented. Rapid self‐healing of conductivity, ...customer‐designed LEDs with complex micropatterns, and foldable stretchable LEDs are demonstrated.
All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability ...in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.
Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn extensive attention for wearable electronics and electronic skins. Printable inks that contain ...liquid metal are strong candidates for these applications, but the insulating oxide skin that forms around the liquid metal particles limits their conductivity. This study reveals that hydrogen doping introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirmed the hydrogen doping, and first-principles calculations were used to rationalize the obtained conductivity. The printed circuit lines show a metallic conductivity (25,000 S cm
), excellent electromechanical decoupling at a 500% uniaxial stretching, mechanical resistance to scratches and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows the direct printing of three-dimensional circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.
The fabrication of deformable devices has been explored by interconnecting nonstretchable unit devices with stretchable conductors or by developing stretchable unit devices consisting of all ...stretchable device components such as electrodes, active channels, and dielectric layers. Most researches have followed the first approach so far, and the researches based on the second approach are at the very beginning stage. This paper discusses the perspectives of the second approach, specifically focusing on the polymer semiconductor channel layers, that is expected to facilitate high density device integration in addition to large area devices including polymer solar cells and light-emitting diodes. Three different routes are suggested as separate sections according to the principles imparting stretchability to polymer semiconductor layers: structural configurations of rigid semiconductors, two-dimensional network structure of semiconductors on elastomer substrates, and ductility enhancement of semiconductor films. Each section includes two subsections divided by the methodological difference. This Perspective ends with discussion on the future works for the routes and the challenges related to other device components.
PDF
