There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

https://cyberleninka.org/article/n/768408.pdf

Mechanical properties of graphene and graphene-based nanocomposites

The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

Flexible and stretchable physical sensors that can measure and quantify electrical signals generated by human activities are attracting a great deal of attention as they have unique characteristics, such as ultrathinness, low modulus, light weight, high flexibility, and stretchability. These flexible and stretchable physical sensors conformally attached on the surface of organs or skin can provide a new opportunity for human-activity monitoring and personal healthcare. Consequently, in recent years there has been considerable research effort devoted to the development of flexible and stretchable physical sensors to fulfill the requirements of future technology, and much progress has been achieved. Here, the most recent developments of flexible and stretchable physical sensors are described, including temperature, pressure, and strain sensors, and flexible and stretchable sensor-integrated platforms. The latest successful examples of flexible and stretchable physical sensors for the detection of temperature, pressure, and strain, as well as their novel structures, technological innovations, and challenges, are reviewed first. In the next section, recent progress regarding sensor-integrated wearable platforms is overviewed in detail. Some of the latest achievements regarding self-powered sensor-integrated wearable platform technologies are also reviewed. Further research direction and challenges are also proposed to develop a fully sensor-integrated wearable platform for monitoring human activity and personal healthcare in the near future.

Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare.

A new method, embedded-3D printing (e-3DP), is reported for fabricating strain sensors within highly conformal and extensible elastomeric matrices. e-3DP allows soft sensors to be created in nearly arbitrary planar and 3D motifs in a highly programmable and seamless manner. Several embodiments are demonstrated and sensor performance is characterized.

Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers

Highly stretchable graphene-nanocellulose composite nanopaper is fabricated for strain-sensor applications. Three-dimensional macroporous nanopaper from crumpled graphene and nanocellulose is embedded in elastomer matrix to achieve stretchability up to 100%. The stretchable graphene nanopaper is demonstrated for efficient human-motion detection applications.

Highly Stretchable Piezoresistive Graphene–Nanocellulose Nanopaper for Strain Sensors

Stretchable and wearable WO3 electrochromic devices on silver nanowire (AgNW) elastic conductors are reported. The stretchable devices are mechanically robust and can be stretched, twisted, folded, and crumpled without performance failure. Fast coloration (1 s) and bleaching (4 s) time and good cyclic stability (81% retention after 100 cycles) were achieved at relaxed state. Proper functioning at stretched state (50% strain) was also demonstrated. The electrochromic devices were successfully implanted onto textile substrates for potential wearable applications. As most existing electrochromic devices are based on rigid technologies, the innovative devices in their soft form hold the promise for next-generation electronics such as stretchable, wearable, and implantable display applications.

Stretchable and Wearable Electrochromic Devices

Fabrication of intrinsically stretchable nanowire photodetectors based on fully embedded structures is reported. A lithographic filtration method is used to integrate different functional layers into the photodetectors, which exhibit excellent stretchability up to 100%. The fully embedded structure enables excellent stability against repeated stretching, mechanical scratching, and adhesive forces.

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An Intrinsically Stretchable Nanowire Photodetector with a Fully Embedded Structure

DOI: 10.1002/aenm.201301396 Stretchable electronics represent a class of unconventional electronic devices based on soft substrates, in contrast to existing technology that is based on rigid substrates such as Si wafers. [ 1 ] Stretchable electronics can conform to complex nonplanar surfaces, such as human organs, and provide unique functionalities. The soft form of electronics also allows the possibility to build up next-generation wearable and implantable electronics including health monitoring patches, hemispherical electronic eyes, smart clothes, and sensory skin. [ 2–6 ] One of the most challenging issues for the development of complete and independent stretchable system is the fabrication of stretchable power sources. Wireless coils were used to power several pioneer stretchable devices, [ 2,7 ] but it is highly desirable to develop stretchable forms of available power sources such as supercapacitors, [ 8–10 ] batteries, [ 7,11–14 ] and solar cells [ 15–17 ] to meet the imperative demands of stretchable, wearable, and implantable electronics. Silver-zinc batteries, which are one of the most mature battery systems, hold promise as high-performance, safe, and green solutions to power stretchable devices. They have been widely used on a small scale such as in coin cells for watches and on a large scale for military and aerospace applications. Silver-zinc batteries have comparable specifi c energy to current market leader Li-ion batteries but can deliver much higher specifi c power. More importantly, silver-zinc batteries are inherently safe due to the use of water-based electrolytes and they are free from the fl ammability problems that have plagued the Li-ion batteries. They are also environmentally benign with the usage of non-toxic elements. The restricted application of silverzinc battery in common consumer electronics such as laptops and cellphones is primarily due to the relatively high cost of silver. However, they are regaining interest as concerns over safety and environmental impact increase. For example, safety issue would become a larger consideration than cost when the batteries are used for wearable and implantable electronics. Moreover, they are promising candidates for applications in printed batteries for easy, low cost integration into future 2D and 3D printing processes for stretchable electronics due to their air stability, which is a key advantage compared to Li-ion batteries for printable device applications. Here, we present the fabrication of silver-zinc battery based on stretchable Ag nanowire (AgNW) electrodes with embedded structures. The AgNW electrodes were fabricated via a lithographic fi ltration method and possess ideal bifunctionality by simultaneously providing electroactive materials and current collectors. The high-performance electrodes maintained their functions even when stretched up to 80% and are stable over 1000 cycles. Fully stretchable batteries are also demonstrated with both Ag and Zn electrodes and are stretchable. The full battery prototypes function properly in both relaxed and stretched states. Schematic illustrations of the lithographic fi ltration method for stretchable AgNW electrode fabrication are shown in Figure 1 a. Patterned fi ltration mask was prepared from cured polydimethylsiloxane (PDMS) substrate (thickness ≈ 1 mm), and they are used to selectively expose certain areas and obtain NW fi lms with desired patterns, analogous to the role of ultraviolet (UV) masks in photolithography. AgNW dispersion (1 mg mL −1 in ethanol) was fi ltered to obtain very uniform NW fi lms due to the simultaneous extraction of solvents from the evenly distributed holes in the track-etched polycarbonate (PC) membrane (Figure S1, Supporting Information). The thicknesses and hence resistances of the NW fi lms can be controlled by the volume of NW dispersion added. Note that the soft PDMS mask was in intimate contact with the PC membrane due to the vacuum suction force and the sticky surface of PDMS, which is critical to prevent liquid spreading and obtain well-defi ned patterns. The PDMS mask was removed after fi ltration and then liquid PDMS was poured on top of the PC fi lter membrane with NW patterns. The liquid PDMS was cured and peeled off from the fi lter membrane. AgNW patterns were successfully transferred into the PDMS matrix and stretchable electrodes with embedded structures were obtained. Examples of the soft, stretchable AgNW electrodes are shown in Figure 1 b (see also Figure S2, Supporting Information). Structural characterizations of the stretchable AgNW electrodes are shown in Figure 2 . The AgNWs embedded in PDMS elastomer matrix exhibit ideal bifunctionalities for battery applications by simultaneously providing electroactive layer and electron conducting layer. Because PDMS is not permeable to aqueous electrolyte, [ 18 ] only the surfi cial AgNW layer in contact with electrolyte (active layer) contributes to energy storage. Those AgNWs beneath the active layer serve as conducting layer to extract the electrons to external circuits. Scanning electron microscopy (SEM) characterizations of the AgNWs used can be found in Figure S3, Supporting Information. Figure 2 b shows a top view of the AgNW/PDMS layer showing that the AgNWs are embedded in the elastomer matrix with part of the

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Stretchable Silver‐Zinc Batteries Based on Embedded Nanowire Elastic Conductors

Stretchable devices with good transparency offer exciting new applications over the existing technologies, but remarkable difficulties remain in the fabrication of transparent and stretchable devices. In this paper, we report an effective method to fabricate transparent elastic photodetectors which combines the merits of the transparent polydimethylsiloxane (PDMS) polymer with its stretchability and the Zn2SnO4 nanowire (NW) with its photodetection functionality. Zonyl fluorosurfactant is found to be critical which improves the bonding between the functional NWs and the PDMS matrix, thus enabling the high efficient transfer of NW structures into PDMS. Highly conductive and thin percolating AgNW films were successfully embedded into PDMS mixed with ∼11% Zonyl which are otherwise not achievable with pure PDMS. Transparent and stretchable photodetectors were fabricated with the developed method. The photocurrent was found to be reciprocal to the square of the channel length, Iph ∼ 1/l2. The chemically bonded sensing materials in the PDMS matrix allow more NW exposure to air. This lead to a fast switching operation of the photodetectors with a response time below 0.8 s and a reset time around 3 s, which is significantly improved compared to reported stretchable NW photodetectors fully embedded in the polymer matrix.

Wenbin Kang

Papers

Highly Stretchable Piezoresistive Graphene–Nanocellulose Nanopaper for Strain Sensors

Stretchable and Wearable Electrochromic Devices

An Intrinsically Stretchable Nanowire Photodetector with a Fully Embedded Structure

Stretchable Silver‐Zinc Batteries Based on Embedded Nanowire Elastic Conductors

High-efficiency transfer of percolating nanowire films for stretchable and transparent photodetectors