A transparent and stretchable all-graphene multifunctional electronic-skin sensor matrix is developed. Three different functional sensors are included in this matrix: humidity, thermal, and pressure sensors. These are judiciously integrated into a layer-by-layer geometry through a simple lamination process.

conventional technology is gradually reaching its limits in terms of further enhancement of sensor performance. Hence, tremendous efforts have been taken to explore the applicability of MXenes in various sensor technologies, including chemical, biological, mechanical, and optical sensors. The large specific surface area, high electrical conductivity, [1,2] and water dispersibility of MXenes, among their various excellent properties, constitute essential characteristics of a sensor material. Particularly, the 2D structure of MXene, which is conducive to functionalization using various terminal groups, provides a large number of active surface sites. These sites can serve as a highly responsive sensory platform for various external stimuli. Furthermore, the high electrical conductivity of MXenes is desirable for achieving low noise in sensory responses. Therefore, these characteristics demonstrate MXenes as a highly promising alternative sensor material for achieving high sensitivity, exceptionally low limit of detection (LOD), and minimum detectable amount of analytes in various sensor applications. Finally, the water dispersibility of MXenes facilitates environment-friendly fabrication and modification treatments; thus, they are further advantageous in processing.Because the fundamental properties of MXenes satisfy the requirements for an alternative sensor material, MXenebased sensor technology has been rapidly evolving over the last few years. The field of MXene-based sensors is facing conventional challenging problems as hurdles to commercialization: achieving reliably high performance, high stability, multifunctionality, and realizing homogeneous and reproducible scale-up processing (Figure 1a) of MXene-based sensors. Several recent studies on MXene-based sensors were aimed at establishing various structural and electrical approaches to utilize the excellent properties of MXene, thus boosting the sensor performance. For example, the macrostructuring of MXenes can significantly increase the sensitivity and lower the LOD of the resultant fabricated sensor. [3,4] Furthermore, the functionalization of MXene surfaces can impart useful properties of the secondary component to the MXene-based sensor to obtain higher performance than that of pristine MXenebased sensors. [5] These approaches have been highly effective and reliable, and thus significantly accelerated the progress of MXene-based sensor research. Figure 1b shows the network of co-occurring keywords in 2375 MXene-based research papers published since the discovery of MXenes in 2011. In an attempt to introduce new low-cost, high-performance, and Various fields of study consider MXene a revolutionary 2D material. Particularly in the field of sensors, the metal-like high electrical conductivity and large surface area of MXenes are desirable characteristics as an alternative sensor material that can transcend the boundaries of existing sensor technology. This critical review provides a comprehensive overview of recent advances in MXene-based sensor technology...

Here, the fabrication of nonwoven fabric by blow spinning and its application to smart textronics are demonstrated. The blow-spinning system is composed of two parallel concentric fluid streams: i) a polymer dissolved in a volatile solvent and ii) compressed air flowing around the polymer solution. During the jetting process with pressurized air, the solvent evaporates, which results in the deposition of nanofibers in the direction of gas flow. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) dissolved in acetone is blowspun onto target substrate. Conductive nonwoven fabric is also fabricated from a blend of single-walled carbon nanotubes (SWCNTs) and PVdF-HFP. An all-fabric capacitive strain sensor is fabricated by vertically stacking the PVdF-HFP dielectric fabric and the SWCNT/PVdF-HFP conductive fabric. The resulting sensor shows a high gauge factor of over 130 and excellent mechanical durability. The hierarchical morphology of nanofibers enables the development of superhydrophobic fabric and their electrical and thermal conductivities facilitate the application to a wearable heater and a flexible heat-dissipation sheet, respectively. Finally, the conductive nonwoven fabric is successfully applied to the detection of various biosignals. The demonstrated facile and cost-effective fabrication of nonwoven fabric by the blow-spinning technique provides numerous possibilities for further development of technologies ranging from wearable electronics to textronics.

Hydrogels and liquid metals have been emerging as potential materials for use in self-healing electronics. This paper presents a simple fabrication procedure for a custom-designed hydrogel−liquid metal composite and its various applications. The hydrogel is patterned using three-dimensional printed molds for creating an electrical pathway, which is subsequently filled with liquid metal. The lifetime and self-healing property of the hydrogel improve drastically through coating of its surface with a moisture protectant layer and via the formation of an oxidized layer of liquid metal, respectively. Three joined units of the resulting hydrogel−liquid metal composite are successfully applied as self-healable electrodes in a customizable multimodular sensor system consisting of a photoresistor, a thermistor, and a tilt switch. The composite is also used as an electrode for biosignal (electromyogram, electrocardiogram, and electrodermal activity) detection, and its sensing ability is found to be comparable to that of a conventional Ag/AgCl electrode. The demonstrated hydrogel−liquid metal composite provides wide scope for researchers to achieve practical advances in self-healing electronics.

In the field of bionics, sophisticated and multifunctional electronic skins with a mechanosensing function that are inspired by nature are developed. Here, an energy-harvesting electronic skin (energy-E-skin), i.e., a pressure sensor with energy-harvesting functions is demonstrated, based on fingerprintinspired conducting hierarchical wrinkles. The conducting hierarchical wrinkles, fabricated via 2D stretching and subsequent Ar plasma treatment, are composed of polydimethylsiloxane (PDMS) wrinkles as the primary microstructure and embedded Ag nanowires (AgNWs) as the secondary nanostructure. The structure and resistance of the conducting hierarchical wrinkles are deterministically controlled by varying the stretching direction, Ar plasma power, and treatment time. This hierarchical-wrinkle-based conductor successfully harvests mechanical energy via contact electrification and electrostatic induction and also realizes self-powered pressure sensing. The energy-E-skin delivers an average output power of 3.5 mW with an open-circuit voltage of 300 V and a short-circuit current of 35 µA; this power is sufficient to drive commercial light-emitting diodes and portable electronic devices. The hierarchical-wrinkle-based conductor is also utilized as a self-powered tactile pressure sensor with a sensitivity of 1.187 mV Pa -1 in both contact-separation mode and the single-electrode mode. The proposed energy-E-skin has great potential for use as a next-generation multifunctional artificial skin, self-powered human-machine interface, wearable thin-film power source, and so on.

Electronic skin based on a multimodal sensing array is ready to detect various stimuli in different categories by utilizing highly sensitive materials, sophisticated geometry designs, and integration of multifunctional sensors. However, it is still difficult to distinguish multiple and complex mechanical stimuli in a local position by conventional multimodal E-skin, which is significantly important in the signals’ feedback of robotic fine motions and human–machine interactions. Here, we present a transparent, flexible, and self-powered multistage sensation matrix based on piezoelectric nanogenerators constructed in a crossbar design. Each sensor cell in the matrix comprises a layer of piezoelectric polymer sandwiched between two graphene electrodes. The simple lamination design allows sequential multistage sensation in one sensing cell, including compressive/tensile strain and detaching/releasing area. Further structure engineering on PDMS substrate allows the sensor cell to be highly sensitive to the applied pressures, representing the minimum sensing pressure below 800 Pa. As the basic combinations of compressive/tensile strains or detaching/releasing represent individual output signals, the proposed multistage sensors are capable of decoding to distinguish external complex motions. The proposed self-powering multistage sensation matrix can be used universally as an autonomous invisible sensory system to detect complex motions of the human body in local position, which has promising potential in movement monitoring, human–computer interaction, humanoid robots, and E-skins.

We reported the development of a transparent stretchable crack-enhanced microfluidic capacitive sensor array for use in E-skin applications. The microfluidic sensor was fabricated through a simple lamination process involving two silver nanowire (AgNW)-embedded rubbery microfluidic channels arranged in a crisscross fashion. The sensing performance was optimized by testing a variety of sensing liquids injected into the channels. External mechanical stimuli applied to the sensor induced the liquid to penetrate the deformed microcracks on the rubber channel surface. The increased interfacial contact area between the liquid and the nanowire electrodes increased the capacitance of the sensor. The device sensitivity was strongly related to both the initial fluid interface between the liquid and crack wall and the change in the contact length of the liquid and crack wall, which were simulated using the finite element method. The microfluidic sensor was shown to detect a wide range of pressures, 0.1-140 kPa. Ordinary human motions, including substantial as well as slight muscle movements, could be successively detected, and 2D color mappings of simultaneous external load sensing were collected. Our simple method of fabricating the microfluidic channels and the application of these channels to stretchable e-skin sensors offers an excellent sensing platform that is highly compatible with emerging medical and electronic applications.

Interestingly, the petals of flowering plants display unique hierarchical structures, in which surface relief gratings (SRGs) are conformably coated on a curved surface with a large radius of curvature (hereafter referred to as wavy surface). However, systematic studies on the interplay between the diffractive modes and the wavy surface have not yet been reported, due to the absence of deterministic nanofabrication methods capable of generating combinatorially diverse SRGs on a wavy surface. Here, by taking advantage of the recently developed nanofabrication composed of evaporative assembly and photofluidic holography inscription, we were able to achieve (i) combinatorially diverse petal-inspired SRGs with controlled curvatures, periodicities, and dimensionalities, and (ii) systematic optical studies of the relevant diffraction modes. Furthermore, the unique diffraction modes of the petal-inspired SRGs were found to be useful for the enhancement of the outcoupling efficiency of an organic light emitting diode (OLED). Thus, our systematic analysis of the interplay between the diffractive modes and the petal-inspired SRGs provides a basis for making more informed decisions in the design of petal-inspired diffractive grating and its applications to optoelectronics.