25th anniversary article: A soft future: from robots and sensor skin to energy harvesters

Bioinspired Adhesive Hydrogel Driven by Adenine and Thymine

Bioinspired strategies have drawn much attention for designing intelligent hydrogels with promising performance. Herein, we present a bioinspired adhesive hydrogel driven by adenine and thymine, which are the basic units of DNA. The adhesive hydrogel exhibited promising adhesive property for the surface of various solid materials, including muscle tissues, plastics, rubbers, glasses, metals, ceramics, carnelians, and woods. The maximum peeling strength of hydrogels was 330 N m^-1 on aluminum, superior to that of PAAm hydrogels with 70 N m^-1. The strong adhesive behavior remained more than 30 times repeated peeling tests. Moreover, the swelling behavior, morphological structure, mechanical strength, and peeling adhesive strength were also investigated and confirmed the formation and various characteristics of adhesive hydrogels driven by adenine and thymine. Thus, the biomimetic strategy to design promising adhesive hydrogels can provide various opportunities in tissue engineering, such as wound dressing, bioglues, and tissue adhesives.

Stretchable Thermoelectric Generators Metallized with Liquid Alloy

Conventional thermoelectric generators (TEGs) are normally hard, rigid, and flat. However, most objects have curvy surfaces, which require soft and even stretchable TEGs for maximizing efficiency of thermal energy harvesting. Here, soft and stretchable TEGs using conventional rigid Bi₂Te₃ pellets metallized with a liquid alloy is reported. The fabrication is implemented by means of a tailored layer-by-layer fabrication process. The STEGs exhibit an output power density of 40.6 μW/cm² at room temperature. The STEGs are operational after being mechanically stretched-and-released more than 1000 times, thanks to the compliant contact between the liquid alloy interconnects and the rigid pellets. The demonstrated interconnect scheme will provide a new route to the development of soft and stretchable energy-harvesting avenues for a variety of emerging electronic applications.

Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors

In this study, we designed and fabricated a high-performance humidity sensor based on a donor-acceptor polymer transistor. To improve its sensing performance, a polymeric semiconductor film with macroporous structure was prepared using a facilitated phase-separation method. The relationship between the sensing performance and the pore size was systematically investigated by testing the humidity-sensing performance. The results suggested that the sensitivity of the sensor was improved with increasing pore size within a certain range. The sensor based on the macroporous film with an average pore size of 154 nm exhibited a sensitivity of 415 and a response time of 0.68 s, as the low relative humidity (RH) changed from 32% RH (9146 ppm) to 69% RH (20 036 ppm). These sensitivity values are better than those obtained by other reported humidity sensors based on organic field-effect transistors.

Stencil Lithography for Scalable Micro- and Nanomanufacturing

In this paper, we review the current development of stencil lithography for scalable micro- and nanomanufacturing as a resistless and reusable patterning technique. We first introduce the motivation and advantages of stencil lithography for large-area micro- and nanopatterning. Then we review the progress of using rigid membranes such as SiNx and Si as stencil masks as well as stacking layers. We also review the current use of flexible membranes including a compliant SiNx membrane with springs, polyimide film, polydimethylsiloxane (PDMS) layer, and photoresist-based membranes as stencil lithography masks to address problems such as blurring and non-planar surface patterning. Moreover, we discuss the dynamic stencil lithography technique, which significantly improves the patterning throughput and speed by moving the stencil over the target substrate during deposition. Lastly, we discuss the future advancement of stencil lithography for a resistless, reusable, scalable, and programmable nanolithography method.

Buckling of elastomer sheets under non-uniform electro-actuation

Dielectric elastomer sheets undergo in-plane expansion when stimulated by a transverse electric field. We study experimentally how dielectric plates subjected to a non-uniform voltage distribution undergo buckling instabilities. Two different configurations involving circular plates are investigated: plates freely floating on a bath of water, and plates clamped on a frame. We describe theoretically the out-of-plane deformation of the plates within the framework of weakly non-linear plate equations. This study constitutes a first step of a route to control the 3D activation of dielectric elastomers.

Electronic Skin with Multifunction Sensors Based on Thermosensation

A multifunctional electronic skin (e-skin) with multimodal sensing capabilities of perceiving mechanical and thermal stimuli, discriminating matter type, and sensing wind is developed using the thermosensation of a platinum ribbon array, whose temperature varies with conductive or convective heat transfer toward the surroundings. Pressure is perceived by a porous elastomer covering on the heated platinum ribbon, which bears mechanical-thermal conversion to allow high integration with other sensors.

A Transparent, Self-Healing, Highly Stretchable Ionic Conductor

Self-healing materials can repair damage caused by mechanical wear, thereby extending lifetime of devices. A transparent, self-healing, highly stretchable ionic conductor is presented that autonomously heals after experiencing severe mechanical damage. The design of this self-healing polymer uses ion-dipole interactions as the dynamic motif. The unique properties of this material when used to electrically activate transparent artificial muscles are demonstrated.

Dielectric Elastomer Generator with Improved Energy Density and Conversion Efficiency Based on Polyurethane Composites

Dielectric elastomer generators (DEGs), which follow the physics of variable capacitors and harvest electric energy from mechanical work, have attracted intensive attention over the past decade. The lack of ideal dielectric elastomers, after nearly two decades of research, has become the bottleneck for DEGs' practical applications. Here, we fabricated a series of polyurethane-based ternary composites and estimated their potential as DEGs to harvest electric energy for the first time. Thermoplastic polyurethane (PU) with high relative permittivity (∼8) was chosen as the elastic matrix. Barium titanate (BT) nanoparticles and dibutyl phthalate (DBP) plasticizers, which were selected to improve the permittivity and mechanical properties, respectively, were blended into the PU matrix. As compared to pristine PU, the resultant ternary composite films fabricated through a solution casting approach showed enhanced permittivity, remarkably reduced elastic modulus, and relatively good electrical breakdown strength, dielectric loss, and strain at break. Most importantly, the harvested energy density of PU was significantly enhanced when blended with BT and DBP. A composite film containing 25 phr of BT and 60 phr of DBP with the harvested energy density of 1.71 mJ/cm³ was achieved, which is about 4 times greater than that of pure PU and 8 times greater than that of VHB adhesives. Remarkably improved conversion efficiency of mechano-electric energy was also obtained via cofilling BT and DBP into PU. The results shown in this work strongly suggest compositing is a very promising way to provide better dielectric elastomer candidates for forthcoming practical DEGs.

Extremely Stretchable Strain Sensors Based on Conductive Self-Healing Dynamic Cross-Links Hydrogels for Human-Motion Detection

Extremely stretchable self-healing strain sensors based on conductive hydrogels are successfully fabricated. The strain sensor can achieve autonomic self-heal electrically and mechanically under ambient conditions, and can sustain extreme elastic strain (1000%) with high gauge factor of 1.51. Furthermore, the strain sensors have good response, signal stability, and repeatability under various human motion detections.

Hydraulic hydrogel actuators and robots optically and sonically camouflaged in water

Sea animals such as leptocephali develop tissues and organs composed of active transparent hydrogels to achieve agile motions and natural camouflage in water. Hydrogel-based actuators that can imitate the capabilities of leptocephali will enable new applications in diverse fields. However, existing hydrogel actuators, mostly osmotic-driven, are intrinsically low-speed and/or low-force; and their camouflage capabilities have not been explored. Here we show that hydraulic actuations of hydrogels with designed structures and properties can give soft actuators and robots that are high-speed, high-force, and optically and sonically camouflaged in water. The hydrogel actuators and robots can maintain their robustness and functionality over multiple cycles of actuations, owing to the anti-fatigue property of the hydrogel under moderate stresses. We further demonstrate that the agile and transparent hydrogel actuators and robots perform extraordinary functions including swimming, kicking rubber-balls and even catching a live fish in water.

A survey on dielectric elastomer actuators for soft robots

Conventional industrial robots with the rigid actuation technology have made great progress for humans in the fields of automation assembly and manufacturing. With an increasing number of robots needing to interact with humans and unstructured environments, there is a need for soft robots capable of sustaining large deformation while inducing little pressure or damage when maneuvering through confined spaces. The emergence of soft robotics offers the prospect of applying soft actuators as artificial muscles in robots, replacing traditional rigid actuators. Dielectric elastomer actuators (DEAs) are recognized as one of the most promising soft actuation technologies due to the facts that: i) dielectric elastomers are kind of soft, motion-generating materials that resemble natural muscle of humans in terms of force, strain (displacement per unit length or area) and actuation pressure/density; ii) dielectric elastomers can produce large voltage-induced deformation. In this survey, we first introduce the so-called DEAs emphasizing the key points of working principle, key components and electromechanical modeling approaches. Then, different DEA-driven soft robots, including wearable/humanoid robots, walking/serpentine robots, flying robots and swimming robots, are reviewed. Lastly, we summarize the challenges and opportunities for the further studies in terms of mechanism design, dynamics modeling and autonomous control.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Flexible and Stretchable Energy Storage: Recent Advances and Future Perspectives

Energy-storage technologies such as lithium-ion batteries and supercapacitors have become fundamental building blocks in modern society. Recently, the emerging direction toward the ever-growing market of flexible and wearable electronics has nourished progress in building multifunctional energy-storage systems that can be bent, folded, crumpled, and stretched while maintaining their electrochemical functions under deformation. Here, recent progress and well-developed strategies in research designed to accomplish flexible and stretchable lithium-ion batteries and supercapacitors are reviewed. The challenges of developing novel materials and configurations with tailored features, and in designing simple and large-scaled manufacturing methods that can be widely utilized are considered. Furthermore, the perspectives and opportunities for this emerging field of materials science and engineering are also discussed.

Electric Eel-Skin-Inspired Mechanically Durable and Super-Stretchable Nanogenerator for Deformable Power Source and Fully Autonomous Conformable Electronic-Skin Applications

Electric eel-skin-inspired mechanically durable and super-stretchable nanogenerator is demonstrated for the first time by using triboelectric effect. This newly designed nanogenerator can produce electricity by touch or tapping despite under various extreme mechanical deformations or even after experiencing damage. This device can be used not only as deformable and wearable power source but also as fully autonomous and self-sufficient adaptive electronic skin system.

Highly Sensitive, Transparent, and Durable Pressure Sensors Based on Sea-Urchin Shaped Metal Nanoparticles

Highly sensitive, transparent, and durable pressure sensors are fabricated using sea-urchin-shaped metal nanoparticles and insulating polyurethane elastomer. The pressure sensors exhibit outstanding sensitivity (2.46 kPa^-1 ), superior optical transmittance (84.8% at 550 nm), fast response/relaxation time (30 ms), and excellent operational durability. In addition, the pressure sensors successfully detect minute movements of human muscles.

Stretchable Organic Semiconductor Devices

Stretchable electronics are essential for the development of intensely packed collapsible and portable electronics, wearable electronics, epidermal and bioimplanted electronics, 3D surface compliable devices, bionics, prosthesis, and robotics. However, most stretchable devices are currently based on inorganic electronics, whose high cost of fabrication and limited processing area make it difficult to produce inexpensive, large-area devices. Therefore, organic stretchable electronics are highly attractive due to many advantages over their inorganic counterparts, such as their light weight, flexibility, low cost and large-area solution-processing, the reproducible semiconductor resources, and the easy tuning of their properties via molecular tailoring. Among them, stretchable organic semiconductor devices have become a hot and fast-growing research field, in which great advances have been made in recent years. These fantastic advances are summarized here, focusing on stretchable organic field-effect transistors, light-emitting devices, solar cells, and memory devices.

One Single Graphene Oxide Film for Responsive Actuation

Graphene, because of its superior electrical/thermal conductivity, high surface area, excellent mechanical flexibility, and stability, is currently receiving significant attention and benefit to fabricate actuator devices. Here, a sole graphene oxide (GO) film responsive actuator with an integrated self-detecting sensor has been developed. The film exhibits an asymmetric surface structure on its two sides, creating a promising actuation ability triggered by multistimuli, such as moisture, thermals, and infrared light. Meanwhile, the built-in laser-writing reduced graphene oxide (rGO) sensor in the film can detect its own deformation in real time. Smart perceptual fingers in addition to rectangular-shaped and even four-legged walking robots have been developed based on the responsive GO film.

Developing the Surface Chemistry of Transparent Butyl Rubber for Impermeable Stretchable Electronics

Transparent butyl rubber is a new elastomer that has the potential to revolutionize stretchable electronics due to its intrinsically low gas permeability. Encapsulating organic electronic materials and devices with transparent butyl rubber protects them from problematic degradation due to oxygen and moisture, preventing premature device failure and enabling the fabrication of stretchable organic electronic devices with practical lifetimes. Here, we report a methodology to alter the surface chemistry of transparent butyl rubber to advance this material from acting as a simple device encapsulant to functioning as a substrate primed for direct device fabrication on its surface. We demonstrate a combination of plasma and chemical treatment to deposit a hydrophilic silicate layer on the transparent butyl rubber surface to create a new layered composite that combines Si-OH surface chemistry with the favorable gas-barrier properties of bulk transparent butyl rubber. We demonstrate that these surface Si-OH groups react with organosilanes to form self-assembled monolayers necessary for the deposition of electronic materials, and furthermore demonstrate the fabrication of stretchable gold wires using nanotransfer printing of gold films onto transparent butyl rubber modified with a thiol-terminated self-assembled monolayer. The surface modification of transparent butyl rubber establishes this material as an important new elastomer for stretchable electronics and opens the way to robust, stretchable devices.

Self-Healing of Bulk Polyelectrolyte Complex Material as a Function of pH and Salt

Self-healing materials are an emerging class of modern materials gaining importance due to environmental and energy concerns. Materials based on the complexation of oppositely charged polyelectrolytes, usually in the form of coatings and films, have been shown to have water activated self-healing properties. In this work, the self-healing of bulk branched poly(ethylene imine) and poly(acrylic acid) (BPEI/PAA) complex is studied as a function of the aqueous solutions used to activate the self-healing. Specifically, exposure to different salt solutions and solutions of different pH was examined including sodium and copper ion containing solutions as well as acidic and basic solutions. By applying NaCl treatment, especially followed by exposure to DI water, the self-healing ability of the BPEI/PAA complex was enhanced. In contrast, after treated by CuCl₂, the BPEI/PAA complex lost its self-healing ability, showing an ability to modulate the ability to self-heal as a function of external stimulus. In addition to improving the ability to self-heal using salt as compared to using DI water alone, acidic and basic solutions can also improve the ability to self-heal. The self-healing is caused by chain mobility at the cut interface of the polyelectrolyte complex material which is controlled by charge density along the polyelectrolyte backbone as well as ionic cross-link density, and correlation between this mobility to rheological behavior is made. Tensile testing and determination of fracture toughness were used to characterize self-healing.

Sensing surface mechanical deformation using active probes driven by motor proteins

Studying mechanical deformation at the surface of soft materials has been challenging due to the difficulty in separating surface deformation from the bulk elasticity of the materials. Here, we introduce a new approach for studying the surface mechanical deformation of a soft material by utilizing a large number of self-propelled microprobes driven by motor proteins on the surface of the material. Information about the surface mechanical deformation of the soft material is obtained through changes in mobility of the microprobes wandering across the surface of the soft material. The active microprobes respond to mechanical deformation of the surface and readily change their velocity and direction depending on the extent and mode of surface deformation. This highly parallel and reliable method of sensing mechanical deformation at the surface of soft materials is expected to find applications that explore surface mechanics of soft materials and consequently would greatly benefit the surface science.

SU8 etch mask for patterning PDMS and its application to flexible fluidic microactuators

Over the past few decades, polydimethylsiloxane (PDMS) has become the material of choice for a variety of microsystem applications, including microfluidics, imprint lithography, and soft microrobotics. For most of these applications, PDMS is processed by replication molding; however, new applications would greatly benefit from the ability to pattern PDMS films using lithography and etching. Metal hardmasks, in conjunction with reactive ion etching (RIE), have been reported as a method for patterning PDMS; however, this approach suffers from a high surface roughness because of metal redeposition and limited etch thickness due to poor etch selectivity. We found that a combination of LOR and SU8 photoresists enables the patterning of thick PDMS layers by RIE without redeposition problems. We demonstrate the ability to etch 1.5-μm pillars in PDMS with a selectivity of 3.4. Furthermore, we use this process to lithographically process flexible fluidic microactuators without any manual transfer or cutting step. The actuator achieves a bidirectional rotation of 50° at a pressure of 200 kPa. This process provides a unique opportunity to scale down these actuators as well as other PDMS-based devices.

Ultra-Stretchable and Force-Sensitive Hydrogels Reinforced with Chitosan Microspheres Embedded in Polymer Networks

An ultra-stretchable and force-sensitive hydrogel with surface self-wrinkling microstructure is demonstrated by in situ synthesizing polyacrylamide (PAAm) and polyaniline (PANI) in closely packed swollen chitosan microspheres, exhibiting ultra-stretchability (>600%), high sensitivity (0.35 kPa^-1 ) for subtle pressures (<1 kPa), and can detect force in a broad range (10² Pa-10¹ MPa) with excellent electrical stability and rapid response speed, potentially finding applications for E-skin.

Design Methodology for Magnetic Field-Based Soft Tri-Axis Tactile Sensors

Tactile sensors are essential if robots are to safely interact with the external world and to dexterously manipulate objects. Current tactile sensors have limitations restricting their use, notably being too fragile or having limited performance. Magnetic field-based soft tactile sensors offer a potential improvement, being durable, low cost, accurate and high bandwidth, but they are relatively undeveloped because of the complexities involved in design and calibration. This paper presents a general design methodology for magnetic field-based three-axis soft tactile sensors, enabling researchers to easily develop specific tactile sensors for a variety of applications. All aspects (design, fabrication, calibration and evaluation) of the development of tri-axis soft tactile sensors are presented and discussed. A moving least square approach is used to decouple and convert the magnetic field signal to force output to eliminate non-linearity and cross-talk effects. A case study of a tactile sensor prototype, MagOne, was developed. This achieved a resolution of 1.42 mN in normal force measurement (0.71 mN in shear force), good output repeatability and has a maximum hysteresis error of 3.4%. These results outperform comparable sensors reported previously, highlighting the efficacy of our methodology for sensor design.

Temperature and layer thickness dependent in situ investigations on epindolidione organic thin-film transistors

We report on in situ performance evaluations as a function of layer thickness and substrate temperature for bottom-gate, bottom-gold contact epindolidione organic thin-film transistors on various gate dielectrics. Experiments were carried out under ultra-high vacuum conditions, enabling quasi-simultaneous electrical and surface analysis. Auger electron spectroscopy and thermal desorption spectroscopy (TDS) were applied to characterize the quality of the substrate surface and the thermal stability of the organic films. Ex situ atomic force microscopy (AFM) was used to gain additional information on the layer formation and surface morphology of the hydrogen-bonded organic pigment. The examined gate dielectrics included SiO2, in its untreated and sputtered forms, as well as the spin-coated organic capping layers poly(vinyl-cinnamate) (PVCi) and poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE, from the class of polynorbornenes). TDS and AFM revealed Volmer-Weber island growth dominated film formation with no evidence of a subjacent wetting layer. This growth mode is responsible for the comparably high coverage required for transistor behavior at 90-95% of a monolayer composed of standing molecules. Surface sputtering and an increased sample temperature during epindolidione deposition augmented the surface diffusion of adsorbing molecules and therefore led to a lower number of better-ordered islands. Consequently, while the onset of charge transport was delayed, higher saturation mobility was obtained. The highest, bottom-contact configuration, mobilities of approximately 2.5 × 10-3cm2/Vs were found for high coverages (50 nm) on sputtered samples. The coverage dependence of the mobility showed very different characteristics for the different gate dielectrics, while the change of the threshold voltage with coverage was approximately the same for all systems. An apparent decrease of the mobility with increasing coverage on the less polar PNDPE was attributed to a change in molecular orientation from upright standing in the thin-film phase to tilted in the bulk phase. From temperature-dependent mobility measurements we calculated activation barriers for the charge transport between 110 meV and 160 meV, depending on the dielectric configuration.

Highly Sensitive, Flexible, and Wearable Pressure Sensor Based on a Giant Piezocapacitive Effect of Three-Dimensional Microporous Elastomeric Dielectric Layer

We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pressure-sensing device for pulse monitoring at the human wrist. Finally, we demonstrate a pressure sensor array pad for the recognition of spatially distributed pressure information on a plane. Our sensor, with its excellent pressure-sensing performance, marks the realization of a true tactile pressure sensor presenting highly sensitive responses to the entire tactile pressure range, from ultralow-force detection to high weights generated by human activity.

Stretchable Hydrogel Electronics and Devices

Stretchable hydrogel electronics and devices are designed by integrating stretchable conductors, functional chips, drug-delivery channels, and reservoirs into stretchable, robust, and biocompatible hydrogel matrices. Novel applications include a smart wound dressing capable of sensing the temperatures of various locations on the skin, delivering different drugs to these locations, and subsequently maintaining sustained release of drugs.

Light-Activated Rapid-Response Polyvinylidene-Fluoride-Based Flexible Films

The design strategy and mechanical response mechanism of light-activated, rapid-response, flexible films are presented. Practical applications as a microrobot and a smart spring are demonstrated.

Silk Fibroin for Flexible Electronic Devices

Flexible electronic devices are necessary for applications involving unconventional interfaces, such as soft and curved biological systems, in which traditional silicon-based electronics would confront a mechanical mismatch. Biological polymers offer new opportunities for flexible electronic devices by virtue of their biocompatibility, environmental benignity, and sustainability, as well as low cost. As an intriguing and abundant biomaterial, silk offers exquisite mechanical, optical, and electrical properties that are advantageous toward the development of next-generation biocompatible electronic devices. The utilization of silk fibroin is emphasized as both passive and active components in flexible electronic devices. The employment of biocompatible and biosustainable silk materials revolutionizes state-of-the-art electronic devices and systems that currently rely on conventional semiconductor technologies. Advances in silk-based electronic devices would open new avenues for employing biomaterials in the design and integration of high-performance biointegrated electronics for future applications in consumer electronics, computing technologies, and biomedical diagnosis, as well as human-machine interfaces.

Bending of Responsive Hydrogel Sheets Guided by Field-Assembled Microparticle Endoskeleton Structures

Hydrogel composites that respond to stimuli can form the basis of new classes of biomimetic actuators and soft robotic components. Common latex microspheres can be assembled and patterned by AC electric fields within a soft thermoresponsive hydrogel. The field-oriented particle chains act as endoskeletal structures, which guide the macroscopic bending pattern of the actuators.

Stretchable Spin Valve with Stable Magnetic Field Sensitivity by Ribbon-Patterned Periodic Wrinkles

A strain-relief structure by combining the strain-engineered periodic wrinkles and the parallel ribbons was employed to fabricate flexible dual spin valves onto PDMS substrates in a direct sputtering method. The strain-relief structure can accommodate the biaxial strain accompanying with stretching operation (the uniaxial applied tensile strain and the induced transverse compressive strain due to the Poisson effect), thus significantly reducing the influence of the residual strain on the giant magnetoresistance (GMR) performance. The fabricated GMR dual spin-valve sensor exhibits the nearly unchanged MR ratio of 9.9%, magnetic field sensitivity up to 0.69%/Oe, and zero-field resistance in a wide range of stretching strain, making it promising for applications on a conformal shape or a movement part.

Self-Powered Analogue Smart Skin

The progress of smart skin technology presents unprecedented opportunities for artificial intelligence. Resolution enhancement and energy conservation are critical to improve the perception and standby time of robots. Here, we present a self-powered analogue smart skin for detecting contact location and velocity of the object, based on a single-electrode contact electrification effect and planar electrostatic induction. Using an analogue localizing method, the resolution of this two-dimensional smart skin can be achieved at 1.9 mm with only four terminals, which notably decreases the terminal number of smart skins. The sensitivity of this smart skin is remarkable, which can even perceive the perturbation of a honey bee. Meanwhile, benefiting from the triboelectric mechanism, extra power supply is unnecessary for this smart skin. Therefore, it solves the problems of batteries and connecting wires for smart skins. With microstructured poly(dimethylsiloxane) films and silver nanowire electrodes, it can be covered on the skin with transparency, flexibility, and high sensitivity.

Flexible Hall sensors based on graphene

The excellent electronic and mechanical properties of graphene provide a perfect basis for high performance flexible electronic and sensor devices. Here, we present the fabrication and characterization of flexible graphene based Hall sensors. The Hall sensors are fabricated on 50 μm thick flexible Kapton foil using large scale graphene grown by chemical vapor deposition technique on copper foil. Voltage and current normalized sensitivities of up to 0.096 V VT(-1) and 79 V AT(-1) were measured, respectively. These values are comparable to the sensitivity of rigid silicon based Hall sensors and are the highest values reported so far for any flexible Hall sensor devices. The sensitivity of the Hall sensor shows no degradation after being bent to a minimum radius of 4 mm, which corresponds to a tensile strain of 0.6%, and after 1000 bending cycles to a radius of 5 mm.

Mogul-Patterned Elastomeric Substrate for Stretchable Electronics

A mogul-patterned stretchable substrate with multidirectional stretchability and minimal fracture of layers under high stretching is fabricated by double photolithography and soft lithography. Au layers and a reduced graphene oxide chemiresistor on a mogul-patterned poly(dimethylsiloxane) substrate are stable and durable under various stretching conditions. The newly designed mogul-patterned stretchable substrate shows great promise for stretchable electronics.

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension. Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries. Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( ̴500 mN/m to near zero). Furthermore, this method requires only a very modest potential (< 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

Versatile Soft Grippers with Intrinsic Electroadhesion Based on Multifunctional Polymer Actuators

A highly versatile soft gripper that can handle an unprecedented range of object types is developed based on a new design of dielectric elastomer actuators employing an interdigitated electrode geometry, simultaneously maximizing both electroadhesion and electrostatic actuation while incorporating self-sensing. The multifunctionality of the actuator leads to a highly integrated, lightweight, fast, soft gripper with simplified structure and control.

A Wearable All-Solid Photovoltaic Textile

A solution is developed to power portable electronics in a wearable manner by fabricating an all-solid photovoltaic textile. In a similar way to plants absorbing solar energy for photosynthesis, humans can wear the as-fabricated photovoltaic textile to harness solar energy for powering small electronic devices.

Ultrafast Dynamic Piezoresistive Response of Graphene-Based Cellular Elastomers

Ultralight graphene-based cellular elastomers are found to exhibit nearly frequency-independent piezoresistive behaviors. Surpassing the mechanoreceptors in the human skin, these graphene elastomers can provide an instantaneous and high-fidelity electrical response to dynamic pressures ranging from quasi-static up to 2000 Hz, and are capable of detecting ultralow pressures as small as 0.082 Pa.

Dielectric Elastomer Based "Grippers" for Soft Robotics

The use of few stiff fibers to control the deformation of dielectric elastomer actuators, in particular to break the symmetry of equi-biaxial lateral strain in the absence of prestretch, is demonstrated. Actuators with patterned fibers are shown to evolve into unique shapes upon electrical actuation, enabling novel designs of gripping actuators for soft robotics.

A Shape-Memory Supercapacitor Fiber

A shape-memory, fiber-shaped supercapacitor is developed by winding aligned carbon nanotube sheets on a shape-memory polyurethane substrate. Despite its flexibility and stretchability, the deformed shapes under bending and stretching can be "frozen" as expected and recovered to the original state when required. Its electrochemical performances are well-maintained during deformation, at the deformed state and after the recovery.

Optimizing pentacene thin-film transistor performance: Temperature and surface condition induced layer growth modification

In this work we present in situ electrical and surface analytical, as well as ex situ atomic force microscopy (AFM) studies on temperature and surface condition induced pentacene layer growth modifications, leading to the selection of optimized deposition conditions and entailing performance improvements. We prepared p++-silicon/silicon dioxide bottom-gate, gold bottom-contact transistor samples and evaluated the pentacene layer growth for three different surface conditions (sputtered, sputtered + carbon and unsputtered + carbon) at sample temperatures during deposition of 200 K, 300 K and 350 K. The AFM investigations focused on the gold contacts, the silicon dioxide channel region and the highly critical transition area. Evaluations of coverage dependent saturation mobilities, threshold voltages and corresponding AFM analysis were able to confirm that the first 3-4 full monolayers contribute to the majority of charge transport within the channel region. At high temperatures and on sputtered surfaces uniform layer formation in the contact-channel transition area is limited by dewetting, leading to the formation of trenches and the partial development of double layer islands within the channel region instead of full wetting layers. By combining the advantages of an initial high temperature deposition (well-ordered islands in the channel) and a subsequent low temperature deposition (continuous film formation for low contact resistance) we were able to prepare very thin (8 ML) pentacene transistors of comparably high mobility.

Flexible high power-per-weight perovskite solar cells with chromium oxide-metal contacts for improved stability in air

Photovoltaic technology requires light-absorbing materials that are highly efficient, lightweight, low cost and stable during operation. Organolead halide perovskites constitute a highly promising class of materials, but suffer limited stability under ambient conditions without heavy and costly encapsulation. Here, we report ultrathin (3 μm), highly flexible perovskite solar cells with stabilized 12% efficiency and a power-per-weight as high as 23 W g(-1). To facilitate air-stable operation, we introduce a chromium oxide-chromium interlayer that effectively protects the metal top contacts from reactions with the perovskite. The use of a transparent polymer electrode treated with dimethylsulphoxide as the bottom layer allows the deposition-from solution at low temperature-of pinhole-free perovskite films at high yield on arbitrary substrates, including thin plastic foils. These ultra-lightweight solar cells are successfully used to power aviation models. Potential future applications include unmanned aerial vehicles-from airplanes to quadcopters and weather balloons-for environmental and industrial monitoring, rescue and emergency response, and tactical security applications.

Fingertip skin-inspired microstructured ferroelectric skins discriminate static/dynamic pressure and temperature stimuli

In human fingertips, the fingerprint patterns and interlocked epidermal-dermal microridges play a critical role in amplifying and transferring tactile signals to various mechanoreceptors, enabling spatiotemporal perception of various static and dynamic tactile signals. Inspired by the structure and functions of the human fingertip, we fabricated fingerprint-like patterns and interlocked microstructures in ferroelectric films, which can enhance the piezoelectric, pyroelectric, and piezoresistive sensing of static and dynamic mechanothermal signals. Our flexible and microstructured ferroelectric skins can detect and discriminate between multiple spatiotemporal tactile stimuli including static and dynamic pressure, vibration, and temperature with high sensitivities. As proof-of-concept demonstration, the sensors have been used for the simultaneous monitoring of pulse pressure and temperature of artery vessels, precise detection of acoustic sounds, and discrimination of various surface textures. Our microstructured ferroelectric skins may find applications in robotic skins, wearable sensors, and medical diagnostic devices.