To accurately probe the tactile information on soft skin, it is critical for the pressure sensing array to be free of noise and inter-taxel crosstalk, irrespective of the measurement condition. However, on dynamically moving and soft surfaces, which are common conditions for on-skin and robotic applications, obtaining precise measurement without compromising the sensing performance is a significant challenge due to mechanical coupling between the sensors and with the moving surface. In this work, multi-level architectural design of micro-pyramids and trapezoid-shaped mechanical barrier array was implemented to enable accurate spatiotemporal tactile sensing on soft surfaces under dynamic deformations. Trade-off relationship between limit of detection and bending insensitivity was discovered, which was overcome by employing micropores in barrier structures. Finally, in-situ pressure mapping on dynamically moving soft surfaces without signal distortion is demonstrated while human skin and/or soft robots are performing complicated tasks such as reading Braille and handling the artificial organs.

Flexible and transparent thin-film silicon solar cells were fabricated and optimized for building-integrated photovoltaics and bifacial operation. A laser lift-off method was developed to avoid thermal damage during the transfer of light-scattering structures onto colorless polyimide substrates and thus enhance front-incidence photocurrent, while a dual n-type rear window layer was introduced to reduce optical losses, facilitate electron transport for rear incidence, and thus enhance performance during bifacial operation. The introduction of the window layer increased the rear-to-front power conversion efficiency ratio to ~86%. The optimized bifacial power conversion efficiency for front and rear irradiances of 1 and 0.3 sun, respectively, equaled 6.15%, and the average transmittance within 500–800 nm equaled 36.9%. Additionally, the flexible and transparent solar cells fabricated using laser lift-off exhibited good mechanical reliability (i.e., sustained 500 cycles at a bending radius of 6 mm) and were therefore suitable for building-integrated photovoltaics.

Wearable electronics with miniaturization and high-power density call for devices with advanced thermal management capabilities, outstanding flexibility, and excellent permeability. However, it is difficult to achieve these goals simultaneously due to the conflict between high thermal conductivity and permeability and flexibility. Here, we report an approach to fabricate flexible, breathable composites with advanced thermal management capability by coating the boron nitride nanosheets (BNNSs) layer with high thermal conductivity on the grids of patterned electrospun thermoplastic polyurethane (TPU) fibrous mats. The composite exhibited a significant enhancement of thermal conductivity and preserved instinctive breathability simultaneously. When the composite was integrated into flexible devices, its saturating operating temperature dropped significantly compared to that of pure Ecoflex packaging. Moreover, the surface temperature fluctuation was less than 0.5 °C during more than 2000 cycles bending-releasing process. Finally, a prototype to fabricate wearable electronics with advanced thermal management capability was proposed.

Until now, conventional nanogenerators could only produce electric pulses with relatively low-power densities. Herein, we invent a novel controllable growth technique for two-dimensional (2-D) cuprous oxide (p-Cu2O) single-crystal films, and on this basis, a new concept of 2-D single-crystal film flexoelectric nanogenerators (FENGs) are rationally designed and constructed for the first time, which has the characteristics of long-range order lattice, few grain boundaries and defects. More importantly, the accumulated built-in polarization potential in the bent 2-D p-Cu2O single-crystal film FENGs is in the same orientation as the output electricity, resulting in the first nanogenerator that can output continuous and stable electric signals with high voltage (Voc of 2.8 V), current (Jsc of 11.5 μA·cm−2) and power density (14.4 μW·cm−2), exhibiting great practical application potential for power generation and motion capture. This research breaks new ground and establishes a precedent for high-performance and continuous-output nanogenerators, as well as smart wearable sensors.

Challenges in the understanding of three-dimensional (3D) brain networks by simultaneously recording both surface and intracortical areas of brain signals remain due to the difficulties of constructing mechanical design and spatial limitations of the implanted sites. Here, we present a foldable and flexible 3D neural prosthetic that facilitates the 3D mapping of complex neural circuits with high spatiotemporal dynamics from the intracortical to cortical region. This device is the tool to map the 3D neural transmission through sophisticatedly designed four flexible penetrating shanks and surface electrode arrays in one integrated system. We demonstrate the potential possibilities of identifying correlations of neural activities from the intracortical area to cortical regions through continuous monitoring of electrophysiological signals. We also exploited the structural properties of the device to record synchronized signals of single spikes evoked by unidirectional total whisker stimulation. This platform offers opportunities to clarify unpredictable 3D neural pathways and provides a next-generation neural interface.

The solubility of luminescent materials is a key parameter to improve the electroluminescent performances of solution-processed organic light-emitting diodes (OLEDs). The through-space charge transfer (TSCT) materials provide an alternative to introduce the solubilizing groups (SGs) to the linker. Herein, the tert-butyl and n-hexyl groups are introduced as SGs at C7 positions of spiro structure, named C6-DMB and tBu-DMB, away from the acceptor. This has no influence on the photophysical properties of the parent TSCT molecule. Highly efficient solution-processed OLEDs were demonstrated with the maximum external quantum efficiencies of 21.0% and 21.7%, respectively. To the best of our knowledge, these are champions in the state-of-the-art solution-processed OLEDs with TSCT emitters. This work confirmed our conjecture of constructing highly efficient soluble emitters by transforming an outstanding TSCT material from thermal evaporation to solution-processed OLEDs with SGs simply integrated on the ‘bridge’ linker.

A flexible full-color micro-LED display with high mechanical robustness was fabricated by printing quantum dots (QDs) on a blue micro-LED array using standard photolithography. The red and green colors yielded from QDs exhibit a better color gamut than conventional color filters. The light conversion efficiency was enhanced by adding TiO2 nanoparticles to the QD-photoresist composite. This full-color micro-LED display was successfully mounted on various unusual substrates such as curved glass, fabrics, and human skin, enabling diverse optoelectronic applications. In addition, wireless multi-channel visible light communication (VLC) based on the wavelength-division-multiplexing orthogonal-frequency-division-multiplexing (WDM-OFDM) technique was demonstrated using a QD-based color micro-LED panel. A high data transmission rate of 1.9 Gbps was successfully obtained owing to the high electrical–optical modulation bandwidth of the QD-based micro-LED panel.

Flexible perovskite solar cells (PSCs) have drawn increasing attention due to their promising applications for wearable electronics and aerospace applications. However, the efficiency and stability of flexible PSCs still lag behind their rigid counterparts. Here, we use N,N-dimethyl acrylamide (DMAA) to in situ synthesize cross-linking polymer for flexible Sn–Pb mixed PSCs. DMAA can gather at grain boundary as a scaffold to regulate the crystallization of perovskite and reduce defects. The rigid and flexible Sn–Pb mixed PSCs showed efficiencies of 16.44% and 15.44%, respectively. In addition, the flexible Sn–Pb mixed PSCs demonstrated excellent bending durability, which retained over 80% of the original efficiency after 5000 bending cycles at a radius of 5 mm.

Paper-based electronics have attracted much attention due to their softness, degradability, and low cost. However, paper-based sensors are difficult to apply to high-humidity environments or even underwater. Here, we report a fully paper-integrated piezoresistive sensing system that exhibits flexibility, waterproofing, air permeability, and biocompatibility. This system consists of hydrophobic paper as the substrate and encapsulation layer, conductive paper with a double ‘zig-zag’ and dotted surface structure as the sensing layer, and silver paste films as the interconnects. The structural design of the sensing layer helps to increase the contact area in adjacent layers under pressure and further improves the pressure sensitivity. The piezoresistive system can be worn on human skin in the ambient environment, wet environment, and water for real-time monitoring of physiological signals with air permeability and waterproofing due to its hydrophobic fiber structure. Such a device provides a reliable, economical, and eco-friendly solution to wearable technologies.

Understanding the stress-induced phenomena is essential for improving the long-term application of flexible solar cells to non-flat surfaces. Here, we investigated the electronic band structure and carrier transport mechanism of Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic devices under mechanical stress. Highly efficient flexible CZTSSe devices were fabricated controlling the Na incorporation. The electronic structure of CZTSSe was deformed with stress as the band gap, valence band edge, and work function changed. Electrical properties of the bent CZTSSe surface were probed by Kelvin probe force microscopy and the CZTSSe with Na showed less degraded carrier transport compared to the CZTSSe without Na. The local open-circuit voltage (VOC) on the bent CZTSSe surface decreased due to limited carrier excitation. The reduction of local VOC occurred larger with convex bending than in concave bending, which is consistent with the degradation of device parameters. This study paves the way for understanding the stress-induced optoelectronic changes in flexible photovoltaic devices.

Lightweight and flexible photovoltaic solar cells and modules are promising technologies that may result in the wide usage of light-to-electricity energy conversion devices. This communication presents the prospects of Cu(In,Ga)Se2 (CIGS)-based lightweight and flexible photovoltaic devices. The current status of flexible CIGS minimodules with photovoltaic efficiency values greater than 18% and future directions to enhance their efficiency values toward >20% are discussed. The effects of cell separation edges, which are formed through a mechanical, laser, or photolithography scribing process used to fabricate solar cells and modules, on the device performance are also discussed. We found that mechanically scribed CIGS device edges, which are present in conventional solar cells and modules, cause deterioration of device performance. In other words, further improvement is expected with appropriate passivation/termination treatment of the edges or replacing mechanical scribing with a damage-free separation process.

The certified power conversion efficiency (PCE) of organic photovoltaics (OPV) fabricated in laboratories has improved dramatically to over 19% owing to the rapid development of narrow-bandgap small-molecule acceptors and wide bandgap polymer donor materials. The next pivotal question is how to translate small-area laboratory devices into large-scale commercial applications. This requires the OPV to be solution-processed and flexible to satisfy the requirements of high-throughput and large-scale production such as roll-to-roll printing. This review summarizes and analyzes recent progress in solution-processed flexible OPV. After a detailed discussion from the perspective of the behavior of the narrow bandgap small-molecule acceptor and wide bandgap polymer donor active layer in solution-processed flexible devices, the existing challenges and future directions are discussed.

X-ray detectors are of pivotal importance for the scientific and technological progress in a wide range of medical, industrial, and scientific applications. Here, we take advantage of the printability of perovskite-based semiconductors and achieve a high X-ray sensitivity combined with the potential of an exceptional high spatial resolution by our origami-inspired folded perovskite X-ray detector. The high performance of our device is reached solely by the folded detector architecture and does not require any photolithography. The design and fabrication of a foldable perovskite sensor array is presented and the detector is characterized as a planar and as a folded device. Exposed to 50 kVp−150 kVp X-ray radiation, the planar detector reaches X-ray sensitivities of 25−35 μC/(Gyaircm2), whereas the folded detector achieves remarkably increased X-ray sensitivities of several hundred μC/(Gyaircm2) and a record value of 1409 μC/(Gyaircm2) at 150 kVp without photoconductive gain. Finally, the potential of an exceptional high spatial resolution of the folded detector of more than 20 lp/mm under 150 kVp X-ray radiation is demonstrated.

There are great concerns for sensing using flexible acoustic wave sensors and lab-on-a-chip, as mechanical strains will dramatically change the sensing signals (e.g., frequency) when they are bent during measurements. These strain-induced signal changes cannot be easily separated from those of real sensing signals (e.g., humidity, ultraviolet, or gas/biological molecules). Herein, we proposed a new strategy to minimize/eliminate the effects of mechanical bending strains by optimizing off-axis angles between the direction of bending deformation and propagation of acoustic waves on curved surfaces of layered piezoelectric film/flexible glass structure. This strategy has theoretically been proved by optimization of bending designs of off-axis angles and acoustically elastic effect. Proof-of-concept for humidity and ultraviolet-light sensing using flexible SAW devices with negligible interferences are achieved within a wide range of bending strains. This work provides the best solution for achieving high-performance flexible acoustic wave sensors under deformed/bending conditions.

The incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 based solar cells is shown. The fabrication used an industry scalable lithography technique—nanoimprint lithography (NIL)—for a 15 × 15 cm2 dielectric layer patterning. Devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. The NIL patterned device shows similar performance to the EBL patterned device.The impact of the lithographic processes in the rigid solar cells’ performance were evaluated via X-ray Photoelectron Spectroscopy and through a Solar Cell Capacitance Simulator. The device on stainless-steel showed a slightly lower performance than the rigid approach, due to additional challenges of processing steel substrates, even though scanning transmission electron microscopy did not show clear evidence of impurity diffusion. Notwithstanding, time-resolved photoluminescence results strongly suggested elemental diffusion from the flexible substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device.

Tactile sensing has been a key challenge in robotic haptics. Inspired by how human skin sense the stress field with layered structure and distributed mechanoreceptors, we herein propose a design for modular multi-parameter perception electronic skin. With the stress field sensing concept, complex tactile signals can be transformed into field information. By analyzing the stress field, the real-time three-dimensional forces can be resolved with 1.8° polar angle resolution and 3.5° azimuthal angle resolution (achieved up to 71 folds of improvement in spatial resolution), we can also detect the hardness of object in contact with the electronic skin. Moreover, we demonstrate random assembly of the sensing arrays and integration of our electronic skin onto differently curved surfaces do not lead to any measurement variation of the stress field. This result reveals that the sensing elements in our electronic skin system can be modularly made and exchanged for specific applications.

The recent advances of wearable sensors are remarkable but there are still limitations that they need to be refabricated to tune the sensor for target signal. However, biological sensory systems have the inherent potential to adjust their sensitivity according to the external environment, allowing for a broad and enhanced detection. Here, we developed a Tunable, Ultrasensitive, Nature-inspired, Epidermal Sensor (TUNES) that the strain sensitivity was dramatically increased (GF ~30k) and the pressure sensitivity could be tuned (10–254 kPa−1) by preset membrane tension. The sensor adjusts the sensitivity to the pressure regime by preset tension, so it can measure a wide range (0.05 Pa–25 kPa) with the best performance: from very small signals such as minute pulse to relatively large signals such as muscle contraction and respiration. We verified its capabilities as a wearable health monitoring system by clinical trial comparing with pressure wire which is considered the current gold standard of blood pressure (r = 0.96) and home health care system by binary classification of Old’s/Young’s pulse waves via machine learning (accuracy 95%).

In modern technology, recent advances in multi-functional devices are rapidly developed for the diverse demands of human beings. Meanwhile, durability and adaptability to extreme environmental conditions are also required. In this study, a flexible magnetoelectric (ME) heterostructure based on CoFe2O4/Pb(Zr,Ti)O3 composite thin film on muscovite is presented, with two geometries of the constituents, namely laminar heterostructure, and vertical nanostructure, adopted for the comparison. On the other hand, credited to the mechanical flexibility of muscovite, the impact of flexibility on ME properties is also discussed with a series of bending tests. Moreover, the ME response sustains for 10,000 times bending without significant decrease, validating the mechanical durability of this heterostructure on muscovite. With these advantages, a flexible proximity sensor based on this heterostructure is demonstrated for motion detection. It is expected to offer a pathway for creating the next-generational flexible devices, showing potential for future practical application.

Activities and physical effort have been commonly estimated using a metabolic rate through indirect calorimetry to capture breath information. The physical effort represents the work hardness used to optimize wearable robotic systems. Thus, personalization and rapid optimization of the effort are critical. Although respirometry is the gold standard for estimating metabolic costs, this method requires a heavy, bulky, and rigid system, limiting the system’s field deployability. Here, this paper reports a soft, flexible bioelectronic system that integrates a wearable ankle-foot exoskeleton, used to estimate metabolic costs and physical effort, demonstrating the potential for real-time wearable robot adjustments based on biofeedback. Data from a set of activities, including walking, running, and squatting with the biopatch and exoskeleton, determines the relationship between metabolic costs and heart rate variability root mean square of successive differences (HRV-RMSSD) (R = −0.758). Collectively, the exoskeleton-integrated wearable system shows potential to develop a field-deployable exoskeleton platform that can measure wireless real-time physiological signals.

Skin-integrated haptic interfaces that can relay a wealth of information from the machine to the human are of great interest. However, existing haptic devices are not yet able to produce haptic cues that are compatible with the skin. In this work, we present the stretchable soft actuators for haptic feedback, which can match the perception range, spatial resolution, and stretchability of the skin. Pressure-amplification structures are fabricated using a scalable self-assembly process to ensure an output pressure beyond the skin perception threshold. Due to the minimized device size, the actuator array can be fabricated with a sufficiently high spatial resolution, which makes the haptic device applicable for skin locations with the highest spatial acuity. A haptic feedback system is demonstrated by employing the developed soft actuators and highly sensitive pressure sensors. Two proof-of-concept applications are developed to illustrate the capability of transferring information related to surface textures and object shapes acquired at the robot side to the user side.