Transient charge-driven 3D conformal printing via pulsed-plasma impingement.

Seamless integration of microstructures and circuits on three-dimensional (3D) complex surfaces is of significance and is catalyzing the emergence of many innovative 3D curvy electronic devices. However, patterning fine features on arbitrary 3D targets remains challenging. Here, we propose a facile charge-driven electrohydrodynamic 3D microprinting technique that allows micron- and even submicron-scale patterning of functional inks on a couple of 3D-shaped dielectrics via an atmospheric-pressure cold plasma jet. Relying on the transient charging of exposed sites arising from the weakly ionized gas jet, the specified charge is programmably deposited onto the surface as a virtual electrode with spatial and time spans of ~mm in diameter and ~µs in duration to generate a localized electric field accordantly. Therefore, inks with a wide range of viscosities can be directly drawn out from micro-orifices and deposited on both two-dimensional (2D) planar and 3D curved surfaces with a curvature radius down to ~1 mm and even on the inner wall of narrow cavities via localized electrostatic attraction, exhibiting a printing resolution of ~450 nm. In addition, several conformal electronic devices were successfully printed on 3D dielectric objects. Self-aligned 3D microprinting, with stacking layers up to 1400, is also achieved due to the electrified surfaces. This microplasma-induced printing technique exhibits great advantages such as ultrahigh resolution, excellent compatibility of inks and substrates, antigravity droplet dispersion, and omnidirectional printing on 3D freeform surfaces. It could provide a promising solution for intimately fabricating electronic devices on arbitrary 3D surfaces.

Impact Monitoring for Aircraft Smart Composite Skins Based on a Lightweight Sensor Network and Characteristic Digital Sequences.

Due to the growing use of composite materials in aircraft structures, Aircraft Smart Composite Skins (ASCSs) which have the capability of impact monitoring for large-scale composite structures need to be developed. However, the impact of an aircraft composite structure is a random transient event that needs to be monitored on-line continuously. Therefore, the sensor network of an ASCS and the corresponding impact monitoring system which needs to be installed on the aircraft as an on-board device must meet the requirements of light weight, low power consumption and high reliability. To achieve this point, an Impact Region Monitor (IRM) based on piezoelectric sensors and guided wave has been proposed and developed. It converts the impact response signals output from piezoelectric sensors into Characteristic Digital Sequences (CDSs), and then uses a simple but efficient impact region localization algorithm to achieve impact monitoring with light weight and low power consumption. However, due to the large number of sensors of ASCS, the realization of lightweight sensor network is still a key problem to realize an applicable ASCS for on-line and continuous impact monitoring. In this paper, three kinds of lightweight piezoelectric sensor networks including continuous series sensor network, continuous parallel sensor network and continuous heterogeneous sensor network are proposed. They can greatly reduce the lead wires of the piezoelectric sensors of ASCS and they can also greatly reduce the monitoring channels of the IRM. Furthermore, the impact region localization methods, which are based on the CDSs and the lightweight sensor networks, are proposed as well so that the lightweight sensor networks can be applied to on-line and continuous impact monitoring of ASCS with a large number of piezoelectric sensors. The lightweight piezoelectric sensor networks and the corresponding impact region localization methods are validated on the composite wing box of an unmanned aerial vehicle. The accuracy rate of impact region localization is higher than 92%.

Buckling-driven self-assembly of self-similar inspired micro/nanofibers for ultra-stretchable electronics.

Self-similar structures are capable of highly enhancing the deformability of stretchable electronics. We presented a self-assembly method based on the tunable buckling of serpentine fiber-based interconnects (FiberBIs), which are deposited using our presented helix electrohydrodynamic printing (HE-printing) technique, to fabricate self-similar structures with enhanced stretchability (up to 250%). It provides a low-cost, printing-based approach for the generation of large-scale self-similar FiberBIs. Distinct buckling behaviors and modes occur under specific conditions. To elucidate the mechanics governing this phenomenon, we present detailed experimental and theoretical studies of the buckling mechanics of serpentine microfibers on compliant substrates. Firstly, the effect of the magnitude and direction of prestrain on the buckling behavior of a fiber-on-substrate is discussed. Secondly, the critical geometry of a serpentine fiber as a key parameter for fabricating uniform self-similar fibers is also figured out. Finally, the cross-sectional geometry of the fiber as a judgment criterion for determining the in-surface or out-of-surface buckling of the fiber is established. The investigation can guide the fabrication process of large-scale self-similar structures for high-performance electronic devices with extreme stretchability.

Stretchable, Highly Durable Ternary Nanocomposite Strain Sensor for Structural Health Monitoring of Flexible Aircraft.

Harmonious developments of electrical and mechanical performances are crucial for stretchable sensors in structural health monitoring (SHM) of flexible aircraft such as aerostats and morphing aircrafts. In this study, we prepared a highly durable ternary conductive nanocomposite made of polydimethylsiloxane (PDMS), carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) to fabricate stretchable strain sensors. The nanocomposite has excellent electrical and mechanical properties by intensively optimizing the weight percentage of conducting fillers as well as the ratio of PDMS pre-polymer and curing agent. It was found that the nanocomposite with homogeneous hybrid filler of 1.75 wt % CB and 3 wt % MWCNTs exhibits a highly strain sensitive characteristics of good linearity, high gauge factor (GF ~ 12.25) and excellent durability over 105 stretching-releasing cycles under a tensile strain up to 25% when the PDMS was prepared at the ratio of 12.5:1. A strain measurement of crack detection for the aerostats surface was also employed, demonstrating a great potential of such ternary nanocomposite used as stretchable strain sensor in SHM.

Elasticity of fractal inspired interconnects.

The use of fractal-inspired geometric designs in electrical interconnects represents an important approach to simultaneously achieve large stretchability and high aerial coverage of active devices for stretchable electronics. The elastic stiffness of fractal interconnects is determined analytically in this paper. Specifically, the elastic energy and the tensile stiffness for an order n fractal interconnect of arbitrary shape are obtained, and are verified by the finite element analysis and experiments.

Process optimization of mechano-electrospinning by response surface methodology.

In this paper, mechano-electrospinning (MES) is presented to write the polyvinylidene fluoride (PVDF) solution into fibers directly, and the effects of the process parameters on the fiber are investigated experimentally based on response surface methodology. The different width of the fiber is obtained by adjusting the individual process parameters (velocity of the substrate, applied voltage and nozzle-to-substrate distance). Considering the continuous jet and stable Taylor-cone, the operation field is selected for investigating the complicated relationship between the process parameters on the width of the fiber by using the response surface methodology. The experiment results show that the predicted width of the fiber is in good agreement with the actual width of the fiber. Based on the analysis of the importance of the terms in the equation, a simple model can be used to predict the width of the fiber. Depending on this model, a large number of calibration experiments can be subducted. Additionally, the principle of the selection of the process parameters is presented by optimizing parameters, which can give a guideline for obtaining the desired fiber in the experiment.

Marangoni-driven deterministic formation of softer, hollow microstructures for sensitivity-enhanced tactile system.

Microengineering the dielectric layers with three-dimensional microstructures has proven effective in enhancing the sensitivity of flexible pressure sensors. However, the widely employed geometrical designs of solid microstructures exhibit limited sensitivity over a wide range of pressures due to their inherent but undesired structural compressibility. Here, a Marangoni-driven deterministic formation approach is proposed for fabricating hollow microstructures, allowing for greater deformation while retarding structural stiffening during compression. Fluid convective deposition enables solute particles to reassemble in template microstructures, controlling the interior cavity with a void ratio exceeding 90%. The hollow micro-pyramid sensor exhibits a 10-fold sensitivity improvement across wider pressure ranges over the pressure sensor utilizing solid micro-pyramids, and an ultra-low detect limit of 0.21 Pa. With the advantages of facilitation, scalability, and large-area compatibility, such an approach for hollow microstructures can be expanded to other sensor types for superior performance and has considerable potential in robotic tactile and epidermal devices.

Laser-Guided, Self-Confined Graphitization for High-Conductivity Embedded Electronics.

Facile fabrication of highly conductive and self-encapsulated graphene electronics is in urgent demand for carbon-based integrated circuits, field effect transistors, optoelectronic devices, and flexible sensors. The current fabrication of these electronic devices is mainly based on layer-by-layer techniques (separate circuit preparation and encapsulation procedures), which show multistep fabrication procedures, complicated renovation/repair procedures, and poor electrical property due to graphene oxidation and exfoliation. Here, we propose a laser-guided interfacial writing (LaserIW) technique based on self-confined, nickel-catalyzed graphitization to directly fabricate highly conductive, embedded graphene electronics inside multilayer structures. The doped nickel is used to induce chain carbonization, which firstly enhances the photothermal effect to increase the confined temperature for initial carbonization, and the generated carbon further increases the light-absorption capacity to fabricate high-quality graphene. Meanwhile, the nickel atoms contribute to the accelerated connection of carbon atoms. This interfacial carbonization inherently avoids the exfoliation and oxidation of the as-formed graphene, resulting in an 8-fold improvement in electrical conductivity (~20,000 S/m at 7,958 W/cm2 and 2 mm/s for 20% nickel content). The LaserIW technique shows excellent stability and reproducibility, with ±2.5% variations in the same batch and ±2% variations in different batches. Component-level wireless light sensors and flexible strain sensors exhibit excellent sensitivity (665 kHz/(W/cm2) for passive wireless light sensors) and self-encapsulation (<1% variations in terms of waterproof, antifriction, and antithermal shock). Additionally, the LaserIW technique allows for one-step renovation of in-service electronics and nondestructive repair of damaged circuits without the need to disassemble encapsulation layers. This technique reverses the layer-by-layer processing mode and provides a powerful manufacturing tool for the fabrication, modification, and repair of multilayer, multifunctional embedded electronics, especially demonstrating the immense potential for in-space manufacturing.

Programmable, High-resolution Printing of Spatially Graded Perovskites for Multispectral Photodetectors.

Micro/nanostructured perovskites with spatially graded compositions and bandgaps are promising in filter-free, chip-level multispectral, and hyperspectral detection. However, achieving high-resolution patterning of perovskites with controlled graded compositions is challenging. Here, a programmable mixed electrohydrodynamic printing (M-ePrinting) technique is presented to realize the one-step direct-printing of arbitrary spatially graded perovskite micro/nanopatterns for the first time. M-ePrinting enables in situ mixing and ejection of solutions with controlled composition/bandgap by programmatically varying driving voltage applied to a multichannel nozzle. Composition can be graded over a single dot, line or complex pattern, and the printed feature size is down to 1 µm, which is the highest printing resolution of graded patterns to the knowledge. Photodetectors based on micro/nanostructured perovskites with halide ions gradually varying from Br to I are constructed, which successfully achieve multispectral detection and full-color imaging, with a high detectivity and responsivity of 3.27 × 1015 Jones and 69.88 A W-1, respectively. The presented method provides a versatile and competitive approach for such miniaturized bandgap-tunable perovskite spectrometer platforms and artificial vision systems, and also opens new avenues for the digital fabrication of composition-programmable structures.

Laser-driven hierarchical "gas-needles" for programmable and high-precision proximity transfer printing of microchips.

Micro-transfer printing (µTP) techniques are essential for advanced electronics. However, current contact/noncontact µTP techniques fail to simultaneously achieve high selectivity and transfer accuracy. Here, a laser projection proximity transfer (LaserPPT) technique is presented, which assembles the microchips in an approach-and-release manner, combining high-precision parallelism with individual chip control. An embedded carbon layer with a thin gas layer is generated by an ultraviolet laser, followed by absorbing heat from the infrared laser, to enable the sequential expansion of hierarchical "gas-needles." The level 1 large gas-needle with a substantially growing height can reduce the gap between the microchip and the receiver. Then, the level 2 small gas-needles enable the gentle release of a chip. Therefore, the LaserPPT can obtain a strong adhesion modulation (~1000 times), excellent size scalability (<100 micrometers), and high transfer accuracy of ~4 micrometers. Last, the assembly of a micro-light-emitting diode display demonstrates the capabilities for deterministic assembly of microarrays.

Primary intracranial leiomyosarcoma: review of the literature and presentation of a case.

BACKGROUND: Primary intracranial leiomyosarcoma (LMS) is a rare tumor of the central nervous system and therefore has only been reported sporadically. METHODS: The MEDLINE database was searched for relevant case reports and series published in English from 1969 to March 2012. These papers were reviewed to identify clinical and histopathological features, treatment modalities, and prognoses of patients with primary intracranial LMS. RESULTS: We reviewed 37 patients, including our patient, with primary intracranial LMS. Of these cases, 16 (43.2%) involved immunosuppression, 4 (10.8%) involved radiation-induced LMS, 3 (8.1%) cases involved a potential leiomyosarcomatous transformation of a brain lesion, and 7 (18.9%) cases, including the presented case, had no specific medical history. The treatment for these cases included resection (33/37, 89.2%), postoperative radiotherapy (20/37, 54.1%), and chemotherapy (7/37, 18.9%). CONCLUSIONS: The best method for the treatment of LMS could not be determined due to the limited number of cases that have been reported. However, optimized surgical resection, postoperative radiotherapy, and sarcoma-based chemotherapy may improve treatment outcomes.

Flexible hybrid electronics: Enabling integration techniques and applications.

The conventional electronic systems enabled by rigid electronic are prone to malfunction under deformation, greatly limiting their application prospects. As an emerging platform for applications in healthcare monitoring and human-machine interface (HMI), flexible electronics have attracted growing attention due to its remarkable advantages, such as stretchability, flexibility, conformability, and wearing comfort. However, to realize the overall electronic systems, rigid components are also required for functions such as signal acquisition and transmission. Therefore, flexible hybrid electronics (FHE), which simultaneously possesses the desirable flexibility and enables the integration of rigid components for functionality, has been emerging as a promising strategy. This paper reviews the enabling integration techniques for FHE, including technologies for two-dimensional/three-dimensional (2D/3D) interconnects, bonding of rigid integrated circuit (IC) chips to soft substrate, stress-isolation structures, and representative applications of FHE. In addition, future challenges and opportunities involved in FHE-based systems are also discussed.

A Snakeskin-Inspired, Soft-Hinge Kirigami Metamaterial for Self-Adaptive Conformal Electronic Armor.

A majority of soft-body creatures evolve armor or shells to protect themselves. Similar protection demand is for flexible electronics working in complex environments. Existing works mainly focus on improving the sensing capabilities such as electronic skin (E-skin). Inspired by snakeskin, a novel electronic armor (E-armor) is proposed, which not only possesses mechanical flexibility and electronic functions similar to E-skin, but is also able to protect itself and the underlying soft body from external physical damage. The geometry of the kirigami mechanical metamaterial (Kiri-MM) ensures auxetic stretchability and meanwhile large areal coverage for sufficient protection. Moreover, to suppress the inherent but undesired out-of-plane buckling of conventional Kiri-MMs for conformal applications, soft hinges are used to form a distinct soft (hinges)-rigid (tiles) configuration. Analytical, computational, and experimental studies of the mechanical behaviors of the soft-hinge Kiri-MM E-armor demonstrate the merits of this design, i.e., stretchability, conformability, and protectability, as applied to flexible electronics. Deploying a conductive soft material at the hinges enables facile wiring strategies for large-scale circuit arrays. Functional E-armor systems for controllable display and sensing purposes provide simple examples of a wide spectrum of applications of this concept.

Flexible Metamaterial Electronics.

Over the last decade, extensive efforts have been made on utilizing advanced materials and structures to improve the properties and functionalities of flexible electronics. While the conventional ways are approaching their natural limits, a revolutionary strategy, namely metamaterials, is emerging toward engineering structural materials to break the existing fetters. Metamaterials exhibit supernatural physical behaviors, in aspects of mechanical, optical, thermal, acoustic, and electronic properties that are inaccessible in natural materials, such as tunable stiffness or Poisson's ratio, manipulating electromagnetic or elastic waves, and topological and programmable morphability. These salient merits motivate metamaterials as a brand-new research direction and have inspired extensive innovative applications in flexible electronics. Here, such a groundbreaking interdisciplinary field is first coined as "flexible metamaterial electronics," focusing on enhancing and innovating functionalities of flexible electronics via the design of metamaterials. Herein, the latest progress and trends in this infant field are reviewed while highlighting their potential value. First, a brief overview starts with introducing the combination of metamaterials and flexible electronics. Then, the developed applications are discussed, such as self-adaptive deformability, ultrahigh sensitivity, and multidisciplinary functionality, followed by the discussion of potential prospects. Finally, the challenges and opportunities facing flexible metamaterial electronics to advance this cutting-edge field are summarized.

High density, addressable electrohydrodynamic printhead made of a silicon plate and polymer nozzle structure.

Electrohydrodynamic (EHD) printing is a promising micro/nanofabrication technique, due to its ultra-high resolution and wide material applicability. However, it suffers from low printing efficiency which urgently calls for a high density and addressable nozzle array. This paper presents a nozzle array chip made of a silicon plate and polymer nozzle structure, where the large silicon plate is conducive to a uniform spatial electric field distribution, and the polymer SU8 nozzle can inhibit tip discharge due to its insulating character and liquid flooding as SU8 is hydrophobic. By carefully designing the nozzle array structure via simulation, and fabricating it through MEMS technology, a high-density nozzle array chip has been achieved which can generate very uniform dots without crosstalk. Meanwhile, by adding extractors underneath the nozzle array, and utilizing a digital switch array to tune their on/off state, addressable printing has been realized. This novel printhead design has solved the discharge, liquid flooding, and crosstalk behavior in EHD nozzle arrays, and is compatible with traditional silicon-based MEMS technology, which will promote the practical applications of EHD printing in micro/nanoelectronics, biomedical/energy devices, etc.

Mode-tunable, micro/nanoscale electrohydrodynamic deposition techniques for optoelectronic device fabrication.

The rapid development of fascinating new optoelectronic materials and devices calls for the innovative production of micro/nanostructures in a high-resolution, large-scale, low-cost fashion, preferably compatible with flexible/wearable applications. Powerful electrohydrodynamic (EHD) deposition techniques, which generate micro/nanostructures using high electrical forces, exhibit unique advantages in high printing resolution (<1 µm), tunable printing modes (electrospray for films, electrospinning for fibers and EHD jet printing for dots), and wide material applicability (viscosity 1-10 000 cps), making them attractive in the fabrication of high-density and high-tech optoelectronic devices. This review highlights recent advances related to EHD-deposited optoelectronics, ranging from solar cells, photodetectors, and light-emitting diodes, to transparent electrodes, with detailed descriptions of the EHD-based jetting mechanism, ink formulation requirements and corresponding jetting modes to obtain functional micro/nanostructures. Finally, a brief summary and an outlook on the future perspectives are proposed.

Critical Size/Viscosity for Coffee-Ring-Free Printing of Perovskite Micro/Nanopatterns.

Inkjet printing is the most encouraging method for patterning and integrating perovskite materials into microminiature application scenarios. However, it is still challenging to achieve high-resolution, coffee-ring-free, and perfect crystallized patterns. Here, a strategy based on powerful electrohydrodynamic printing and droplet viscosity-size coordinate regulation is developed to solve the above problems. By adding a long-chain polymer poly(vinylpyrrolidone) (PVP) into perovskite precursor to tune ink viscosity and introducing electrohydrodynamic printing to print the high-viscosity ink into droplets of different sizes, we can manipulate the inside flowing resistance and outside evaporation rate of a droplet, thus revealing a critical size/viscosity under which the coffee ring effect is inhibited, showing immense potential and significance for high-quality patterning. In addition, the long-chain polymer benefits droplet spatial limitation and uniform crystallization. The as-printed luminous patterns demonstrate high resolution (structure size ∼1 µm), excellent brightness, pleasant uniformity, and fascinating compatibility with flexible substrates, which is promising for future perovskite optoelectronic device applications.

3D Interfacing between Soft Electronic Tools and Complex Biological Tissues.

Recent developments in soft functional materials have created opportunities for building bioelectronic devices with tissue-like mechanical properties. Their integration with the human body could enable advanced sensing and stimulation for medical diagnosis and therapies. However, most of the available soft electronics are constructed as planar sheets, which are difficult to interface with the target organs and tissues that have complex 3D structures. Here, the recent approaches are highlighted to building 3D interfaces between soft electronic tools and complex biological organs and tissues. Examples involve mesh devices for conformal contact, imaging-guided fabrication of organ-specific electronics, miniaturized probes for neurointerfaces, instrumented scaffold for tissue engineering, and many other soft 3D systems. They represent diverse routes for reconciling the interfacial mismatches between electronic tools and biological tissues. The remaining challenges include device scaling to approach the complexity of target organs, biological data acquisition and processing, 3D manufacturing techniques, etc., providing a range of opportunities for scientific research and technological innovation.

Highly Robust and Wearable Facial Expression Recognition via Deep-Learning-Assisted, Soft Epidermal Electronics.

The facial expressions are a mirror of the elusive emotion hidden in the mind, and thus, capturing expressions is a crucial way of merging the inward world and virtual world. However, typical facial expression recognition (FER) systems are restricted by environments where faces must be clearly seen for computer vision, or rigid devices that are not suitable for the time-dynamic, curvilinear faces. Here, we present a robust, highly wearable FER system that is based on deep-learning-assisted, soft epidermal electronics. The epidermal electronics that can fully conform on faces enable high-fidelity biosignal acquisition without hindering spontaneous facial expressions, releasing the constraint of movement, space, and light. The deep learning method can significantly enhance the recognition accuracy of facial expression types and intensities based on a small sample. The proposed wearable FER system is superior for wide applicability and high accuracy. The FER system is suitable for the individual and shows essential robustness to different light, occlusion, and various face poses. It is totally different from but complementary to the computer vision technology that is merely suitable for simultaneous FER of multiple individuals in a specific place. This wearable FER system is successfully applied to human-avatar emotion interaction and verbal communication disambiguation in a real-life environment, enabling promising human-computer interaction applications.

Template-Free Construction of Tin Oxide Porous Hollow Microspheres for Room-Temperature Gas Sensors.

Porous hollow microsphere (PHM) materials represent ideal building blocks for realizing diverse functional applications such as catalysis, energy storage, drug delivery, and chemical sensing. This has stimulated intense efforts to construct metal oxide PHMs for achieving highly sensitive and low-power-consumption semiconductor gas sensors. Conventional methods for constructing PHMs rely on delicate reprogramming of templates and may suffer from the structural collapse issue during the removal of templates. Here, we propose a template-free method for the construction of tin oxide (SnO2) PHMs via the competition between the solvent evaporation rate and the phase separation dynamics of colloidal SnO2 quantum wires. The SnO2 PHMs (typically 3 ± 0.5 µm diameter and approximately 200 nm shell thickness) exhibit desirable structural stability with desirable processing compatibility with various substrates. This enables the realization of NO2 gas sensors having a superior response and recovery process at room temperature. The superior NO2-sensing characteristic is attributed to the effective gas adsorption competition on solid surfaces benefiting from efficient diffusion channels, enhancing the interaction of metal oxide solids with gas molecules in terms of the receptor function, transducer function, and utility factor. In addition, the one-step deposition of SnO2 PHMs directly onto device substrates simplifies the fabrication conditions for semiconductor gas sensors. The desirable structural stability of PHMs combined with the functional diversity of metal oxides may open new opportunities for the design of functional materials and devices.