Injectable ultrasonic sensor for wireless monitoring of intracranial signals.

Direct and precise monitoring of intracranial physiology holds immense importance in delineating injuries, prognostication and averting disease1. Wired clinical instruments that use percutaneous leads are accurate but are susceptible to infection, patient mobility constraints and potential surgical complications during removal2. Wireless implantable devices provide greater operational freedom but include issues such as limited detection range, poor degradation and difficulty in size reduction in the human body3. Here we present an injectable, bioresorbable and wireless metastructured hydrogel (metagel) sensor for ultrasonic monitoring of intracranial signals. The metagel sensors are cubes 2 × 2 × 2 mm3 in size that encompass both biodegradable and stimulus-responsive hydrogels and periodically aligned air columns with a specific acoustic reflection spectrum. Implanted into intracranial space with a puncture needle, the metagel deforms in response to physiological environmental changes, causing peak frequency shifts of reflected ultrasound waves that can be wirelessly measured by an external ultrasound probe. The metagel sensor can independently detect intracranial pressure, temperature, pH and flow rate, realize a detection depth of 10 cm and almost fully degrade within 18 weeks. Animal experiments on rats and pigs indicate promising multiparametric sensing performances on a par with conventional non-resorbable wired clinical benchmarks.

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.

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.

Flexible sensors with zero Poisson's ratio.

Flexible sensors have been developed for the perception of various stimuli. However, complex deformation, usually resulting from forces or strains from multi-axes, can be challenging to measure due to the lack of independent perception of multiaxial stimuli. Herein, flexible sensors based on the metamaterial membrane with zero Poisson's ratio (ZPR) are proposed to achieve independent detection of biaxial stimuli. By deliberately designing the geometric dimensions and arrangement parameters of elements, the Poisson's ratio of an elastomer membrane can be modulated from negative to positive, and the ZPR membrane can maintain a constant transverse dimension under longitudinal stimuli. Due to the accurate monitoring of grasping force by ZPR sensors that are insensitive to curvatures of contact surfaces, rigid robotic manipulators can be guided to safely grasp deformable objects. Meanwhile, the ZPR sensor can also precisely distinguish different states of manipulators. When ZPR sensors are attached to a thermal-actuation soft robot, they can accurately detect the moving distance and direction. This work presents a new strategy for independent biaxial stimuli perception through the design of mechanical metamaterials, and may inspire the future development of advanced flexible sensors for healthcare, human-machine interfaces and robotic tactile sensing.

Preparation of high-resolution micro/nano dot array by electrohydrodynamic jet printing with enhanced uniformity.

The high-resolution array is the basic structure of most kinds of microelectronics. Electrohydrodynamic jet (E-Jet) printing technology is widely applied in manufacturing array structures with high resolution, high material compatibility and multi-modal printing. It is still challenging to acquire high uniformity of printed array with micro-nanometer resolution, which greatly influences the performance and lifetime of the microelectronics. In this paper, to improve the uniformity of the printed array, the influence of each parameter on the uniformity of the E-jet printed dot array is studied on the cobuilt NEJ-E/P200 experimental platform, finding the applied voltage plays the most important role in maintaining the uniformity of the printed array. By appropriately adjusting the printing parameters, the dot arrays with different resolutions from 500 pixels per inch (PPI) to 17,000 PPI are successfully printed. For arrays below and over 10,000 PPI, the deviations of the uniformity are within 5% and 10% respectively. In this work, the dot array over 15,000 PPI is first implemented using E-jet printing. The conclusions acquired by experimental analysis of dot array printing process are of great importance in high resolution array printing as it provides practical guidance for parameters adjustment.

Magnetic soft microfiberbots for robotic embolization.

Cerebral aneurysms and brain tumors are leading life-threatening diseases worldwide. By deliberately occluding the target lesion to reduce the blood supply, embolization has been widely used clinically to treat cerebral aneurysms and brain tumors. Conventional embolization is usually performed by threading a catheter through blood vessels to the target lesion, which is often limited by the poor steerability of the catheter in complex neurovascular networks, especially in submillimeter regions. Here, we propose magnetic soft microfiberbots with high steerability, reliable maneuverability, and multimodal shape reconfigurability to perform robotic embolization in submillimeter regions via a remote, untethered, and magnetically controllable manner. Magnetic soft microfiberbots were fabricated by thermal drawing magnetic soft composite into microfibers, followed by magnetizing and molding procedures to endow a helical magnetic polarity. By controlling magnetic fields, magnetic soft microfiberbots exhibit reversible elongated/aggregated shape morphing and helical propulsion in flow conditions, allowing for controllable navigation through complex vasculature and robotic embolization in submillimeter regions. We performed in vitro embolization of aneurysm and tumor in neurovascular phantoms and in vivo embolization of a rabbit femoral artery model under real-time fluoroscopy. These studies demonstrate the potential clinical value of our work, paving the way for a robotic embolization scheme in robotic settings.

Lattice Boltzmann Study of Microdroplet Evaporation Process in Pixel Pits.

Inkjet printing has the advantages of high material utilization, low cost, and large-area production and is a promising manufacturing technology for organic light-emitting diode (OLED) displays. However, the droplet evaporation in micron-size pixel pits is highly influenced by the pit wall. Such a process is extremely difficult to control, leading to the appearance of defects such as the coffee ring in the printing process of OLED displays. In this work, a multiphase thermal lattice Boltzmann (LB) model based on multiple distribution functions is established to study the evaporation process of micron-size droplets in pits. According to the characteristics of the largest number of the three-phase contact line (TCL) appearing in the evaporation process, the evaporation modes can be divided into three types, i.e., one, two, and three TCLs. In the 1-TCL mode, the droplet stays in constant contact radius (CCR) for the shortest time; in 2-TCL and 3-TCL modes, the liquid film fracture behavior of evaporating droplets in the pit is well captured. The effects of the pit height and the contact angle on the droplet evaporation mode are investigated in detail. The phase diagrams of evaporation modes with different parameters are also established. The revealed evaporation mechanism is supposed to be useful for regulating the droplet evaporation behavior and controlling the cured film shape in the OLED printing process.

On-skin biosensors for noninvasive monitoring of postoperative free flaps and replanted digits.

Severe soft tissue defects and amputated digits are clinically common injuries. Primary treatments include surgical free flap transfer and digit replantation, but these can fail because of vascular compromise. Postoperative monitoring is therefore crucial for timely detection of vessel obstruction and survival of replanted digits and free flaps. However, current postoperative clinical monitoring methods are labor intensive and highly dependent on the experience of nurses and surgeons. Here, we developed on-skin biosensors for noninvasive and wireless postoperative monitoring based on pulse oximetry. The on-skin biosensor was made of polydimethylsiloxane with gradient cross-linking to create a self-adhesive and mechanically robust substrate that interfaces with skin. The substrate was shown to exhibit appropriate adhesion on one side for both high-fidelity measurements of the sensor and low risk of peeling injury to delicate tissues. The other side demonstrated mechanical integrity to facilitate flexible hybrid integration of the sensor. Validation studies using a model of vascular obstruction in rats demonstrated the effectiveness of the sensor in vivo. Clinical studies indicated that the on-skin biosensor was accurate and more responsive than current clinical monitoring methods in identifying microvascular conditions. Comparisons with existing monitoring techniques, including laser Doppler flowmetry and micro-lightguide spectrophotometry, further verified the sensor's accuracy and ability to identify both arterial and venous insufficiency. These findings suggest that this on-skin biosensor may improve postoperative outcomes in free flap and replanted digit surgeries by providing sensitive and unbiased data directly from the surgical site that can be remotely monitored.

Apollo: Adaptive Polar Lattice-Based Local Obstacle Avoidance and Motion Planning for Automated Vehicles.

The motion planning module is the core module of the automated vehicle software system, which plays a key role in connecting its preceding element, i.e., the sensing module, and its following element, i.e., the control module. The design of an adaptive polar lattice-based local obstacle avoidance (APOLLO) algorithm proposed in this paper takes full account of the characteristics of the vehicle's sensing and control systems. The core of our approach mainly consists of three phases, i.e., the adaptive polar lattice-based local search space design, the collision-free path generation and the path smoothing. By adjusting a few parameters, the algorithm can be adapted to different driving environments and different kinds of vehicle chassis. Simulations show that the proposed method owns strong environmental adaptability and low computation complexity.

A high-adaptability nozzle-array printing system based on a set covering printing planning model for printed display manufacturing.

Inkjet printing technology is expected to enhance printed display mass production technology in the future. Nozzle-array printheads form the basis for printed display mass production applications. However, jet instability caused by air bubble entrapment and nozzle wettability changes during the printing process is a major challenge in the application of this technology. To adapt to possible nozzle abnormalities, a high-adaptability nozzle-array printing system based on a set covering printing planning (SCPP) model for printed display manufacturing is designed in this study. The study consists of two parts. First, a printing system based on multistep visual inspection and closed-loop feedback is proposed to accurately detect and screen abnormal nozzle positions. Notably, the inkjet printing system can identify nozzles with abnormal ejection characteristics and ensure that the remaining nozzles work accurately and stably. Then, an SCPP model is established for display pixel printing planning by using the remaining normal nozzles on the nozzle-array printhead. This model can output the most efficient printing path and nozzle printing action and can adapt to any pixel pattern, nozzle type, and abnormal nozzle distribution. The system and technology are highly adaptable and scalable for fabricating large-area printed display devices.

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.

Fin-TS and Fix-TS on fractional quaternion delayed neural networks with uncertainty via establishing a new Caputo derivative inequality approach.

This paper investigates the finite time synchronization (Fin-TS) and fixed time synchronization (Fix-TS) issues on Caputo quaternion delayed neural networks (QDNNs) with uncertainty. A new Caputo fractional differential inequality is constructed, then Fix-TS settling time of the positive definite function is estimated, which is very convenient to derive Fix-TS condition to Caputo QDNNs. By designing the appropriate self feedback and adaptive controllers, the algebraic discriminant conditions to achieve Fin-TS and Fix-TS on Caputo QDNNs are proposed based on quaternion direct method, Lyapunov stability theory, extended Cauchy Schwartz inequality, Jensen inequality. Finally, the correctness and validity of the presented results under the different orders are verified by two numerical examples.

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.

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.

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.

Monitoring the delicate operations of surgical robots via ultra-sensitive ionic electronic skin.

The arrival of surgical robots in high-end medical equipment is a landmark, and the realization of tactile sensation a major challenge in this important cutting-edge research field. Aiming to address this issue, we present ultra-sensitive ionic electronic skin in the form of flexible capacitive pressure sensors, which incorporate multistage bionic microstructures in ion gels for the purpose of monitoring the delicate operations of surgical robots. Significantly, the ionic skin exhibits an ultra-high sensitivity of 9484.3 kPa-1 (<15 kPa), and the sensitivity remains higher than 235 kPa-1 in the wide range of 15-155 kPa. The device has also achieved a detection limit as low as 0.12 Pa or, equivalently, 0.31 mg, fast response within 24 ms, and high robustness (loading/unloading for 5000 cycles without fatigue). The sensor facilitates the challenging task of tele-operated robotic threading, which exceeds the human tactile perception limit when threading a needle. We have also confirmed that ionic skin can be used in robot-assisted invasive surgery, such as incision/resection of tissues and suturing of wounds, providing tactile information to surgeons to improve operation success rates. The flexible ionic skin is capable of conforming to the various shapes of robotic manipulators, thus has great promise for applications in robotic dexterous manipulation, prosthetics and human-machine interfaces.

Multidimensional Force Sensors Based on Triboelectric Nanogenerators for Electronic Skin.

The ability to detect multidimensional forces is highly desired for electronic skin (E-skin) sensors. Here, based on single-electrode-mode triboelectric nanogenerators (S-TENGs), fully elastic E-skin that can simultaneously sense normal pressure and shear force has been proposed. With the hemispherical curve-structure design and further structural optimization, the pressure sensor exhibits a high linearity and sensitivity of 144.8 mV/kPa in the low-pressure region. By partitioning the lower tribolayer into two symmetric parts, a multidimensional force sensor has been fabricated in which the output voltage sum and ratio of the two S-TENGs can be used for normal pressure and shear force sensing, respectively. When the multidimensional force sensors are mounted at a two-fingered robotic manipulator, the change of the grabbing state can be recognized, indicating that the sensor may have great application potential in tactile sensing for robotic manipulation, human-robot interactions, environmental awareness, and object recognition.

Positioning Error Limit for the Last Droplet Deposition into a Microcavity in the Manufacture of Printed OLEDs.

In the manufacture of the emissive layer and the encapsulation layer of organic light-emitting diode panels, inkjet printing has the advantages of high material utilization, low cost, flexibility in patterning, and large-area production. Especially for emissive layer printing, the micro-pixel array brings a higher requirement of droplet positioning accuracy and volume of the liquid in a pixel. To achieve a uniform deposit morphology, several droplets are usually needed in the inkjet printing of emissive layers. As the printing process continues, these droplets coalesce, and its equilibrium outcome can be roughly approximated by a section of an ellipsoidal cap under the interaction of the surface tension and gravity. The existence of the "ellipsoidal cap" enlarges the spread, and the maximum allowable out-of-pixel spreading length is decreased because of the "ellipsoidal cap" in the neighboring pixel. In this research, the volume of fluid method is used to study the behavior of the last droplet deposition into the wetted microcavity. The effects of wettability, droplet deposition speed, and initial volume of the liquid in the pixel on the printable region are investigated, and printing parameter spaces that result in successful printing are established.

Weighted Feature Histogram of Multi-Scale Local Patch Using Multi-Bit Binary Descriptor for Face Recognition.

Most face recognition methods employ single-bit binary descriptors for face representation. The information from these methods is lost in the process of quantization from real-valued descriptors to binary descriptors, which greatly limits their robustness for face recognition. In this study, we propose a novel weighted feature histogram (WFH) method of multi-scale local patches using multi-bit binary descriptors for face recognition. First, to obtain multi-scale information of the face image, the local patches are extracted using a multi-scale local patch generation (MSLPG) method. Second, with the goal of reducing the quantization information loss of binary descriptors, a novel multi-bit local binary descriptor learning (MBLBDL) method is proposed to extract multi-bit local binary descriptors (MBLBDs). In MBLBDL, a learned mapping matrix and novel multi-bit coding rules are employed to project pixel difference vectors (PDVs) into the MBLBDs in each local patch. Finally, a novel robust weight learning (RWL) method is proposed to learn a set of robust weights for each patch to integrate the MBLBDs into the final face representation. In RWL, a codebook is first constructed by clustering MBLBDs on each local patch to extract a feature histogram. Then, considering that different parts of the face have different degrees of robustness to local changes, a set of weights is learned to concatenate the feature histograms of all local patches into the final representation of a face image. In addition, to further improve the performance for heterogeneous face recognition, a coupled WFH (C-WFH) method is proposed. C-WFH maintains the similarity of the corresponding MBLBDs and feature histograms for a pair of heterogeneous face images by means of a novel coupled feature learning (CFL) method to reduce the modality gap. A series of experiments are conducted on widely used face datasets to analyze the performance of WFH and C-WFH. Extensive experimental results show that WFH and C-WFH outperform state-of-the-art face recognition methods.