Breathable Electronic Skins for Daily Physiological Signal Monitoring

With the aging of society and the increase in people’s concern for personal health,long-term physiological signal monitoring in daily life is in demand.In recent years,electronic skin(e-skin)for daily health monitoring applications has achieved rapid development due to its advantages in high-quality physiological signals monitoring and suitability for system integrations.Among them,the breathable e-skin has developed rapidly in recent years because it adapts to the long-term and high-comfort wear requirements of monitoring physiological signals in daily life.In this review,the recent achievements of breathable e-skins for daily physiological monitoring are systematically introduced and discussed.By dividing them into breathable e-skin electrodes,breathable e-skin sensors,and breathable e-skin systems,we sort out their design ideas,manufacturing processes,performances,and applications and show their advantages in long-term physiological signal monitoring in daily life.In addition,the development directions and challenges of the breathable e-skin are discussed and prospected.

Control of GaN inverted pyramids growth on c-plane patterned sapphire substrates

Growth of gallium nitride(GaN)inverted pyramids on c-plane sapphire substrates is benefit for fabricating novel devices as it forms the semipolar facets.In this work,GaN inverted pyramids are directly grown on c-plane patterned sapphire substrates(PSS)by metal organic vapor phase epitaxy(MOVPE).The influences of growth conditions on the surface morphol-ogy are experimentally studied and explained by Wulff constructions.The competition of growth rate among{0001},{1011},and{1122}facets results in the various surface morphologies of GaN.A higher growth temperature of 985 ℃ and a lowerⅤ/Ⅲratio of 25 can expand the area of{}facets in GaN inverted pyramids.On the other hand,GaN inverted pyramids with almost pure{}facets are obtained by using a lower growth temperature of 930℃,a higherⅤ/Ⅲratio of 100,and PSS with pattern arrangement perpendicular to the substrate primary flat.

Multistable Mechanical Metamaterials:A Brief Review

Over the past decade,multistable mechanical metamaterials have been widely investigated because of their novel shape reconfigurability and programmable energy landscape.The ability to reversibly reshape among diverse stable states with different energy levels represents the most important feature of the multistable mechanical metamaterials.We summarize main design strategies of multistable mechanical metamaterials,including those based on self-assembly scheme,snap-through instability,structured mechanism and geometrical frustration,with a focus on the number and controllability of accessible stable states.Then we concentrate on unusual mechanical properties of these multistable mechanical metamaterials,and present their applications in a wide range of areas,including tunable electromagnetic devices,actuators,robotics,and mechanical logic gates.Finally,we discuss remaining challenges and open opportunities of designs and applications of multistable mechanical metamaterials.

Flexible inorganic bioelectronics

Flexible inorganic bioelectronics represent a newly emerging and rapid developing research area.With its great power in enhancing the acquisition,management and utilization of health information,it is expected that these flexible and stretchable devices could underlie the new solutions to human health problems.Recent advances in this area including materials,devices,integrated systems and their biomedical applications indicate that through conformal and seamless contact with human body,the measurement becomes continuous and convenient with yields of higher quality data.This review covers recent progresses in flexible inorganic bio-electronics for human physiological parameters’monitoring in a wearable and continuous way.Strategies including materials,structures and device design are introduced with highlights toward the ability to solve remaining challenges in the measurement process.Advances in measuring bioelectrical signals,i.e.,the electrophysiological signals(including EEG,ECoG,ECG,and EMG),biophysical signals(including body temperature,strain,pressure,and acoustic signals)and biochemical signals(including sweat,glucose,and interstitial fluid)have been summarized.In the end,given the application property of this topic,the future research directions are outlooked.

Recent progress of morphable 3D mesostructures in advanced materials

Soft robots complement the existing efforts of miniaturizing conventional,rigid robots,and have the potential to revolutionize areas such as military equipment and biomedical devices.This type of system can accomplish tasks in complex and time-varying environments through geometric reconfiguration induced by diverse external stimuli,such as heat,solvent,light,electric field,magnetic field,and mechanical field.Approaches to achieve reconfigurable mesostructures are essential to the design and fabrication of soft robots.Existing studies mainly focus on four key aspects:reconfiguration mechanisms,fabrication schemes,deformation control principles,and practical applications.This review presents a detailed survey of methodologies for morphable mesostructures triggered by a wide range of stimuli,with a number of impressive examples,demonstrating high degrees of deformation complexities and varied multi-functionalities.The latest progress based on the development of new materials and unique design concepts is highlighted.An outlook on the remaining challenges and open opportunities is provided.

Subsurface damage and bending strength analysis for ultra-thin and flexible silicon chips

Subsurface damage(SSD) is an unavoidable problem in the precision mechanical grinding for preparing ultra-thin and flexible silicon chips. At present, there are relatively few studies on the relationship between SSD and the bending strength of ultra-thin chips under different grinding parameters. In this study, SSD including amorphization and dislocation is observed using a transmission electron microscope. Theoretical predictions of the SSD depth induced by different processing parameters are in good agreement with experimental data. The main reasons for SSD depth increase include the increase of grit size, the acceleration of feed rate, and the slowdown of wheel rotation speed. Three-point bending test is adopted to measure the bending strength of ultra-thin chips processed by different grinding conditions. The results show that increasing wheel rotation speed and decreasing grit size and feed rate will improve the bending strength of chips, due to the reduction of SSD depth. Wet etching and chemical mechanical polishing(CMP) are applied respectively to remove the SSD induced by grinding, and both contribute to providing a higher bending strength, but in comparison, CMP works better due to a smooth surface profile. This research aims to provide some guidance for optimizing the grinding process and fabricating ultra-thin chips with higher bending strength.

Hydrogen bond modulation in 1,10-phenanthroline derivatives for versatile electron transport materials with high thermal stability,large electron mobility and excellent n-doping ability

4,7-Bisphenyl-1,10-phenanthroline(BPhen)is a promising electron transport material(ETM)and has been widely used in organic light-emitting diodes(OLEDs)because of the large electron mobility and easy fabrication process.However,its low glass transition temperature would lead to poor device stability.In the past decades,various attempts have been carried out to improve its thermal stability though always be accomplished by the reduced electron mobility.Here,we present a molecular engineering to modulate the properties of BPhen,and through which,a versatile BPhen derivative(4,7-bis(naphthaleneb-yl)-1,10-phenanthroline,b-BNPhen)with high thermal stability(glass transition temperature=111.9℃),large electron mobility(7.8×10-4 cm2/(V s)under an electrical field of 4.5×105 V/cm)and excellent n-doping ability with an air-stable metal of Ag is developed and used as multifunctional layers to improve the efficiency and enhance the stability of OLEDs.This work elucidates the great importance of our molecular engineering methodology and device structure optimization strategy,unlocking the potential of 1,10-phenanthroline derivatives towards practical applications.

Recent progress in three-dimensional flexible physical sensors

Three-dimensional(3D)functional systems are of rapidly growing interest over the past decade,from the perspective of both the fundamental and applied research.In particular,tremendous efforts have been devoted to the developments of 3D flexible,physical sensors,partly because of their substantial advantages over planar counterparts in many specific performances.In this review,we summarize recent advances in diverse categories of 3D flexible physical sensors,covering the photoelectric,mechanical,temperature,magnetic,and other physical sensors.This review mainly focuses on their design strategies,working principles and applications.Finally,we offer an outlook on the future developments,and provide perspectives on the remaining challenges and opportunities in this area.

Tunable seesaw-like 3D capacitive sensor for force and acceleration sensing

To address the resource-competing issue between high sensitivity and wide working range for a stand-alone sensor,development of capacitive sensors with an adjustable gap between two electrodes has been of growing interest.While several approaches have been developed to fabricate tunable capacitive sensors,it remains challenging to achieve,simultaneously,a broad range of tunable sensitivity and working range in a single device.In this work,a 3D capacitive sensor with a seesaw-like shape is designed and fabricated by the controlled compressive buckling assembly,which leverages the mechanically tunable configuration to achieve high-precision force sensing(resolution~5.22 nN)and unprecedented adjustment range(by~33 times)of sensitivity.The mechanical tests under different loading conditions demonstrate the stability and reliability of capacitive sensors.Incorporation of an asymmetric seesaw-like structure design in the capacitive sensor allows the acceleration measurement with a tunable sensitivity.These results suggest simple and low-cost routes to high-performance,tunable 3D capacitive sensors,with diverse potential applications in wearable electronics and biomedical devices.

Implementing fractional Fourier transform using SH_(0) wave computational metamaterials in space domain

Through the design of artificial structures, metamaterials can control the incident wave in the sub-wavelength range to achieve functions that natural materials cannot achieve, such as high absorption, negative refraction, cloaking [1–3],asymmetric propagation [4,5], and holography [6–8]. In recent years, the new concept of computational metamaterials[9–12] has opened up a new direction for analog computing。

Distributed parameter circuit model for wideband photodiodes with inductive coplanar waveguide electrodes

An equivalent circuit model including multi-section distributed parameters is proposed to analyze wideband photodiodes(PDs)with coplanar waveguide(CPW)electrodes.The model helps extract CPW parameters as well as intrinsic bandwidth parameters so that the influence of theCPW structure can be investigated,making it valuable for the design of high-performance PDs.PDs with an inductive 115Ωimpedance CPW are fabricated,and the 3 dB bandwidth is improved from 28 GHz to 37.5 GHz compared with PDs with a conventional 50Ωimpedance CPW.

Implementing fractional Fourier transform and solving partial differential equations using acoustic computational metamaterials in space domain

Metamaterials can control incident waves in the sub-wavelength range through the design of artificial structures, and realize the functions that natural materials cannot achieve. The study of metamaterials has important theoretical value and application prospects. In recent years, the proposal of computational metamaterials has opened up a brand-new direction for analog computing, providing high-throughput, energy-free computing methods for special computing tasks. However, the development of acoustic computing metamaterials is relatively preliminary, and it is necessary to develop design theories. There is no work to solve partial differential equations and realize fractional Fourier transform in spatial domain acoustic computing metamaterials. In this paper, the acoustic wave computational metamaterial is designed, and the simulation realizes the spatial domain fractional Fourier transform and partial differential equation calculation. It is expected that acoustic computational metamaterials will enable new capabilities in signal acquisition and processing, network computing, and drive new applications of sound wave.

Emerging Applications of Optical Fiber‑Based Devices for Brain Research

Research in neuroscience and neuroengineering has attracted tremendous interest in the past decades.However,the complexity of the brain tissue,in terms of its structural,chemical,mechanical,and optical properties,makes the interrogation of biophysical and biochemical signals within the brain of living animals extremely challenging.As a viable and versatile tool for brain studies,optical fber based technologies have provided exceptional opportunities to unravel the mysteries of the brain and open the door for clinical applications in the treatment,diagnosis,and prevention of neurological diseases.Typically,optical fbers with diameters from 10 to 1000μm are capable of guiding and delivering light to deep levels of the living tissue.Moreover,small dimensions of such devices along with their fexibility and light weight paved the way for understanding the complex behaviours of living and freely moving mammals.This article provides a review of the emerging applications of optical fbers in neuroscience,specifcally in the mammalian brain.Representative utilities,including optogenetics,fuorescence sensing,drug administration and phototherapy,are highlighted.We also discuss other biological applications of such implantable fbers,which may provide insights into the future study of brain.It is envisioned that these and other optical fber based techniques ofer a powerful platform for multi-functional neural activity sensing and modulation.

Suppressing Competitive Coordination Reaction for Ohmic Cathode Contact Using Amino-Substituted Organic Ligands and Air-Stable Metals

Ohmic cathode contact can be formed readily via coordination-activated n-doping(CAN),by co-evaporating air-stable metals(e.g.,silver)and organic ligands with coordination sites.It has been proposed that increasing the nucleophilicity of the main binding site of a ligand is essential for reducing the work function of the doped films.

Skin-integrated,biocompatible,and stretchable silicon microneedle electrode for long-term EMG monitoring in motion scenario

Electromyography(EMG)signal is the electrical potential generated by contracting muscle cells.Long-term and accurate EMG monitoring is desirable for neuromuscular function assessment in clinical and the human–computer interfaces.Herein,we report a skin-integrated,biocompatible,and stretchable silicon microneedle electrode(SSME)inspired by the plant thorns.The silicon microneedles are half encapsulated by the polyimide(PI)to enhance the adaptability to deformation and resistance to fatigue.Thorn-like SSME is realized by the semi-additive method with a stretchability of not less than 36%.The biocompatibility of SSME has been verified using cytotoxicity tests.EMG monitoring in motion and long-term has been conducted to demonstrate the feasibility and performance of the SSME,which is compared with a commercial wet electrode.Hopefully,the strategies reported here can lead to accurate and long-term EMG monitoring,facilitating an effective and reliable human–computer interface.

Negative Charge Management to Make Fragile Bonds Less Fragile toward Electrons for Robust Organic Optoelectronic Materials

The operational stability of organic(opto)electronic devices largely depends on the intrinsic stability of organic materials on service.For organic light-emitting diode(OLED)materials,a key parameter of their intrinsic stability is the bond-dissociation energy of the most fragile bond(BDE_(f)).Although rarely involved,many OLED molecules have the lowest BDE_(f) in anionic states[BDE_(f)(−)∼1.6–2.5 eV],which could be a fatal short-slab for device stability.Herein,we separated BDE_(f)(−)from other parameters and confirmed the clear relationship between BDE_(f)(−),intrinsic material stability and device lifetime.Based on thermodynamic principles,we developed a general and effective strategy to greatly improve BDE_(f)(−)by introducing a negative charge manager within the molecule.The manager must combine an electron-withdrawing group(EWG)with a delocalizing structure,so that it can firmly confine the negative charge and hinder the charge redistribution toward fragile bonds.Consequently,the use of this manager can substantially promote BDE_(f)(−)by∼1 eV for various fragile bonds and outperform the effect reported from solely employing EWGs or delocalizing structures.This effect was verified in typical phosphine-oxide and carbazole derivatives and backed up by newly designed molecules with multiple fragile bonds.This strategy provides a new way to transform vulnerable building blocks into robust organic(opto)electronic materials and devices.

Novel implantable devices delivering electrical cues for tissue regeneration and functional restoration

Millions of people suffer from tissue diseases and organ dysfunction,such as bone or nerve defects,spinal cord injuries and arrhythmia,which often leads to morbidity and disability.Electrical stimulation as a promising nonpharmacological technique has been proven to be effective in promoting tissue regeneration and functional restoration.Nevertheless,existing clinical electrical therapies are often limited to intraoperative window or percutaneous stimulation that suffer from insufficient time frame and potential infection risks.To overcome these challenges,innovative electrical stimulation implants with miniaturized,self-powered,flexible or biodegradable features have been proposed.This review summarizes recent advances of novel materials strategies and device schemes for tissue regeneration and/or functional restoration of bones,nerves,gastrointestinal tracts,cardiac systems,etc.Insights on future directions of electrical stimulation devices are given at the end.

Morphable three-dimensional electronic mesofliers capable of on-demand unfolding

Development of miniaturized three-dimensional(3 D)fliers with integrated functional components has important implications to a diverse range of engineering areas.Among the various active and passive miniaturized 3 D fliers reported previously,a class of 3 D electronic fliers inspired by wind-dispersed seeds show promising potentials,owing to the lightweight and noiseless features,aside from the stable rotational fall associated with a low falling velocity.While on-demand shape-morphing capabilities are essential for those 3 D electronic fliers,the realization of such miniaturized systems remains very challenging,due to the lack of fast-response 3 D actuators that can be seamlessly integrated with 3 D electronic fliers.Here we develop a type of morphable3 D mesofliers with shape memory polymer(SMP)-based electrothermal actuators,capable of large degree of actuation deformations,with a fast response(e.g.,~1 s).Integration of functional components,including sensors,controllers,and chip batteries,enables development of intelligent 3 D mesoflier systems that can achieve the on-demand unfolding,triggered by the processing of real-time sensed information(e.g.,acceleration and humidity data).Such intelligent electronic mesofliers are capable of both the low-air-drag rising and the low-velocity falling,and thereby,can be used to measure the humidity fields in a wide 3 D space by simple hand throwing,according to our demonstrations.The developed electronic mesofliers can also be integrated with other types of physical/chemical sensors for uses in different application scenarios.

Imperfection sensitivity of mechanical properties in soft network materials with horseshoe microstructures

Developments of soft network materials with rationally distributed wavy microstructures have enabled many promising applications in bio-integrated electronic devices,due to their abilities to reproduce precisely nonlinear mechanical properties of human tissues/organs.In practical applications,the soft network materials usually serve as the encapsulation layer and/or substrate of bio-integrated electronic devices,where deterministic holes can be utilized to accommodate hard chips,thereby increasing the filling ratio of the device system.Therefore,it is essential to understand how the hole-type imperfection affects the stretchability of soft network materialswith various geometric constructions.Thiswork presents a systematic investigation of the imperfection sensitivity of mechanical properties in soft network materials consisting of horseshoe microstructures,through combined computational and experimental studies.A factor of imperfection insensitivity of stretchability is introduced to quantify the influence of hole imperfections,as compared to the case of perfect soft network materials.Such factor is shown to have different dependences on the arc angle and normalized width of horseshoe microstructures for triangular network materials.The soft triangular and Kagome network materials,especially with the arc angle in the range of(30?,60?),are found to be much more imperfection insensitive than corresponding traditional lattice materials with straight microstructures.Differently,the soft honeycomb network materials are not as imperfection insensitive as traditional honeycomb lattice materials.

Optoelectronic sensing of biophysical and biochemical signals based on photon recycling of a micro-LED

Conventional bioelectrical sensors and systems integrate multiple power harvesting,signal amplification and data transmission components for wireless biological signal detection.This paper reports the real-time biophysical and biochemical activities can be optically captured using a microscale light-emitting diode(micro-LED),eliminating the need for complicated sensing circuit.Such a thin-film diode based device simultaneously absorbs and emits photons,enabling wireless power harvesting and signal transmission.Additionally,owing to its strong photon-recycling effects,the micro-LED^photoluminescence(PL)emission exhibits a superlinear dependence on the external conductance.Taking advantage of these unique mechanisms,instantaneous biophysical signals including galvanic skin response,pressure and temperature,and biochemical signals like ascorbic acid concentration,can be optically monitored,and it demonstrates that such an optoelectronic sensing technique outperforms a traditional tethered,electrically based sensing circuit,in terms of its footprint,accuracy and sensitivity.This presented optoelectronic sensing approach could establish promising routes to advanced biological sensors.