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Bowel sounds (BSs) have important clinical value in the auxiliary diagnosis of digestive diseases, but due to the inconvenience of long-term monitoring and too much interference from environmental noise, they have not been well studied. Most of the current electronic stethoscopes are hard and bulky without the function of noise reduction, and their application for long-term wearable monitoring of BS in noisy clinical environments is very limited. In this paper, a flexible dual-channel digital auscultation patch with active noise reduction is designed and developed, which is wireless, wearable, and conformably attached to abdominal skin to record BS more accurately. The ambient noise can be greatly reduced through active noise reduction based on the adaptive filter. At the same time, some nonstationary noises appearing intermittently (e.g., frictional noise) can also be removed from BS by the cross validation of multichannel simultaneous acquisition. Then, two kinds of typical BS signals are taken as examples, and the feature parameters of the BS in the time domain and frequency domain are extracted through the time-frequency analysis algorithm. Furthermore, based on the short-term energy ratio between the four channels of dual patches, the two-dimensional localization of BS on the abdomen mapping plane is realized. Finally, the continuous wearable monitoring of BS for patients with postoperative ileus (POI) in the noisy ward from pre-operation (POD0) to postoperative Day 7 (POD7) was carried out. The obtained change curve of the occurrence frequency of BS provides guidance for doctors to choose a reasonable feeding time for patients after surgery and accelerate their recovery. Therefore, flexible dual-channel digital auscultation patches with active noise reduction will have promising applications in the clinical auxiliary diagnosis of digestive diseases.

Advanced capabilities in monitoring sunlight and temperature, significantly associated with human health, are essential for avoiding diseases (e.g., skin cancer), treating neonatal jaundice, and regulating the human circadian rhythm. Here, we show an ultrathin flexible inorganic device integrated with photodetectors across wide ranges of optical wavelengths from ultraviolet (UV) to infrared (IR) and a temperature sensor for simultaneous long-term monitoring of light intensity and temperature during the daytime. The flexible device is fabricated by nanodiamond thinning technology, and the performances are demonstrated by continuously monitoring light intensity and temperature in the daytime from 8 A.M. to 5 P.M. As a result, the ambient parameters of the human living environment are provided by the proposed device, which has a great potential in clinical application and provides suggestions of sunlight exposure for regular users.

Hypoxia refers to insufficient oxygen amounts at the tissue level unable to maintain adequate homeostasis. Severe hypoxia may occur in the absence of subjective breathlessness due to respiratory failure. Precise monitoring of low blood oxygen saturation is crucially desired, catering to the clinical requirements. However, current pulse oximeters cannot function well in monitoring peripheral oxygen saturation limited by the weak peripheral blood circulation at a low oxygen level. In this work, we propose a flexible hybrid electronic (FHE) with a compact structure and high sensitivity for conveniently monitoring hypoxia. This FHE is composed of 10-µm thickness semiconductors with different materials, functionalities, and sizes. Its performance is demonstrated by monitoring arterial blood oxygen saturation (SaO2) at the body's different arteries. The absolute error is less than 2% within a SaO2 ranging from 99% to 63%. The efficient techniques presented in this work may bring light to the next-generation flexible hybrid electronics and provide potential widespread use in research and clinical applications, especially for emergency treatment.

Nowadays, controllable drug release is a vitally important strategy for cancer treatment and usually realized using implanting biocompatible devices. However, these devices need to be removed by another surgery after the function fails, which brings the risks of inflammation or potential death. In this article, a biodegradable flexible electronic device with controllable drug (paclitaxel) release was proposed for cancer treatment. The device is powered by an external alternating magnetic field to generate internal resistance heat and promote drug release loaded on the substrate. Moreover, the device temperature can even reach to 65 °C, which was sufficient for controllable drug release. This device also has similar mechanical properties to human tissues and can autonomously degrade due to the structure design of the circuit and degradable compositions. Finally, it is confirmed that the device has a good inhibitory effect on the proliferation of breast cancer cells (MCF-7) and could be completely degraded in vitro. Thus, its great biodegradability and conformity can relieve patients of second operation, and the device proposed in this paper provides a promising solution to complete conquest of cancer in situ.

Advances in materials and mechanics designs have led to the development of flexible electronics, which have important applications to human healthcare due to their good biocompatibility and conformal integration with biological tissue. Material innovation and mechanics design have played a key role in designing the substrates and encapsulations of flexible electronics for various bio-integrated systems. This review first introduces the inorganic materials and novel organic materials used for the substrates and encapsulation of flexible electronics, and summarizes their mechanics properties, permeability and optical transmission properties. The structural designs of the substrates are then introduced to ensure the reliability of flexible electronics, including the patterned and pre-strained designs to improve the stretchability, and the strain-isolation and -limiting substrates to reduce the deformation. Some emerging encapsulations are presented to protect the flexible electronics from degradation, environmental erosion or contamination, though they may slightly reduce the stretchability of flexible electronics.

Flexible electronics have brought considerable benefits to healthcare advancement by their powerful functions of mimicking human skin to quantitatively transform external stimuli into electronic si...

According to the statistics of the World Health Organization, an estimated 17.9 million people die from cardiovascular diseases each year, representing 31% of all global deaths. Continuous non-invasive arterial pressure (CNAP) is essential for the management of cardiovascular diseases. However, it is difficult to achieve long-term CNAP monitoring with the daily use of current devices due to irritation of the skin as well as the lack of motion artifacts suppression. Here, we report a high-performance skin-like optoelectronic system integrated with ultra-thin flexible circuits to monitor CNAP. We introduce a theoretical model via the virtual work principle for predicting the precise blood pressure and suppressing motion artifacts, and propose optical difference in the frequency domain for stable optical measurements in terms of skin-like devices. We compare the results with the blood pressure acquired by invasive (intra-arterial) blood pressure monitoring for >1500 min in total on 44 subjects in an intensive care unit. The maximum absolute errors of diastolic and systolic blood pressure were ±7/±10 mm Hg, respectively, in immobilized, and ±10/±14 mm Hg, respectively, in walking scenarios. These strategies provide advanced blood pressure monitoring techniques, which would directly address an unmet clinical need or daily use for a highly vulnerable population., We demonstrate skin-like optoelectronic systems that can monitor continuous non-invasive arterial pressure, which would directly address a clinical need or daily use for the population.

Next‐generation robots are being designed to function autonomously in complex and unstructured environments. In particular, based on the real‐time measurement and differentiation of normal pressure and shear force, robots can be equipped with the capabilities of damage‐free grasp within minimum force limits, as well as dexterous operation through surface roughness and slip information. Herein, a flexible tactile sensor with a small cylinder protrusion and four arc‐shaped protrusions is developed. Due to its center symmetry and axisymmetry characteristics, the normal pressure and shear force can be decoupled from the complex stress without any interference from torsion. The flexible tactile sensor exhibits good linearity and superior cycling stability and is capable of determining the magnitude and direction of the applied force accurately. The flexible tactile sensor is comfortable to wear, and it is integrated onto the manipulator to realize various delicate and dexterous tasks, such as pressure detection, interaction with fragile objects, and roughness identification. Moreover, intelligent recognition of the sliding and stationary states can be achieved by decoding signals of sliding friction and static friction from the feedback information, leading to real time and precise adjustment of the grasping state of the manipulator.

The real-time characterization of thin film properties can provide insights into the behavior of film material during process such as phase-transition, hydration, and chemical reaction. The shift of reasonant frequency in structural vibration serves as the basis of an effective approach to determine film properties, but encounters the difficulty that multiple to-be-determined quantites (e.g., film modulus and density) are often related to the resonant frequency simultaneously and therefore cannot be determined by a structure with fixed shape and vibration mode. Determinsitic mechanical buckling provides an effective route for the vibrational structure to rapidly switch between designed shapes and vibration modes. Here, we adopt a ribbon structure in the flat state and buckled state to yield two distinct vibration modes. Theoretical models of the natural frequencies are established for first-order out-of-plane modes of the ribbon with patterned thin films in these two states, respectively. The model suggests that with optimized film pattern the sensitivity of the natural frequencies to the film modulus and density can be partially decoupled. The results lead to a simple and effective method based on tunable vibration to characterize the thin film modulus and density at small scale.

Description, A flexible device is capable of simultaneous acoustic energy transfer and communication, for implantable medical application., The use of implantable medical devices, including cardiac pacemakers and brain pacemakers, is becoming increasingly prevalent. However, surgically replacing batteries owing to their limited lifetime is a drawback of those devices. Such an operation poses a risk to patients—a problem that, to date, has not yet been solved. Furthermore, current devices are large and rigid, potentially causing patient discomfort after implantation. To address this problem, we developed a thin, battery-free, flexible, implantable system based on flexible electronic technology that can not only achieve wireless recharging and communication simultaneously via ultrasound but also perform many current device functions, including in vivo physiological monitoring and cardiac pacing. To prove this, an animal experiment was conducted involving creating a cardiac arrest model and powering the system by ultrasound. The results showed that it automatically detected abnormal heartbeats and responded by electrically stimulating the heart, demonstrating the device’s potential clinical utility for emergent treatment.

Cement is one of the primary barriers in a wellbore and critical to well integrity. Radial cracking is a pervasive failure mode in cement due to the temperature and pressure variation during drilling, completion, or production. This work presents a comprehensive analysis of radial cracking in cement under various loading events. The proposed model estimates the stress intensity factor and fracture surface displacement as indicators for crack propagation and opening, respectively, through a distributed dislocation technique. Three types of radial cracks, divided by their tips terminating at the casing–cement interface, inside cement, or at the cement–formation interface, are considered. Based on this model, we conduct a parametric study for radial cracking under typical loading events such as steam injection, CO2 injection, and high-pressure and high-temperature (HPHT) drilling. Results indicate that the crack near the casing–cement interface has an increased risk for steam injection and HPHT drilling, while all three types of radial cracks are destructive during CO2 injection. The thermal expansion coefficient of cement is a significant parameter for steam and CO2 injection wells. The fluid pressure and the cement’s thickness are crucial to radial cracking under HPHT conditions. Stiffer cement could promote crack opening for steam injection but prohibit the crack deformation for CO2 injection or HPHT wells. Thicker cement would accelerate radial cracking under the three loading events. These findings are helpful in designing cement to maintain long-term integrity.

Flexible pressure sensors have attracted considerable attention because of their potential applications in healthcare monitoring and human-machine interactions. However, the complicated fabrication process and the cos of sensing materials limit their widespread applications in practice. Herein, a flexible pressure sensor with outstanding performances is presented through an extremely simple and cost-efficient fabrication process. The sensing materials of the sensor are based on low-cost carbon black (CB)@airlaid paper (AP) composites, which are just prepared by drop-casting CB solutions onto APs. Through simply stacking multiple CB@APs with an irregular surface and a fiber-network structure, the obtained pressure sensor demonstrates an ultrahigh sensitivity of 51.23 kPa-1 and an ultralow detection limit of 1 Pa. Additionally, the sensor exhibits fast response time, wide working range, good stability, as well as excellent flexibility and biocompatibility. All the comprehensive and superior performances endow the sensor with abilities to precisely detect weak air flow, wrist pulse, phonation, and wrist bending in real time. In addition, an array electronic skin integrated with multiple CB@AP sensors has been designed to identify spatial pressure distribution and pressure magnitude. Through a biomimetic structure inspired by blooming flowers, a sensor with the open-petal structure has been designed to recognize the wind direction. Therefore, our study, which demonstrates a flexible pressure sensor with low cost, simple preparation, and superior performances, will open up for the exploration of cost-efficient pressure sensors in wearable devices.

The conformability of epidermal electronics bonded onto bio-tissue is the key element to realize long-term and real-time monitoring physiological characteristics of human body. Currently, mechanics models of the epidermal electronics conforming to the bio-tissue are only applicable for the epidermal electronics whose thickness is larger than or close to the amplitude parameter of the bio-tissue (i.e., infinitesimal deflection theory). But an analytical model with moderately large deflection theory is proposed in this paper to predict the fully conformal, partially conformal and fully non-conformal states between the epidermal electronics and the bio-tissue through energy minimization, which combines the effects of epidermal electronics and bio-tissue geometries, work of adhesion and modulus of epidermal electronics. The model is verified by the experimental observations, and thereby can provide some guidance to a general design strategy for epidermal electronics bonded onto bio-tissue.

Wrinkling and buckling of stiff film/soft substrate system has great potential applications in many areas, especially in stretchable electronics. Inspired by the micro-patterns to tune the interfacial adhesion strength, a novel way to tune three buckling configurations (i.e., wrinkling, upward and downward buckling) by using the micro-patterns on the soft substrate was found experimentally. The established governing equations to distinguish the three buckling configurations are consistent with the experimental observations. Results show that the geometries of the micro-patterns, the material properties and the interfacial adhesion between the stiff film and the micro-patterned soft substrate can tremendously influence the final buckling configurations. The proposed analytical governing equations can not only clarify the mechanism of the micro-patterns on the tuning of different buckling configurations, but also be used to design the buckling configurations as desired.

Skin-interfaced medical devices are critically important for diagnosing disease, monitoring physiological health and establishing control interfaces with prosthetics, computer systems and wearable robotic devices. Skin-like epidermal electronic technologies can support these use cases in soft and ultrathin materials that conformally interface with the skin in a manner that is mechanically and thermally imperceptible. Nevertheless, schemes so far have limited the overall sizes of these devices to less than a few square centimetres. Here, we present materials, device structures, handling and mounting methods, and manufacturing approaches that enable epidermal electronic interfaces that are orders of magnitude larger than previously realized. As a proof-of-concept, we demonstrate devices for electrophysiological recordings that enable coverage of the full scalp and the full circumference of the forearm. Filamentary conductive architectures in open-network designs minimize radio frequency-induced eddy currents, forming the basis for structural and functional compatibility with magnetic resonance imaging. We demonstrate the use of the large-area interfaces for the multifunctional control of a transhumeral prosthesis by patients who have undergone targeted muscle-reinnervation surgery, in long-term electroencephalography, and in simultaneous electroencephalography and structural and functional magnetic resonance imaging.

Digital image correlation (DIC) method has been widely used for measurement in high temperature environment, where speckle pattern that can withstand high temperature and maintain good stability is required. In this work we develop a simple method to fabricate speckle pattern for high temperature application. The curing temperature of the speckle is controlled under 350 °C to avoid potential damage to the substrate material. The speckle can resist temperature up to 1050 °C for minimum 0.5 h. Three point bending experiment was carried out at 700 °C to test the speckle. Results show that this high temperature speckle exhibits excellent adhesion to the substrate. This non-invasive speckle shows great potential for improving the measurement reliability at high temperature in engineering application.

Three-dimensional (3D) porous nanostructure materials have promising applications in pressure sensors or other situations. However, the low sensing sensitivity of these materials restricts precise detection of physiological signals, and it is still a challenge to manufacture highly pressure-sensitive materials, which simultaneously possess other versatile properties. Herein, a simple and cost-efficient strategy is proposed to fabricate versatile 3D carbon nanofiber networks (CNFNs) with superior pressure-sensitivity through electrospinning and thermal treatment. The pressure sensitivity of the CNFNs is 1.41 kPa−1, which is much higher than that of similar 3D porous materials. Unlike traditional carbonaceous materials, the CNFNs exhibit excellent flexibility, stable resilience, and super compressibility (>95%), because of the nano-reinforce of Al2O3. Benefiting from the robust mechanical and piezoresistive properties of the CNFNs, a pressure sensor designed with the CNFNs is able to monitor human physiological signals, such as phonation, pulse, respiration and human activities. An arch-array platform for direction identification of tangential forces and an artificial electronic skin bioinspired by human's hairy skin have been ingeniously designed. The CNFNs also present other versatile characteristics as well, including ultralight density, hydrophobicity, low thermal conductivity, and low infrared emissivity. Therefore, the CNFNs have promising potential in a wide range of applications.

Direct-current generators, especially those based on the Schottky contact between conductive polymers and metal electrodes, are efficient in converting mechanical stimuli into electrical energy. In contrast to triboelectric and piezoelectric generators, direct-current generators readily produce direct-current outputs and high currents that are crucial for integrating multiple energy-harvesting units in large scale and driving some types of devices. To further increase the current/voltage output of direct-current generator, here, we systematically investigate the effects of various conductive polymers and electrodes on the outputs by both computations and experiments. Particularly, the direct-current generator based on Cu/PANI/Ag is found to exhibit overwhelmingly current and voltage outputs over previous ones (current: 800 µA, 4 times larger; voltage: 3.5 V, 5 times larger.) under the same mechanical deformation. The preparation method of electrochemical deposition endows the generators flexibility, the linear relationship of current/voltage output vs. strain applied on the generators, combined with the large outputs offer advantages for the generator to work as flexible sensors. Furthermore, a mechanosensation-active matrix array based on direct-current generator for the strain monitoring demonstrated its promising prospects in flexible electronics. The direct-current generators with improved performance could serve as a stream new blood for versatile sensory systems and human-machine interactive interfaces.

An extensive range of nanocarbon, such as OD fullerene, 1D carbon nanotube, 2D graphene, and 3D carbon aerogels, has attracted significant attention. Graphene, being an outstanding electrode material has already been evidenced in its performance for fabricating supercapacitors. Direct laser writing was reported to be an innovative methodology that enabled patterning graphene electrodes within a single-step. Laser scribed graphene (LSG) is produced on polyimide (PI) substrate, which is not stretchable. Most researchers report transferring LSG from PI onto the stretchable substrate to fabricate stretchable LSG sensors and supercapacitors. Transfer printing is a critical procedure for LSG embedding on a stretchable substrate. But no one has indeed investigated the transfer printing of LSG. In this work, we report a laser-assisted method to improve the success rate of transfer printing. The resulting LSG/PDMS is utilized in fabricating the in-plane stretchable supercapacitor. Its current density was improved from 0.02 to 0.6 m.A/cm2 with the rising scan rate from 10 to 200 mV/s. The highest capacitance was calculated as 18 mF/cm2 at the consistent current density of 0.02 ma/cm2, Also, the LSG supercapacitor showed excellent cycling stability after 1000 cycles. The laser-assisted strategy has permitted the successful promises for developing LSG stretchable wearable electronics.

As the processing engines for the flexible electronics, ultrathin chips have received extensive attention in recent years. This paper studies the fabrication of the ultrathin silicon chips by thinning and grinding, in which temporary support, release process, pick-up method and transfer technology have been recreated to thin the chips of different substrates. The electrical and mechanical properties of the prepared ultra-thin chip were characterized, and its performance did not significantly decrease compared with that before thinning.

Recent advances in the material and structural design of skin‐like electronics have enhanced the interactions between the virtual and physical worlds and between people and objects. Herein, the existing flexible sensing materials and microstructures, including flexible stimuli‐responsive materials and response‐ and stretchability‐enhanced microstructures, are reviewed. Five typical skin‐like electronics with sensitivities analogous to the human senses (olfactory, visual, auditory, tactile, and gustatory senses) and their corresponding applications (gas, light, chemical composition, sound, and mechanical signal monitoring) are introduced. Finally, it is concluded with some perspectives on challenges and opportunities for future research in flexible hybrid electronics.

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This paper studies the failure mode and failure mechanism of the underfill fillet cracks of a flexible surface-mount device, where the device is produced by mounting the stiff components directly onto the surface of a flexible printed circuit board; the underfill encapsulants are used to reduce the stresses around the solder joints. The underfill fillet cracks are observed in failed devices by the scanning electron microscope. The mechanics behind how the underfill geometry and the material property affect the reliability of the device is investigated by studying the stress singularity near the interfacial edge of the underfill fillet/flexible substrate. It is found that there exists a strong stress concentration near the interfacial edge of the underfill fillet/flexible substrate, due to the large mechanical and geometry mismatch between the underfill fillet and the flexible substrate. The results present the method to modulate the level of stress singularity by changing the wedge angle of the underfill fillet and the material property of the underfill encapsulants.

In this paper, an effective general method is proposed to determine the optimal virtual fields and extract the elastic constants and the thermal expansion coefficients of orthotropic composite materials from the coupled thermo-mechanical deformation fields simultaneously. First, a practical analysis procedure is implemented based on the assumed material parameters with orthotropic in-plane properties to determine the effective virtual fields for three-point bending test configuration. Six thermo-mechanical parameters of three kinds of orthotropic composite materials are decoupled and identified by the determined effective virtual fields to verify its independence on material parameters. Second, the influences of specimen dimensions and external loadings on identified material parameters are investigated to validate the applicability of effective virtual fields under different test configurations. Finally, the optimal virtual fields are determined by means of a numerical method and its capacity for decreasing the noise influence is investigated quantitatively. The research work will provide an effective method to determine the optimal virtual fields for decoupling the thermo-mechanical coupled deformation of orthotropic composite materials.

Twining plants, using their special attachment structure to cling to a support and grow upward to capture more sunshine without losing stability, have fascinated many scholars. A mechanics model to interpret this natural selection phenomenon based on the finite deformation theory is developed here. Results show the pitch angle of the twiner, the elongation in the stem and the radius ratio of the twiner to its support play an important role in the twining habit. To get more sunshine overhead, twiners tend to increase the initial pitch angle but to decrease the final twining angle to maintain their positions and stability. By taking into account of the tuning of the interfacial strength with the microstructure, the maximum values of the elongation and the support radius are determined, over which twiners will lose their ability to wind around. The proposed model and the results can not only clarify the mechanism of climbing habits and attachment, but also show the potential applications for the twining-mimic designs.

Noninvasive methods for precise characterization of the thermal properties of soft biological tissues such as the skin can yield vital details about physiological health status including at critical intervals during recovery following skin injury. Here, we introduce quantitative measurement and characterization methods that allow rapid, accurate determination of the thermal conductivity of soft materials using thin, skin-like resistive sensor platforms. Systematic evaluations of skin at eight different locations and of six different synthetic skin-mimicking materials across sensor sizes, measurement times, and surface geometries (planar, highly curvilinear) validate simple scaling laws for data interpretation and parameter extraction. As an example of the possibilities, changes in the thermal properties of skin (volar forearm) can be monitored during recovery from exposure to ultraviolet radiation (sunburn) and to stressors associated with localized heating and cooling. More generally, the results described here facilitate rapid, non-invasive thermal measurements on broad classes of biological and non-biological soft materials., GRAPHICAL ABSTRACT

Cuff-based blood pressure measurement is the gold standard method for blood pressure monitoring. However, the wall of the sclerotic arteries (stiffen or thicken) generates extra resistance to the cuff pressure in the cuff-based blood pressure measurement, which tends to overestimate the blood pressure. This paper uses a collapse model to study the arterial deformation under the cuff pressure in the cuff-based blood pressure measurement, which gives the effects of arterial stiffness on measured systolic and diastolic blood pressure by cuff-based method, respectively. The results have general utility for future work in improving the accuracy of cuff-based BP measurement.

Transfer printing is a pivotal technology in the fabrication of flexible electronics, which is used to pick up the chiplet from the donor substrate and print the chiplet onto the receiver substrate. Transfer printing by stamp with micropillar has been widely used in the manufacturing process due to the adjustability of adhesion force. However, there is no adequate theoretical analysis on the regulation mechanism of the adhesion for the stamp with micropillar in the transfer printing. Herein, an analytical model of tunable elastomeric micropillar adhesion is established for pressure driven transfer printing process. With this model, the delamination length and maximum adhesion force between the stamp and the chiplet can be predicted. Meanwhile, different factors, including parameters, pressure and displacement of the stamp, were studied in this paper, to reveal their influence on the interface delamination length and adhesion force. The maximum adhesion force predicted by analytical model is verified through experiments. These conclusions can be used to guide the design of the stamp to assist the transfer printing process.