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Abstract Twenty-four hours continuous intraocular pressure (IOP) monitoring is beneficial for glaucoma care. Contact lens sensors using LC technology can achieve non-invasive continuous IOP measuring around the clock and are relatively simple in principle and structure, thus dominating the mainstream. Among them, stretchable inductive LC contact lens sensors constructed with liquid metal have advantages in signal quality as well as wearing comfort, but currently its sensitivity is slightly insufficient. Here, we propose an LC sensor that constructed with liquid metal to form the stretchable inductance and capacitance further. The capacitive plate and inductive coil can response to IOP changes simultaneously, thus able to enhance the sensitivity in principle. We modeled the sensing mechanism and conducted design, fabrication, as well as various tests. The device exhibits good characteristics, including reliability, good signal quality, etc. Especially, it has a threefold increase in sensitivity, exceeding the current state-of-the-art contact lens sensors.

Abstract Transfer printing is a key technology in the fabrication of flexible electronics. Transfer printing method based on curvature provides a simple and effective way to transfer films onto weakly adhesive or even adhesiveless surfaces, overcoming the shortage of traditional transfer printing methods that it is difficult to print functional materials from strong interface to weak interface and can only be applicable to surfaces with certain interfacial strength. So far, the theoretical principle of the curvature‐based transfer printing method has not yet been developed. In this article, a dual‐interface competitive fracture model is established to analyze the mechanism of printing and picking up processes and quantitatively provide the critical transfer printing radius in terms of material and geometric properties of the transfer printing system. This model is verified to be both correct and widely applicable by rich experimental results, providing a new and reliable theory for the fabrication of inorganic flexible electronics.

Abstract 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.

Abstract Brain-computer interfaces (BCIs) have attracted considerable attention in motor and language rehabilitation. Most devices use cap-based non-invasive, headband-based commercial products or microneedle-based invasive approaches, which are constrained for inconvenience, limited applications, inflammation risks and even irreversible damage to soft tissues. Here, we propose in-ear visual and auditory BCIs based on in-ear bioelectronics, named as SpiralE, which can adaptively expand and spiral along the auditory meatus under electrothermal actuation to ensure conformal contact. Participants achieve offline accuracies of 95% in 9-target steady state visual evoked potential (SSVEP) BCI classification and type target phrases successfully in a calibration-free 40-target online SSVEP speller experiment. Interestingly, in-ear SSVEPs exhibit significant 2nd harmonic tendencies, indicating that in-ear sensing may be complementary for studying harmonic spatial distributions in SSVEP studies. Moreover, natural speech auditory classification accuracy can reach 84% in cocktail party experiments. The SpiralE provides innovative concepts for designing 3D flexible bioelectronics and assists the development of biomedical engineering and neural monitoring.

The problem that the thermal safety of flexible electronic devices is difficult to evaluate in real time is addressed in this study by establishing a BP neural network (GA-BPNN) temperature prediction model based on genetic algorithm optimisation. The model uses a BP neural network to fit the functional relationship between the input condition and the steady-state temperature of the equipment and uses a genetic algorithm to optimise the parameter initialisation problem of the BP neural network. To overcome the challenge of the high cost of obtaining experimental data, finite element analysis software is used to simulate the temperature results of the equipment under different working conditions. The prediction variance of the GA-BPNN model does not exceed 0.57 °C and has good robustness, as the model is trained according to the simulation data. The study conducted thermal validation experiments on the temperature prediction model for this flexible electronic device. The device reached steady state after 1200 s of operation at rated power. The error between the predicted and experimental results was less than 0.9 °C, verifying the validity of the model’s predictions. Compared with traditional thermal simulation and experimental methods, this model can quickly predict the temperature with a certain accuracy and has outstanding advantages in computational efficiency and integrated application of hardware and software.

Abstract Low‐flow removal of refractory ascites is critical to treating cirrhosis and digestive system tumor, and thus, commercial ascites pump emerged lately. The rigid structure of clinically available pumps rises complication rate and lack of flow rate monitoring hinders early warning of abnormalities. Herein, a soft artificial system was proposed inspired by lymph for interactive ascites transfer with great biocompatibility. The implantable system is composed of pump cavity, valves and tubes, which are soft and flexible made by silica gel. Therefore, the system possesses similar modulus to tissues and can naturally fit surrounding tissues. The cavity with magnetic tablet embedded is driven by extracorporeal magnetic field. Subsequently, the system can drain ascites with a top speed of 23 mL min−1, much higher than that of natural lymphatic system and state‐of‐art devices. Moreover, integrated flexible sensors enable wireless, real‐time flow rate monitoring, serving as proof of treatment adjustment, detection and locating of malfunction at early stage. The liver function of experimental objects was improved, and no severe complications occurred for 4 weeks, which proved its safety and benefit to treatment. This artificial lymphatic system can serve as a bridge to recovery and pave the way for further clinical research.

Abstract Soft bioelectronic devices have potential in medical applications, especially comprehensive diagnoses that rely on immovable equipment to acquire multiple physiological signals. Challenges for bioelectronic devices lie in the opposite modulus demand for multifunctional integration and biocompatibility, and shape compatibility for reducing implantation trauma and covering target tissue. Here, this work reports a multifunctional bioelectronic device endowed with switchable rigidity and reconfigurable shapes by a fast thermal response shape memory polymer substrate. The customizable substrate has a melting temperature around body temperature (≈38 °C) and an ≈100 times modulus drop (from ≈100 MPa to ≈700 kPa). The switchable rigidity realizes stacked layers fabrication in the rigid state and self‐adaptive contact in the soft state. Shape reconfiguration allows the device to be implanted through small incisions and recover in limited spaces to envelop biosurfaces. Multiple physical, biochemical, and electrophysiological signal sensing functions are integrated in the device, realized by temperature and pH electrodes with linear, stable, and fast responses and a low‐impedance potential electrode. For some epilepsy and pericardial effusion types that a single physiological parameter fails to distinguish, animal experiments demonstrate efficiency in their comprehensive diagnosis with electrocorticogram assisted by brain temperature and electrocardiogram assisted by pericardial fluid pH, respectively.

Limited by the effective launch capacity of a rocket, the deployable antenna is very important in the design of spaceborne antenna array. Compared to traditional deployable antenna, flexible coilable antenna array has higher surface precision and better vibration control and therefore is more suitable for high frequency communication. In order to minimize the weight of satellite and reduce cost of its launch, a design guideline to the geometry parameters of flexible coilable antenna array is crucial. Existing models cannot be directly applied to interaction and large deformation between coilable membrane and conical spiral antenna in the flexible coilable antenna array. Hence, the geometry parameters of the conical spiral structure and the thickness of the coilable membrane in the flexible coilable antenna array have not been optimized yet. In this paper, the interaction between the coilable membrane and the concial spiral antenna is analyzed in the antenna array. A concise formula is derived to predict the critical force that flattens the conical spiral antenna by a coiling scroll. Combined with a theoretical model to predict the deformation of the membrane, the model provides an important theoretical support for the lightweight design and mechanical design of flexible coilable antenna array, such as the thickness of the coilable membrane. The proposed design is validated by experiments. The above findings have potential applications in the effective reduction of antenna array weight and satellite launch costs.

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.

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.

Packers based on shape memory polymers (SMPs) are an emerging technology that have the advantages of compact structure, easy manufacture, and adaptability to complex wells. This paper proposes a finite element model to simulate the setting process and mechanical response of an SMP packer. The investigated material is an epoxy-based thermal responsive SMP, whose relaxation modulus and thermal expansion coefficient were measured at different temperatures. Based on the experimental data, the model describes the viscoelastic behavior of the SMP using the generalized Maxwell model. The results show that the SMP packer could provide sufficient contact stress under downhole conditions, even after the stress was relaxed. A further parametric study revealed that the most significant factor in sealing effects is the wellbore pressure, followed by the interference between the packer and the annular, the seal length, the pre-compression, and the setting temperature. High downhole pressures require more significant contact stress and increase the risk of slip between the packer and casing wall by promoting shear stress. Increasing the seal length and interference enhances the contact stress and mitigates the shear stress to improve the seal performance. Pre-compression and setting temperatures are minor factors that have little influence on sealability.

Cement provides zonal isolation and mechanical support, and its integrity is critical to the safety and efficiency of the CO2 injection process for geologic carbon storage. This work focuses on interfacial debonding at wellbore interfaces and radial cracking in cement during CO2 injection. It adopts the definition of the energy release rate (ERR) to characterize the propagation of cracks. Based on the finite element method, the proposed model estimates the ERRs of both types of cracks with practical wellbore configurations and injection parameters. Further parametric studies reveal the effects of cement’s mechanical and thermal properties and the crack geometry on crack propagation. Simulation results show that the ERRs of interfacial and radial cracks would surpass 100 J/m2 with typical cement properties. The cement’s thermal expansion coefficient is the most influential factor on the ERR, followed by its Young’s modulus, Poisson’s ratio, and thermal conductivity. The initial sizes and positions of the cracks are also important parameters for controlling crack propagation. Moreover, non-uniform in situ stresses would accelerate crack propagation at the interfaces. These findings are valuable and could help to optimize cement sheath design in order to ensure the long-term integrity of wells for geological carbon storage.

Continuous monitoring of blood pressure, an essential measure of health status, typically requires complex, costly, and invasive techniques that can expose patients to risks of complications. Continuous, cuffless, and noninvasive blood pressure monitoring methods that correlate measured pulse wave velocity (PWV) to the blood pressure via the Moens-Korteweg (MK) and Hughes Equations, offer promising alternatives. The MK Equation, however, involves two assumptions that do not hold for human arteries, and the Hughes Equation is empirical, without any theoretical basis. The results presented here establish a relation between the blood pressure P and PWV that does not rely on the Hughes Equation nor on the assumptions used in the MK Equation. This relation degenerates to the MK Equation under extremely low blood pressures, and it accurately captures the results of in vitro experiments using artificial blood vessels at comparatively high pressures. For human arteries, which are well characterized by the Fung hyperelastic model, a simple formula between P and PWV is established within the range of human blood pressures. This formula is validated by literature data as well as by experiments on human subjects, with applicability in the determination of blood pressure from PWV in continuous, cuffless, and noninvasive blood pressure monitoring systems. [ABSTRACT FROM AUTHOR]

An effective correction model is proposed to eliminate the refraction error effect caused by an optical window of a furnace in digital image correlation (DIC) deformation measurement under high-temperature environment. First, a theoretical correction model with the corresponding error correction factor is established to eliminate the refraction error induced by double-deck optical glass in DIC deformation measurement. Second, a high-temperature DIC experiment using a chromium–nickel austenite stainless steel specimen is performed to verify the effectiveness of the correction model by the correlation calculation results under two different conditions (with and without the optical glass). Finally, both the full-field and the divisional displacement results with refraction influence are corrected by the theoretical model and then compared to the displacement results extracted from the images without refraction influence. The experimental results demonstrate that the proposed theoretical correction model can effectively improve the measurement accuracy of DIC method by decreasing the refraction errors from measured full-field displacements under high-temperature environment. [ABSTRACT FROM AUTHOR]

The interaction between different shapes of inclusions and the mode I matrix crack was studied experimentally using the photoelasticity method. First, the stress intensity factor (SIF) at the matrix crack tip in the neighborhood of the inclusions was derived based on transformation toughening theory and the Eshelby equivalent inclusion method. Then, photoelastic experiments were conducted using specimens with different shapes of inclusions, and the typical isochromatic fringe patterns around the crack tip near the inclusions were obtained. Finally, a numerical simulation was conducted for the same problem using ABAQUS, and the stress intensity factors computed from the finite-element method were compared with the experimental results. The results show that the stress intensity factors extracted from isochromatic fringe patterns agree well with the numerical and theoretical results. [ABSTRACT FROM AUTHOR]

Conformal modulus sensors (CMS) incorporate PZT nanoribbons as mechanical actuators and sensors to achieve reversible conformal contact with the human skin for non-invasive, in vivo measurements of skin modulus. An analytic model presented in this paper yields expressions that connect the sensor output voltage to the Young moduli of the epidermis and dermis, the thickness of the epidermis, as well as the material and geometrical parameters of the CMS device itself and its encapsulation layer. Results from the model agree well with in vitro experiments on bilayer structures of poly(dimethylsiloxane). [ABSTRACT FROM AUTHOR]

A stiff thin film bonded to a pre-strained, compliant substrate wrinkles into a sinusoidal form upon release of the pre-strain. Many analytical models developed for the critical pre-strain for wrinkling assume that the substrate is semi-infinite. This critical pre-strain is actually much smaller than that for a substrate with finite thickness (Ma Y et al. 2016 Adv. Funct. Mater. (doi:10.1002/adfm.201600713)). An analytical solution of the critical pre-strain for a system of a stiff film bonded to a pre-strained, finite-thickness, compliant substrate is obtained, and it agrees well with the finite-element analysis. The finite-thickness effect is significant when the substrate tensile stiffness cannot overwhelm the film tensile stiffness. [ABSTRACT FROM AUTHOR]

In this paper, buckle-driven delamination between thermal barrier coatings (TBCs) and a polynomial curved substrate is theoretically analyzed under thermal loading. First, based on the one-dimensional flat blister mode, a theoretical model for buckle-driven delamination of a polynomial curved coating blister is established. Second, a finite element model of the TBCs with an interface initial crack on different polynomial curved substrates is built to investigate the influence of temperature and the initial substrate morphology on the energy release rate and mode mixity, which agrees well with the results from the theoretical prediction. Finally, the influence of crack length, coating thickness and substrate morphology on the buckle-driven mechanism is analyzed and discussed. The results will provide important guidance for characterizing and designing curved TBCs of thermal protection structures. [ABSTRACT FROM AUTHOR]

The article presents a study, which suggests an automated wrap-like transfer printing prototype to address the challenges associated with three-dimensional (3D) curvy electronics. The prototype uses a petal-like stamp with prefabricated planar circuits that are integrated onto the target surface with full coverage, assisted by a gentle and uniform pressure field.

The article focuses on flexible Doppler ultrasound device for the continuous monitoring of the absolute velocity of blood flow in deeply embedded arteries based on the Doppler effect. It mentions studies on ultrasound phantoms and human subjects demonstrate accurate measurement of the flow velocity and wearable Doppler device has the potential to enhance the quality of care of patients. It also mentions thrombosis and restenosis after vascular reconstruction.

The article focuses on flexible, stretchable system for simultaneous acoustic energy transfer and communication. Topics icnlude the use of implantable medical devices, including cardiac pacemakers and brain pacemakers, is becoming increasingly prevalent, and the surgically replacing batteries owing to their limited lifetime is a drawback of those devices.