Analytical Higher-Order Model for Flexible and Stretchable Sensors

The stretchable sensor wrapped around a foldable airfoil or embedded inside of it has great potential for use in the monitoring of the structural status of the foldable airfoil.The design methodology is important to the development of the stretchable sensor for status monitoring on the foldable airfoil.According to the requirement of mechanical flexibility of the sensor,the combined use of a layered flexible structural formation and a strain isolation layer is implemented.An analytical higher-order model is proposed to predict the stresses of the strain-isolation layer based on the shear-lag model for the safe design of the flexible and stretchable sensors.The normal stress and shear stress equations in the constructed structure of the sensors are obtained by the proposed model.The stress distribution in the structure is investigated when bending load is applied to the structures.The numerical results show that the proposed model can predict the variation of normal stress and shear stress along the thickness of the strain-isolation（polydimethylsiloxane）layer accurately.The results by the proposed model are in good agreement with the finite element method,in which the normal stress is variable while the shear stress is invariable along the thickness direction of strain-isolation layer.The high-order model is proposed to predict the stresses of the layered structure of the flexible and stretchable sensor for monitoring the status of the foldable airfoil.

Vasculature-on-a-chip with stretchable sensor for recapitulating hemodynamics and electrochemical monitoring of endothelium

Vascular endothelium can perceive fluid shear stress(FSS)and cyclic circumferential stretch(CCS)caused by the pulsatile blood flow,and translate the hemodynamics into biochemical signals to regulate vascular pathophysiology.However,existing methods provide little information about the real-time biochemical responses of endothelium when exposed to dynamic FSS and CCS.Herein,a vasculature-on-a-chip integrated with stretchable sensing is engineered for recapitulating the hemodynamic milieus and in-situ monitoring biochemical responses of endothelial monolayer.The integrated device is developed by sandwiching a robust stretchable electrode between an upper fluidic channel and a lower pneumatic channel.The fluidic and pneumatic channels enable the simultaneous recapitulation of both FSS and CCS,and the integrated sensor exhibits excellent cell-adhesive capacity and electrochemical sensing stability even after long-term hemodynamic exposure.These allow real-time monitoring of hemodynamic form-and duration-dependent endothelium responses,and further efficacy investigation about a recommended drug for COVID-19,demonstrating the great potential in vascular disease and drug screening.

Functionalized Hydrogel-Based Wearable Gas and Humidity Sensors

Breathing is an inherent human activity;however,the composition of the air we inhale and gas exhale remains unknown to us.To address this,wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks,and for the early detection and treatment of diseases for home healthcare.Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable.Functionalized hydrogels are intrinsically conductive,self-healing,self-adhesive,biocompatible,and room-temperature sensitive.Compared with traditional rigid vapor sensors,hydrogel-based gas and humidity sensors can directly fit human skin or clothing,and are more suitable for real-time monitoring of personal health and safety.In this review,current studies on hydrogel-based vapor sensors are investigated.The required properties and optimization methods of wearable hydrogel-based sensors are introduced.Subsequently,existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized.Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented.Moreover,the potential of hydrogels in the field of vapor sensing is elucidated.Finally,the current research status,challenges,and future trends of hydrogel gas/humidity sensing are discussed.

Self-Healing,Self-Adhesive and Stable Organohydrogel-Based Stretchable Oxygen Sensor with High Performance at Room Temperature

With the advent of the 5G era and the rise of the Internet of Things,various sensors have received unprecedented attention,especially wearable and stretchable sensors in the healthcare field.Here,a stretchable,self-healable,self-adhesive,and room-temperature oxygen sensor with excellent repeatability,a full concentration detection range(0-100%),low theoretical limit of detection(5.7 ppm),high sensitivity(0.2%/ppm),good linearity,excellent temperature,and humidity tolerances is fabricated by using polyacrylamide-chitosan(PAM-CS)double network(DN)organohydrogel as a novel transducing material.The PAM-CS DN organohydrogel is transformed from the PAM-CS composite hydrogel using a facile soaking and solvent replacement strategy.Compared with the pristine hydrogel,the DN organohydrogel displays greatly enhanced mechanical strength,moisture retention,freezing resistance,and sensitivity to oxygen.Notably,applying the tensile strain improves both the sensitivity and response speed of the organohydrogel-based oxygen sensor.Furthermore,the response to the same concentration of oxygen before and after self-healing is basically the same.Importantly,we propose an electrochemical reaction mechanism to explain the positive current shift of the oxygen sensor and corroborate this sensing mechanism through rationally designed experiments.The organohydrogel oxygen sensor is used to monitor human respiration in real-time,verifying the feasibility of its practical application.This work provides ideas for fabricating more stretchable,self-healable,self-adhesive,and high-performance gas sensors using ion-conducting organohydrogels.

Laser-Assisted Reduction of Highly Conductive Circuits Based on Copper Nitrate for Flexible Printed Sensors

Stretchable electronic sensing devices are defining the path toward wearable electronics. High-performance flexible strain sensors attached on clothing or human skin are required for potential applications in the entertainment,health monitoring, and medical care sectors. In this work,conducting copper electrodes were fabricated onpolydimethylsiloxane as sensitive stretchable microsensors by integrating laser direct writing and transfer printing approaches. The copper electrode was reduced from copper salt using laser writing rather than the general approach of printing with pre-synthesized copper or copper oxide nanoparticles. An electrical resistivity of 96 l X cm was achieved on 40-lm-thick Cu electrodes on flexible substrates. The motion sensing functionality successfully demonstrated a high sensitivity and mechanical robustness.This in situ fabrication method leads to a path toward electronic devices on flexible substrates.

Review of flexible microelectromechanical system sensors and devices

Today,the vast majority of microelectromechanical system(MEMS)sensors are mechanically rigid and therefore suffer from disadvantages when used in intimately wearable or bio-integrated applications.By applying new engineering strategies,mechanically bendable and stretchable MEMS devices have been successfully demonstrated.This article reviews recent progress in this area,focusing on high-performance flexible devices based on inorganic thin films.We start with the common design and fabrication strategies for flexibility and stretchability,summarize the recent application-oriented flexible devices,and conclude with criteria and opportunities for the future development of flexible MEMS sensors.

Design of AI-Enhanced and Hardware-Supported Multimodal E-Skin for Environmental Object Recognition and Wireless Toxic Gas Alarm

Post-earthquake rescue missions are full of challenges due to the unstable structure of ruins and successive aftershocks.Most of the current rescue robots lack the ability to interact with environments,leading to low rescue efficiency.The multimodal electronic skin(e-skin)proposed not only reproduces the pressure,temperature,and humidity sensing capabilities of natural skin but also develops sensing functions beyond it—perceiving object proximity and NO2 gas.Its multilayer stacked structure based on Ecoflex and organohydrogel endows the e-skin with mechanical properties similar to natural skin.Rescue robots integrated with multimodal e-skin and artificial intelligence(AI)algorithms show strong environmental perception capabilities and can accurately distinguish objects and identify human limbs through grasping,laying the foundation for automated post-earthquake rescue.Besides,the combination of e-skin and NO2 wireless alarm circuits allows robots to sense toxic gases in the environment in real time,thereby adopting appropriate measures to protect trapped people from the toxic environment.Multimodal e-skin powered by AI algorithms and hardware circuits exhibits powerful environmental perception and information processing capabilities,which,as an interface for interaction with the physical world,dramatically expands intelligent robots’application scenarios.

Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors.However,traditional humidity sensors suffer from a trade-off between deformability,sensitivity,and transparency,and thus the development of high-performance,stretchable,and low-cost humidity sensors is urgently needed as wearable electronics.Here,ultrasensitive,highly deformable,and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels.Concomitantly,a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area,which can be applied to different polymer networks and facilitate the development of flexible integrated electronics.In addition,sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network,exhibiting excellent water adsorption capacity.Through the synergistic optimization of structure and composition,the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH,which is unprecedented.Moreover,the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity.As a proof of concept,we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission,enabling real-time monitoring of human health status.This work provides a general strategy to construct high-performance,stretchable,and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Engineering Smart Composite Hydrogels for Wearable Health Monitoring

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely.During the health monitoring process,different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

Ultrasmall barium titanate nanoparticles modulated stretchable dielectric elastomer sensors with large deformability and high sensitivity

Large deformability and high sensitivity is difficult to be realized simultaneously in flexible sensors.Herein,taking advantage of the high permittivity and highly active surfaces of the ultrasmall barium titanate nanoparticles(BT NPs)and the high stretchability of the p(BA-GMA)elastomer matrix,we propose a high-performance soft stretchable sensor.The addition of the ultrasmall BT NPs can not only increase the permittivity and capacitance of polyacrylate-matrix composite dielectric material to obtain a high sensitivity,but also basically maintains the excellent mechanical properties of the polymer matrix.The dielectric constants of the composite films increase from 5.68 to 13.13 at 10 kHz with the increase of BT NPs content from 0 to 15 vol.%,which results in a high capacitance of 236.16 pF for 15 vol.%BT/p(BA-GMA)sensor.Combining the high permittivity and the large deformability(a maximal deformation of 87.2%),the 15 vol.%BT/p(BA-GMA)sensor has high sensitivity and shows high linearity and stable output even if under dynamic measurement.The dual-mode sensor that utilizes the orthogonality of capacitance-resistance is designed,which shows excellent performance in monitoring human body movements and noncontact measurement.The results present that the BT/p(BA-GMA)-based sensor has high stability and reliability not exceed 65C,which can meet the application requirements in dynamic monitoring.

3D Printing of Stretchable Strain Sensor Based on Continuous Fiber Reinforced Auxetic Structure

Stretchable strain sensors play a key role in motion detection and human-machine interface functionality,and deformation control.However,their sensitivity is often limited by the Poisson effect of elastic substrates.In this study,a stretchable strain sensor based on a continuous-fiber-reinforced auxetic structure was proposed and fabricated using a direct ink writing(DIW)3D printing process.The application of multi-material DIW greatly simplifies the fabrication process of a sensor with an auxetic structure(auxetic sensor).The auxiliary auxetic struc-ture was innovatively printed using a continuous-fiber-reinforced polydimethylsiloxane composite(Fiber-PDMS)to balance the rigidity and flexibility of the composite.The increase in stiffness enhances the negative Poisson’s ratio effect of the auxetic structure,which can support the carbon nanotube-polydimethylsiloxane composite(CNT-PDMS)stretchable sensor to produce a significant lateral expansion when stretched.It is shown that the structural Poisson’s ratio of the sensor decreased from 0.42 to−0.33 at 20%tensile strain,and the bidirectional tensile strain increases the sensor sensitivity by 2.52 times(gage factor to 18.23).The Fiber-PDMS composite maintains the excellent flexibility of the matrix material.The auxetic sensor exhibited no structural damage af-ter 150 cycles of tension and the signal output exhibited high stability.In addition,this study demonstrates the significant potential of auxetic sensors in the field of deformation control.

Piezoresistive behavior of elastomer composites with segregated network of carbon nanostructures and alumina

Electrically conductive elastomer composites(CECs)with segregated networks of conductive nanofillers show high potential in stretchable strain sensors due to balanced mechanical and electrical properties,yet the sensitivity at low strain is generally insufficient for practical application.Herein,we report an easy and effective way to improve the resistive response to low strain for CECs with segregated network structure via adding stiff alumina into carbon nanostructures(CNS).The CEC containing 0.7 wt%CNS and 5 wt%Al_(2)O_(3) almost sustains the same elasticity(elongation at break of~900%)and conductivity(0.8 S/m)as the control,while the piezoresistive sensitivity is significantly improved.Thermoplastic polyurethane(TPU)composites with a segregated network of hybrid nanofillers(CNS and Al_(2)O_(3))show much higher strain sensitivity(Gauge factor,GF-566)at low strain(45%strain)due to a local stress concentration effect,this sensitivity is superior to that of TPU/CNS composites(GF-11).Such a local stress concentration effect depends on alumina content and its distribution at the TPU particle interface.In addition,CECs with hybrid fillers show better reproducibility in cyclic piezoresistive behavior testing than the control.This work offers an easy method for fabricating CECs with a segregated filler network offering stretchable strain sensors with a high strain sensitivity.

Highly stretchable polymer/silver nanowires composite sensor for human health monitoring

Flexible strain sensors exhibit outstanding advantages in terms of sensitivity and stability by detecting changes in physical signals.It can be easily attached to human skin and clothed to achieve monitoring of human motion and health.However,general sensing material shows low stretchability and cannot respond to signals under large deformation.In this work,a highly stretchable polymer composite was developed by adding small amount(0.17 wt.%)of silver nanowires(AgNWs)in stretchable conductive polymer materials.The conductivity of polymer/AgNWs composite is 1.3 S/m with the stretchability up to 500%.The stretchable strain sensor based on the polymer/AgNWs composite can respond to strain signals in real time,even for 1%strain response,and shows excellent stability over 1,000 loading/unloading cycles.Moreover,the strain sensor can be attached to human skin and clothed to monitor joints,throat and pulse of the human body.The human body electrocardiogram(ECG)signal was detected successfully with the polymer/AgNWs electrode,which is comparable to the signal obtained by the commercial electrode.Overall,the sensors enable monitoring of human movement and health.These advantages make it a potential application in wearable devices and electronic skin.

MWCNTs based flexible and stretchable strain sensors

Carbon nanotubes have potential applications in flexible and stretchable devices due to their remarkable electromechanical properties.Flexible and stretchable strain sensors of multi-walled carbon nanotubes（MWCNTs）with aligned or random structures were fabricated on poly-dimethylsiloxane（PDMS） substrate with different techniques.It was observed that the spraycoatedtechniquebased strain sensor fabricated on PDMS substrate showed higher sensitivity higher stretchability,better linearity and excellent longer time stability than the sensor fabricated with other methods presented in this work.The scanning electron microscopy images indicated the spray coating technique can produce a better uniform and compact CNT network,which is the important role affecting the performance of CNT-based flexible strain sensors.

Hydrogel-elastomer-based stretchable strain sensor fabricated by a simple projection lithography method

Stretchable strain sensor detects a wide range of strain variation and is therefore a key component in various applications.Unlike traditional ones made of elastomers doped with conductive components or fabricated with liquid conductors,ionically conductive hydrogel-based strain sensors remain conductive under large deformations and are biocompatible.However,dehydration is a challenging issue for the latter.Researchers have developed hydrogel-elastomer-based strain sensors where an elastomer matrix encapsulates a hydrogel circuit to prevent its dehydration.However,the reported multistep approaches are generally time-consuming.Our group recently reported a multimaterial 3D printing approach that enables fast fabrication of such sensors,yet requires a self-built digital-light-processing-based multimaterial 3D printer.Here,we report a simple projection lithography method to fabricate hydrogel-elastomer-based stretchable strain sensors within 5 minutes.This method only requires a UV projector/lamp with photomasks;the chemicals are commercially available;the protocols for preparing the polymer precursors are friendly to users without chemistry background.Moreover,the manufacturing flexibility allows users to readily pattern the sensor circuit and attach the sensor to a 3D printed soft pneumatic actuator to enable strain sensing on the latter.The proposed approach paves a simple and versatile way to fabricate hydrogel-elastomer-based stretchable strain sensors and flexible electronic devices.

高灵敏度和高线性度的可拉伸应变传感器的分层结构设计

对柔性电子设备日益增长的需求使得开发具有高灵敏度和高线性度的传感器更加迫切.由于拉伸应变下不可逆结构损伤诱导电阻线性急剧增加,现有的可拉伸应变传感器难以完美地同时实现这两个特性.针对这一问题,本文提出了一种兼具表面褶皱和体相梯度孔隙的新型分级互连结构,利用水热活化机制精确控制纳米级褶皱间距,通过调控相分离中热力学和动力学行为构筑出体相梯度多孔结构,通过改变器件两侧曲率实现各向异性特征,深入研究各种设计的功效、量化几何结构对灵敏度的有效贡献和追踪形态演变.基于器件显著的灵敏度和各向异性,所制备的传感器能够有效监测静态和动态位移、表面运动、二维应变信号变化以及预测液位随时间变化.本工作为实现高质量的感知能力提供了一种广泛适用、适应性强、可扩展且具有成本效益的方法.

用于紫外线成像和无线报警的水凝胶基可伸缩、高稳定性和高性能的光电传感器

柔性可穿戴的紫外线探测器在个体紫外线暴露监测、智能仿生眼和军事领域受到广泛关注.然而,传统不可拉伸的材料和器件结构设计限制了设备在可动态变形的人体皮肤和非平面表面上的精确监测应用和稳定附着.在本研究中,我们提出了基于氧化铜/碳-水凝胶-氧化锌/碳的新型应变隔离异质结构的本征可拉伸(拉伸应变或弯曲高达150%)紫外线探测器,该探测器具有高灵敏度(开/关比为2100%)、高稳定性(大于4000次循环),以及快速响应和恢复时间(分别为1.5和4 s).值得注意的是,该探测器通过水凝胶桥在氧化铜/碳和氧化锌/碳电极之间引入的p-n结能带结构,以及离子导电水凝胶中I^(-)/I_(3)^(-)的电化学反应机制,显著提高了其载流子效率和紫外线响应强度.本文通过集成传感器与所设计的电路板,进一步开发了一种无线紫外线报警系统,用于紫外线强度超标警报.此外,本文还设计了一种光电传感器阵列,并将其用作视网膜假体,以实现对环境紫外线强度的实时监测和成像.本文基于水凝胶基异质结构为开发可穿戴、可拉伸的光电紫外线传感器单元和成像阵列提供了一种新的可行方法.

The stretchable carbon black-based strain fiber with a remarkable linearity in a wide sensing range

Ascribed to its wide sensing range,high sensitivity,and low stiff-ness to match target objects with complex 3D shapes,the stretch-able strain sensor has shown its promising applications in various fields,ranging from healthcare,bodynet,and intelligent traffic system,to the robotic system.This paper presents a low-cost and straightforward fabrication technology for the stretchable strain fiber with the combined attributes of a wide sensing range,excep-tional linearity,and high durability.The hybrid composite consist-ing of carbon black and silicone is utilized as the functional material to respond to the external mechanical deformation due to the piezoresistive effect.To address the remarkable hysteresis of the CB-silicone composites,the latex tubes with excellent mechanical robustness and a considerable accessible tensile strain are intro-duced as the outer supporting components.After injecting the conductive CB-silicone composite into these tubes,the stretchable strain fibers are successfully prepared.Notably,the stretchable strain sensor exhibits linearity(R^(2)=0.9854)in a wide sensing range(0-400%)and remarkable durability even after the 2500 cycles under 100%tension.Additionally,the potential of this stretchable strain fiber as the wearable strain sensor and the realtime feedback is demonstrated by detecting the body motion and the expansion devices.