Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing.

Luminescent Lanthanide Metal Organic Frameworks (LnMOFs): A Versatile Platform towards Organomolecule Sensing

Mechanoluminescence, a smart luminescence phenomenon in which light energy is directly produced by a mechanical force, has recently received significant attention because of its important applications in fields such as visible strain sensing and structural health monitoring. Up to present, hundreds of inorganic and organic mechanoluminescent smart materials have been discovered and studied. Among them, strontium‐aluminate‐based materials are an important class of inorganic mechanoluminescent materials for fundamental research and practical applications attributed to their extremely low force/pressure threshold of mechanoluminescence, efficient photoluminescence, persistent afterglow, and a relatively low synthesis cost. This paper presents a systematic and comprehensive review of strontium‐aluminate‐based luminescent materials’ mechanoluminescence phenomena, mechanisms, material synthesis techniques, and related applications. Besides of summarizing the early and the latest research on this material system, an outlook is provided on its environmental, energy issue and future applications in smart wearable devices, advanced energy‐saving lighting and displays.

Smart Mechanoluminescent Phosphors: A Review of Strontium‐Aluminate‐Based Materials, Properties, and Their Advanced Application Technologies

Tactile sensors with multimode sensing ability are cornerstones of artificial skin for applications in humanoid robotics and smart prosthetics. However, the intuitive and interference-free reading of multiple tactile signals without involving complex algorithms and calculations remains a challenge. Herein a pressure-temperature bimodal tactile sensor without any interference is demonstrated by combining the fundamentally different sensing mechanisms of optics and electronics, enabling the simultaneous and independent sensing of pressure and temperature with the elimination of signal separation algorithms and calculations. The bimodal sensor comprises a mechanoluminescent hybrid of ZnS-CaZnOS and a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thermoresistant material, endowing the unambiguous transduction of pressure and temperature into optical and electrical signals, respectively. This device exhibits the highest temperature sensitivity of -0.6% °C-1 in the range of 21-60 °C and visual sensing of the applied forces at a low limitation of 2 N. The interference-free and light-emitting characteristics of this device permit user-interactive applications in robotics for encrypted communication as well as temperature and pressure monitoring, along with wireless signal transmission. This work provides an unexplored solution to signal interference of multimodal tactile sensors, which can be extended to other multifunctional sensing devices.

Bimodal Tactile Sensor without Signal Fusion for User-Interactive Applications.

Achieving Remote Stress and Temperature Dual-Modal Imaging by Double-Lanthanide-Activated Mechanoluminescent Materials

Proportionally converting the applied mechanical energy into photons by individual mechanoluminescent (ML) micrometer‐sized particles opens a new way to develop intelligent electronic skins as it promises high‐resolution stress distribution visualization and fast response. However, a big challenge for ML sensing technology is its low sensitivity in detecting stress. In this work, a novel stress distribution sensor with the detection sensitivity enhanced by two orders of magnitude is developed by combining a proposed near‐distance ML imaging scheme with an improved mechano‐to‐photon convertor. The enhanced sensitivity is the main contributor to the realization of a maximum photon harvesting rate of ≈80% in the near‐distance ML imaging scheme. The developed near‐distance ML sensor shows a high sensitivity with a detection limit down to ≈kPa level, high spatial resolution of 254 dpi, and fast response with an interval of 3.3 ms, which allows for high‐resolution and real‐time visualization of complex mechanical actions such as irregular solid contacts or fluid impacts, and thus enables use in intelligent electronic skin, structural health monitoring, and human–computer interaction.

Visualizing Dynamic Mechanical Actions with High Sensitivity and High Resolution by Near‐Distance Mechanoluminescence Imaging

Ratiometric fluorescence detection of 2,6-pyridine dicarboxylic acid with a dual-emitting lanthanide metal-organic framework (MOF)

Modeling Polyhedron Distortion for Mechanoluminescence in Mixed-Anion Compounds RE2O2S:Ln3+

The hot tensile deformation behavior of 7075 aluminum alloy sheet was studied using Gleeble-3800 thermal simulation machine at deformation temperatures of 400∼475°C and strain rates of 0.001∼1s-1. Microstructure evolution and fracture morphology were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hyperbolic sine model is used to establish the constitutive equation of the relationship between peak stress and Z parameter. The results show that the hyperbolic sine Arrhenius constitutive equation can well characterize the flow behavior of the studied alloy, and the hot deformation activation energy is 132.52kJ/mol. The average relative error between the predicted and experimental values is only 4.58%, and r2 is as high as 0.988, indicating that there is good consistency between the experimental value and the predicted value. When the strain rate keeps unchanged, the elongation of the studied alloy shows an upward trend in the lower deformation temperature range(400∼425°C). When the deformation temperature continued to rise to 475°C, the elongation of the studied alloy decreased sharply. Based on the analysis of fracture morphology, a large number of dimples can be observed at 450°C/0.001s-1, which is a typical ductile fracture. With increasing of deformation temperatures, the fracture mechanism gradually changes from ductile fracture to brittle fracture. Under the condition of 425°C/0.01s-1, there are a lot of dislocations in the alloy, and the dislocation walls are formed. With the increase of deformation temperatures or the decrease of strain rates, the dislocation density gradually decreases and dynamic recrystallization occurs. The softening mechanism of the studied alloy changes from dynamic recovery to dynamic recrystallization. 

Hot tensile deformation behavior and microstructure evolution of 7075 aluminum alloy sheet

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