Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Highly stretchable, adhesive and conductive hydrogel for flexible and stable bioelectrocatalytic sensing layer of enzyme-based amperometric glucose biosensor

Highly stretchable, adhesive and conductive triblock hydrogel was synthesized and utilized as a flexible and stable bioelectrocatalytic sensing layer of enzyme-based amperometric glucose biosensor. The hydrogel was prepared through one-pot polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid, methacrylamide, and hydroxyethyl methacrylate. The physical and chemical properties of the hydrogel were characterized with X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and electrochemical techniques. Glucose oxidase (GOx) and chitosan (CTS) embedded hydrogel was drop-coated on glassy carbon electrode (GCE) and screen printed graphite electrode (SPGE). The resulting GOx/CTS/hydrogel-GCE and GOx/CTS/hydrogel-SPGE exhibited excellent mediated bioelectrocatalytic oxidation current for glucose. The calibration curve of glucose by the GOx/CTS/hydrogel-GCE showed the linear range from 0.25 to 15 mM with the sensitivity of 27.0 µA mM-1 cm-2. This GOx/CTS/hydrogel-based sensing layer coated on the SPGE was stable against bending, and the response to glucose was almost same irrespective of the bending angles (0, 30, 60, and 90 degree). In addition, the response to glucose was not interfered by various organic and inorganic interfering species, allowed to detect glucose in goat serum. Furthermore, the GOx/CTS/hydrogel-GCE kept its original activity of 99.64 % during 30 days' storage under dry state in refrigerator.

Challenges and Opportunities: Nanomaterials in Epilepsy Diagnosis

Epilepsy is a common neurological disorder characterized by a significant rate of disability. Accurate early diagnosis and precise localization of the epileptogenic zone are essential for timely intervention, seizure prevention, and personalized treatment. However, over 30% of patients with epilepsy exhibit negative results on electroencephalography and magnetic resonance imaging (MRI), which can lead to misdiagnosis and subsequent delays in treatment. Consequently, enhancing diagnostic methodologies is imperative for effective epilepsy management. The integration of nanomaterials with biomedicine has led to the development of diagnostic tools for epilepsy. Key advancements include nanomaterial-enhanced neural electrodes, contrast agents, and biochemical sensors. Nanomaterials improve the quality of electrophysiological signals and broaden the detection range of electrodes. In imaging, functionalized magnetic nanoparticles enhance MRI sensitivity, facilitating localization of the epileptogenic zone. NIR-II nanoprobes enable tracking of seizure-related biomarkers with deep tissue penetration. Furthermore, nanomaterials enhance the sensitivity of biochemical sensors for detecting epilepsy biomarkers, which is crucial for early detection. These advancements significantly increase diagnostic sensitivity and specificity. However, challenges remain, particularly regarding biosafety, quality control, and the scalability of fabrication processes. Overcoming these obstacles is essential for successful clinical translation. Artificial-intelligence-based big data analytics can facilitate the development of diagnostic tools by screening nanomaterials with specific properties. This approach may help to address current limitations and improve both effectiveness and safety. This review explores the application of nanomaterials in the diagnosis and detection of epilepsy, with the objective of inspiring innovative ideas and strategies to enhance diagnostic effectiveness.

Biosensors and Biomarkers for the Detection of Motion Sickness

Motion sickness (MS) is a prevalent syndrome that predominantly occurs during transportation and virtual reality (VR). The absence of reliable indicators and detection methods makes precise diagnosis difficult. Biomarker concentrations and trends may imply individual susceptibility, symptom classification, and the specific progression of MS. It is therefore essential to explore biosensors capable of providing sensitive, accurate, and real-time monitoring of biomarkers. This review provides a summary of the pathogenesis and biological pathways underlying MS, followed by an examination of biomarkers and their research progress. The most recent electrochemical biosensors developed for the non-invasive detection of representative biomarkers (e.g., cortisol, α-amylase, and estrogen) are comprehensively summarized. The effectiveness of these biosensors in practical application is discussed. It is anticipated that electrochemical biosensors can be gradually improved from the sampling methods, multimodal combinations, and data processing, which can facilitate the detection of MS toward individuation, refinement, and intelligence.

Trends in Flexible Sensing Technology in Smart Wearable Mechanisms-Materials-Applications

Flexible sensors are revolutionizing our lives as a key component of intelligent wearables. Their pliability, stretchability, and diverse designs enable foldable and portable devices while enhancing comfort and convenience. Advances in materials science have provided numerous options for creating flexible sensors. The core of their application in areas like electronic skin, health medical monitoring, motion monitoring, and human-computer interaction is selecting materials that optimize sensor performance in weight, elasticity, comfort, and flexibility. This article focuses on flexible sensors, analyzing their "sensing mechanisms-materials-applications" framework. It explores their development trajectory, material characteristics, and contributions in various domains such as electronic skin, health medical monitoring, and human-computer interaction. The article concludes by summarizing current research achievements and discussing future challenges and opportunities. Flexible sensors are expected to continue expanding into new fields, driving the evolution of smart wearables and contributing to the intelligent development of society.

Electrospun multifunctional nanofibers for advanced wearable sensors

The multifunctional extension of fiber-based wearable sensors determines their integration and sustainable development, with electrospinning technology providing reliable, efficient, and scalable support for fabricating these sensors. Despite numerous studies on electrospun fiber-based wearable sensors, further attention is needed to leverage composite structural engineering for functionalizing electrospun fibers. This paper systematically reviews the research progress on fiber-based multifunctional wearable sensors in terms of design concept, device fabrication, mechanism exploration, and application potential. Firstly, the basics of electrospinning are briefly introduced, including its development, principles, parameters, and material selection. Tactile sensors, as crucial components of wearable sensors, are discussed in detail, encompassing their performance parameters, transduction mechanisms, and preparation strategies for pressure, strain, temperature, humidity, and bioelectrical signal sensors. The main focus of the article is on the latest research progress in multifunctional sensing design concepts, multimodal decoupling mechanisms, sensing mechanisms, and functional extensions. These extensions include multimodal sensing, self-healing, energy harvesting, personal thermal management, EMI shielding, antimicrobial properties, and other capabilities. Furthermore, the review assesses existing challenges and outlines future developments for multifunctional wearable sensors, highlighting the need for continued research and innovation.

Flexible Pressure, Humidity, and Temperature Sensors for Human Health Monitoring

The rapid advancements in artificial intelligence, micro-nano manufacturing, and flexible electronics technology have unleashed unprecedented innovation and opportunities for applying flexible sensors in healthcare, wearable devices, and human-computer interaction. The human body's tactile perception involves physical parameters such as pressure, temperature, and humidity, all of which play an essential role in maintaining human health. Inspired by the sensory function of human skin, many bionic sensors have been developed to simulate human skin's perception to various stimuli and are widely applied in health monitoring. Given the urgent requirements for sensing performance and integration of flexible sensors in the field of wearable devices and health monitoring, here is a timely overview of recent advances in pressure, humidity, temperature, and multi-functional sensors for human health monitoring. It covers the fundamental components of flexible sensors and categorizes them based on different response mechanisms, including resistive, capacitive, voltage, and other types. Specifically, the application of these flexible tactile sensors in the area of human health monitoring is highlighted. Based on this, an extended overview of recent advances in dual/triple-mode flexible sensors integrating pressure, humidity, and temperature tactile sensing is presented. Finally, the challenges and opportunities of flexible sensors are discussed.

Silk Flocked Flexible Sensor Capable of Wide-Range and Sensitive Pressure Perception

In recent years, there have been advancements in high-performance soft sensors with simultaneous moderate sensitivity and wide linearity. However, it remains challenging to combine high-efficiency production and high performance for soft sensors. The skin and hair structure provide an elegantly simple sensing model, where hair acts as signal receptors and basal skin acts as signal processors. Herein, we used efficient electrostatic flocking and extrusion printing to engineer comb-shaped electrodes with conductive polydimethylsiloxane (PDMS) flocked with conductive silk fibers as a biomimetic flexible sensor. The mechanical and electrical properties of modified PDMS and silk fibers were characterized to optimize the functionalization process. The sensor unit exhibited a high linear range up to 2,000 kPa and a competitively good sensitivity of 0.0285 kPa-1 in the contact mode with conductive materials, as well as good resolution in the noncontact mode. Such sensors and a sensor array demonstrated potential applications for detecting pressure disturbances from acoustic activity and for human-robot interactions. We anticipate that the straightforward design and facile fabrication of soft sensors with vertical fibrous morphology for perception will open new avenues for the next generation of high-performance soft sensors integrated with artificial intelligence.

High‐Resolution Carbon‐Based Tactile Sensor Array for Dynamic Pulse Imaging

With the development of modern medicine, the importance of continuous and reliable pulse wave monitoring has increased significantly in physiological evaluation and disease diagnosis. Among them, the 3D reconstruction of the pulse wave is indispensable, and needs rely on ultra‐high resolution sensor arrays, that is, high spatial resolution, temporal resolution, and force resolution. Herein, a flexible high‐density 32 × 32 tactile sensor array based on pressure‐sensitive tunneling mechanism is develpoed. Conformal graphene nanowalls (GNWs) pattern arrays are deposited on micro‐pyramidal structural Si substrate via mask‐assisted plasma enhanced chemical vapor deposition (PECVD) method and are adopted as pressure‐sensitive electrode, exhibiting a spatial resolution of 64 dots/cm2, high sensitivity (222.36 kPa−1) and short response time (2 ms). More importantly, HfO2 tunneling layer can effectively suppress noise current, which made it sense weak pressure signals with 1/1000 force resolution and SNR of 36.32 dB. By leveraging its high‐resolution array, more holistic pulse signals are acquired and the 3D shape of the pulse wave are successfully replicated. This work shows high‐resolution sensors have significant promise for applications in remote intelligent diagnostics.

Topic Editorial on Flexible Electronics

Fields such as the Internet of Things (IoT), smart healthcare, and intelligent manufacturing are at the forefront of technological advancement, involving the extensive deployment of numerous sophisticated electronic systems and devices [...].

A physiological perspective of the relevance of sweat biomarkers and their detection by wearable microfluidic technology: A review

The great majority of published microfluidic wearable platforms for sweat sensing focus on the development of the technology to fabricate the device, the integration of sensing materials and actuators and the fluidics of sweat within the device. However, very few papers have discussed the physiological relevance of the metabolites measured using these novel approaches. In fact, some of the analytes present in sweat, which serve as biomarkers in blood, do not show a correlation with blood levels. This discrepancy can be attributed to factors such as contamination during measurements, the metabolism of sweat glands, or challenges in obtaining significant samples. The objective of this review is to present a critical and meaningful insight into the real applicability and potential use of wearable technology for improving health and sport performance. It also discusses the current limitations and future challenges of microfluidics, aiming to provide accurate information about the actual needs in this field. This work is expected to contribute to the future development of more suitable wearable microfluidic technology for health and sports science monitoring, using sweat as the biofluid for analysis.

Ultrasensitive Flexible Strain Sensor Made with Carboxymethyl-Cellulose-Anchored Carbon Nanotubes/MXene for Machine-Learning-Assisted Handwriting Recognition

The combination of wearable sensors with machine learning enables intelligent perception in human-machine interaction and healthcare, but achieving high sensitivity and a wide working range in flexible strain sensors for signal acquisition and accurate recognition remains challenging. Herein, we introduced carboxymethyl cellulose (CMC) into a carbon nanotubes (CNTs)/MXene hybrid network, forming tight anchoring among the conductive materials and, thus, bringing enhanced interaction. The silicone-rubber-encapsulated CMC-anchored CNTs/MXene (CCM) strain sensor exhibits an excellent sensitivity (maximum gauge factor up to 71 294), wide working range (200%), ultralow detection limit (0.05%), and outstanding durability (over 10 000 cycles), which is superior to most of the recently reported counterparts also based on a conductive composite film. Moreover, the sensor achieves seamless integration with human skin with the help of a poly(acrylic acid) adhesive layer, successfully obtaining stable and clear waveforms with meaningful profiles from the human body. On this basis, we proposed and realized a novel in-air handwriting recognition method via extracting multiple features of high-quality strain signals assisted by deep neural networks, achieving a high classification accuracy of 98.00 and 94.85% for Arabic numerals and letters, respectively. Our work provides an effective approach for significantly improving strain sensing performance, thereby facilitating innovative applications of flexible sensors.

Luminescence Probes in Bio-Applications: From Principle to Practice

Bioanalysis based on optical imaging has gained significant progress in the last few decades. Luminescence probes are capable of detecting, monitoring, and tracing particular biomolecules in complex biological systems to figure out the roles of these molecules in organisms. Considering the rapid development of luminescence probes for bio-applications and their promising future, we have attempted to explore the working principles and recent advances in bio-applications of luminescence probes, in the hope of helping readers gain a detailed understanding of luminescence probes developed in recent years. In this review, we first focus on the current widely used luminescence probes, including fluorescence probes, bioluminescence probes, chemiluminescence probes, afterglow probes, photoacoustic probes, and Cerenkov luminescence probes. The working principles for each type of luminescence probe are concisely described and the bio-application of the luminescence probes is summarized by category, including metal ions detection, secretion detection, imaging, and therapy.

Implementation of an epicardial implantable MEMS sensor for continuous and real-time postoperative assessment of left ventricular activity in adult minipigs over a short- and long-term period

The sensing of left ventricular (LV) activity is fundamental in the diagnosis and monitoring of cardiovascular health in high-risk patients after cardiac surgery to achieve better short- and long-term outcome. Conventional approaches rely on noninvasive measurements even if, in the latest years, invasive microelectromechanical systems (MEMS) sensors have emerged as a valuable approach for precise and continuous monitoring of cardiac activity. The main challenges in designing cardiac MEMS sensors are represented by miniaturization, biocompatibility, and long-term stability. Here, we present a MEMS piezoresistive cardiac sensor capable of continuous monitoring of LV activity over time following epicardial implantation with a pericardial patch graft in adult minipigs. In acute and chronic scenarios, the sensor was able to compute heart rate with a root mean square error lower than 2 BPM. Early after up to 1 month of implantation, the device was able to record the heart activity during the most important phases of the cardiac cycle (systole and diastole peaks). The sensor signal waveform, in addition, closely reflected the typical waveforms of pressure signal obtained via intraventricular catheters, offering a safer alternative to heart catheterization. Furthermore, histological analysis of the LV implantation site following sensor retrieval revealed no evidence of myocardial fibrosis. Our results suggest that the epicardial LV implantation of an MEMS sensor is a suitable and reliable approach for direct continuous monitoring of cardiac activity. This work envisions the use of this sensor as a cardiac sensing device in closed-loop applications for patients undergoing heart surgery.

High-Performance Dual-Responsive Sensing Skin Enabled by Bioinspired Transduction of Coplanar Square-Loop Electrodes

Advancements in intelligent robots and human-machine interaction necessitate a shift in artificial skins toward multimodal perception. Dual-responsive skins that can detect proximity and pressure information are significant to establishing continuous sensing of interaction processes and extending interactive application scenarios. To address the current limitations of inadequate dual-mode performance, such as limited proximal response change and low tactile sensitivity, this paper presents a bioinspired complementary gradient architecture-enabled (CGA) transduction design and a high-performance dual-responsive skin based on coplanar square-loop electrodes. Through systematic investigation into the transduction of various electrode configurations, comparative results reveal the remarkable potential of coplanar electrodes to deliver superior dual-mode performance without compromise. Simulations and experiments prove that the proposed CGA response mechanism can capture local interface deformation and overall compression signals, further enhancing response sensitivity. The final developed artificial skin is sensitive to external pressure and the approach of objects simultaneously, exhibiting a long detection distance (∼40 mm), a high proximity response (>0.4), and outstanding touch sensitivity (0.131 kPa-1). Furthermore, we demonstrate proof-of-concept applications for the proposed sensing skin in a dual-mode teleoperation interface and adaptive grasping interactions.

Normal-Direction Graded Hemispheres for Ionic Flexible Sensors with a Record-High Linearity in a Wide Working Range

Flexible pressure sensors developed rapidly with increased sensitivity, a fast response time, high stability, and excellent deformability. These progresses have expanded the application of wearable electronics under high-pressure backgrounds while also bringing new challenges. In particular, the nonlinearity and narrow working range lead to a gradually insensitive response, principally because the microstructure deforms inconsistently on the device interfaces in the whole working range. Herein, we report an ionic flexible sensor with a record-high linearity (R2 = 0.99994) in a wide working range (up to 600 kPa). The linearity response comes from the normal-direction graded hemisphere (GH) microstructure. It is prepared from poly(dimethylsiloxane) (PDMS)/carbon nanotubes (CNTs)/Au into flexible and deformable electrodes, and its geometry is precisely designed from the linear elastic theory and optimized through finite element simulation. The sensor can achieve a high sensitivity of S = 165.5 kPa-1, a response-relaxation time of <30 ms, and superb consistency, allowing the device to detect vibration signals. Our sensor has been assembled with circuits and capsulation in order to monitor the function state of players in underwater sports in the frequency domain. This work deepens the theory of linearized design of microstructures and provides a strategy to make flexible pressure sensors that have combined the performances of ultrahigh linearity, high sensitivity, and a wide working range.