A flexible organic reflectance oximeter array

Self-Driven Perovskite/Organic Quasi-Tandem Photodetectors Operating in Both Narrowband and Broadband Regimes

Dual-band photodetectors (PDs) have attracted extensive research attention due to their great potential for diverse and refreshing application scenarios in full-color imaging, optical communication, and imaging detection. Here, a self-driven dual-band PD without filters and other auxiliary equipment to achieve a narrowband response in Mode 1 and a broadband response in Mode 2 was designed based on carrier-selective transmission narrowing (CSTN). The polymer material poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), which has the appropriate energy level, was selected to be the carrier-selective transmission layer. In Mode 1, the dual-band PD exhibits a near-infrared (NIR) narrowband response in 750-900 nm, which indicates a responsivity of 360 mA/W, a full-width at half-maximum (fwhm) of 81 nm, and a specific detectivity (D*) of 7.49 × 1010 Jones at 810 nm. Simultaneously, in Mode 2, the dual-band PD exhibits a UV-visible-NIR broadband responsivity of 180 mA/W and a specific detectivity (D*) of 3.8 × 1010 Jones at 520 nm. Our study provides a reliable idea for the commercial applications of dual-function photodetectors.

Quantum-Dot Light-Emitting Fiber Toward All-In-One Clothing-Type Health Monitoring

In the Fourth Industrial Revolution, as the connection between objects and people becomes increasingly important, interest in wearable optoelectronic device-based medical diagnosis is on the rise. Pulse oximetry sensors based on a fiber platform, which is the smallest unit of clothing, could be considered an attractive candidate for this application. In this study, red and green quantum-dot light-emitting fibers (QDLEFs) based on a 250 μm-diameter 1-dimensional fiber were successfully implemented, achieving high current efficiencies of approximately 22.46 mW/sr/A and 23.6 mW/sr/A and narrow full-width at half-maximum (FWHM) of about 33 nm, respectively. In addition, its omnidirectional flexibility was confirmed through a vertical and lateral bending test with 0.92% strain. By employing a transparent and flexible elastomer, a wearable pulse oximeter incorporating QDLEFs was successfully demonstrated for oxygen saturation level (SpO2) monitoring on finger and wrist. It was demonstrated to be washable, and could be operated for up to about 18 h. Due to the elastomer and bottom emission, it exhibited excellent wear resistance characteristics in a 50 cycle reciprocating test conducted at about 2180.43 kPa with 220-grit abrasive paper sheet. A theoretical investigation based on modified photon diffusion analysis (MPDA) modeling also determined that using narrow FWHM light sources, such as QDLEFs, improves the resolution and accuracy of SpO2 monitoring. Accordingly, the proposed QDLEF showed distinguished potential as an all-in-one clothing type pulse oximetry.

Stretchable OLEDs based on a hidden active area for high fill factor and resolution compensation

Stretchable organic light-emitting diodes (OLEDs) have emerged as promising optoelectronic devices with exceptional degree of freedom in form factors. However, stretching OLEDs often results in a reduction in the geometrical fill factor (FF), that is the ratio of an active area to the total area, thereby limiting their potential for a broad range of applications. To overcome these challenges, we propose a three-dimensional (3D) architecture adopting a hidden active area that serves a dual role as both an emitting area and an interconnector. For this purpose, an ultrathin OLED is first attached to a 3D rigid island array structure through quadaxial stretching for precise, deformation-free alignment. A portion of the ultrathin OLED is concealed by letting it 'fold in' between the adjacent islands in the initial, non-stretched condition and gradually surfaces to the top upon stretching. This design enables the proposed stretchable OLEDs to exhibit a relatively high FF not only in the initial state but also after substantial deformation corresponding to a 30% biaxial system strain. Moreover, passive-matrix OLED displays that utilize this architecture are shown to be configurable for compensation of post-stretch resolution loss, demonstrating the efficacy of the proposed approach in realizing the full potential of stretchable OLEDs.

Flexible Organic Transistors for Biosensing: Devices and Applications

Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible-OTFT-based biosensors and their broad applications, herein, a timely and comprehensive review is provided. It starts with a detailed introduction to the features of various OTFTs including organic field-effect transistors and organic electrochemical transistors, and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, the applications of flexible-OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces are detailed. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, the remaining challenges and the future opportunities in this field are summarized.

All-solution-processed ultraflexible wearable sensor enabled with universal trilayer structure for organic optoelectronic devices

All-solution-processed organic optoelectronic devices can enable the large-scale manufacture of ultrathin wearable electronics with integrated diverse functions. However, the complex multilayer-stacking device structure of organic optoelectronics poses challenges for scalable production. Here, we establish all-solution processes to fabricate a wearable, self-powered photoplethysmogram (PPG) sensor. We achieve comparable performance and improved stability compared to complex reference devices with evaporated electrodes by using a trilayer device structure applicable to organic photovoltaics, photodetectors, and light-emitting diodes. The PPG sensor array based on all-solution-processed organic light-emitting diodes and photodetectors can be fabricated on a large-area ultrathin substrate to achieve long storage stability. We integrate it with a large-area, all-solution-processed organic solar module to realize a self-powered health monitoring system. We fabricate high-throughput wearable electronic devices with complex functions on large-area ultrathin substrates based on organic optoelectronics. Our findings can advance the high-throughput manufacture of ultrathin electronic devices integrating complex functions.

Effects of Deuterium Isotopes on Pt(II) Complexes and Their Impact on Organic NIR Emitters

Insight into effect of deuterium isotopes on organic near-IR (NIR) emitters was explored by the use of self-assembled Pt(II) complexes H-3-f and HPh-3-f, and their deuterated analogues D-3-f and DPh-3-f, respectively (Scheme 2). In vacuum deposited thin film, albeit having nearly identical emission spectral feature maximized at ~810 nm, H-3-f and D-3-f exhibit remarkable difference in photoluminescence quantum yield (PLQY) of 29 % and 50 %, respectively. Distinction in PLQY is also observed for HPh-3-f (800 nm, 50 %) and DPh-3-f (798 nm, 67 %). We then elucidated the theoretical differences in the impact on near-infrared (NIR) luminescence between Pt(II) complexes and organic small molecules upon deuteration. The results establish a general guideline for the deuteration on NIR emission efficiency. From a perspective of practical application, NIR OLEDs based on D-3-f and DPh-3-f emitters attain EQEmax of 15.5 % (radiance 31,287 mW Sr-1  m-2 ) and 16.6 % (radiance of 32,279 mW Sr-1  m-2 ) at 764 nm and 796 nm, respectively, both of which set new records for NIR OLEDs of >750 nm.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

Indoor Self-Powered Perovskite Optoelectronics with Ultraflexible Monochromatic Light Source

Self-powered skin optoelectronics fabricated on ultrathin polymer films is emerging as one of the most promising components for the next-generation Internet of Things (IoT) technology. However, a longstanding challenge is the device underperformance owing to the low process temperature of polymer substrates. In addition, broadband electroluminescence (EL) based on organic or polymer semiconductors inevitably suffers from periodic spectral distortion due to Fabry-Pérot (FP) interference upon substrate bending, preventing advanced applications. Here, ultraflexible skin optoelectronics integrating high-performance solar cells and monochromatic light-emitting diodes using solution-processed perovskite semiconductors is presented. n-i-p perovskite solar cells and perovskite nanocrystal light-emitting diodes (PNC-LEDs), with power-conversion and current efficiencies of 18.2% and 15.2 cd A-1 , respectively, are demonstrated on ultrathin polymer substrates with high thermal stability, which is a record-high efficiency for ultraflexible perovskite solar cell. The narrowband EL with a full width at half-maximum of 23 nm successfully eliminates FP interference, yielding bending-insensitive spectra even under 50% of mechanical compression. Photo-plethysmography using the skin optoelectronic device demonstrates a signal selectivity of 98.2% at 87 bpm pulse. The results presented here pave the way to inexpensive and high-performance ultrathin optoelectronics for self-powered applications such as wearable displays and indoor IoT sensors.

Wearable and Wavelength-Tunable Near-Infrared Organic Light-Emitting Diodes for Biomedical Applications

Near-infrared organic light-emitting diodes (NIR OLEDs) have significant potential for wearable phototherapeutic applications because of the unique properties of the OLEDs, including their free-form electronics and the excellent biomedical effects of NIR emission. In spite of their tremendous promise, given that the majority of NIR OLEDs in previous research have relied on the utilization of an intrinsically brittle indium tin oxide (ITO) electrode, their practicality in the field of wearable electronics is inherently constrained. Here, we report wearable and wavelength-tunable NIR OLEDs that employ a high-performance NIR emitter and an innovative architecture by replacing the ITO with a silver (Ag) electrode. The NIR OLEDs permit wavelength tuning of emissions from 700 to 800 nm and afford stable operation even under repeated bending conditions. The NIR OLEDs provide a lowered device temperature of 37.5 °C even during continuous operation under several emission intensities. In vitro experiments were performed with freshly fabricated NIR OLEDs. The outcomes were evaluated against experimental results performed using the same procedure utilizing blue, green, and red OLEDs. When exposed to NIR light irradiation, the promoting effect of cell proliferation surpassed the proliferative responses observed under the influence of visible light irradiation. The proliferation effect of human hair follicle dermal papilla cells is clearly related to the irradiation wavelength and time, thus underscoring the potential of wavelength-tunable NIR OLEDs for efficacious phototherapy. This work will open novel avenues for wearable NIR OLEDs in the field of biomedical application.

Accuracy of Heart Rate Measurement with Wrist-Worn Wearable Devices in Various Skin Tones: a Systematic Review

BACKGROUND

Wearable consumer technology allows for the collection of a growing amount of personal health data. Through the analysis of reflected LED light on the skin, heart rate measurement and arrhythmia detection can be performed. Given that melanin alters skin light absorption, this study seeks to summarize the accuracy of cardiac data from wrist-worn wearable devices for participants of varying skin tones.

METHODS

We conducted a systematic review, searching Embase, MEDLINE, CINAHL, and Cochrane for original studies that stratified heart rate and rhythm data for consumer wearable technology according to participant race and/or skin tone.

RESULTS

A total of 10 studies involving 469 participants met inclusion criteria. The frequency-weighted Fitzpatrick score for skin tone was reported in six studies (n = 293), with a mean participant score of 3.5 (range 1-6). Overall, four of the ten studies reported a significant reduction in accuracy of heart rate measurement with wearable devices in darker-skinned individuals, compared to participants with lighter skin tones. Four studies noted no effect of user skin tone on accuracy. The remaining two studies showed mixed results.

CONCLUSIONS

Preliminary evidence is inconclusive, but some studies suggest that wearable devices may be less accurate for detecting heart rate in participants with darker skin tones. Higher quality evidence is necessary, with larger sample sizes and more objective stratification of participants by skin tone, in order to characterize potential racial bias in consumer devices.

Near-Infrared Organic Photodetectors toward Skin-Integrated Photoplethysmography-Electrocardiography Multimodal Sensing System

In the fast-evolving landscape of decentralized and personalized healthcare, the need for multimodal biosensing systems that integrate seamlessly with the human body is growing rapidly. This presents a significant challenge in devising ultraflexible configurations that can accommodate multiple sensors and designing high-performance sensing components that remain stable over long periods. To overcome these challenges, ultraflexible organic photodetectors (OPDs) that exhibit exceptional performance under near-infrared illumination while maintaining long-term stability are developed. These ultraflexible OPDs demonstrate a photoresponsivity of 0.53 A W-1 under 940 nm, shot-noise-limited specific detectivity of 3.4 × 1013 Jones, and cut-off response frequency beyond 1 MHz at -3 dB. As a result, the flexible photoplethysmography sensor boasts a high signal-to-noise ratio and stable peak-to-peak amplitude under hypoxic and hypoperfusion conditions, outperforming commercial finger pulse oximeters. This ensures precise extraction of blood oxygen saturation in dynamic working conditions. Ultraflexible OPDs are further integrated with conductive polymer electrodes on an ultrathin hydrogel substrate, allowing for direct interface with soft and dynamic skin. This skin-integrated sensing platform provides accurate measurement of photoelectric and biopotential signals in a time-synchronized manner, reproducing the functionality of conventional technologies without their inherent limitations.

Applications of flexible electronics related to cardiocerebral vascular system

Ensuring accessible and high-quality healthcare worldwide requires field-deployable and affordable clinical diagnostic tools with high performance. In recent years, flexible electronics with wearable and implantable capabilities have garnered significant attention from researchers, which functioned as vital clinical diagnostic-assisted tools by real-time signal transmission from interested targets in vivo. As the most crucial and complex system of human body, cardiocerebral vascular system together with heart-brain network attracts researchers inputting profuse and indefatigable efforts on proper flexible electronics design and materials selection, trying to overcome the impassable gulf between vivid organisms and rigid inorganic units. This article reviews recent breakthroughs in flexible electronics specifically applied to cardiocerebral vascular system and heart-brain network. Relevant sensor types and working principles, electronics materials selection and treatment methods are expounded. Applications of flexible electronics related to these interested organs and systems are specially highlighted. Through precedent great working studies, we conclude their merits and point out some limitations in this emerging field, thus will help to pave the way for revolutionary flexible electronics and diagnosis assisted tools development.

The 2023 wearable photoplethysmography roadmap

Photoplethysmography is a key sensing technology which is used in wearable devices such as smartwatches and fitness trackers. Currently, photoplethysmography sensors are used to monitor physiological parameters including heart rate and heart rhythm, and to track activities like sleep and exercise. Yet, wearable photoplethysmography has potential to provide much more information on health and wellbeing, which could inform clinical decision making. This Roadmap outlines directions for research and development to realise the full potential of wearable photoplethysmography. Experts discuss key topics within the areas of sensor design, signal processing, clinical applications, and research directions. Their perspectives provide valuable guidance to researchers developing wearable photoplethysmography technology.

Recent Progress in Flexible and Wearable All Organic Photoplethysmography Sensors for SpO2 Monitoring

Flexible and wearable biosensors are the next-generation healthcare devices that can efficiently monitor human health conditions in day-to-day life. Moreover, the rapid growth and technological advancements in wearable optoelectronics have promoted the development of flexible organic photoplethysmography (PPG) biosensor systems that can be implanted directly onto the human body without any additional interface for efficient bio-signal monitoring. As an example, the pulse oximeter utilizes PPG signals to monitor the oxygen saturation (SpO2 ) in the blood volume using two distinct wavelengths with organic light emitting diode (OLED) as light source and an organic photodiode (OPD) as light sensor. Utilizing the flexible and soft properties of organic semiconductors, pulse oximeter can be both flexible and conformal when fabricated on thin polymeric substrates. It can also provide highly efficient human-machine interface systems that can allow for long-time biological integration and flawless measurement of signal data. In this work, a clear and systematic overview of the latest progress and updates in flexible and wearable all-organic pulse oximetry sensors for SpO2 monitoring, including design and geometry, processing techniques and materials, encapsulation and various factors affecting the device performance, and limitations are provided. Finally, some of the research challenges and future opportunities in the field are mentioned.

Organic Electronics-Microfluidics/Lab on a Chip Integration in Analytical Applications

Organic electronics (OE) technology has matured in displays and is advancing in solid-state lighting applications. Other promising and growing uses of this technology are in (bio)chemical sensing, imaging, in vitro cell monitoring, and other biomedical diagnostics that can benefit from low-cost, efficient small devices, including wearable designs that can be fabricated on glass or flexible plastic. OE devices such as organic LEDs, organic and hybrid perovskite-based photodetectors, and organic thin-film transistors, notably organic electrochemical transistors, are utilized in such sensing and (bio)medical applications. The integration of compact and sensitive OE devices with microfluidic channels and lab-on-a-chip (LOC) structures is very promising. This survey focuses on studies that utilize this integration for a variety of OE tools. It is not intended to encompass all studies in the area, but to present examples of the advances and the potential of such OE technology, with a focus on microfluidics/LOC integration for efficient wide-ranging sensing and biomedical applications.

Implantable Hydrogel-Protective DNA Aptamer-Based Sensor Supports Accurate, Continuous Electrochemical Analysis of Drugs at Multiple Sites in Living Rats

The ability to track the levels of specific molecules, such as drugs, metabolites, and biomarkers, in the living body, in real time and for long durations, would improve our understanding of health and our ability to diagnose, treat, and monitor disease. To this end, we are developing electrochemical aptamer-based (EAB) biosensors, a general platform supporting high-frequency, real-time molecular measurements in the living body. Here we report that the use of an agarose hydrogel protective layer for EAB sensors significantly improves their signaling stability when deployed in the complex, highly time-varying environments found in vivo. The improved stability is sufficient that these hydrogel-protected sensors achieved good baseline stability and precision when deployed in situ in the veins, muscles, bladder, or tumors of living rats without the use of the drift correction approaches traditionally required in such placements. Finally, our implantable gel-protective EAB sensors achieved good biocompatibility when deployed in vivo in the living rats without causing any severe inflammation.

Recent advances in wearable sensors and data analytics for continuous monitoring and analysis of biomarkers and symptoms related to COVID-19

The COVID-19 pandemic has changed the lives of many people around the world. Based on the available data and published reports, most people diagnosed with COVID-19 exhibit no or mild symptoms and could be discharged home for self-isolation. Considering that a substantial portion of them will progress to a severe disease requiring hospitalization and medical management, including respiratory and circulatory support in the form of supplemental oxygen therapy, mechanical ventilation, vasopressors, etc. The continuous monitoring of patient conditions at home for patients with COVID-19 will allow early determination of disease severity and medical intervention to reduce morbidity and mortality. In addition, this will allow early and safe hospital discharge and free hospital beds for patients who are in need of admission. In this review, we focus on the recent developments in next-generation wearable sensors capable of continuous monitoring of disease symptoms, particularly those associated with COVID-19. These include wearable non/minimally invasive biophysical (temperature, respiratory rate, oxygen saturation, heart rate, and heart rate variability) and biochemical (cytokines, cortisol, and electrolytes) sensors, sensor data analytics, and machine learning-enabled early detection and medical intervention techniques. Together, we aim to inspire the future development of wearable sensors integrated with data analytics, which serve as a foundation for disease diagnostics, health monitoring and predictions, and medical interventions.

Boosting the Performance of Organic Photodetectors with a Solution-Processed Integration Circuit toward Ubiquitous Health Monitoring

Organic photodetectors, as an emerging wearable photoplethysmographic (PPG) technology, offer exciting opportunities for next-generation photonic healthcare electronics. However, the mutual restraints among photoresponse, structure complexity, and fabrication cost have intrinsically limited the development of organic photodetectors for ubiquitous health monitoring in daily activities. Here, an effective route to dramatically boost the performance of organic photodetectors with a solution-processed integration circuit for health monitoring application is reported. Through creating an ideal metal-semiconductor junction interface that minimizes the trap states within the device, solution-printed organic field-effect transistors (OFETs) are achieved with an ultrahigh signal amplification efficiency of 37.1 S A-1 , approaching the theoretical thermionic limit. Consequently, monolithic integration of the OFET with an organic photoconductor enables the remarkable amplification of photoresponse signal-to-noise ratio by more than four orders of magnitude from 5.5 to 4.6 × 105 , which is able to meet the demand for accurately extracting physiological information from the PPG waveforms. This work offers an effective and versatile approach to greatly enhance the photodetector performance, promising to revolutionize health monitoring technologies.

Enhanced sub-1 eV detection in organic photodetectors through tuning polymer energetics and microstructure

One of the key challenges facing organic photodiodes (OPDs) is increasing the detection into the infrared region. Organic semiconductor polymers provide a platform for tuning the bandgap and optoelectronic response to go beyond the traditional 1000-nanometer benchmark. In this work, we present a near-infrared (NIR) polymer with absorption up to 1500 nanometers. The polymer-based OPD delivers a high specific detectivity D^* of 1.03 × 10¹⁰ Jones (-2 volts) at 1200 nanometers and a dark current J_d of just 2.3 × 10^-6 ampere per square centimeter at -2 volts. We demonstrate a strong improvement of all OPD metrics in the NIR region compared to previously reported NIR OPD due to the enhanced crystallinity and optimized energy alignment, which leads to reduced charge recombination. The high D^* value in the 1100-to-1300-nanometer region is particularly promising for biosensing applications. We demonstrate the OPD as a pulse oximeter under NIR illumination, delivering heart rate and blood oxygen saturation readings in real time without signal amplification.

On-skin biosensors for noninvasive monitoring of postoperative free flaps and replanted digits

Severe soft tissue defects and amputated digits are clinically common injuries. Primary treatments include surgical free flap transfer and digit replantation, but these can fail because of vascular compromise. Postoperative monitoring is therefore crucial for timely detection of vessel obstruction and survival of replanted digits and free flaps. However, current postoperative clinical monitoring methods are labor intensive and highly dependent on the experience of nurses and surgeons. Here, we developed on-skin biosensors for noninvasive and wireless postoperative monitoring based on pulse oximetry. The on-skin biosensor was made of polydimethylsiloxane with gradient cross-linking to create a self-adhesive and mechanically robust substrate that interfaces with skin. The substrate was shown to exhibit appropriate adhesion on one side for both high-fidelity measurements of the sensor and low risk of peeling injury to delicate tissues. The other side demonstrated mechanical integrity to facilitate flexible hybrid integration of the sensor. Validation studies using a model of vascular obstruction in rats demonstrated the effectiveness of the sensor in vivo. Clinical studies indicated that the on-skin biosensor was accurate and more responsive than current clinical monitoring methods in identifying microvascular conditions. Comparisons with existing monitoring techniques, including laser Doppler flowmetry and micro-lightguide spectrophotometry, further verified the sensor's accuracy and ability to identify both arterial and venous insufficiency. These findings suggest that this on-skin biosensor may improve postoperative outcomes in free flap and replanted digit surgeries by providing sensitive and unbiased data directly from the surgical site that can be remotely monitored.

Multifunctional and Multicolor Perovskite-CdSe Quantum Dots Diodes for Pulse Oximetry

A conventional pulse oximeter system is composed of two light sources with different peak emission wavelengths and a photodetector. Integrating these three independent components into one single device will absolutely simplify the system design and create a miniaturized size of the product. Here, we demonstrate a bilayer perovskite-CdSe quantum dot (hereafter perovskite-QD) diode capable of voltage-tunable green/red emission and photodetection. The proposed diode also presents an intriguing feature of simultaneous light emission and detection, which is explored as regards the diode being used as a photoconductor when the positive bias is larger than the built-in voltage. The multifunctional and multicolor diode is further employed in a reflective pulse oximeter system, as either the multicolor light sources or the sensing unit in the system provide accepted and trustful results for heart rate and arterial blood oxygenation. Our work provides a possible avenue for the simplification of the pulse oximetry with a compact and miniaturized design in the future.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al₂O₃ by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al₂O₃ layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al₂O₃ treatment reaches a remarkable level of 2.5 nW cm^-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 10⁸% and 1.85 × 10⁶ A W^-1, respectively, outperforming all other polymer-based PM-OPDs.

Fiber-based quantum-dot pulse oximetry for wearable health monitoring with high wavelength selectivity and photoplethysmogram sensitivity

Increasing demand for real-time healthcare monitoring is leading to advances in thin and flexible optoelectronic device-based wearable pulse oximetry. Most previous studies have used OLEDs for this purpose, but did not consider the side effects of broad full-width half-maximum (FWHM) characteristics and single substrates. In this study, we performed SpO₂ measurement using a fiber-based quantum-dot pulse oximetry (FQPO) system capable of mass production with a transferable encapsulation technique, and a narrow FWHM of about 30 nm. Based on analyses we determined that uniform angular narrow FWHM-based light sources are important for accurate SpO₂ measurements through multi-layer structures and human skin tissues. The FQPO was shown to have improved photoplethysmogram (PPG) signal sensitivity with no waveguide-mode noise signal, as is typically generated when using a single substrate (30-50%). We successfully demonstrate improved SpO₂ measurement accuracy as well as all-in-one clothing-type pulse oximetry with FQPO.

Vitality surveillance at distance using thin-film tandem-like narrowband near-infrared photodiodes with light-enhanced responsivity

Remote measurement of vital sign parameters like heartbeat and respiration rate represents a compelling challenge in monitoring an individual's health in a noninvasive way. This could be achieved by large field-of-view, easy-to-integrate unobtrusive sensors, such as large-area thin-film photodiodes. At long distances, however, discriminating weak light signals from background disturbance demands superior near-infrared (NIR) sensitivity and optical noise tolerance. Here, we report an inherently narrowband solution-processed, thin-film photodiode with ultrahigh and controllable NIR responsivity based on a tandem-like perovskite-organic architecture. The device has low dark currents (<10^-6 mA cm^-2), linear dynamic range >150 dB, and operational stability over time (>8 hours). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically filtered silicon-based sensors. We demonstrate its potential in remote monitoring by measuring the heart rate and respiration rate from distances up to 130 cm in reflection.

Perovskite Solar Cell-Gated Organic Electrochemical Transistors for Flexible Photodetectors with Ultrahigh Sensitivity and Fast Response

Photodetectors (PDs) are the building block of various imaging and sensing applications. However, commercially available PDs based on crystalline inorganic semiconductors cannot meet the requirements of emerging wearable/implantable applications due to their rigidity and fragility, which creates the need for flexible devices. Here, a high-performance flexible PD is presented by gating an organic electrochemical transistor (OECT) with a perovskite solar cell. Due to the ultrahigh transconductance of the OECT, the device demonstrates a high gain of ≈10⁶ , a fast response time of 67 µs and an ultrahigh detectivity of 6.7 × 10¹⁷ Jones to light signals under a low working voltage (≤0.6 V). Thanks to the ultrahigh sensitivity and fast response, the device can track photoplethysmogram signals and peripheral oxygen saturation under ambient light and even provide contactless remote sensing, offering a low-power and convenient way for continuous vital signs monitoring. This work offers a novel strategy for realizing high-performance flexible PDs that are promising for low-power, user-friendly and wearable optoelectronics.

Stable and Dynamic Multiparameter Monitoring on Chests Using Flexible Skin Patches with Self-Adhesive Electrodes and a Synchronous Correlation Peak Extraction Algorithm

Advances in wearable bioelectronics interfacing directly with skin offer important tools for non-invasive measurements of physiological parameters. However, wearable monitoring devices majorly conduct static sensing to avoid signal disturbance and unreliable contact with the skin. Dynamic multiparameter sensing is challenging even with the advanced flexible skin patches. This epidermal electronics system with self-adhesive conductive electrodes to supply stable skin contact and a unique synchronous correlation peak extraction (SCPE) algorithm to minimize motion artifacts in the photoplethysmogram (PPG) signals. The skin patch system can simultaneously and precisely monitor electrocardiogram (ECG), PPG, body temperature, and acceleration on chests undergoing daily activities. The low latency between the ECG and the PPG signals enables the SCPE algorithm that leads to reduced errors in deduced heart rates and improved performance in oxygen level determination than conventional adaptive filtering and wavelet transformation approaches. Dynamic multiparameter recording over 24 h by the system can reflect the circadian patterns of the wearers with low disturbance from motion artifacts. This demonstrated system may be applied for health monitoring in large populations to alleviate pressure on medical systems and assist management of public health crisis.

Organic photodiodes: device engineering and applications

Organic photodiodes (OPDs) have shown great promise for potential applications in optical imaging, sensing, and communication due to their wide-range tunable photoelectrical properties, low-temperature facile processes, and excellent mechanical flexibility. Extensive research work has been carried out on exploring materials, device structures, physical mechanisms, and processing approaches to improve the performance of OPDs to the level of their inorganic counterparts. In addition, various system prototypes have been built based on the exhibited and attractive features of OPDs. It is vital to link the device optimal design and engineering to the system requirements and examine the existing deficiencies of OPDs towards practical applications, so this review starts from discussions on the required key performance metrics for different envisioned applications. Then the fundamentals of the OPD device structures and operation mechanisms are briefly introduced, and the latest development of OPDs for improving the key performance merits is reviewed. Finally, the trials of OPDs for various applications including wearable medical diagnostics, optical imagers, spectrometers, and light communications are reviewed, and both the promises and challenges are revealed.

A photoacoustic patch for three-dimensional imaging of hemoglobin and core temperature

Electronic patches, based on various mechanisms, allow continuous and noninvasive monitoring of biomolecules on the skin surface. However, to date, such devices are unable to sense biomolecules in deep tissues, which have a stronger and faster correlation with the human physiological status than those on the skin surface. Here, we demonstrate a photoacoustic patch for three-dimensional (3D) mapping of hemoglobin in deep tissues. This photoacoustic patch integrates an array of ultrasonic transducers and vertical-cavity surface-emitting laser (VCSEL) diodes on a common soft substrate. The high-power VCSEL diodes can generate laser pulses that penetrate >2 cm into biological tissues and activate hemoglobin molecules to generate acoustic waves, which can be collected by the transducers for 3D imaging of the hemoglobin with a high spatial resolution. Additionally, the photoacoustic signal amplitude and temperature have a linear relationship, which allows 3D mapping of core temperatures with high accuracy and fast response. With access to biomolecules in deep tissues, this technology adds unprecedented capabilities to wearable electronics and thus holds significant implications for various applications in both basic research and clinical practice.

Detection of fluorescence-labeled DNA with in-plane organic optoelectronic devices

We present a system efficiency analysis of a monolithic integrated organic optoelectronic unit for the detection of fluorescence labeled single-stranded DNA (ssDNA) for veterinary disease testing. The side-by-side integration of an organic light emitting diode (OLED) and an organic photodetector (OPD) with 0.5 mm by 0.5 mm device sizes has the potential to enable compact and low-cost fluorescence point-of-care (POC) devices for decentral multiplex biomedical testing. Here, we used two 6-FAM and BHQ1 labeled complementary ssDNA strands to form the Förster resonance transfer (FRET) upon the hybridization of the DNA. In this work we successfully show ssDNA hybridization sensing with samples diluted in TE buffer and investigate the detection of covalently bound 6-FAM-ssDNA on a glass surface for multiplex biomarker measurements.

A Sparse Sampling Sensor Front-End IC for Low Power Continuous SpO 2 & HR Monitoring

Photoplethysmography (PPG) is an attractive method to acquire vital signs such as heart rate and blood oxygenation and is frequently used in clinical and at-home settings. Continuous operation of health monitoring devices demands a low power sensor that does not restrict the device battery life. Silicon photodiodes (PD) and LEDs are commonly used as interface devices in PPG sensors; however, using of flexible organic devices can enhance the sensor conformality and reduce the cost of fabrication. In most PPG sensors, most of system power consumption is concentrated in powering LEDs, traditionally consuming mWs. Using organic devices further increases this power demand since these devices exhibit larger parasitic capacitances and typically need higher drive voltages.This work presents a sensor IC for continuous SpO ₂ and HR monitoring that features an on-chip reconstruction-free sparse sampling algorithm to reduce the overall system power consumption by  ∼ 70% while maintaining the accuracy of the output information. The designed frontend is compatible with a wide range of devices from silicon PDs to organic PDs with parasitic capacitances up to 10 nF. Implemented in a 40 nm HV CMOS process, the chip occupies 2.43 mm ² and consumes 49.7 μW and 15.2 μW of power in continuous and sparse sampling modes respectively. The performance of the sensor IC has been verified in vivo with both types of devices and the results are compared against a clinical grade reference. Less than 1 bpm and 1% mean absolute errors were achieved in both continuous and sparse modes of operation.

Miniaturizing a Chip-Scale Spectrometer Using Local Strain Engineering and Total-Variation Regularized Reconstruction

A wafer-thin chip-scale portable spectrometer suitable for wearable applications based on a reconstructive algorithm was demonstrated. A total of 16 spectral encoders that simultaneously functioned as photodetectors were monolithically integrated on a chip area of 0.16 mm² by applying local strain engineering in compressively strained InGaN/GaN multiple quantum well heterostructures. The built-in GaN pn junction enabled a direct photocurrent measurement. A non-negative least-squares (NNLS) algorithm with total-variation regularization and a choice of a proper kernel function was shown to deliver a decent spectral reconstruction performance in the wavelength range of 400-645 nm. The accuracies of spectral peak positions and intensity ratios between peaks were found to be 0.97% and 10.4%, respectively. No external optics, such as collimation optics and apertures, were used, enabled by angle-insensitive light-harvesting structures, including an array of cone-shaped backreflectors fabricated on the underside of the sapphire substrate.

Advances in Flexible Organic Photodetectors: Materials and Applications

Future electronics will need to be mechanically flexible and stretchable in order to enable the development of lightweight and conformal applications. In contrast, photodetectors, an integral component of electronic devices, remain rigid, which prevents their integration into everyday life applications. In recent years, significant efforts have been made to overcome the limitations of conventional rigid photodetectors, particularly their low mechanical deformability. One of the most promising routes toward facilitating the fabrication of flexible photodetectors is to replace conventional optoelectronic materials with nanomaterials or organic materials that are intrinsically flexible. Compared with other functional materials, organic polymers and molecules have attracted more attention for photodetection applications due to their excellent photodetection performance, cost-effective solution-fabrication capability, flexible design, and adaptable manufacturing processes. This article comprehensively discusses recent advances in flexible organic photodetectors in terms of optoelectronic, mechanical properties, and hybridization with other material classes. Furthermore, flexible organic photodetector applications in health-monitoring sensors, X-ray detection, and imager devices have been surveyed.

Ultra-Thin Flexible Encapsulating Materials for Soft Bio-Integrated Electronics

Recently, bioelectronic devices extensively researched and developed through the convergence of flexible biocompatible materials and electronics design that enables  more precise diagnostics and therapeutics in human health care and opens up the potential to expand into various fields, such as clinical medicine and biomedical research. To establish an accurate and stable bidirectional bio-interface, protection against the external environment and high mechanical deformation is essential for wearable bioelectronic devices. In the case of implantable bioelectronics, special encapsulation materials and optimized mechanical designs and configurations that provide electronic stability and functionality are required for accommodating various organ properties, lifespans, and functions in the biofluid environment. Here, this study introduces recent developments of ultra-thin encapsulations with novel materials that can preserve or even improve the electrical performance of wearable and implantable bio-integrated electronics by supporting safety and stability for protection from destruction and contamination as well as optimizing the use of bioelectronic systems in physiological environments. In addition, a summary of the materials, methods, and characteristics of the most widely used encapsulation technologies is introduced, thereby providing a strategic selection of appropriate choices of recently developed flexible bioelectronics.

Bulk-Heterojunction Adjustment Enables a Self-filtering Organic Photodetector with a Narrowband Response

Image-sensor technology is the foundation of many emerging applications, where the photodetector is designed to interact with incoming photons that have specific colors or wavelengths. A color filter is therefore crucial to enable the selective spectral response of the photodetector and to eliminate the crosstalk interference resulting from ambient lights. Unfortunately, a reduced detection sensitivity of the photodetector is inevitable due to an imperfect light filtering, which greatly limits the practical applications of selective-response photodetectors. Herein, we demonstrate a bulk-heterojunction (BHJ) organic composite featuring a self-filtering light responsive characteristic. Through a careful optimization of the BHJ film, the organic photodetector (OPD) demonstrates a high-selective spectral response to the infrared (IR) radiation without the need of applying a color filter. As a result, the self-filtering top-illuminated OPD exhibits a narrowband external quantum efficiency (EQE) of 53% with a narrow full width at half-maximum (fwhm) of 56 nm centering at 1080 nm. A high responsivity of 0.46 A W^-1 is also achieved at 1080 nm wavelength due to the self-filtering characteristic.

Accuracy enhancement in reflective pulse oximetry by considering wavelength-dependent pathlengths

Objective. Noninvasive measurement of oxygen saturation (SpO₂) using pulse oximetry based on transmissive photoplethysmography (tPPG) is clinically accepted and widely employed. However, reflective photoplethysmography (rPPG) - present in smartwatches - has not become equally accepted, partially because the pathlengths of the red and infrared PPGs are patient-dependent. Thus, even the most popular "Ratio of Modulation" (R) method requires patient-dependent calibration to reduce the errors in the measurement of SpO2 using rPPGs.Approach. In this paper, a correction factor or "pathlength ratio" β is introduced in an existing calibration-free algorithm that compensates the patient-dependent pathlength variations, and improved accuracy is obtained in the measurement of SpO2 using rPPGs. The proposed β is derived through the analytical model of a rPPG signal. Using the new expression and data obtained from a human hypoxia study wherein arterial oxygen saturation values acquired through Blood Gas Analysis were employed as a reference, β is determined.Main results. The results of the analysis show that a specific combination of the β and the measurements on the pulsating part of the natural logarithm of the red and infrared PPG signals yields a reduced root-mean-square error (RMSE). It is shown that the average RMSE in measuring SpO2 values reduces to 1 %.Significance. The human hypoxia study data used for this work, obtained in a previous study, coversSpO₂values in the range from 70 % to 100 %, and thus shows that the pathlength ratio β proposed here works well in the range of clinical interest. This work demonstrates that the calibration-free method applicable for transmission type PPGs can be extended to determineSpO₂using reflective PPGs with the incorporation of the correction factor β. Our algorithm significantly reduces the number of parameters needed for the estimation, while keeping the RMSE below the clinically accepted 2 %.

Enhanced performance and stability in InGaZnO NIR phototransistors with alumina-infilled quantum dot solid

The optimized ALD infilling process for depositing Al₂O₃ in the vertical direction of PbS QDs enhances the photoresponsivity, relaxation rate and the air stability of PbS QDs hybrid IGZO NIR phototransistors. Infilled Al₂O₃, which is gradually deposited from the top of PbS QDs to the PbS/IGZO interface (1) passivates the trap sites up to the interface of PbS/IGZO without disturbing charge transfer and (2) prevents QDs deterioration caused by outside air. Therefore, an Al₂O₃ infilled PbS QD/IGZO hybrid phototransistor (AI-PTs) exhibited enhanced photoresponsivity from 96.4 A/W to 1.65 × 10² A/W and a relaxation time decrease from 0.52 to 0.03 s under NIR light (880 nm) compared to hybrid phototransistors without Al₂O₃ (RF-PTs). In addition, AI-PTs also showed improved shelf stability over 4 months compared to RF-PTs. Finally, all devices we manufactured have the potential to be manufactured in an array, and this ALD technique is a means of fabricating robust QDs/metal oxide hybrids for optoelectronic devices.

High performance visible-SWIR flexible photodetector based on large-area InGaAs/InP PIN structure

Mechanically flexible optoelectronic devices and systems can enable a much broader range of applications than what their rigid counterparts can do, especially for novel bio-integrated optoelectronic systems, flexible consumer electronics and wearable sensors. Inorganic semiconductor could be a good candidate for the flexible PD when it can keep its high performance under the bending condition. Here, we demonstrate a III–V material-based flexible photodetector operating wavelength from 640 to 1700 nm with the high detectivity of 5.18 × 10¹¹ cm‧Hz^1/2/W and fast response speed @1550 nm by using a simply top-to-down fabrication process. The optoelectrical performances are stable as the PDs are exposed to bending cycles with a radius of 15 mm up to 1000 times. Furthermore, the mechanical failure mode of the PD is also investigated, which suggests that the cracking and delamination failure mode are dominant in bending up and bending down direction, respectively. Such a flexible III–V material-based PD and design with stable and high performance could be a promising strategy for the application of the flexible broad spectrum detection.

Stretchable PPG sensor with light polarization for physical activity-permissible monitoring

Skin-attachable sensors, which represent the ultimate form of wearable electronic devices that ensure conformal contact with skin, suffer from motion artifact limitations owing to relative changes in position between the sensor and skin during physical activities. In this study, a polarization-selective structure of a skin-conformable photoplethysmographic (PPG) sensor was developed to decrease the amount of scattered light from the epidermis, which is the main cause of motion artifacts. The motion artifacts were suppressed more than 10-fold in comparison with those of rigid sensors. The developed sensor-with two orthogonal polarizers-facilitated successful PPG signal monitoring during wrist angle movements corresponding to high levels of physical activity, enabling continuous monitoring of daily activities, even while exercising for personal health care.

The Current State of Optical Sensors in Medical Wearables

Optical sensors play an increasingly important role in the development of medical diagnostic devices. They can be very widely used to measure the physiology of the human body. Optical methods include PPG, radiation, biochemical, and optical fiber sensors. Optical sensors offer excellent metrological properties, immunity to electromagnetic interference, electrical safety, simple miniaturization, the ability to capture volumes of nanometers, and non-invasive examination. In addition, they are cheap and resistant to water and corrosion. The use of optical sensors can bring better methods of continuous diagnostics in the comfort of the home and the development of telemedicine in the 21st century. This article offers a large overview of optical wearable methods and their modern use with an insight into the future years of technology in this field.

Flexible Electronics and Devices as Human-Machine Interfaces for Medical Robotics

Medical robots are invaluable players in non-pharmaceutical treatment of disabilities. Particularly, using prosthetic and rehabilitation devices with human-machine interfaces can greatly improve the quality of life for impaired patients. In recent years, flexible electronic interfaces and soft robotics have attracted tremendous attention in this field due to their high biocompatibility, functionality, conformability, and low-cost. Flexible human-machine interfaces on soft robotics will make a promising alternative to conventional rigid devices, which can potentially revolutionize the paradigm and future direction of medical robotics in terms of rehabilitation feedback and user experience. In this review, the fundamental components of the materials, structures, and mechanisms in flexible human-machine interfaces are summarized by recent and renowned applications in five primary areas: physical and chemical sensing, physiological recording, information processing and communication, soft robotic actuation, and feedback stimulation. This review further concludes by discussing the outlook and current challenges of these technologies as a human-machine interface in medical robotics.

Wearable Sensors for Vital Signs Measurement: A Survey

With the outbreak of coronavirus disease-2019 (COVID-19) worldwide, developments in the medical field have aroused concerns within society. As science and technology develop, wearable medical sensors have become the main means of medical data acquisition. To analyze the intelligent development status of wearable medical sensors, the current work classifies and prospects the application status and functions of wireless communication wearable medical sensors, based on human physiological data acquisition in the medical field. By understanding its working principles, data acquisition modes and action modes, the work chiefly analyzes the application of wearable medical sensors in vascular infarction, respiratory intensity, body temperature, blood oxygen concentration, and sleep detection, and reflects the key role of wearable medical sensors in human physiological data acquisition. Further exploration and prospecting are made by investigating the improvement of information security performance of wearable medical sensors, the improvement of biological adaptability and biodegradability of new materials, and the integration of wearable medical sensors and intelligence-assisted rehabilitation. The research expects to provide a reference for the intelligent development of wearable medical sensors and real-time monitoring of human health in the follow-up medical field.

Ultrathin Optics-Free Spectrometer with Monolithically Integrated LED Excitation

A semiconductor spectrometer chip with a monolithically integrated light-emitting diode was demonstrated. The spectrometer design was based on a computational reconstruction algorithm and a series of absorptive spectral filters directly built in to the photodetectors' active regions. The result is the elimination of the need to employ external optics to control the incident angle of light. In the demonstration, an array of gallium nitride (GaN) based photodetectors with wavelength selectivity generated via the principle of local strain engineering were designed and fabricated. Additionally, a GaN based LED was monolithically integrated. An optical blocking structure was used to suppress the LED-photodetector interference and was shown to be essential for the spectroscopic functionality. A proof of concept using a reflection spectroscopy configuration was experimentally conducted to validate the feasibly of simultaneously operating the LED excitation light source and the photodetectors. Spectral reconstruction using a non-negative least squares (NNLS) algorithm enhanced with orthogonal matching pursuit was shown to reconstruct the signal from the reflection spectroscopy. Optics-free operation was also demonstrated.

Perovskite multifunctional logic gates via bipolar photoresponse of single photodetector

The explosive demand for a wide range of data processing has sparked interest towards a new logic gate platform as the existing electronic logic gates face limitations in accurate and fast computing. Accordingly, optoelectronic logic gates (OELGs) using photodiodes are of significant interest due to their broad bandwidth and fast data transmission, but complex configuration, power consumption, and low reliability issues are still inherent in these systems. Herein, we present a novel all-in-one OELG based on the bipolar spectral photoresponse characteristics of a self-powered perovskite photodetector (SPPD) having a back-to-back p+-i-n-p-p+ diode structure. Five representative logic gates ("AND", "OR", "NAND", "NOR", and "NOT") are demonstrated with only a single SPPD via the photocurrent polarity control. For practical applications, we propose a universal OELG platform of integrated 8 × 8 SPPD pixels, demonstrating the 100% accuracy in five logic gate operations irrelevant to current variation between pixels.

A Dual-Channel PPG Readout System With Motion-Tolerant Adaptability for OLED-OPD Sensors

An adaptive PPG (Photoplethysmography) readout system for a dual-channel OLED-OPD flexible sensor is designed and developed with motion artifact (<1Hz) and ambient lighting interference successfully compensated without any additional motion sensors. The compensation is made possible by adopting multi-feedbacks and an additional reference OPD channel to cancel effectively DC drifts. In result, the quality of measured PPG is improved to the level such that long-time, continuous quality monitoring of bio-sign such as heart rate (HR) is possible. The readout is designed with an auto-programmable band-pass trans-impedance amplifier (TIA) of a 100dbΩ gain with a continuous-type DC-current cancellation loop. The rest of the readout consists of a 0.5 Hz low-pass filter, an additional second-order band-pass filter (0.1-10Hz), a difference amplifier, a motion reference channel, an analog multiplexer, a programmable gain amplifier (PGA), a digital control and a programmable DAC-PWM based auto-intensity tuned OLED driver. The readout is fabricated in an area of 9 mm² via the TSMC 180nm process. The experiment result shows that the developed OLED-OPD readout senses well as small as 1nA current, with a measured dynamic range >90dB (1nA to 100 µA) and input-referred noise of 0.26 nA/√H, with power consumption of 460µW. The DC drift is successfully reduced to 1% of its average. The accuracy for heart rate is 96%.

Monolithic Integrated OLED–OPD Unit for Point-of-Need Nitrite Sensing

In this study, we present a highly integrated design of organic optoelectronic devices for Point-of-Need (PON) nitrite (NO2−) measurement. The spectrophotometric investigation of nitrite concentration was performed utilizing the popular Griess reagent and a reflection-based photometric unit with an organic light emitting diode (OLED) and an organic photodetector (OPD). In this approach a nitrite concentration dependent amount of azo dye is formed, which absorbs light around ~540 nm. The organic devices are designed for sensitive detection of absorption changes caused by the presence of this azo dye without the need of a spectrometer. Using a green emitting TCTA:Ir(mppy)3 OLED (peaking at ~512 nm) and a DMQA:DCV3T OPD with a maximum sensitivity around 530 nm, we successfully demonstrated the operation of the OLED–OPD pair for nitrite sensing with a low limit of detection 46 µg/L (1.0 µM) and a linearity of 99%. The hybrid integration of an OLED and an OPD with 0.5 mm × 0.5 mm device sizes and a gap of 0.9 mm is a first step towards a highly compact, low cost and highly commercially viable PON analytic platform. To our knowledge, this is the first demonstration of a fully organic-semiconductor-based monolithic integrated platform for real-time PON photometric nitrite analysis.

Contact-Resistance-Free Stretchable Strain Sensors with High Repeatability and Linearity

Most of the existing stretchable strain sensors are based on the contact-resistance mechanism, where the stretchability and resistance variation depend on the change of the contact relationship of the conductive microstructures. These sensors usually exhibit large sensing ranges and gauge factors but unsatisfactory repeatability and linearity of the electrical responses because the contact is unstable. Here, we report a completely different design for stretchable strain sensors based on a contact-resistance-free structure, i.e., the off-axis serpentine sandwich structure (OASSS), with the mechanism of the stretch-bending-stretch transformation (SBST). Neither unstable contact resistance nor nonlinear constitutive and geometric behaviors occur for the OASSS while the sensor undergoes a large applied strain (50%), which guarantees high repeatability (repeatability error = 1.58%) and linearity (goodness-of-fit &gt;0.999). Owing to such performances, the present sensors are not only applied to monitoring human activities and medical surgery but also to the ground tests of Tianwen-1, China's first Mars exploration mission.

Recent Advances in Materials and Flexible Sensors for Arrhythmia Detection

Arrhythmias are one of the leading causes of death in the United States, and their early detection is essential for patient wellness. However, traditional arrhythmia diagnosis by expert evaluation from intermittent clinical examinations is time-consuming and often lacks quantitative data. Modern wearable sensors and machine learning algorithms have attempted to alleviate this problem by providing continuous monitoring and real-time arrhythmia detection. However, current devices are still largely limited by the fundamental mismatch between skin and sensor, giving way to motion artifacts. Additionally, the desirable qualities of flexibility, robustness, breathability, adhesiveness, stretchability, and durability cannot all be met at once. Flexible sensors have improved upon the current clinical arrhythmia detection methods by following the topography of skin and reducing the natural interface mismatch between cardiac monitoring sensors and human skin. Flexible bioelectric, optoelectronic, ultrasonic, and mechanoelectrical sensors have been demonstrated to provide essential information about heart-rate variability, which is crucial in detecting and classifying arrhythmias. In this review, we analyze the current trends in flexible wearable sensors for cardiac monitoring and the efficacy of these devices for arrhythmia detection.

Narrowband organic photodetectors - towards miniaturized, spectroscopic sensing

Omnipresent quality monitoring in food products, blood-oxygen measurement in lightweight conformal wrist bands, or data-driven automated industrial production: Innovation in many fields is being empowered by sensor technology. Specifically, organic photodetectors (OPDs) promise great advances due to their beneficial properties and low-cost production. Recent research has led to rapid improvement in all performance parameters of OPDs, which are now on-par or better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors, in several aspects. In particular, it is possible to directly design OPDs for specific wavelengths. This makes expensive and bulky optical filters obsolete and allows for miniature detector devices. In this review, recent progress of such narrowband OPDs is systematically summarized covering all aspects from narrow-photo-absorbing materials to device architecture engineering. The recent challenges for narrowband OPDs, like achieving high responsivity, low dark current, high response speed, and good dynamic range are carefully addressed. Finally, application demonstrations covering broadband and narrowband OPDs are discussed. Importantly, several exciting research perspectives, which will stimulate further research on organic-semiconductor-based photodetectors, are pointed out at the very end of this review.

A wireless optoelectronic skin patch for light delivery and thermal monitoring

Wearable optoelectronic devices can interface with the skin for applications in continuous health monitoring and light-based therapy. Measurement of the thermal effect of light on skin is often critical to track physiological parameters and control light delivery. However, accurate measurement of light-induced thermal effects is challenging because conventional sensors cannot be placed on the skin without obstructing light delivery. Here, we report a wearable optoelectronic patch integrated with a transparent nanowire sensor that provides light delivery and thermal monitoring at the same location. We achieve fabrication of a transparent silver nanowire network with >92% optical transmission that provides thermoresistive sensing of skin temperature. By integrating the sensor in a wireless optoelectronic patch, we demonstrate closed-loop regulation of light delivery as well as thermal characterization of blood flow. This light delivery and thermal monitoring approach may open opportunities for wearable devices in light-based diagnostics and therapies.