Ultratrace Measurement of Acetone from Skin Using Zeolite: Toward Development of a Wearable Monitor of Fat Metabolism

Real-time quantitative characterization of ion channel activities for automated control of a lipid bilayer system

This paper describes a novel signal processing method to characterize the activity of ion channels on a lipid bilayer system in a real-time and quantitative manner. Lipid bilayer systems, which enable single-channel level recordings of ion channel activities against physiological stimuli in vitro, are gaining attention in various research fields. However, the characterization of ion channel activities has heavily relied on time-consuming analyses after recording, and the inability to return the quantitative results in real time has long been a bottleneck to incorporating the system into practical products. Herein, we report a lipid bilayer system that integrates real-time characterization of ion channel activities and real-time response based on the characterization result. Unlike conventional batch processing, an ion channel signal is divided into short segments and processed during the recording. After optimizing the system to maintain the same characterization accuracy as conventional operation, we demonstrated the usability of the system with two applications. One is quantitative control of a robot based on ion channel signals. The velocity of the robot was controlled every second, which was around tens of times faster than the conventional operation, in proportion to the stimulus intensity estimated from changes in ion channel activities. The other is the automation of data collection and characterization of ion channels. By constantly monitoring and maintaining the functionality of a lipid bilayer, our system enabled continuous recording of ion channels over 2 h without human intervention, and the time of manual labor has been reduced from conventional 3 h to 1 min at a minimum. We believe the accelerated characterization and response in the lipid bilayer systems presented in this work will facilitate the transformation of lipid bilayer technology toward a practical level, finally leading to its industrialization.

Towards skin-acetone monitors with selective sensitivity: Dynamics of PANI-CA films

Most research aimed at measuring biomarkers on the skin is only concerned with sensing chemicals in sweat using electrical signals, but these methods are not truly non-invasive nor non-intrusive because they require substantial amounts of sweat to get a reading. This project aims to create a truly non-invasive wearable sensor that continuously detects the gaseous acetone (a biomarker related to metabolic disorders) that ambiently comes out of the skin. Composite films of polyaniline and cellulose acetate, exhibiting chemo-mechanical actuation upon exposure to gaseous acetone, were tested in the headspaces above multiple solutions containing acetone, ethanol, and water to gauge response sensitivity, selectivity, and repeatability. The bending of the films in response to exposures to these environments was tracked by an automatic video processing code, which was found to out-perform an off-the-shelf deep neural network-based tracker. Using principal component analysis, we showed that the film bending is low dimensional with over 90% of the shape changes being captured with just two parameters. We constructed forward models to predict shape changes from the known exposure history and found that a linear model can explain 40% of the observed variance in film tip angle changes. We constructed inverse models, going from third order fits of shape changes to acetone concentrations where about 45% of the acetone variation and about 30% of ethanol variation are captured by linear models, and non-linear models did not perform substantially better. This suggests there is sufficient sensitivity and inherent selectivity of the films. These models, however, provide evidence for substantial hysteretic or long-time-scale responses of the PANI films, seemingly due to the presence of water. Further experiments will allow more accurate discrimination of unknown exposure environments. Nevertheless, the sensor will operate with high selectivity in low sweat body locations, like behind the ear or on the nails.

Graphene-enabled wearable sensors for healthcare monitoring

Wearable sensors in healthcare monitoring have recently found widespread applications in biomedical fields for their non- or minimal-invasive, user-friendly and easy-accessible features. Sensing materials is one of the major challenges to achieve these superiorities of wearable sensors for healthcare monitoring, while graphene-based materials with many favorable properties have shown great efficiency in sensing various biochemical and biophysical signals. In this paper, we review state-of-the-art advances in the development and modification of graphene-based materials (i.e., graphene, graphene oxide and reduced graphene oxide) for fabricating advanced wearable sensors with 1D (fibers), 2D (films) and 3D (foams/aerogels/hydrogels) macroscopic structures. We summarize the structural design guidelines, sensing mechanisms, applications and evolution of the graphene-based materials as wearable sensors for healthcare monitoring of biophysical signals (e.g., mechanical, thermal and electrophysiological signals) and biochemical signals from various body fluids and exhaled gases. Finally, existing challenges and future prospects are presented in this area.

Detection of transdermal biomarkers using gradient-based colorimetric array sensor

Accurate assessment of dietary macronutrients intake is critical for the effective management of multiple diseases, such as obesity, diabetes, cardiovascular disease, metabolic disease, and cancer. Conventional self-reporting method is burdensome, inaccurate, and often biased. Though blood analysis and breath analysis can provide evidence-based information, they are either invasive or subject to human errors. Here we reported a wearable transdermal volatile biomarkers detection system based on novel colorimetric sensing technology for dietary macronutrients intake assessment. This technique quantifies the emission rates of transdermal volatile biomarkers via a gradient-based colorimetric array sensor (GCAS). The optical system of the GCAS device tracks the localized color development associated with the chemical reaction between the volatile biomarkers and the porous sensing probes, and determines the biomarkers emission rates through image processing algorithms. The localized chemical reaction and the image-based signal processing also make the GCAS capable for multiplexed detection of multiple analytes simultaneously. The GCAS sensor has been applied for transdermal acetone detection on 5 subjects in a keto diet intervention. The study indicates that the transdermal acetone increases after the subjects consuming keto diets and it decreases to basal level after intaking carb-rich diets. The transdermal acetone response from the GCAS sensor correlates well with breath acetone concentration in the range between 0 and 40 ppm and the correlation factor (R²) is as high as 0.8877. This method provides a noninvasive, low-cost, and wearable tool for assessing dietary macronutrients intake outside of lab or hospital settings. It could be widely applied in disease management, weight control, and nutrition management.

Future Trends in Semiconducting Gas-Selective Sensing Probes for Skin Diagnostics

This paper presents sensor nanotechnologies that can be used for the skin-based gas “smelling” of disease. Skin testing may provide rapid and reliable results, using specific “fingerprints” or unique patterns for a variety of diseases and conditions. These can include metabolic diseases, such as diabetes and cholesterol-induced heart disease; neurological diseases, such as Alzheimer’s and Parkinson’s; quality of life conditions, such as obesity and sleep apnea; pulmonary diseases, such as cystic fibrosis, asthma, and chronic obstructive pulmonary disease; gastrointestinal tract diseases, such as irritable bowel syndrome and colitis; cancers, such as breast, lung, pancreatic, and colon cancers; infectious diseases, such as the flu and COVID-19; as well as diseases commonly found in ICU patients, such as urinary tract infections, pneumonia, and infections of the blood stream. Focusing on the most common gaseous biomarkers in breath and skin, which is nitric oxide and carbon monoxide, and certain abundant volatile organic compounds (acetone, isoprene, ammonia, alcohols, sulfides), it is argued here that effective discrimination between the diseases mentioned above is possible, by capturing the relative sensor output signals from the detection of each of these biomarkers and identifying the distinct breath print for each disease.

Gas Sensor Array Using a Hybrid Structure Based on Zeolite and Oxide Semiconductors for Multiple Bio-Gas Detection

Semiconductor-type gas sensors, composed of metal-oxide semiconductors and porous zeolite materials, are attractive devices for bio-gas detection, particularly when used as bio-gas sensors such as electronic nose application. Previous studies have shown such detection can be obtained with a separate gas concentrator and a sensor device using zeolites and oxide semiconductors of WO₃ nanoparticles. By applying the gas concentrator, porous molecular structures alter both the gas sensitivity and the selectivity, and even can be used to define the sensor characteristics. Based on such a gas sensor design, we investigated the properties of an array of three sensors made of a layer of WO₃ nanoparticles coated with zeolites with different interactions between gas molecule adsorption and desorption. The array was tested with four volatile organic compounds, each measured at different concentrations. The results confirm that the features of individual zeolites combined with the hybrid gas sensor behavior, along with the differences among the sensors, are sufficient for enabling the discrimination of volatile compounds when disregarding their concentration.

Highly selective MXene/V2O5/CuWO4-based ultra-sensitive room temperature ammonia sensor

A Schottky junction based on Ti₃C₂T_x MXene sheet integrated with marigold flower-like V₂O₅/CuWO₄ heterojunction was designed and fabricated for robust ammonia sensing by monitoring the electrical resistance changes in air and ammonia. The electron transport behavior of the sensor was investigated by electrochemical analysis, ultraviolet photoelectron spectroscopy and reflection electron energy loss spectroscopy. Besides, negative zeta potential obtained for sensor components was in consistent with surface functional groups (e.g. OH and F) observed by XPS analysis helping better understanding of the ammonia sensing mechanism. The results desirably confirmed high sensitivity, selectivity, linear range (1-160 ppm), the limit of quantification, repeatability, long-term stability, very short response time (few seconds) and low working temperature (25 °C) of the sensor. The measurements on the resistance changes of the MXene/V₂O₅/CuWO₄-based sensor under the exposure to various types of analytes (Ammonia, Acetone, Benzene, Chloroform, DMF, Ethanol, humidity (80%), Methanol and Toluene as well as NO, NO₂, H₂S, SO₂, CO and CH₄) at different concentrations revealed that the fabricated sensor is excellently selective to ammonia with ultra-high sensitivity. Intra-day stability (7 runs a day) and long-term stability (every 10 days over 70 days) as important sensor characteristics were investigated at 51 ppm and ambient temperature, which showed very good repeatability and recoverability in both short and long periods for sensing the ammonia. Overall, MXene/V₂O₅/CuWO₄ was shown to be cost-effective, easy to handle and suitably applicable for simple, ultrafast and extremely efficient trace ammonia detection, which could be of high interest for future exhaled breath analysis and the development of a novel noninvasive diagnostic strategy to monitor chronic kidney disease to stop a large measure of unnecessary invasive testing.

External ears for non-invasive and stable monitoring of volatile organic compounds in human blood

Volatile organic compounds (VOCs) released through skin (transcutaneous gas) has been increasing in importance for the continuous and real-time assessment of diseases or metabolisms. For stable monitoring of transcutaneous gas, finding a body part with little interference on the measurement is essential. In this study, we have investigated the possibility of external ears for stable and real-time measurement of ethanol vapour by developing a monitoring system that consisted with an over-ear gas collection cell and a biochemical gas sensor (bio-sniffer). The high sensitivity with the broad dynamic range (26 ppb–554 ppm), the high selectivity to ethanol, and the capability of the continuous measurement of the monitoring system uncovered three important characteristics of external ear-derived ethanol with alcohol intake for the first time: there is little interference from sweat glands to a sensor signal at the external ear; similar temporal change in ethanol concentration to that of breath with delayed peak time (avg. 13 min); relatively high concentration of ethanol relative to other parts of a body (external ear-derived ethanol:breath ethanol = 1:590). These features indicated the suitability of external ears for non-invasive monitoring of blood VOCs.

Stretchable gas sensors for detecting biomarkers from humans and exposed environments

The recent advent of stretchable gas sensors demonstrates their capabilities to detect not only gaseous biomarkers from the human body but also toxic gas species from the exposed environment. To ensure accurate gas detection without device breakdown from the mechanical deformations, the stretchable gas sensors often rely on the direct integration of gas-sensitive nanomaterials on the stretchable substrate or fibrous network, as well as being configured into stretchable structures. The nanomaterials in the forms of nanoparticles, nanowires, or thin-films with nanometer thickness are explored for a variety of sensing materials. The commonly used stretchable structures in the stretchable gas sensors include wrinkled structures from a pre-strain strategy, island-bridge layouts or serpentine interconnects, strain isolation approaches, and their combinations. This review aims to summarize the recent advancement in novel nanomaterials, sensor design innovations, and new fabrication approaches of stretchable gas sensors.

Adsorption/Combustion-type Micro Gas Sensors: Typical VOC-sensing Properties and Material-design Approach for Highly Sensitive and Selective VOC Detection

Highly sensitive and selective detection of various volatile organic compounds (VOCs) has been most needed in a wide range of fields, such as medical diagnosis, health supervision, industry-process control, and environmental monitoring. Since a semiconductor-type gas sensor is a typical promising candidate among various portable VOC-sensing devices, many efforts on developing these gas sensors are introduced in this article for the first time. Through some development stages, it has been well known that the temperature-modulated operation of gas sensors is one of effective ways to improve the magnitude of VOC responses. On the other hand, catalytic combustion-type gas sensors operated with a mode of pulse-driven heating were developed in the early 2000s, and they are named as "adsorption/combustion-type gas sensors" after their gas-sensing mechanism, based on the combustion of VOC adsorbates on the sensing material. The representative VOC-sensing properties of the adsorption/combustion-type gas sensors and recent material-design approach to achieve highly sensitive and selective VOC detection are summarized in this article.

Transcutaneous Blood VOC Imaging System (Skin-Gas Cam) with Real-Time Bio-Fluorometric Device on Rounded Skin Surface

A skin-gas cam that allows continuous imaging of transcutaneous blood volatile organic compounds (VOCs) emanated from human skin was developed. The skin-gas cam is able to reveal the relationship between the local skin conditions and transcutaneous blood VOCs in the field of volatile metabolomics (volatolomics). A ring-type ultraviolet (UV) light-emitting diode was mounted around a camera lens as an excitation light source, which enabled the simultaneous excitation and imaging of fluorescence. A nicotinamide adenine dinucleotide (NAD)-dependent alcohol dehydrogenase (ADH) was used to detect ethanol as a model sample. When gaseous ethanol was applied to an ADH-immobilized mesh that was wetted with an oxidized NAD solution placed in front of the camera, a reduced form of NAD (NADH) was produced through an ADH-mediated reaction. NADH emits fluorescence by UV excitation, and thus, the concentration distribution of ethanol was visualized by measuring the distribution of the fluorescence light intensity from NADH on the ADH-immobilized mesh surface. In this study, a new gas application method that mimicked the release mechanism of transcutaneous gas for quantification of the transcutaneous gas concentration was evaluated. Also, spatiotemporal changes of transcutaneous ethanol for various body parts were measured. As a result, we revealed a relationship between local skin conditions and VOCs that could not be observed previously. In particular, we demonstrated the facile measurement of transdermal gases from around the ear where capillaries are densely distributed below a thin stratum corneum.

Biochemical Gas Sensors (Biosniffers) Using Forward and Reverse Reactions of Secondary Alcohol Dehydrogenase for Breath Isopropanol and Acetone as Potential Volatile Biomarkers of Diabetes Mellitus

This study describes two biosniffers to determine breath acetone and isopropanol (IPA) levels and applies them for breath measurement in healthy subjects and diabetic patients. Secondary alcohol dehydrogenase (S-ADH) can reduce acetone and oxidize nicotinamide adenine dinucleotide (NADH to NAD⁺) in a weak acid environment. NADH can be excited by 340 nm excitation lights and subsequently emit 490 nm fluorescence. Therefore, acetone can be measured by the decrease in NADH fluorescence intensity. S-ADH can also oxidize IPA and reduce NAD⁺ to NADH when it is in an alkaline environment. Thus, IPA can be detected by the increase of fluorescence. The developed biosniffers show rapid response, high sensitivity and high selectivity. The breath acetone and IPA analysis in healthy subjects shows that the mean values were 750.0 ± 434.4 ppb and 15.4 ± 11.3 ppb. Both acetone and IPA did not show a statistical difference among different genders and ages. The breath acetone analysis for diabetic patients shows a mean value of 1207.7 ± 689.5 ppb, which was higher than that of healthy subjects (p < 1 × 10^-6). In particularly, type-1 diabetic (T1D) patients exhaled a much higher concentration of acetone than type-2 diabetic (T2D) patients (p < 0.01). The breath IPA also had a higher concentration in diabetic patients (23.1 ± 20.1 ppb, p < 0.01), but only T2D patients presented a statistical difference (23.9 ± 21.3 ppb, p < 0.01). These findings are worthwhile in the study of breath biomarkers for diabetes mellitus diagnosis. Additionally, the developed biosniffers provide a new technique for volatolomics research.

Room Temperature Gas Sensing of Two-Dimensional Titanium Carbide (MXene)

Wearable gas sensors have received lots of attention for diagnostic and monitoring applications, and two-dimensional (2D) materials can provide a promising platform for fabricating gas sensors that can operate at room temperature. In the present study, the room temperature gas-sensing performance of Ti₃C₂T_x nanosheets was investigated. 2D Ti₃C₂T_x (MXene) sheets were synthesized by removal of Al atoms from Ti₃AlC₂ (MAX phases) and were integrated on flexible polyimide platforms with a simple solution casting method. The Ti₃C₂T_x sensors successfully measured ethanol, methanol, acetone, and ammonia gas at room temperature and showed a p-type sensing behavior. The fabricated sensors showed their highest and lowest response toward ammonia and acetone gas, respectively. The limit of detection of acetone gas was theoretically calculated to be about 9.27 ppm, presenting better performance compared to other 2D material-based sensors. The sensing mechanism was proposed in terms of the interactions between the majority charge carriers of Ti₃C₂T_x and gas species.

Fluorometric Biosniffer Camera "Sniff-Cam" for Direct Imaging of Gaseous Ethanol in Breath and Transdermal Vapor

Various volatile organic compounds can be found in human transpiration, breath and body odor. In this paper, a novel two-dimensional fluorometric imaging system, known as a "sniffer-cam" for ethanol vapor released from human breath and palm skin was constructed and validated. This imaging system measures ethanol vapor concentrations as intensities of fluorescence through an enzymatic reaction induced by alcohol dehydrogenase (ADH). The imaging system consisted of multiple ultraviolet light emitting diode (UV-LED) excitation sheet, an ADH enzyme immobilized mesh substrate and a high-sensitive CCD camera. This imaging system uses ADH for recognition of ethanol vapor. It measures ethanol vapor by measuring fluorescence of nicotinamide adenine dinucleotide (NADH), which is produced by an enzymatic reaction on the mesh. This NADH fluorometric imaging system achieved the two-dimensional real-time imaging of ethanol vapor distribution (0.5-200 ppm). The system showed rapid and accurate responses and a visible measurement, which could lead to an analysis of metabolism function at real time in the near future.

Binary coded identification of industrial chemical vapors with an optofluidic nose

An artificial nose system for the recognition and classification of gas-phase analytes and its application in identifying common industrial gases is reported. The sensing mechanism of the device comprises the measurement of infrared absorption of volatile analytes inside the hollow cores of optofluidic Bragg fibers. An array of six fibers is used, where each fiber targets a different region of the mid-infrared in the range of 2-14 μm with transmission bandwidths of about 1-3 μm. The quenching in the transmission of each fiber due to the presence of analyte molecules in the hollow core is measured separately and the cross response of the array allows the identification of virtually any volatile organic compound (VOC). The device was used for the identification of seven industrial VOC vapors with high selectivity using a standard blackbody source and an infrared detector. The array response is registered as a unique six digit binary code for each analyte by assigning a threshold value to the fiber transmissions. The developed prototype is a comprehensive and versatile artificial nose that is applicable to a wide range of analytes.