Measuring Impedance Changes Research Articles

IoT-Driven Experimental Framework for Advancing Electrical Impedance Tomography

This research paper focuses on the current emphasis on the latest industrial revolution, particularly the innovative integration of artificial intelligence and the Internet of Things (IoT). The study explores the seamless integration of Electrical Impedance Tomography (EIT) with IoT, presenting a groundbreaking framework where impedance-based sensing plays a vital role in enhancing the dynamic and adaptable qualities of IoT ecosystems. This contribution facilitates intelligent decision-making and real-time monitoring. The research investigates the application of non-invasive Electrical Impedance Tomography for the rapid identification of minor changes in the electrical impedance of the body or a simulated object. Electrodes positioned at the ends of the phantom’s cylinder measure impedance changes through the application of a high-frequency, low-current signal. Image reconstruction employs both forward and inverse solutions, utilizing a triangular finite element method (FEM) mesh to determine conductivity distribution based on recommended phantom models. The integration of IoT enables data capture, enhancing accessibility through remote monitoring. The novel IoT system proves advantageous for various engineering research applications, providing easily monitored parameters in both commercial and clinical contexts.

Dual Mode Strain–Temperature Sensor with High Stimuli Discriminability and Resolution for Smart Wearables

AbstractStrain and temperature are important physiological parameters for health monitoring, providing access to the respiration state, movement of joints, and inflammation processes. The challenge for smart wearables is to unambiguously discriminate strain and temperature using a single sensor element assuring a high degree of sensor integration. Here, a dual‐mode sensor with two electrodes and tubular mechanically heterogeneous structure enabling simultaneous sensing of strain and temperature without cross‐talk is reported. The sensor structure consists of a thermocouple coiled around an elastic strain‐to‐magnetic induction conversion unit, revealing a giant magnetoelastic effect, and accommodating a magnetic amorphous wire. The thermocouple provides access to temperature and its coil structure allows to measure impedance changes caused by the applied strain. The dual‐mode sensor also exhibits interference‐free temperature sensing performance with high coefficient of 54.49 µV °C−1, low strain and temperature detection limits of 0.05% and 0.1 °C, respectively. The use of these sensors in smart textiles to monitor continuously breathing, body movement, body temperature, and ambient temperature is demonstrated. The developed multifunctional wearable sensor is needed for applications in early disease prevention, health monitoring, and interactive electronics as well as for smart prosthetics and intelligent soft robotics.

Development of a Wearable Gesture Recognition System Based on Two-Terminal Electrical Impedance Tomography.

This paper proposes a low-cost, wearable gesture recognition system based on the two-terminal electrical impedance tomography (EIT) technique. The system includes a wearable EIT sensor of eight electrodes, a hardware device, and gesture recognition software running on a PC. Nine different gestures can be stably identified from the measured impedance changes through machine learning algorithms. Experimental results show that the Quadric Discriminator algorithm has the highest recognition rate of 98.49% for the filtered validation set. Besides, the recognition results in the two-terminal mode and transformed four-terminal mode are compared by applying a two-to-four-terminal mapping to the two-terminal EIT system, and the recognition rate decreases with the most classification models in the latter mode. Thus, it is supposed that contact impedance plays an important role in gesture recognition. By analyzing the data characteristics with variance inflation factor (VIF) test and principal component analysis (PCA), the supposition is explained and verified, proving the merit of a two-terminal EIT system in gesture recognition.

Piezoresistive Conductive Microfluidic Membranes for Low-Cost On-Chip Pressure and Flow Sensing.

Over the last two decades, the field of microfluidics has received significant attention from both academia and industry. Each year, researchers report thousands of new prototype devices for use in a broad range of environmental, pharmaceutical, and biomedical engineering applications. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for monitoring fluid flows within the microfluidic devices themselves remains expensive and often cost-prohibitive for researchers interested in starting a microfluidics project. As microfluidic devices become capable of handling complex fluidic systems, low-cost, precise, and real-time pressure and flow rate measurement capabilities have become increasingly important. While many labs use commercial platforms and sensors, these solutions can often cost thousands of dollars and can be too bulky for on-chip use. Here we present a new inexpensive and easy-to-use piezoresistive pressure and flow sensor that can be easily integrated into existing on-chip microfluidic channels. The sensor consists of PDMS–carbon black conductive membranes and uses an impedance analyzer to measure impedance changes due to fluid pressure. The sensor costs several orders of magnitude less than existing commercial platforms and can monitor local fluid pressures and calculate flow rates based on the pressure gradient.

A Bacteriophage-Based Electrochemical Biosensor for Detection of Methicillin-Resistant Staphylococcus aureus

Bacterial pathogens are important targets for detection and early diagnostics of infection in healthcare. Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium that can cause severe complications such as pneumonia and septicemia from surgical site infections. In this work, a carbon nanotube (CNT)-based impedimetric biosensor using SATA-8505, a bacteriophage, as a recognition element was developed for the detection of MRSA USA300, a common strain of MRSA found in surgical infections. The biosensor allows for fast and selective detection of viable MRSA cells by measuring impedance changes on the electrode surface as a result of bacterial capture by the phage-functionalized CNT electrode. The immobilization of the bacteriophage was achieved by inducing an electric-field and using a charge-directed orientation strategy, and activity of the immobilized phage was confirmed by an infectivity study using disk diffusion methods. The biosensing platform showed high selectivity and sensitivity toward MRSA USA300 with a limit of detection of 1.23 × 102 CFU/ml in aqueous solution and 1.29 × 102 CFU/ml in blood plasma. The biosensing platform could potentially be integrated into a lab-on-a-chip platform for point of care use and for other pathogens such as Pseudomonas aeruginosa, Klebsiella and Listeria monocytogenes.

Rapid and Sensitive Detection of Nanomolecules by an AC Electrothermal Flow Facilitated Impedance Immunosensor.

Conventional immunosensors typically rely on passive diffusion dominated transport of analytes for binding reaction and hence, it is limited by low sensitivity and long detection times. We report a simple and efficient impedance sensing method that can be utilized to overcome both sensitivity and diffusion limitations of immunosensors. This method incorporates the structural advantage of nanorod-covered interdigitated electrodes and the microstirring effect of AC electrothermal flow (ACET) with impedance spectroscopy. ACET flow induced by a biased AC electric field can rapidly convect the analyte onto nanorod structured electrodes within a few seconds and enriches the number of binding molecules because of the excessive effective surface area. We performed numerical simulations to investigate the effect of ACET flow on the biosensor performance. The results indicated that AC bias to the side electrodes could induce fast convective flow, which facilitates the transport of the target molecules to the binding region located in the middle as a floating electrode. The temperature rise due to the Joule heating effect was measured using a thermoreflectance imaging method to find the optimum device operation conditions. The change of impedance caused by the receptors-target molecules binding at the sample/electrode interface was experimentally measured and quantified in real-time using the impedance spectroscopy technique. We observed that the impedance sensing method exhibited extremely fast response compared with those under no bias conditions. The measured impedance change can reach saturation in a minute. Compared to the conventional incubation method, the ACET flow enhanced method is faster in its reaction time, and the detection limit can be reduced to 1 ng/mL. In this work, we demonstrate that this sensor technology is promising and reliable for rapid, sensitive, and real-time monitoring of biomolecules in biologically relevant media such as blood, urine, and saliva.

Flex-GO (Flexible graphene oxide) sensor for electrochemical monitoring lactate in low-volume passive perspired human sweat

In this work, a low volume, sweat lactate sensor functioning on passively expressed eccrine sweat was designed, fabricated and tested in human sweat and its performance was benchmarked against a standard reference; Lactate Plus meter. This novel sensor comprises of graphene oxide (GO) nanosheets integrated into a nanoporous flexible electrode system for low-volume (1–5 μL) ultrasensitive impedance based detection of lactate using non-faradaic electron-ionic charge transfer. Lactate oxidase (LOD) enzyme was immobilized on the surface of GO nanosheets towards developing an affinity biosensor specific to the physiological relevant range (4–80 mM) of lactate in perspired human sweat. Sensing was achieved by measuring impedance changes specific to lactate binding along the GO nanosheet interface using electrochemical impedance spectroscopy. The sensor demonstrated a dynamic range from 1 to 100 mM spiked in synthetic and human sweat with a limit of detection of 1 mM. A specificity study conducted using cortisol expressed in sweat revealed a negative response to the lactate oxidase. Continuous lactate sensing studies were performed during which the sensor was responsive to concentrations of lactate up to 138.6 mM. Correlation of the sensor response with actual lactate concentration (1.3–113.4 mM) was found to be 0.955.

Mutual Impedance of Eddy-Current Coils Above a Second-Layer Crack of Finite Length

A model is presented to determine the change in mutual impedance between a driver and pickup coil due to eddy-current interaction with a crack in a conductive two-layer plate system where the plates are separated by a non-conducting layer of constant thickness. Here, the impedance change of interest arises from a through-thickness crack of finite length located in the hidden lower plate. The model assumes that both conducting plates are electromagnetically thin and is therefore valid for large electromagnetic skin-depth. The model predictions are compared with experimental measurements using a pair of cylindrical air-cored coils arranged to simulate an eddy-current sliding probe. Agreement between the calculated and measured impedance change is obtained over the frequency range 10 Hz–3 kHz for which the plates can be treated as electromagnetically thin, and reasonable qualitative agreement was obtained at higher frequencies. A method for computing the induced current streamlines in the presence of the crack is also described and visualisation of the computed streamlines is used to assist in the interpretation of the impedance change for different probe orientations. These results provide a valuable initial insight into the interaction between a buried crack in a multilayer structure and the electromagnetic fields induced and detected by eddy-current sliding probes.

Effects of electrode diameter on the detection sensitivity and frequency characteristics of electric cell-substrate impedance sensing

Cell-based electrochemical impedance spectroscopy (EIS) has emerged as a label-free approach to monitor whole cellular behavior in response to the environmental stimuli in real time. The measured impedance is strongly dependent on the attached cells, the sensing electrode size, and the constriction resistance between the sensing electrode and its counter electrode. However, determining the optimal electrode size for sensitive detection of impedance characteristics of the attached cells remains a challenge. In this study, four different cell lines (MDCK, MCF-7, HUVEC, and MDA-MB-231 cells) were cultured on the sensing electrodes with four different diameters, 250, 100, 50, and 25 μm, and impedance measurements were performed at frequencies ranging from 25 Hz to 64 kHz using electric cell-substrate impedance sensing (ECIS) method. As the diameter of sensing electrode decreases, the peak value (optimal detection sensitivity) of the normalized resistance spectrum decreases and the peak position (optimal detection frequency) shifts to the high-frequency region. These criteria were applied to sensitively measure impedance changes of MDA-MB-231 cells in response to TGF-β1 treatment. The transient increase of measured impedance after TGF-β1 treatment coincided with the augmented formation of the peripheral paxillin-rich sites and the increased cell migration rate, while little difference in cell spreading area was observed. Our results suggest that 250 μm is approximately the optimal electrode diameter for EIS measurement if sensitively monitoring the morphological changes of adherent cells is required.

Lithium fluoride layer formed by thermal evaporation for stable lithium metal anode in rechargeable batteries

In this study, lithium fluoride (LiF) was deposited on lithium (Li) metal by thermal evaporation in order to reduce degradation of Li metal batteries caused by growth of Li dendrites. Li | Li symmetric cells were used to measure impedance changes, overvoltage after cycling at 1 mA/cm2 and 1 mAh/cm2, and surface images after 100 cycles. In addition, the residual capacity and Coulombic efficiency after 0.5C charge/discharge cycling were confirmed using LiCoO2|Li cells. Based on electrochemical impedance spectroscopy measurements, Li | Li symmetric cells made of LiF-coated Li showed no impedance changes, even 12 h after fabrication. During the overvoltage test with Li | Li symmetric cells, the cell with a 300 nm thick LiF layer showed highest stability under repeated cycling. The surface of the 300 nm LiF-coated Li was relatively smooth and almost no abnormal growth of Li was observed. In the LiCoO2|Li cells, 200 nm and 300 nm LiF-coated cells showed the best performance in terms of residual capacity. The 300 nm LiF-coated Li cell exhibited close to 100% Coulombic efficiency and remained more stable than the bare Li cell.

In vivo effects of serotonin and fluoxetine on cardio-ventilatory functions in the shore crab Carcinus maenas (L. 1758)

Serotonin (5-HT) takes a key position in regulating vital functions, such as cardio-ventilatory activity, locomotion and behaviour. Selective serotonin reuptake inhibitors (SSRIs) modulate the serotonergic system and thus affect these functions. Rhythmic behaviours, such as cardio-ventilatory activity, are controlled by central pattern generators, which in turn are regulated by 5-HT. In crustaceans, 5-HT also regulates the synthesis and secretion of crustacean hyperglycaemic hormone, a pleiotropic hormone involved in the mobilisation and release of glucose into the haemolymph, thus stimulating the animal’s activity. As a matter of consequence, SSRIs may affect cardio-ventilatory activity. In order to examine how the SSRIs affect fundamental physiological parameters based on rhythmic behaviours in decapods, cardio-respiratory activity in the shore crab Carcinus maenas was assessed after pericardial injection of a single dose of either 0.5 μM, 0.75 μM or 1 μM fluoxetine, respectively. Simultaneous recordings of heart and scaphognathite movements in both brachial chambers were conducted by measuring impedance changes in the respective body compartments. Injection of 5-HT had an immediate effect on cardio-ventilatory activities and strongly upregulated both cardiac and ventilatory activities. Fluoxetine showed similar effects, entailing moderate tachycardia and increased ventilation rates. Compared to 5-HT, these effects were delayed in time and much less pronounced. Metabolism of fluoxetine into the active compound nor-fluoxetine might account for the delayed action, whereas compensatory regulation of cardio-ventilatory frequencies and amplitudes are likely to explain the attenuation of the responses compared to the strong and immediate increase by 5-HT. Overall, the results suggest increased 5-HT levels in invertebrates following fluoxetine exposure, which are able to disturb physiological functions regulated by 5-HT, such as cardiac and respiratory activity.

Simultaneous lancet-free monitoring of alcohol and glucose from low-volumes of perspired human sweat

A lancet-free, label-free biosensor for simultaneous detection of sweat glucose and alcohol was demonstrated using zinc oxide thin films integrated into a nanoporous flexible electrode system. Sensing was achieved from perspired human sweat at low volumes (1–3 μL), comparable to ambient conditions without external stimulation. Zinc oxide thin film electrodes were surface functionalized with alcohol oxidase enzyme and with glucose oxidase enzyme towards developing an affinity biosensor specific to the physiological relevant range of alcohol comprising of 0–2 drinks (0–50 mg/dl) and physiologically relevant range of glucose ranging from hypo- to hyper-glycaemia (50–130 mg/dl) in perspired human sweat. Sensing was achieved by measuring impedance changes associated with alcohol and glucose binding onto the sensor interface using electrochemical impedance spectroscopy with a dynamic range from 0.01–200 mg/dl and a limit of detection of 0.01 mg/dl for alcohol in human sweat. Sensor calibration in synthetic sweat containing interferents (25–200 mg/dl) and comparison using regression and Bland-Altman analysis of sweat sensor performance was done with BACtrack®. Combinatorial detection of glucose and ethanol in perspired human sweat and comparison of sweat sensor performance with Accu-Chek® blood glucose monitoring system that we expect would be relevant for pre-diabetics and diabetics for monitoring their glucose levels and alcohol consumption.

Wholistic Electrochemical Biosensor for the Combinatorial Detection of Alcohol and Glucose in Perspired Sweat

In this paper, we demonstrate a novel, flexible electrochemical biosensor of the simultaneous detection of glucose and alcohol in perspired human sweat for wearable sensing applications. Evaluation of bodily fluids such as blood, sweat, urine has allowed patients and health-care professionals to track their vital parameters reliably through non-invasive monitoring. Human sweat is one such fluid that contains valuable medical information which can be used to develop real-time, healthcare management devices. Literature studies have shown that sweat alcohol content (SAC) is well correlated to blood alcohol content (BAC). Alcohol consumption modifies blood glucose levels of a diabetic individual depending on their nutrition state inducing either hypoglyceamia or hyperglyceamia. The motivation of this study was to build a sweat based sensor platform for alcohol and glucose management to monitor glucose levels on moderate consumption of alcohol of diabetic social drinkers in low volumes of sweat. The developed sweat biosensor employs a metal- metal oxide electrode stack on a flexible substrate for the non-invasive, low- volume detection of a cohort of glucose and alcohol molecules in sweat through a stable enzyme immunoassay chemistry. Non-faradaic electrochemical impedance spectroscopy (EIS) is the signal transduction mechanism used to detect the presence of glucose and alcohol in sweat. The detection of these molecules is achieved by measuring impedance changes occurring at the electrode- electrolyte interface due to the formation of electrical double layer. Binding events and enzyme catalysis reactions modulate the electrical double layer (EDL) which contribute to the overall impedance change of the system. Detection of sweat glucose levels in the presence of alcohol is demonstrated on the sensor systems at a specific frequency.

Fully electronic urine dipstick probe for combinatorial detection of inflammatory biomarkers.

Aim:An electrochemical urine dipstick probe biosensor has been demonstrated using molybdenum electrodes on nanoporous polyamide substrate for the quantitative detection of two inflammatory protein biomarkers, CRP and IL-6.Materials & methods:The electrode interface was characterized using ζ-potential and Fourier transform infrared spectroscopy. Detection of biomarkers was demonstrated by measuring impedance changes associated with the dose concentrations of the two biomarkers. A proof of feasibility of point-of-care implementation of the biosensor was demonstrated using a portable electronics platform.Results & conclusion:Limit of detection of 1 pg/ml was achieved for CRP and IL-6 in human urine and synthetic urine buffers. The developed portable hardware demonstrated close correlation with benchtop equipment results.

Electrical impedance tomography: Amplitudes of cardiac related impedance changes in the lung are highly position dependent.

BackgroundElectrical impedance tomography (EIT) is used on the thorax to measure impedance changes due to the presence of air and blood in the lung. This experimental study was performed to investigate the effect of posture on cardiac and respiratory related impedance changes.MethodsEIT measurements were performed on 14 healthy subjects in left-, right lateral, prone, supine and upright positions. Simultaneously, tidal volume was recorded with an ultrasonic flowmeter. For image reconstruction, the classic Sheffield back-projection and three variants of the modern GREIT algorithm were applied with two different reference frames. Amplitudes of cardiac- and respiratory impedance changes were extracted and compared between the positions.ResultsWe found significant differences in both cardiac and respiratory amplitudes between postures. Especially, supine and upright positions showed dramatic changes in amplitude. These differences between postures were unaffected by the change of reference frames in all reconstruction methods except of the classic Sheffield back projection. Possible sources that explain the observed posture dependency are discussed.ConclusionResearchers and clinicians need to be aware of this phenomenon when comparing EIT amplitudes in different body positions.

Portable biosensor for monitoring cortisol in low-volume perspired human sweat

A non-faradaic label-free cortisol biosensor was demonstrated using MoS2 nanosheets integrated into a nanoporous flexible electrode system. Low volume (1–5 μL) sensing was achieved through use of a novel sensor stack design comprised of vertically aligned metal electrodes confining semi-conductive MoS2 nanosheets. The MoS2 nanosheets were surface functionalized with cortisol antibodies towards developing an affinity biosensor specific to the physiological relevant range of cortisol (8.16 to 141.7 ng/mL) in perspired human sweat. Sensing was achieved by measuring impedance changes associated with cortisol binding along the MoS2 nanosheet interface using electrochemical impedance spectroscopy. The sensor demonstrated a dynamic range from 1–500 ng/mL with a limit of detection of 1 ng/mL. A specificity study was conducted using a metabolite expressed in human sweat, Ethyl Glucuronide. Continuous dosing studies were performed during which the sensor was able to discriminate between four cortisol concentration ranges (0.5, 5, 50, 500 ng/mL) for a 3+ hour duration. Translatability of the sensor was shown with a portable form factor device, demonstrating a comparable dynamic range and limit of detection for the sensor. The device demonstrated a R2 correlation value of 0.998 when comparing measurements to the reported impedance values of the benchtop instrumentation.

Companion and Point-of-Care Sensor System for Rapid Multiplexed Detection of a Panel of Infectious Disease Markers.

A nanochannel-based electrochemical biosensor has been demonstrated for rapid and multiplexed detection of a panel of three biomarkers associated with rapid detection of sepsis. The label-free biosensor detected procalcitonin (PCT), lipoteichoic acid (LTA), and lipopolysaccharide (LPS) from human whole blood. The biosensor comprises a nanoporous nylon membrane integrated onto a microelectrode sensor platform for nanoconfinement effects. Charge perturbations due to biomarker binding are recorded as impedance changes using electrochemical impedance spectroscopy. The measured impedance change is used to quantitatively determine the concentration of the three biomarkers using antibody receptors from the tested sample. We were successful in detecting and quantifying the three biomarkers from whole blood. The limit of detection was 0.1 ng/mL for PCT and 1 µg/mL for LPS and LTA. The sensor was able to demonstrate a dynamic range of detection from 01.1 ng/mL to 10 µg/mL for PCT and from 1 µg/mL to 1000 µg/mL for LPS and LTA biomarkers. This novel technology has promising preliminary results toward the design of sensors for rapid and sensitive detection of the three panel biomarkers in whole blood toward diagnosis and classification of sepsis.

Companion and Point-of-Care Sensor System for Rapid Multiplexed Detection of a Panel of Infectious Disease Markers

A nanochannel-based electrochemical biosensor has been demonstrated for rapid and multiplexed detection of a panel of three biomarkers associated with rapid detection of sepsis. The label-free biosensor detected procalcitonin (PCT), lipoteichoic acid (LTA), and lipopolysaccharide (LPS) from human whole blood. The biosensor comprises a nanoporous nylon membrane integrated onto a microelectrode sensor platform for nanoconfinement effects. Charge perturbations due to biomarker binding are recorded as impedance changes using electrochemical impedance spectroscopy. The measured impedance change is used to quantitatively determine the concentration of the three biomarkers using antibody receptors from the tested sample. We were successful in detecting and quantifying the three biomarkers from whole blood. The limit of detection was 0.1 ng/mL for PCT and 1 µg/mL for LPS and LTA. The sensor was able to demonstrate a dynamic range of detection from 01.1 ng/mL to 10 µg/mL for PCT and from 1 µg/mL to 1000 µg/mL for LPS and LTA biomarkers. This novel technology has promising preliminary results toward the design of sensors for rapid and sensitive detection of the three panel biomarkers in whole blood toward diagnosis and classification of sepsis.

An Impedance-Based Mold Sensor with on-Chip Optical Reference.

A new miniaturized sensor system with an internal optical reference for the detection of mold growth is presented. The sensor chip comprises a reaction chamber provided with a culture medium that promotes the growth of mold species from mold spores. The mold detection is performed by measuring impedance changes with integrated electrodes fabricated inside the reaction chamber. The impedance change in the culture medium is caused by shifts in the pH (i.e., from 5.5 to 8) as the mold grows. In order to determine the absolute pH value without the need for calibration, a methyl red indicator dye has been added to the culture medium. It changes the color of the medium as the pH passes specific values. This colorimetric principle now acts as a reference measurement. It also allows the sensitivity of the impedance sensor to be established in terms of impedance change per pH unit. Major mold species that are involved in the contamination of food, paper and indoor environments, like Fusarium oxysporum, Fusarium incarnatum, Eurotium amstelodami, Aspergillus penicillioides and Aspergillus restrictus, have been successfully analyzed on-chip.

Noncontact Measurement of Complex Permittivity of Electrically Small Samples at Microwave Frequencies

Noncontact measurements of material parameters have important applications in various fields. In this paper, we propose a noncontact measurement of complex permittivity for electrically small samples at microwave frequencies. Based on Rayleigh approximation, we propose a noncontact measurement approach that can linearly retrieve complex permittivity from the measured impedance change. Using a subwavelength resonance, far-field radiation can be effectively suppressed, and thus the measurement is not notably influenced by unwanted environmental reflection. With the features of simple calibration and noncontact measurement, our method can be widely used in repeated on-site measurements of complex permittivity for small-sized samples at microwave frequencies.