Reverse iontophoresis for non-invasive transdermal monitoring

Continuous glucose monitoring: Minimally and non-invasive technologies

This paper highlights technological advancements in non-invasive blood glucose monitoring against the backdrop of increasing global prevalence of diabetes. Traditional monitoring methods, primarily invasive methods face limitations in providing continuous glucose level data, which is essential for effective and timely diagnosis of disease stage and for determining the optimal therapeutic strategy. Recent non-invasive technologies encompass optical, acoustic, electromagnetic, and chemical approaches. These technologies exploit the intrinsic properties of glucose, such as its optical absorption coefficients, to offer promising avenues for less intrusive blood glucose monitoring. Despite these advancements, challenges in achieving high accuracy persist due to interference from substances like water and other blood components. This underlines the need for sophisticated algorithms and sensor designs for accurate glucose estimation. Further research is required to integrate various sensing techniques and advanced data processing to enhance accuracy and user-friendliness. In conclusion, while significant progress has been made, developing a reliable, convenient, and accessible method for non-invasive glucose monitoring is crucial for transforming diabetes management and improving patients' quality of life.

Improving Electrical Stimulation Effectiveness and Versatility for Non-Invasive Transdermal Monitoring Applications via an Innovative Mixed-Signal Electronic Interface

Electrical stimulation can be used in several applications such as fatigue reduction, muscle rehabilitation, neurorehabilitation, neuro-prosthesis and pain relief. Moreover, electrical stimulation can be used for drug delivery applications or body fluids extraction (e.g., sweat and interstitial fluid) to successively monitor several parameters, such as glucose, lactate, etc. All these applications are performed using electrical stimulator devices capable of applying constant voltage pulses or constant current pulses via electrodes to human tissues. Usually, constant current stimulators are most widely used because of their safety, stability, and repeatability. Thus, the aim of this work was to design, realize and test a mixed-signal electronic interface capable of producing current pulses with custom amplitude, duration, frequency, polarity and symmetry with extended voltage compliance. To achieve this result, we developed a high-voltage current stimulator suitable for iontophoresis applications. Current stimuli can be applied setting the intensity, frequency and duty cycle of the stimulation patterns through a µC. A custom electronic interface was designed to allow the control of the injected current in real time and to prevent electrical injuries to the patient by avoiding potential unwanted short circuits. Moreover, the system was tested in a simulated environment demonstrating its effectiveness and applicability for transdermal monitoring applications. The obtained results show that the device is able to apply monophasic and biphasic pulses, ranging from 0.1 to 10 mA, with a maximum error of about 10% at the minimum intensity; in addition, current stimuli can be applied up to a maximum frequency of 100 kHz with a voltage compliance of 120 V.

On-body non-invasive glucose monitoring sensor based on high figure of merit (FoM) surface plasmonic microwave resonator

High-figure of merit (FoM) plasmonic microwave resonator is researched as a non-invasive on-body sensor to monitor the human body's blood glucose variation rate in adults for biomedical applications, e.g., diabetic patients. The resonance frequencies of the proposed sensor are measured to be around [Formula: see text] GHz and [Formula: see text] GHz over the frequency band of DC to 6GHz which are suitable for monitoring interstitial fluid (ISF) changing rate. The [Formula: see text] sensor is experimentally wrapped on the human body arm to monitor the blood glucose changing rate via amplitude and frequency variations of the sensor. Amplitude variation and frequency shift are measured to be around 7 dB and 30 MHz, respectively. The measured results demonstrate the high precision of the proposed approach to depict a valid diagram for glucose changing rate due to good impedance matching of the designed microwave sensor and human body. The sensor is shown to enhance the sensitivity by a factor of 5 compared to the conventional ones.

Effect of interstitial fluid pH on transdermal glucose extraction by reverse iontophoresis

Reverse iontophoresis (RI) is a promising technology in the field of continuous glucose monitoring (CGM), offering significant advantages such as finger-stick-free operation, wearability, and non-invasiveness. In the glucose extraction process based on RI, the pH of the interstitial fluid (ISF) is a critical factor that needs further investigation, as it directly influences the accuracy of transdermal glucose monitoring. In this study, a theoretical analysis was conducted to investigate the mechanism by which pH affects the glucose extraction flux. Modeling and numerical simulations performed at different pH conditions indicated that the zeta potential was significantly impacted by the pH, thereby altering the direction and flux of the glucose iontophoretic extraction. A screen-printed glucose biosensor integrated with RI extraction electrodes was developed for ISF extraction and glucose monitoring. The accuracy and stability of the ISF extraction and glucose detection device were demonstrated with extraction experiments using different subdermal glucose concentrations ranging from 0 to 20 mM. The extraction results for different ISF pH values exhibited that at 5 mM and 10 mM subcutaneous glucose, the extracted glucose concentration was increased by 0.08212 mM and 0.14639 mM for every 1 pH unit increase, respectively. Furthermore, the normalized results for 5 mM and 10 mM glucose demonstrated a linear correlation, indicating considerable potential for incorporating a pH correction factor in the blood glucose prediction model used to calibrate glucose monitoring.

Non-invasive electrochemical immunosensor for reverse iontophoretic determination of cardiac troponins (cTnT & cTnI) in a simulated artificial skin model. Significance of raw DPV and CV data for chemometric discrimination

Electrochemical immunosensors coupled with reverse iontophoresis (RI) for noninvasive determination of cardiac troponins were developed and validated according to ICH Q2 (R1) guideline. Linearity was in 0.01-10 and 100-500 ng/mL ranges. LODs (ng/mL) were in 6-25 × 10-4, while LOQs (μg/mL) were in 18-7.5 × 10-4 range. Chemometric evaluation was performed on raw data simply by principle component analysis and cluster analysis to discriminate stages of immunosensors. This is the first demonstration of RI determination of cardiac troponins so far. Findings of the current manuscript have great potential to develop point of care diagnostic systems for major cardiac events, where high sensitivity and specificity are required.

Reverse iontophoresis with the development of flexible electronics: A review

Skin-centric diagnosis techniques, such as epidermal physiological parameter monitoring, have developed rapidly in recent years. The analysis of interstitial fluid (ISF), a body liquid with abundant physiological information, is a promising method to obtain health status because ISF is easily assessed by implanted or percutaneous measurements. Reverse iontophoresis extracts ISF by applying an electric field onto the skin, and it is a promising method to noninvasively obtain ISF, which, in turn, enables noninvasive epidermal physiological parameter monitoring. However, the development of reverse iontophoresis was relatively slow around the 2010s due to the rigidity and low biocompatibility of the applied devices. With the rapid development of flexible electronic technology in recent years, new progress has been made in the field of reverse iontophoresis, especially in the field of blood glucose monitoring and drug monitoring. This review summarizes the recent advances and discusses the challenges and opportunities of reverse iontophoresis.

Non-invasive monitoring of interstitial fluid lactate through an epidermal iontophoretic device

The development of a non-invasive sensing technology that allows collection of interstitial fluid (ISF) lactate and its subsequent analysis without exertion requirement, could enable lactate monitoring from rested individuals. Here, we describe a wearable, soft epidermal adhesive patch that integrates a reverse iontophoretic (RI) system, and an amperometric lactate biosensor placed on the anodic electrode with a porous hydrogel reservoir, for simultaneous ISF lactate extraction and quantification via electrochemical sensing, respectively. The iontophoretic system includes agarose hydrogels for preventing skin electrocution, while a porous polyvinyl alcohol-based hydrogel facilitates the effective transport of lactate from skin to the biosensor. The flexible skin-worn device tested on healthy individuals at rest showed rapid lactate collection from the ISF after 10 min of reverse iontophoresis with no evidence of discomfort or irritation to the skin. Detailed characterization of the enzymatic biosensor before and during on-body trials along with relevant control experiments confirmed the efficient extraction and selective detection of ISF lactate. Such an epidermal technology represents the first demonstration of an all-in-one platform that integrates non-invasive collection and subsequent analysis of lactate from iontophoretically extracted ISF toward point-of-care operation.

A Simple Reversed Iontophoresis-Based Sensor to Enable In Vivo Multiplexed Measurement of Plant Biomarkers Using Screen-Printed Electrodes

The direct quantification of plant biomarkers in sap is crucial to enhancing crop production. However, current approaches are inaccurate, involving the measurement of non-specific parameters such as colour intensity of leaves, or requiring highly invasive processes for the extraction of sap. In addition, these methods rely on bulky and expensive equipment, and they are time-consuming. The present work reports for the first time a low-cost sensing device that can be used for the simultaneous determination of sap K+ and pH in living plants by means of reverse iontophoresis. A screen-printed electrode was modified by deposition of a K+-selective membrane, achieving a super-Nernstian sensitivity of 70 mV Log[K+]−1 and a limit of detection within the micromolar level. In addition, the cathode material of the reverse iontophoresis device was modified by electrodeposition of RuOx particles. This electrode could be used for the direct extraction of ions from plant leaves and the amperometric determination of pH within the physiological range (pH 3−8), triggered by the selective reaction of RuOx with H+. A portable and low-cost (<£60) microcontroller-based device was additionally designed to enable its use in low-resource settings. The applicability of this system was demonstrated by measuring the changes in concentration of K+ and pH in tomato plants before and after watering with deionised water. These results represent a step forward in the design of affordable and non-invasive devices for the monitoring of key biomarkers in plants, with a plethora of applications in smart farming and precision agriculture among others.

Pilot study in human healthy volunteers on the use of magnetohydrodynamics in needle-free continuous glucose monitoring

The benefits of continuous glucose monitoring (CGM) in diabetes management are extensively documented. Yet, the broader adoption of CGM systems is limited by their cost and invasiveness. Current CGM devices, requiring implantation or the use of hypodermic needles, fail to offer a convenient solution. We have demonstrated that magnetohydrodynamics (MHD) is effective at extracting dermal interstitial fluid (ISF) containing glucose, without the use of needles. Here we present the first study of ISF sampling with MHD for glucose monitoring in humans. We conducted 10 glucose tolerance tests on 5 healthy volunteers and obtained a significant correlation between the concentration of glucose in ISF samples extracted with MHD and capillary blood glucose samples. Upon calibration and time lag removal, the data indicate a Mean Absolute Relative Difference (MARD) of 12.9% and Precision Absolute Relative Difference of 13.1%. In view of these results, we discuss the potential value and limitations of MHD in needle-free glucose monitoring.

A Comprehensive Review of the Recent Developments in Wearable Sweat-Sensing Devices

Sweat analysis offers non-invasive real-time on-body measurement for wearable sensors. However, there are still gaps in current developed sweat-sensing devices (SSDs) regarding the concerns of mixing fresh and old sweat and real-time measurement, which are the requirements to ensure accurate the measurement of wearable devices. This review paper discusses these limitations by aiding model designs, features, performance, and the device operation for exploring the SSDs used in different sweat collection tools, focusing on continuous and non-continuous flow sweat analysis. In addition, the paper also comprehensively presents various sweat biomarkers that have been explored by earlier works in order to broaden the use of non-invasive sweat samples in healthcare and related applications. This work also discusses the target analyte's response mechanism for different sweat compositions, categories of sweat collection devices, and recent advances in SSDs regarding optimal design, functionality, and performance.

Highly integrated watch for noninvasive continual glucose monitoring

This article reports a highly integrated watch for noninvasive continual blood glucose monitoring. The watch employs a Nafion-coated flexible electrochemical sensor patch fixed on the watchband to obtain interstitial fluid (ISF) transdermally at the wrist. This reverse iontophoresis-based extraction method eliminates the pain and inconvenience that traditional fingerstick blood tests pose in diabetic patients' lives, making continual blood glucose monitoring practical and easy. All electronic modules, including a rechargeable power source and other modules for signal processing and wireless transmission, are integrated onto a watch face-sized printed circuit board (PCB), enabling comfortable wearing of this continual glucose monitor. Real-time blood glucose levels are displayed on the LED screen of the watch and can also be checked with the smartphone user interface. With 23 volunteers, the watch demonstrated 84.34% clinical accuracy in the Clarke error grid analysis (zones A + B). In the near future, commercial products could be developed based on this lab-made prototype to provide the public with noninvasive continual glucose monitoring.

Reverse Iontophoretic Extraction of Skin Cancer-Related Biomarkers

Non-invasive methods for early diagnosis of skin cancer are highly valued. One possible approach is to monitor relevant biomarkers such as tryptophan (Trp) and kynurenine (Kyn), on the skin surface. The primary aim of this in vitro investigation was, therefore, to examine whether reverse iontophoresis (RI) can enhance the extraction of Trp and Kyn, and to demonstrate how the Trp/Kyn ratio acquired from the skin surface reflects that in the epidermal tissue. The study also explored whether the pH of the receiver medium impacted on extraction efficiency, and assessed the suitability of a bicontinuous cubic liquid crystal as an alternative to a simple buffer solution for this purpose. RI substantially enhanced the extraction of Trp and Kyn, in particular towards the cathode. The Trp/Kyn ratio obtained on the surface matched that in the viable skin. Increasing the receiver solution pH from 4 to 9 improved extraction of both analytes, but did not significantly change the Trp/Kyn ratio. RI extraction of Trp and Kyn into the cubic liquid crystal was comparable to that achieved with simple aqueous receiver solutions. We conclude that RI offers a potential for non-invasive sampling of low-molecular weight biomarkers and further investigations in vivo are therefore warranted.

Electrical aspects of skin as a pathway to engineering skin devices

Skin is one of the indispensable organs for life. The epidermis at the outermost surface provides a permeability barrier to infectious agents, chemicals, and excessive loss of water, while the dermis and subcutaneous tissue mechanically support the structure of the skin and appendages, including hairs and secretory glands. The integrity of the integumentary system is a key for general health, and many techniques have been developed to measure and control this protective function. In contrast, the effective skin barrier is the major obstacle for transdermal delivery and detection. Changes in the electrical properties of skin, such as impedance and ionic activity, is a practical indicator that reflects the structures and functions of the skin. For example, the impedance that reflects the hydration of the skin is measured for quantitative assessment in skincare, and the current generated across a wound is used for the evaluation and control of wound healing. Furthermore, the electrically charged structure of the skin enables transdermal drug delivery and chemical extraction. This paper provides an overview of the electrical aspects of the skin and summarizes current advances in the development of devices based on these features.

Recent Progress in Intelligent Wearable Sensors for Health Monitoring and Wound Healing Based on Biofluids

The intelligent wearable sensors promote the transformation of the health care from a traditional hospital-centered model to a personal portable device-centered model. There is an urgent need of real-time, multi-functional, and personalized monitoring of various biochemical target substances and signals based on the intelligent wearable sensors for health monitoring, especially wound healing. Under this background, this review article first reviews the outstanding progress in the development of intelligent, wearable sensors designed for continuous, real-time analysis, and monitoring of sweat, blood, interstitial fluid, tears, wound fluid, etc. Second, this paper reports the advanced status of intelligent wound monitoring sensors designed for wound diagnosis and treatment. The paper highlights some smart sensors to monitor target analytes in various wounds. Finally, this paper makes conservative recommendations regarding future development of intelligent wearable sensors.

Energy Autonomous Sweat-Based Wearable Systems

The continuous operation of wearable electronics demands reliable sources of energy, currently met through Li-ion batteries and various energy harvesters. These solutions are being used out of necessity despite potential safety issues and unsustainable environmental impact. Safe and sustainable energy sources can boost the use of wearables systems in diverse applications such as health monitoring, prosthetics, and sports. In this regard, sweat- and sweat-equivalent-based studies have attracted tremendous attention through the demonstration of energy-generating biofuel cells, promising power densities as high as 3.5 mW cm-2 , storage using sweat-electrolyte-based supercapacitors with energy and power densities of 1.36 Wh kg-1 and 329.70 W kg-1 , respectively, and sweat-activated batteries with an impressive energy density of 67 Ah kg-1 . A combination of these energy generating, and storage devices can lead to fully energy-autonomous wearables capable of providing sustainable power in the µW to mW range, which is sufficient to operate both sensing and communication devices. Here, a comprehensive review covering these advances, addressing future challenges and potential solutions related to fully energy-autonomous wearables is presented, with emphasis on sweat-based energy storage and energy generation elements along with sweat-based sensors as applications.

The role of microneedle arrays in drug delivery and patient monitoring to prevent diabetes induced fibrosis

Diabetes affects approximately 450 million adults globally. If not effectively managed, chronic hyperglycaemia causes tissue damage that can develop into fibrosis. Fibrosis leads to end-organ complications, failure of organ systems occurs, which can ultimately cause death. One strategy to tackle end-organ complications is to maintain normoglycaemia. Conventionally, insulin is administered subcutaneously. Whilst effective, this delivery route shows several limitations, including pain. The transdermal route is a favourable alternative. Microneedle (MN) arrays are minimally invasive and painless devices that can enhance transdermal drug delivery. Convincing evidence is provided on MN-mediated insulin delivery. MN arrays can also be used as a diagnostic tool and monitor glucose levels. Furthermore, sophisticated MN array-based systems that integrate glucose monitoring and drug delivery into a single device have been designed. Therefore, MN technology has potential to revolutionise diabetes management. This review describes the current applications of MN technology for diabetes management and how these could prevent diabetes induced fibrosis.

Sampling of fluid through skin with magnetohydrodynamics for noninvasive glucose monitoring

Out of 463 million people currently with diabetes, 232 million remain undiagnosed. Diabetes is a threat to human health, which could be mitigated via continuous self-monitoring of glucose. In addition to blood, interstitial fluid is considered to be a representative sample for glucose monitoring, which makes it highly attractive for wearable on-body sensing. However, new technologies are needed for efficient and noninvasive sampling of interstitial fluid through the skin. In this report, we introduce the use of Lorentz force and magnetohydrodynamics to noninvasively extract dermal interstitial fluid. Using porcine skin as an ex-vivo model, we demonstrate that the extraction rate of magnetohydrodynamics is superior to that of reverse iontophoresis. This work seeks to provide a safe, effective, and noninvasive sampling method to unlock the potential of wearable sensors in needle-free continuous glucose monitoring devices that can benefit people living with diabetes.

Selective colorimetric urine glucose detection by paper sensor functionalized with polyaniline nanoparticles and cell membrane

For the diabetes diagnosis, noninvasive methods are preferred to invasive methods; urine glucose measurement is an example of a noninvasive method. However, conventional noninvasive methods for urine glucose measurement are not intuitive. Furthermore, such methods exhibit low selectivity because they can detect interfering molecules in addition to glucose. Herein, we fabricate a noninvasive, intuitive, and highly selective paper sensor consisting of polyaniline nanoparticles (PAni-NPs) and red blood cell membranes (RBCMs). The PAni-NPs (adsorbed on the paper) are highly sensitive to hydrogen ions and change color from emeraldine blue to emeraldine green within a few seconds. The RBCM (coated on the PAni-NP-adsorbed paper) having the glucose transporter-1 protein plays the role of a smart filter that transports glucose but rejects other interfering molecules. In particular, the selectivity of the RBCM-coated PAni-NP-based paper sensor was approximately improved ∼85%, compared to the uncoated paper sensors. The paper sensor could detect urine glucose over the range of 0-10 mg/mL (0-56 mM), with a limit of detection of 0.54 mM. The proposed paper sensor will facilitate the development of a highly selective and colorimetric urine glucose monitoring system.

From Diagnosis to Treatment: Recent Advances in Patient-Friendly Biosensors and Implantable Devices

Patient-friendly medical diagnostics and treatments have been receiving a great deal of interest due to their rapid and cost-effective health care applications with minimized risk of infection, which has the potential to replace conventional hospital-based medical procedures. In particular, the integration of recently developed materials into health care devices allows the rapid development of point-of-care (POC) sensing platforms and implantable devices with special functionalities. In this review, the recent advances in biosensors for patient-friendly diagnosis and implantable devices for patient-friendly treatment are discussed. Comprehensive analysis of portable and wearable biosensing platforms for patient-friendly health monitoring and disease diagnosis is provided, including topics such as materials selection, device structure and integration, and biomarker detection strategies. Moreover, specific challenges related to each biological fluid for wearable biosensor-based POC applications are presented. Also, advances in implantable devices, including recent materials development and wireless communication strategies, are discussed. Furthermore, various patient-friendly surgical and treatment approaches are reviewed, such as minimally invasive insertion and mounting, in vivo electrical and optical modulations, and post-operation health monitoring. Finally, the challenges and future perspectives toward the development of the patient-friendly diagnosis and treatment are provided.

A thermal activated and differential self-calibrated flexible epidermal biomicrofluidic device for wearable accurate blood glucose monitoring

This paper reports a flexible electronics-based epidermal biomicrofluidics technique for clinical continuous blood glucose monitoring, overcoming the drawback of the present wearables, unreliable measurements. A thermal activation method is proposed to improve the efficiency of transdermal interstitial fluid (ISF) extraction, enabling extraction with a low current density to notably reduce skin irritation. An Na⁺ sensor and a correction model are proposed to eliminate the effect of individual differences, which leads to fluctuations in the amount of ISF extraction. An electrochemical sensor with a 3D nanostructured working electrode surface is designed to enable precise in situ glucose measurement. A differential structure is proposed to eliminate the effect of passive perspiration, which leads to inaccurate blood glucose prediction. Fabrications of the epidermal biomicrofluidic device including formation of flexible electrodes, nanomaterial modification, and enzyme immobilization are fully realized by inkjet printing to enable facile manufacturing with low cost, which benefits practical production.

Optimization of Extraction Parameters of Reverse Iontophoretic Determination of Blood Glucose in an Artificial Skin Model

Background:

Reverse İontophoresis (RI) is one of the promising non-invasive technologies. It relies on the transition of low magnitude current through the skin and thus glucose measurement becomes possible as it is extracted from the surface during this porter current flow.

Objective:

This paper deals with the development and optimization of an RI determination method for glucose. CE dialysis membrane based artificial skin model was developed and the dependence of RI extraction on various experimental parameters was investigated.

Method:

Dependence of RI extraction performance on noble electrodes (platinum, silver, palladium, ruthenium, rhodium) was checked with CA, CV and DPV, in a wide pH and ionic strength range. Optimizations on inter-electrode distance, potential type and magnitude, extraction time, gel type, membrane MWCO, usage frequency, pretreatment, artificial body fluids were performed.

Results:

According to the optimized results, the inter-electrode distance was 7.0 mm and silver was the optimum noble metal. Optimum pH and ionic strength were achieved with 0.05M PBS at pH 7.4. Higher glucose yields were obtained with DPV, while CA and CV achieved almost the same levels. During CA, +0.5V achieved the highest glucose yield and higher potential even caused a decrease. Glucose levels could be monitored for 24 hours. CMC gel was the optimum collection media. Pretreated CE membrane with 12kD MWCO was the artificial skin model. Pretreatment affected the yields while its condition caused no significant difference. Except PBS solution (simulated as artificial plasma), among the various artificial simulated body fluids, intestinal juice formulation (AI) and urine formulation U2 were the optimum extraction media, respectively.

Conclusion:

In this study, various experimental parameters (pretereatment procedure, type and MWCO values of membranes, inter-electrode distance, electrode material, extraction medium solvents, ionic strength and pH, collection medium gel type, extraction potential type and magnitude, extraction time and etc) were optimized for the non-invasive RI determination of glucose in a CE dialysis membrane-based artificial skin model and various simulated artificial body fluids.

Evaluation of the clinical impact of repeat application of hydrogel-forming microneedle array patches

Hydrogel-forming microneedle array patches (MAPs) have been proposed as viable clinical tools for patient monitoring purposes, providing an alternative to traditional methods of sample acquisition, such as venepuncture and intradermal sampling. They are also undergoing investigation in the management of non-melanoma skin cancers. In contrast to drug or vaccine delivery, when only a small number of MAP applications would be required, hydrogel MAPs utilised for sampling purposes or for tumour eradication would necessitate regular, repeat applications. Therefore, the current study was designed to address one of the key translational aspects of MAP development, namely patient safety. We demonstrate, for the first time in human volunteers, that repeat MAP application and wear does not lead to prolonged skin reactions or prolonged disruption of skin barrier function. Importantly, concentrations of specific systemic biomarkers of inflammation (C-reactive protein (CRP); tumour necrosis factor-α (TNF-α)); infection (interleukin-1β (IL-1β); allergy (immunoglobulin E (IgE)) and immunity (immunoglobulin G (IgG)) were all recorded over the course of this fixed study period. No biomarker concentrations above the normal, documented adult ranges were recorded over the course of the study, indicating that no systemic reactions had been initiated in volunteers. Building upon the results of this study, which serve to highlight the safety of our hydrogel MAP, we are actively working towards CE marking of our MAP technology as a medical device.

Recent Developments of Flexible and Stretchable Electrochemical Biosensors

The skyrocketing popularity of health monitoring has spurred increasing interest in wearable electrochemical biosensors. Compared with the traditionally rigid and bulky electrochemical biosensors, flexible and stretchable devices render a unique capability to conform to the complex, hierarchically textured surfaces of the human body. With a recognition element (e.g., enzymes, antibodies, nucleic acids, ions) to selectively react with the target analyte, wearable electrochemical biosensors can convert the types and concentrations of chemical changes in the body into electrical signals for easy readout. Initial exploration of wearable electrochemical biosensors integrates electrodes on textile and flexible thin-film substrate materials. A stretchable property is needed for the thin-film device to form an intimate contact with the textured skin surface and to deform with various natural skin motions. Thus, stretchable materials and structures have been exploited to ensure the effective function of a wearable electrochemical biosensor. In this mini-review, we summarize the recent development of flexible and stretchable electrochemical biosensors, including their principles, representative application scenarios (e.g., saliva, tear, sweat, and interstitial fluid), and materials and structures. While great strides have been made in the wearable electrochemical biosensors, challenges still exist, which represents a small fraction of opportunities for the future development of this burgeoning field.

Wearable and flexible electronics for continuous molecular monitoring

Wearable biosensors have received tremendous attention over the past decade owing to their great potential in predictive analytics and treatment toward personalized medicine. Flexible electronics could serve as an ideal platform for personalized wearable devices because of their unique properties such as light weight, low cost, high flexibility and great conformability. Unlike most reported flexible sensors that mainly track physical activities and vital signs, the new generation of wearable and flexible chemical sensors enables real-time, continuous and fast detection of accessible biomarkers from the human body, and allows for the collection of large-scale information about the individual's dynamic health status at the molecular level. In this article, we review and highlight recent advances in wearable and flexible sensors toward continuous and non-invasive molecular analysis in sweat, tears, saliva, interstitial fluid, blood, wound exudate as well as exhaled breath. The flexible platforms, sensing mechanisms, and device and system configurations employed for continuous monitoring are summarized. We also discuss the key challenges and opportunities of the wearable and flexible chemical sensors that lie ahead.

Extraction of levetiracetam for therapeutic drug monitoring by transdermal reverse iontophoresis

Recently, transdermal reverse iontophoresis across the skin has been investigated as a novel technology for the purpose of diagnosis as well as therapeutic drug monitoring. Accordingly, the objective of this study was to investigate ex vivo and in vivo transdermal extraction of levetiracetam, an antiepileptic drug, across the pig ear skin by reverse iontophoresis. Reverse iontophoresis experiments were performed using three chambered diffusion cells. Extractions profiles were generated in phosphate buffers at different current intensities, pH and ionic strength as well donor drug concentrations. This was followed by ex vivo extraction in gels and in vivo extractions using New Zealand rabbits. Results indicate that levetiracetam was extracted at both anode and cathode. Flux values were unaffected by increase in current intensities (0.5 mA and 0.6 mA) but affected by pH and ionic strength. Neither in cathodal nor in anodal extraction, flux values did show a proportional relationship with the donor drug concentration. At low and medium concentration levels, flux values did not show any major change but the extraction flux at high donor concentration was much higher. In contrast, in vivo experiment with rabbits resulted in wide variation of fluxes with very high fluxes recorded at the cathodal end. Reasons attributed to this difference may include lower current intensity, and species variation. The most significant finding of this study is that measurable amounts of the levetiracetam were extracted at both the ends indicating its feasibility for non-invasive drug monitoring.

Temperature-Corrected Fluidic Glucose Sensor Based on Microwave Resonator

In this paper, a fluidic glucose sensor that is based on a complementary split-ring resonator (CSRR) is proposed for the microwave frequency region. The detection of glucose with different concentrations from 0 mg/dL to 400 mg/dL in a non-invasive manner is possible by introducing a fluidic system. The glucose concentration can be continuously monitored by tracking the transmission coefficient S21 as a sensing parameter. The variation tendency in S21 by the glucose concentration is analyzed with equivalent circuit model. In addition, to eradicate the systematic error due to temperature variation, the sensor is tested in two temperature conditions: the constant temperature condition and the time-dependent varying temperature condition. For the varying temperature condition, the temperature correction function was derived between the temperature and the variation in S21 for DI water. By applying the fitting function to glucose solution, the subsidiary results due to temperature can be completely eliminated. As a result, the S21 varies by 0.03 dB as the glucose concentration increases from 0 mg/dL to 400 mg/dL.

Enhancing glucose flux into sweat by increasing paracellular permeability of the sweat gland

Non-invasive wearable biosensors provide real-time, continuous, and actionable health information. However, difficulties detecting diluted biomarkers in excreted biofluids limit practical applications. Most biomarkers of interest are transported paracellularly into excreted biofluids from biomarker-rich blood and interstitial fluid during normal modulation of cellular tight junctions. Calcium chelators are reversible tight junction modulators that have been shown to increase absorption across the intestinal epithelium. However, calcium chelators have not yet been shown to improve the extraction of biomarkers. Here we show that for glucose, a paracellularly transported biomarker, the flux into sweat can be increased by >10x using citrate, a calcium chelator, in combination with electroosmosis. Our results demonstrate a method of increasing glucose flux through the sweat gland epithelium, thereby increasing the concentration in sweat. Future work should examine if this method enhances flux for other paracellularly transported biomarkers to make it possible to detect more biomarkers with currently available biosensors.

Wearable sensors: modalities, challenges, and prospects

Wearable sensors have recently seen a large increase in both research and commercialization. However, success in wearable sensors has been a mix of both progress and setbacks. Most of commercial progress has been in smart adaptation of existing mechanical, electrical and optical methods of measuring the body. This adaptation has involved innovations in how to miniaturize sensing technologies, how to make them conformal and flexible, and in the development of companion software that increases the value of the measured data. However, chemical sensing modalities have experienced greater challenges in commercial adoption, especially for non-invasive chemical sensors. There have also been significant challenges in making significant fundamental improvements to existing mechanical, electrical, and optical sensing modalities, especially in improving their specificity of detection. Many of these challenges can be understood by appreciating the body's surface (skin) as more of an information barrier than as an information source. With a deeper understanding of the fundamental challenges faced for wearable sensors and of the state-of-the-art for wearable sensor technology, the roadmap becomes clearer for creating the next generation of innovations and breakthroughs.

Skin-like biosensor system via electrochemical channels for noninvasive blood glucose monitoring

Currently, noninvasive glucose monitoring is not widely appreciated because of its uncertain measurement accuracy, weak blood glucose correlation, and inability to detect hyperglycemia/hypoglycemia during sleep. We present a strategy to design and fabricate a skin-like biosensor system for noninvasive, in situ, and highly accurate intravascular blood glucose monitoring. The system integrates an ultrathin skin-like biosensor with paper battery-powered electrochemical twin channels (ETCs). The designed subcutaneous ETCs drive intravascular blood glucose out of the vessel and transport it to the skin surface. The ultrathin (~3 μm) nanostructured biosensor, with high sensitivity (130.4 μA/mM), fully absorbs and measures the glucose, owing to its extreme conformability. We conducted in vivo human clinical trials. The noninvasive measurement results for intravascular blood glucose showed a high correlation (>0.9) with clinically measured blood glucose levels. The system opens up new prospects for clinical-grade noninvasive continuous glucose monitoring.

Electrochemical skin conductance: a systematic review

PURPOSE

Currently available techniques for the evaluation of small fiber neuropathy and related sudomotor function remain suboptimal. Electrochemical skin conductance (ESC) has recently been introduced as a simple noninvasive and fast method for the detection of sudomotor dysfunction. The purpose of this review is to synthesize and appraise research using ESC measurements for sudomotor evaluation in adults.

METHODS

Electronic databases including MEDLINE and Google Scholar were searched (up to March 13, 2017). The search strategy included the following terms: "electrochemical skin conductance," "Sudoscan," and "EZSCAN." Evidence was graded according to defined quality indicators including (1) level of evidence; (2) use of established tests as reference tests (e.g., quantitative sudomotor axon test [QSART], sympathetic skin responses [SSR], thermoregulatory sweat test [TST], and skin biopsies to assess sudomotor and epidermal small fibers); (3) use of consecutive/non-consecutive subjects; and (4) study design (prospective/retrospective).

RESULTS

A total of 24 studies met the inclusion criteria. These were classified into preclinical, normative, comparative/diagnostic, or interventional. ESC measurement properties, diagnostic accuracy, and similarities to and differences from established tests were examined.

CONCLUSIONS

ESC measurements expand the arsenal of available tests for the evaluation of sudomotor dysfunction. The advantages and disadvantages of ESC versus established tests for evaluating sudomotor/small fiber function reviewed herein should be used as evidence to inform future guidelines on the assessment of sudomotor function.

Reverse Iontophoretic Extraction of Metabolites from Living Plants and their Identification by Ion-chromatography Coupled to High Resolution Mass Spectrometry

INTRODUCTION

The identification and characterisation of cellular metabolites has now become an important strategy to obtain insight into functional plant biology. However, the extraction of metabolites for identification and analysis is challenging and, at the present time, usually requires destruction of the plant.

OBJECTIVE

To detect different plant metabolites in living plants with no pre-treatment using the combination of iontophoresis and ion-chromatography with mass spectrometry detection.

METHODOLOGY

In this work, the simple and non-destructive method of reverse iontophoresis has been used to extract in situ multiple plant metabolites from intact Ocimum basilicum leaves. Subsequently, the analysis of these metabolites has been performed with ion chromatography coupled directly to high resolution mass spectrometric detection (IC-MS).

RESULTS

The application of reverse iontophoresis to living plant samples has avoided the need for complex pre-treatments. With this approach, no less than 24 compounds, including organic acids and sugars as well as adenosine triphosphate (ATP) were successfully detected.

CONCLUSION

The research demonstrates that it is feasible to monitor, therefore, a number of important plant metabolites using a simple, relatively fast and non-destructive approach. Copyright © 2016 John Wiley & Sons, Ltd.

Hydrogel-forming microneedle arrays: Potential for use in minimally-invasive lithium monitoring

We describe, for the first time, hydrogel-forming microneedle (s) (MN) arrays for minimally-invasive extraction and quantification of lithium in vitro and in vivo. MN arrays, prepared from aqueous blends of hydrolysed poly(methyl-vinylether-co-maleic anhydride) and crosslinked by poly(ethyleneglycol), imbibed interstitial fluid (ISF) upon skin insertion. Such MN were always removed intact. In vitro, mean detected lithium concentrations showed no significant difference following 30min MN application to excised neonatal porcine skin for lithium citrate concentrations of 0.9 and 2mmol/l. However, after 1h application, the mean lithium concentrations extracted were significantly different, being appropriately concentration-dependent. In vivo, rats were orally dosed with lithium citrate equivalent to 15mg/kg and 30mg/kg lithium carbonate, respectively. MN arrays were applied 1h after dosing and removed 1h later. The two groups, having received different doses, showed no significant difference between lithium concentrations in serum or MN. However, the higher dosed rats demonstrated a lithium concentration extracted from MN arrays equivalent to a mean increase of 22.5% compared to rats which received the lower dose. Hydrogel-forming MN clearly have potential as a minimally-invasive tool for lithium monitoring in outpatient settings. We will now focus on correlation between serum and MN lithium concentrations.

Hydrogel-Forming Microneedle Arrays Allow Detection of Drugs and Glucose In Vivo: Potential for Use in Diagnosis and Therapeutic Drug Monitoring

We describe, for the first time the use of hydrogel-forming microneedle (MN) arrays for minimally-invasive extraction and quantification of drug substances and glucose from skin in vitro and in vivo. MN prepared from aqueous blends of hydrolysed poly(methyl-vinylether-co-maleic anhydride) (11.1% w/w) and poly(ethyleneglycol) 10,000 daltons (5.6% w/w) and crosslinked by esterification swelled upon skin insertion by uptake of fluid. Post-removal, theophylline and caffeine were extracted from MN and determined using HPLC, with glucose quantified using a proprietary kit. In vitro studies using excised neonatal porcine skin bathed on the underside by physiologically-relevant analyte concentrations showed rapid (5 min) analyte uptake. For example, mean concentrations of 0.16 μg/mL and 0.85 μg/mL, respectively, were detected for the lowest (5 μg/mL) and highest (35 μg/mL) Franz cell concentrations of theophylline after 5 min insertion. A mean concentration of 0.10 μg/mL was obtained by extraction of MN inserted for 5 min into skin bathed with 5 μg/mL caffeine, while the mean concentration obtained by extraction of MN inserted into skin bathed with 15 μg/mL caffeine was 0.33 μg/mL. The mean detected glucose concentration after 5 min insertion into skin bathed with 4 mmol/L was 19.46 nmol/L. The highest theophylline concentration detected following extraction from a hydrogel-forming MN inserted for 1 h into the skin of a rat dosed orally with 10 mg/kg was of 0.363 μg/mL, whilst a maximum concentration of 0.063 μg/mL was detected following extraction from a MN inserted for 1 h into the skin of a rat dosed with 5 mg/kg theophylline. In human volunteers, the highest mean concentration of caffeine detected using MN was 91.31 μg/mL over the period from 1 to 2 h post-consumption of 100 mg Proplus^® tablets. The highest mean blood glucose level was 7.89 nmol/L detected 1 h following ingestion of 75 g of glucose, while the highest mean glucose concentration extracted from MN was 4.29 nmol/L, detected after 3 hours skin insertion in human volunteers. Whilst not directly correlated, concentrations extracted from MN were clearly indicative of trends in blood in both rats and human volunteers. This work strongly illustrates the potential of hydrogel-forming MN in minimally-invasive patient monitoring and diagnosis. Further studies are now ongoing to reduce clinical insertion times and develop mathematical algorithms enabling determination of blood levels directly from MN measurements.

Extraction of Iron from the Rabbit Anterior Chamber with Reverse Iontophoresis

Ocular siderosis is a common eye disease caused by retention of an iron-containing intraocular foreign body in the eye. Iron-containing intraocular foreign bodies may cause severe inflammatory reaction and affect visual function. Currently the optimal treatment method of ocular siderosis is a moot point. This study used the reverse iontophoresis technique to noninvasively extract iron from the rabbit anterior chamber. By slit lamp observation and histological examination, reverse iontophoresis treatment has a good effect on ocular siderosis. Reverse iontophoresis seems to be a noninvasive and promising approach to extract iron from the anterior chamber to treat ocular siderosis.

Effect of potassium present in stratum corneum during non-invasive measurement of potassium in human subjects using reverse iontophoresis

PURPOSE

Reverse iontophoresis (RI) is one of the potential techniques used to monitor the concentration of various analytes in body fluids non-invasively. Transdermal extraction of potassium is investigated using RI. In the present work, the effect of potassium on stratum corneum (SC) during RI, feasibility of RI for continuous monitoring of potassium, and use of potassium as internal standard in RI, are investigated.

METHODS

Tape stripping experiment is carried out to find potassium concentration in SC. RI is carried out continuously for 180 min without passive diffusion and after passive diffusion for 60 min. Skin impedance measurements are done at 20 Hz and 20 kHz.

RESULTS

Potassium is found to be in the range 300-650 nmol/cm(2) on SC by tape stripping experiment. Correlation coefficient between blood potassium and extracted potassium through RI after passive diffusion (R(2) = 0.5870) is more than without passive diffusion (R(2) = 0.5117). The skin impedance measurement shows that RI has more effect on SC than superficial layer of SC during RI.

CONCLUSION

The present investigations conclude that it is possible to monitor potassium continuously through RI and using potassium as internal standard in RI.

Technology platforms for remote monitoring of vital signs in the new era of telemedicine

Driven by healthcare cost and home healthcare need, the development of remote monitoring technologies is poised to improve and revolutionize healthcare delivery and accessibility. This paper reviews the recent progress in the field of remote monitoring technologies that may have the potential to become the basic platforms for telemedicine. In particular, key techniques and devices for monitoring cardiorespiratory activity, blood pressure and blood glucose concentration are summarized and discussed. In addition, the US FDA approved remote vital signs monitoring devices currently available on the market are presented.

Trials and tribulations of skin iontophoresis in therapeutics

Iontophoresis is a method of non-invasive transdermal drug delivery based on the transfer of charged molecules using a low-intensity electric current. Both local and systemic administration are possible; however, the skin pharmacokinetics of iontophoretically delivered drugs is complex and difficult to anticipate. The unquestionable theoretical advantages of the technique make it attractive in several potential applications. After a brief review of the factors influencing iontophoresis, we detail the current applications of iontophoresis in therapeutics and the main potential applications under investigation, including systemic and topical drugs and focusing on the treatment of scleroderma-related ulcerations. Finally, we address the issue of safety, which could be a limitation to the routine clinical use of iontophoresis.

In situ detection of salicylate in Ocimum basilicum plant leaves via reverse iontophoresis

We report an in situ quantitative method to measure the concentration of salicylates, from intact, living Ocimum basilicum plant leaves. This simple, non-invasive method utilises iontophoresis in combination with cyclic voltammetry at disposable screen-printed electrodes.

Iontophoretic transdermal sampling of iohexol as a non-invasive tool to assess glomerular filtration rate

PURPOSE

To explore the potential of non-invasive reverse iontophoresis transdermal extraction of iohexol as a marker of glomerular filtration rate.

METHODS

A series of in vitro experiments were undertaken to establish the feasibility of iohexol reverse iontophoresis and to determine the optimal conditions for the approach. Subsequently, a pilot study in paediatric patients was performed to provide proof-of-concept.

RESULTS

The iontophoretic extraction fluxes of iohexol in vitro were proportional to the marker subdermal concentration and the reverse iontophoretic technique was able to track changes dynamically in simulated pharmacokinetic profiles. Reverse iontophoresis sampling was well tolerated by the four paediatric participants. The deduced values of the iohexol terminal elimination rate constant from transdermal reverse iontophoresis sampling agreed with those estimated by conventional blood sampling.

CONCLUSIONS

Reverse iontophoretic transdermal extraction fluxes mirrored the subdermal concentration profiles of iohexol, a relatively large neutral marker of glomerular filtration both in vitro and in vivo. The efficiency of extraction in vivo was well predicted by the in vitro model used.

Microneedle-mediated minimally invasive patient monitoring

BACKGROUND

The emerging field of microneedle-based minimally invasive patient monitoring and diagnosis is reviewed. Microneedle arrays consist of rows of micron-scale projections attached to a solid support. They have been widely investigated for transdermal drug and vaccine delivery applications since the late 1990s. However, researchers and clinicians have recently realized the great potential of microneedles for extraction of skin interstitial fluid and, less commonly, blood, for enhanced monitoring of patient health.

METHODS

We reviewed the journal and patent literature, and summarized the findings and provided technical insights and critical analysis.

RESULTS

We describe the basic concepts in detail and extensively review the work performed to date.

CONCLUSIONS

It is our view that microneedles will have an important role to play in clinical management of patients and will ultimately improve therapeutic outcomes for people worldwide.

Prospects of iontophoresis in cardiovascular drug delivery

Clinical benefits, industry interest, regulatory precedence, and strong market potential have made transdermal research the fastest growth area in drug delivery. As most drugs permeate poorly through skin, a major challenge is achieving the therapeutic level by enhancement of permeation rate. Iontophoresis, utilizing a minimal amount of current, is found to affect the skin permeation process drastically. Ideally suited for protein drugs, attempts have been made to utilize the technology for accelerating the low-molecular-weight drugs for chronic administration. However, because of the difficulty associated with the energy supply, commercialization was not feasible until recent times. Fortunately, the unprecedented growth of microelectronics has bridged this lacuna and brought the technology right into limelight. This article analyses the advantages of electrically assisted drug delivery in relation to passive permeation, with special reference to some cardiovascular drugs, for which there is already a demand in the market.