Noninvasive and minimally invasive methods for transdermal glucose monitoring

A Glycemic Status Classification Model Using a Radiofrequency Noninvasive Blood Glucose Monitor

Despite significant efforts in the development of noninvasive blood glucose (BG) monitoring solutions, delivering an accurate, real-time BG measurement remains challenging. We sought to address this by using a novel radiofrequency (RF) glucose sensor to noninvasively classify glycemic status. The study included 31 participants aged 18-65 with prediabetes or type 2 diabetes and no other significant medical history. During control sessions and oral glucose tolerance test sessions, data were collected from both a RF sensor that rapidly scans thousands of frequencies and concurrently from a venous blood draw measured with an US Food and Drug Administration (FDA)-cleared glucose hospital meter system to create paired observations. We trained a time series forest machine learning model on 80% of the paired observations and reported results from applying the model to the remaining 20%. Our findings show that the model correctly classified glycemic status 93.37% of the time as high, normal, or low.

Noninvasive Glucose Measurements Through Transcutaneous Raman Spectroscopy: A Review

BACKGROUND

People living with diabetes need constant glucose monitoring to avoid health complications. However, they do not monitor their glucose levels as often as recommended, probably because glucose measurement devices can be painful, costly, need testing strips or sensors, require lancing the finger or inserting a sensor with risk of infection, and can be inaccurate or have failures. Therefore, developing new alternatives for noninvasive glucose measurements that overcome these disadvantages is necessary, being Raman spectroscopy (RS) a solution.

OBJECTIVE

This review aims to provide an overview of the current glucose-monitoring technologies and the uses and advantages of RS to improve noninvasive transcutaneously glucose-monitoring devices.

RESULTS

The skin has been used to assess glucose levels noninvasively because it is an accessible tissue where glucose can be measured in the interstitial fluid (ISF) in the epidermis (especially in the stratum corneum). The most selected skin sites to apply RS for noninvasive glucose measurements were the nailfold, finger, and forearm because, in these sites, the penetration depth of the excitation light can reach the stratum corneum (10-20 µm) and the ISF. Studies found that RS is a good optical technique to measure glucose noninvasively by comparing glucose levels obtained by RS with those from invasive methods such as glucose meters with testing strips during an oral glucose tolerance test (OGTT).

CONCLUSIONS

New alternatives for noninvasive glucose measurements that overcome the disadvantages of current devices is necessary, and RS is a possible solution. However, more research is needed to evaluate the stability, accuracy, costs, and acceptance.

Glucose Concentration Measurement by All-Grating-Based System

An accurate, easy setup, low-cost, and time-saving method for measuring glucose concentration was proposed. An all-grating-based glucose concentration measurement system contained moving-grating-based heterodyne interferometry and a grating-based self-align sensor. By combining the first-order diffraction lights from two separated moving gratings by a polarization beam splitter and creating S- and P-polarized light interference by an analyzer, the interference signal could be a heterodyne light source with a heterodyne frequency depending on the relative velocities of the two moving gratings. Next, a grating-based self-align sensor was used to make the optical configuration setup easy and accurate. Moreover, the sensor was deposited on GOx film to improve the measurement sensitivity and specificity for glucose. Finally, the phase change induced by the reaction of the sensor and glucose solutions was detected. The validity of this method was proved, and the measurement resolution can reach 2 mg/dL.

Coronary Artery Disease and Inflammatory Activation Interfere with Peripheral Tissue Electrical Impedance Spectroscopy Characteristics-Initial Report

BACKGROUND

The electrical properties of cells and tissues in relation to energy exposure have been investigated, presenting their resistance and capacitance characteristics. The dielectric response to radiofrequency fields exhibits polarization heterogeneity under pathological conditions. The aim of the study was to analyze the differences in changes in resistance and capacitance measurements in the range from 1 kHz to 1 MHz, combined with an assessment of the correlation between the results of electrical impedance spectroscopy (EIS) and inflammatory activation.

METHODS

In the prospective study, EIS was performed on the non-dominant arm in 29 male patients (median (Q1-Q3) age of 69 (65-72)) with complex coronary artery disease and 10 male patients (median (Q1-Q3) age of 66 (62-69)) of the control group. Blood samples were collected for inflammatory index analysis.

RESULTS

The logistic regression analysis revealed a negative correlation with inflammatory indexes, including neutrophil to lymphocyte ratio (NLR) in the CAD group in the frequency of 30 kHz (p = 0.038, r = -0.317) regarding EIS resistance measurements and a positive correlation in CAD group in the frequency of 10 kHz (p = 0.029, r = -0.354) regarding EIS capacitance.

CONCLUSIONS

The bioelectric characteristics of peripheral tissues measured by resistance and capacitance in EIS differ in patients with coronary artery disease and in the control group. Electrical impedance spectroscopy reveals a statistically significant correlation with inflammatory markers in patients with CAD.

Noninvasive Glucose Monitor Using Dielectric Spectroscopy

OBJECTIVE

The Alertgy noninvasive continuous glucose monitor (ANICGM) is a novel wristband device that reports glucose levels without entailing skin puncture. This study evaluated the performance of the ANICGM compared to a Food and Drug Administration-approved glucose meter in patients with type 2 diabetes.

METHODS

The ANICGM device measures changes in the electromagnetic field generated by its sensor to produce a dielectric spectrum. The data contained within this spectrum are used in tandem with machine learning algorithms to estimate blood glucose levels. Values from the ANICGM were collected, sent to the Alertgy lab, formatted, and compared with fingerstick blood glucose levels, which were measured using the Accuchek Inform II glucometer. Fifteen patients completed three 120-minute sessions. The mean absolute relative difference (MARD) was calculated.

RESULTS

MARD values were compared between study days 2 and 3. The MARD for day 2 was 18.5% (95% CI, 12.8-42.2%), and the MARD for day 3 was 15.3% (95% CI, 12.3-18.4%). The difference in the MARD between days 2 and 3 was not statistically significant (P = .210).

CONCLUSION

The resulting MARDs suggest that further investigation into the use of dielectric spectroscopy for glucose monitoring should be explored.

Roles of Lipids in the Permeability Barriers of Skin and Oral Mucosa

PubMed searches reveal much literature regarding lipids in barrier function of skin and less literature on lipids in barrier function of the oral mucosa. In terrestrial mammals, birds, and reptiles, the skin’s permeability barrier is provided by ceramides, fatty acids, and cholesterol in the outermost layers of the epidermis, the stratum corneum. This layer consists of about 10–20 layers of cornified cells embedded in a lipid matrix. It effectively prevents loss of water and electrolytes from the underlying tissue, and it limits the penetration of potentially harmful substances from the environment. In the oral cavity, the regions of the gingiva and hard palate are covered by keratinized epithelia that much resemble the epidermis. The oral stratum corneum contains a lipid mixture similar to that in the epidermal stratum corneum but in lower amounts and is accordingly more permeable. The superficial regions of the nonkeratinized oral epithelia also provide a permeability barrier. These epithelial regions do contain ceramides, cholesterol, and free fatty acids, which may underlie barrier function. The oral epithelial permeability barriers primarily protect the underlying tissue by preventing the penetration of potentially toxic substances, including microbial products. Transdermal drug delivery, buccal absorption, and lipid-related disease are discussed.

Smartphone-based colorimetric detection systems for glucose monitoring in the diagnosis and management of diabetes

Diabetes is a group of metabolic conditions resulting in high blood sugar levels over prolonged periods that affects hundreds of millions of patients worldwide. Measuring glucose concentration enables patient-specific insulin therapy, and is essential to reduce the severity of the disease, potential complications, and related mortalities. Recent advances and developments in smartphone-based colorimetric glucose detection systems are discussed in this review. The importance of glucose monitoring, data collection, transfer, and analysis, via non-invasive/invasive methods is highlighted. The review also presents various approaches using 3D-printed materials, screen-printed electrodes, polymer templates, designs allowing multiple glucose analysis, bioanalytes and/or nanostructures for glucose detection. The positive effects of advances in improving the performance of smartphone-based platforms are introduced along with future directions and trends in the application of emerging technologies in smartphone-based diagnostics.

Is it possible to constantly and accurately monitor blood sugar levels, in people with Type 1 diabetes, with a discrete device (non-invasive or invasive)?

Real-time continuous glucose monitors using subcutaneous needle-type sensors continue to develop. The limitations of currently available systems, however, include time lag behind changes in blood glucose, the invasive nature of such systems, and in some cases, their accuracy. Non-invasive techniques have been developed, but, to date, no commercial device has been successful. A key research priority for people with Type 1 diabetes identified by the James Lind Alliance was to identify ways of monitoring blood glucose constantly and accurately using a discrete device, invasive or non-invasive. Integration of such a sensor is important in the development of a closed-loop system and the technology must be rapid, selective and acceptable for continuous use by individuals. The present review provides an update on existing continuous glucose-sensing technologies, and an overview of emergent techniques, including their accuracy and limitations.

Blood Glucose Level Monitoring Using an FMCW Millimeter-Wave Radar Sensor

In this article, a novel sensing approach is presented for glucose level monitoring where a robust low-power millimeter(mm)-wave radar system is used to differentiate between blood samples of disparate glucose concentrations in the range 0.5 to 3.5 mg/mL. The proposed radar sensing mechanism shows greater capabilities for remote detection of blood glucose inside test tubes through detecting minute changes in their dielectric properties. In particular, the reflected mm-waves that represent unique signatures for the internal synthesis and composition of the tested blood samples, are collected from the multi-channels of the radar and analyzed using signal processing techniques to identify different glucose concentrations and correlate them to the reflected mm-wave readings. The mm-wave spectrum is chosen for glucose sensing in this study after a set of preliminary experiments that investigated the dielectric permittivity behavior of glucose-loaded solutions across different frequency bands. In this regard, a newly-developed commercial coaxial probe kit (DAK-TL) is used to characterize the electromagnetic properties of glucose-loaded samples in a broad range of frequencies from 300 MHz to 67 GHz using two different 50 Ω open-coaxial probes. This would help to determine the portion of the frequency spectrum that is more sensitive to slight variations in glucose concentrations as indicated by the amount of change in the dielectric constant and loss tangent parameters due to the different concentrations under test. The mm-wave frequency range 50 to 67 GHz has shown to be promising for acquiring both high sensitivity and sufficient penetration depth for the most interaction between the glucose molecules and electromagnetic waves. The processed results have indicated the reliability of using mm-wave radars in identifying changes in blood glucose levels while monitoring trends among those variations. Particularly, blood samples of higher glucose concentrations are correlated with reflected mm-wave signals of greater energy. The proposed system could likely be adapted in the future as a portable non-invasive continuous blood glucose level monitoring for daily use by diabetics.

Effect of electrode geometry on the electrolyte resistance measurement over the surface of a skin phantom in a noninvasive manner

We analyzed the electrode geometry to obtain the potential (E) and current density (J) distributions at the surface of a skin phantom (SP), in this case a planar surface. Two electrode geometries were tested: a circular electrode (CiE) and a rectangular electrode (ReE). First, by a finite element simulation, we calculated the E and J distributions at the surface of the SP. Second, we determined the resistivity properties as a function of the electrochemical impedance. Three- and four-electrode configurations were used to measure the E versus distance between the reference electrodes (d). For the ReE, the electrolyte resistance (R_e) measurements show a linear behavior with respect to "d" if the zone of the linear distribution of E and the homogeneous current density (J_H) is considered. In contrast, the CiE shows nonlinear behavior due to the absence of that zone of the linear distribution of E and J_H in the entire range. For ReE, we deduced that the behavior of R_e versus "d" is related to the material resistivity. Consequently, the ReE geometry improves the R_e measurements on the surface and shows us a way to control the behavior of this element in planar samples such as skin.

Biosensors to Monitor Water Quality Utilizing Insect Odorant-Binding Proteins as Detector Elements

In the developing world, the identification of clean, potable water continues to pose a pervasive challenge, and waterborne diseases due to fecal contamination of water supplies significantly threaten public health. The ability to efficiently monitor local water supplies is key to water safety, yet no low-cost, reliable method exists to detect contamination quickly. We developed an in vitro assay utilizing an odorant-binding protein (OBP), AgamOBP1, from the mosquito, Anopheles gambiae, to test for the presence of a characteristic metabolite, indole, from harmful coliform bacteria. We demonstrated that recombinantly expressed AgamOBP1 binds indole with high sensitivity. Our proof-of-concept assay is fluorescence-based and demonstrates the usefulness of insect OBPs as detector elements in novel biosensors that rapidly detect the presence of bacterial metabolic markers, and thus of coliform bacteria. We further demonstrated that rAgamOBP1 is suitable for use in portable, inexpensive "dipstick" biosensors that improve upon lateral flow technology since insect OBPs are robust, easily obtainable via recombinant expression, and resist detector "fouling." Moreover, due to their wide diversity and ligand selectivity, insect chemosensory proteins have other biosensor applications for various analytes. The techniques presented here therefore represent platform technologies applicable to various future devices.

Effect of Chemical Permeation Enhancers on Skin Permeability: In silico screening using Molecular Dynamics simulations

Breaching of the skin barrier is essential for delivering active pharmaceutical ingredients (APIs) for pharmaceutical, dermatological and aesthetic applications. Chemical permeation enhancers (CPEs) are molecules that interact with the constituents of skin’s outermost and rate limiting layer stratum corneum (SC), and increase its permeability. Designing and testing of new CPEs is a resource intensive task, thus limiting the rate of discovery of new CPEs. In-silico screening of CPEs in a rigorous skin model could speed up the design of CPEs. In this study, we performed coarse grained (CG) molecule dynamics (MD) simulations of a multilayer skin lipid matrix in the presence of CPEs. The CPEs are chosen from different chemical functionalities including fatty acids, esters, and alcohols. A multi-layer in-silico skin model was developed. The CG parameters of permeation enhancers were also developed. Interactions of CPEs with SC lipids was studied in silico at three different CPE concentrations namely, 1% w/v, 3% w/v and 5% w/v. The partitioning and diffusion coefficients of CPEs in the SC lipids were found to be highly size- and structure-dependent and these dependencies are explained in terms of structural properties such as radial distribution function, area per lipid and order parameter. Finally, experimentally reported effects of CPEs on skin from the literature are compared with the simulation results. The trends obtained using simulations are in good agreement with the experimental measurements. The studies presented here validate the utility of in-silico models for designing, screening and testing of novel and effective CPEs.

Transdermal sampling of vitamin D3 and 25-hydroxyvitamin D3

AIM

Transdermal analysis is proposed for vitamin D₃ and its hydroxylated metabolite to overcome problems associated with blood analysis.

METHODS

Vitamin D₃ was extracted directly from skin with solid patches and liquid phases. Deuterium-labeled vitamin D₃ was added to the extraction solutions to compensate for variability and accurately determine the rate of transdermal transfer. Of the different extraction solvents tested, 50:50 octanol:isopropanol showed the best results, with an accuracy of 115% and reproducibility better than 30%.

CONCLUSION

The research shows that transdermal route can be used for analysis of vitamin D₃ in porcine skin. When microneedles are used, accurate measurements were obtained in 1 h. With intact skin, the highest accuracy was obtained when extraction was done for 2 h.

Ultrasound mediated transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injections. However, the stratum corneum serves as a barrier that limits the penetration of substances to the skin. Application of ultrasound (US) irradiation to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. This review presents the main findings in the field of sonophoresis in transdermal drug delivery as well as transdermal monitoring and the mathematical models associated with this field. Particular attention is paid to the proposed enhancement mechanisms and future trends in the fields of cutaneous vaccination and gene therapy.

Glucose monitoring in neonates: need for accurate and non-invasive methods

Neonatal hypoglycaemia can lead to devastating consequences. Thus, constant, accurate and safe glucose monitoring is imperative in neonatal care. However, point-of-care (POC) devices for glucose testing currently used for neonates were originally designed for adults and do not address issues specific to neonates. This review will address currently available monitoring options and describe new methodologies for non-invasive glucose monitoring in newborns.

Integration of biosensors and drug delivery technologies for early detection and chronic management of illness

Recent advances in biosensor design and sensing efficacy need to be amalgamated with research in responsive drug delivery systems for building superior health or illness regimes and ensuring good patient compliance. A variety of illnesses require continuous monitoring in order to have efficient illness intervention. Physicochemical changes in the body can signify the occurrence of an illness before it manifests. Even with the usage of sensors that allow diagnosis and prognosis of the illness, medical intervention still has its downfalls. Late detection of illness can reduce the efficacy of therapeutics. Furthermore, the conventional modes of treatment can cause side-effects such as tissue damage (chemotherapy and rhabdomyolysis) and induce other forms of illness (hepatotoxicity). The use of drug delivery systems enables the lowering of side-effects with subsequent improvement in patient compliance. Chronic illnesses require continuous monitoring and medical intervention for efficient treatment to be achieved. Therefore, designing a responsive system that will reciprocate to the physicochemical changes may offer superior therapeutic activity. In this respect, integration of biosensors and drug delivery is a proficient approach and requires designing an implantable system that has a closed loop system. This offers regulation of the changes by means of releasing a therapeutic agent whenever illness biomarkers prevail. Proper selection of biomarkers is vital as this is key for diagnosis and a stimulation factor for responsive drug delivery. By detecting an illness before it manifests by means of biomarkers levels, therapeutic dosing would relate to the severity of such changes. In this review various biosensors and drug delivery systems are discussed in order to assess the challenges and future perspectives of integrating biosensors and drug delivery systems for detection and management of chronic illness.

A promising solution to enhance the sensocompatibility of biosensors in continuous glucose monitoring systems

BACKGROUND

Continuous glucose monitors (CGMs) measure glucose in real time, making it possible to improve glycemic control. A promising technique involves glucose sensors implanted in subcutaneous tissue measuring glucose concentration in interstitial fluid. A major drawback of this technique is sensor bioinstability, which can lead to unpredictable drift and reproducibility. The bioinstability is partly due to sensor design but is also affected by naturally occurring subcutaneous inflammations. Applying a nonbiofouling coating to the sensor membrane could be a means to enhancing sensocompatibility.

METHODS

This study evaluates the suitability of a polyethylene-glycol-based coating on sensors in CGMs. Methods used include cross hatch, wet paper rub, paper double rub, bending, hydrophilicity, protein adsorption, bio-compatibility, hemocompatibility, and glucose/oxygen permeability testing.

RESULTS

Results demonstrate that coating homogeneity, adhesion, integrity, and scratch resistance are good. The coating repels lysozyme and bovine serum albumin, and only a low level of fibrin and blood platelet adsorption to the coating was recorded when testing in whole human blood. Cytotoxicity, irritation, sensitization, and hemolysis were assessed, and levels suggested good biocompatibility of the coating in subcutaneous tissue. Finally, it was shown that the coating can be applied to cellulose acetate membranes of different porosity without changing their permeability for glucose and oxygen.

CONCLUSIONS

These results suggest that the mechanical properties of the coating are sufficient for the given application, that the coating is effective in preventing protein adsorption and blood clot formation on the sensor surface, and that the coating can be applied to membranes without hindering their glucose and oxygen transport.

Detection of trace glucose on the surface of a semipermeable membrane using a fluorescently labeled glucose-binding protein: a promising approach to noninvasive glucose monitoring

BACKGROUND

Our motivation for this study was to develop a noninvasive glucose sensor for low birth weight neonates. We hypothesized that the underdeveloped skin of neonates will allow for the diffusion of glucose to the surface where it can be sampled noninvasively. On further study, we found that measurable amounts of glucose can also be collected on the skin of adults.

METHOD

Cellulose acetate dialysis membrane was used as surrogate for preterm neonatal skin. Glucose on the surface was collected by saline-moistened swabs and analyzed with glucose-binding protein (GBP). The saline-moistened swab was also tested in the neonatal intensive care unit. Saline was directly applied on adult skin and collected for analysis with two methods: GBP and high-performance anion-exchange chromatography (HPAEC).

RESULTS

The amount of glucose on the membrane surface was found (1) to accumulate with time but gradually level off, (2) to be proportional to the swab dwell time, and (3) the concentration of the glucose solution on the opposite side of the membrane. The swab, however, failed to absorb glucose on neonatal skin. On direct application of saline onto adult skin, we were able to measure by HPAEC and GBP the amount of glucose collected on the surface. Blood glucose appears to track transdermal glucose levels.

CONCLUSIONS

We were able to measure trace amounts of glucose on the skin surface that appear to follow blood glucose levels. The present results show modest correlation with blood glucose. Nonetheless, this method may present a noninvasive alternative to tracking glucose trends.

Measurement of glucose area under the curve using minimally invasive interstitial fluid extraction technology: evaluation of glucose monitoring concepts without blood sampling

BACKGROUND

Monitoring postprandial hyperglycemia is crucial in treating diabetes, although its dynamics make accurate monitoring difficult. We developed a new technology for monitoring postprandial hyperglycemia using interstitial fluid (ISF) extraction technology without blood sampling. The glucose area under the curve (AUC) using this system was measured as accumulated ISF glucose (IG) with simultaneous calibration with sodium ions. The objective of this study was to evaluate this technological concept in healthy individuals.

METHODS

Minimally invasive ISF extraction technology (MIET) comprises two steps: pretreatment with microneedles and ISF accumulation over a specific time by contact with a solvent. The correlation between glucose and sodium ion levels using MIET was evaluated in 12 subjects with stable blood glucose (BG) levels during fasting. BG and IG time courses were evaluated in three subjects to confirm their relationship while BG was fluctuating. Furthermore, the accuracy of glucose AUC measurements by MIET was evaluated several hours after a meal in 30 subjects.

RESULTS

A high correlation was observed between glucose and sodium ion levels when BG levels were stable (R=0.87), indicating that sodium ion is a good internal standard for calibration. The variation in IG and BG with MIET was similar, indicating that IG is an adequate substitute for BG. Finally, we showed a strong correlation (R=0.92) between IG-AUC and BG-AUC after a meal.

CONCLUSIONS

These findings validate the adequacy of glucose AUC measurements using MIET. Monitoring glucose using MIET without blood sampling may be beneficial to patients with diabetes.

Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus

BACKGROUND

Multiple laboratory tests are used to diagnose and manage patients with diabetes mellitus. The quality of the scientific evidence supporting the use of these tests varies substantially.

APPROACH

An expert committee compiled evidence-based recommendations for the use of laboratory testing for patients with diabetes. A new system was developed to grade the overall quality of the evidence and the strength of the recommendations. Draft guidelines were posted on the Internet and presented at the 2007 Arnold O. Beckman Conference. The document was modified in response to oral and written comments, and a revised draft was posted in 2010 and again modified in response to written comments. The National Academy of Clinical Biochemistry and the Evidence-Based Laboratory Medicine Committee of the American Association for Clinical Chemistry jointly reviewed the guidelines, which were accepted after revisions by the Professional Practice Committee and subsequently approved by the Executive Committee of the American Diabetes Association.

CONTENT

In addition to long-standing criteria based on measurement of plasma glucose, diabetes can be diagnosed by demonstrating increased blood hemoglobin A(1c) (HbA(1c)) concentrations. Monitoring of glycemic control is performed by self-monitoring of plasma or blood glucose with meters and by laboratory analysis of HbA(1c). The potential roles of noninvasive glucose monitoring, genetic testing, and measurement of autoantibodies, urine albumin, insulin, proinsulin, C-peptide, and other analytes are addressed.

SUMMARY

The guidelines provide specific recommendations that are based on published data or derived from expert consensus. Several analytes have minimal clinical value at present, and their measurement is not recommended.

Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus

BACKGROUND

Multiple laboratory tests are used to diagnose and manage patients with diabetes mellitus. The quality of the scientific evidence supporting the use of these tests varies substantially.

APPROACH

An expert committee compiled evidence-based recommendations for the use of laboratory testing for patients with diabetes. A new system was developed to grade the overall quality of the evidence and the strength of the recommendations. Draft guidelines were posted on the Internet and presented at the 2007 Arnold O. Beckman Conference. The document was modified in response to oral and written comments, and a revised draft was posted in 2010 and again modified in response to written comments. The National Academy of Clinical Biochemistry and the Evidence Based Laboratory Medicine Committee of the AACC jointly reviewed the guidelines, which were accepted after revisions by the Professional Practice Committee and subsequently approved by the Executive Committee of the American Diabetes Association.

CONTENT

In addition to long-standing criteria based on measurement of plasma glucose, diabetes can be diagnosed by demonstrating increased blood hemoglobin A(1c) (Hb A(1c)) concentrations. Monitoring of glycemic control is performed by self-monitoring of plasma or blood glucose with meters and by laboratory analysis of Hb A(1c). The potential roles of noninvasive glucose monitoring, genetic testing, and measurement of autoantibodies, urine albumin, insulin, proinsulin, C-peptide, and other analytes are addressed.

SUMMARY

The guidelines provide specific recommendations that are based on published data or derived from expert consensus. Several analytes have minimal clinical value at present, and their measurement is not recommended.

Detecting vital signs with wearable wireless sensors

The emergence of wireless technologies and advancements in on-body sensor design can enable change in the conventional health-care system, replacing it with wearable health-care systems, centred on the individual. Wearable monitoring systems can provide continuous physiological data, as well as better information regarding the general health of individuals. Thus, such vital-sign monitoring systems will reduce health-care costs by disease prevention and enhance the quality of life with disease management. In this paper, recent progress in non-invasive monitoring technologies for chronic disease management is reviewed. In particular, devices and techniques for monitoring blood pressure, blood glucose levels, cardiac activity and respiratory activity are discussed; in addition, on-body propagation issues for multiple sensors are presented.

A low frequency electromagnetic sensor for indirect measurement of glucose concentration: in vitro experiments in different conductive solutions

In recent years there has been considerable interest in the study of glucose-induced dielectric property variations of human tissues as a possible approach for non-invasive glycaemia monitoring. We have developed an electromagnetic sensor, and we tested in vitro its ability to estimate variations in glucose concentration of different solutions with similarities to blood (sodium chloride and Ringer-lactate solutions), differing though in the lack of any cellular components. The sensor was able to detect the effect of glucose variations over a wide range of concentrations (∼78–5,000 mg/dL), with a sensitivity of ∼0.22 mV/(mg/dL). Our proposed system may thus be useful in a new approach for non-invasive and non-contact glucose monitoring.

On correlating otoacoustic emissions with blood glucose levels

The long term objective of this research is to develop a new means for diabetic patients to painlessly and non-invasively monitor their blood glucose levels. We propose a novel method for noninvasive glucose monitoring based on measurement and analysis of otoacoustic emissions (OAEs). OAEs are low-intensity sounds generated by the cochlea in response to acoustic stimuli. Evoking and measuring OAEs is done using a tiny speaker and microphone that fit snugly inside the ear canal. The OAE response can be partially masked or reduced in amplitude by presenting competing acoustic stimuli contralaterally (opposite ear), ipsilaterally (same ear), or both. This masking effect is caused by activation of neural efferent pathways from the brain. Neural effects, including evoked responses such as auditory brainstem responses and axonal transmission latencies, are known to correlate with glucose. This suggests that masked OAEs may correlate with glucose since masking is a result of neural activity, and neural activity is affected by glucose levels. Prior to our research, no studies have investigated the correlation of masked OAEs with blood glucose. In this paper we present our preliminary findings, including experimental results that suggest a correlation with blood glucose levels.

Transdermal drug delivery

Transdermal drug delivery has made an important contribution to medical practice, but has yet to fully achieve its potential as an alternative to oral delivery and hypodermic injections. First-generation transdermal delivery systems have continued their steady increase in clinical use for delivery of small, lipophilic, low-dose drugs. Second-generation delivery systems using chemical enhancers, noncavitational ultrasound and iontophoresis have also resulted in clinical products; the ability of iontophoresis to control delivery rates in real time provides added functionality. Third-generation delivery systems target their effects to skin's barrier layer of stratum corneum using microneedles, thermal ablation, microdermabrasion, electroporation and cavitational ultrasound. Microneedles and thermal ablation are currently progressing through clinical trials for delivery of macromolecules and vaccines, such as insulin, parathyroid hormone and influenza vaccine. Using these novel second- and third-generation enhancement strategies, transdermal delivery is poised to significantly increase its impact on medicine.

Transdermal drug delivery

Transdermal drug delivery has made an important contribution to medical practice, but has yet to fully achieve its potential as an alternative to oral delivery and hypodermic injections. First-generation transdermal delivery systems have continued their steady increase in clinical use for delivery of small, lipophilic, low-dose drugs. Second-generation delivery systems using chemical enhancers, noncavitational ultrasound and iontophoresis have also resulted in clinical products; the ability of iontophoresis to control delivery rates in real time provides added functionality. Third-generation delivery systems target their effects to skin's barrier layer of stratum corneum using microneedles, thermal ablation, microdermabrasion, electroporation and cavitational ultrasound. Microneedles and thermal ablation are currently progressing through clinical trials for delivery of macromolecules and vaccines, such as insulin, parathyroid hormone and influenza vaccine. Using these novel second- and third-generation enhancement strategies, transdermal delivery is poised to significantly increase its impact on medicine.

Noninvasive glycaemia monitoring: background, traditional findings, and novelties in the recent clinical trials

PURPOSE OF REVIEW

The noninvasive measurement of glycaemia is a topic of great interest and is rapidly changing; novelties have emerged frequently in the last years. The aim of the present review is to briefly present the main technologies in the field and the devices recently developed based on them.

RECENT FINDINGS

The main technologies for noninvasive glycaemia monitoring can be usually classified into two main categories, 'optical' and 'electric'. Each technology will be briefly presented. Some noninvasive glucose meters, Diasensor (based on near infrared spectroscopy), Pendra (impedance spectroscopy), and GlucoWatch (reverse iontophoresis), which entered either the European or the US market or both in the last years will be described. Unfortunately, none of them is currently in the market. Some new device prototypes, TANGTEST blood glucose meter (optical method), reverse iontophoresis-based glucose monitoring device, Aprise sensor (ultrasounds), have emerged in the last months. In clinical studies on humans, they generally showed a good accuracy in glycaemia measurement.

SUMMARY

The development of reliable methods for noninvasive glycaemia monitoring would have a tremendous impact on diabetes management. The most recent device prototypes appear promising, but because of the failure of previous projects, the assessment of such new devices must be done with extreme caution.

Noninvasive transcutaneous sampling of glucose by electroporation

BACKGROUND

In people with diabetes, blood glucose levels should be monitored regularly to prevent serious complications associated with diabetes. This involves the invasive method of withdrawing blood, which causes inconvenience to patients. The objective of this study was to investigate the efficiency of the noninvasive electroporation and transcutaneous sampling (ETS) technique for predicting blood glucose levels.

METHODS

In vitro studies were carried out in Franz diffusion cells using porcine epidermis to assess the feasibility of transcutaneous sampling of glucose. In vivo, the ETS technique was assessed in the diabetes-induced Sprague-Dawley rat model. Glucose was sampled following the application of 30 electrical pulses of 1 ms duration at 120 V/cm(2), 1 Hz. Clarke error grid analysis was carried out for the venous blood glucose levels that were determined by the ETS with reference to those measured by a glucose meter.

RESULTS

The amount of glucose sampled by the ETS method both in vitro and in vivo was proportional to the dermal glucose concentration. All data points from in vivo studies were in A and B zones of Clarke error grid analysis, and the mean absolute relative error was 12.8%.

CONCLUSION

Results of the present study demonstrate that ETS technique could be developed as a noninvasive method of predicting venous blood glucose levels in people with diabetes.

Current development in non-invasive glucose monitoring

Painless control of blood glycemic levels could improve life quality of diabetes patients, enabling a better regulation of hyper- and hypoglycaemia episodes and thereby avoiding physiological complications. Although research groups have been trying for decades to separate non-invasive glucose information from interference compounds, none of the available commercial devices offers enough precision to replace lancet approaches. Many reviews have already been published on this topic, but the great amount of information available and the fast development of technologies require a continuous update in the research status. Besides the description of current in-vivo methods and the analysis of devices available commercially, one also explains treatment algorithms useful for multivariate analysis.

Recent advances in continuous glucose monitoring: biocompatibility of glucose sensors for implantation in subcutis

Tight glycemic control slows or prevents the development of short- and long-term complications of diabetes mellitus. Continuous glucose measurements provide improved glycemic control and potentially prevent these diabetic complications. Glucose sensors, especially implantable devices, offer an alternative to classical self-monitored blood glucose levels and have shown promising glucose-sensing properties. However, the ultimate goal of implementing the glucose sensor as the glucose-sensing part of a closed loop system (artificial pancreas) is still years ahead because of malfunctions of the implanted sensor. The malfunction is partly a consequence of the subcutaneous inflammatory reaction caused by the implanted sensor. In order to improve sensor measurements and thereby close the loop, it is crucial to understand what happens at the tissue-sensor interface.

Non-invasive glucose monitoring: assessment of technologies and devices according to quantitative criteria

Aim of this review was to describe the main technologies for non-invasive glucose monitoring and the corresponding most relevant devices. The review tries to overcome the limitations of previous reviews on this topic, such as the lack of objective criteria for inclusion or exclusion of technologies or devices, and the poor organization of the information, which often does not allow easy comparison between technologies and devices. In this review, the information is concise and organized into specific categories, and hence it becomes easy to compare advantages and disadvantages of the different technologies and devices. For technologies, the categories of information considered are the technology name, the underlying physical principle, the technology limitations and the measurement sites on the human body. For devices, the categories of information are the device name, its approval condition (FDA Approval and/or CE Mark), the technology on which it is based, a device general description, the tests performed on the device, the corresponding results, safety information, aspects affecting usability, current status of the device and the manufacturer, an Internet reference for the device. A total of 14 technologies and 16 devices are included. Conclusions of the review were that, despite some interesting and promising technologies and devices, a satisfactory solution to the non-invasive glucose monitoring problem still requires further efforts.

Continuous noninvasive glucose monitoring technology based on "occlusion spectroscopy"

BACKGROUND

A truly noninvasive glucose-sensing device could revolutionalize diabetes treatment by leading to improved compliance with recommended glucose levels, thus reducing the long-term complications and cost of diabetes. Herein, we present the technology and evaluate the efficacy of a truly noninvasive device for continuous blood glucose monitoring, the NBM (OrSense Ltd.).

METHODS

In vitro analysis was used to validate the technology and algorithms. A clinical study was performed to quantify the in vivo performance of the NBM device. A total of 23 patients with type 1 (n = 12) and type 2 (n = 11) diabetes were enrolled in the clinical study and participated in 111 sessions. Accuracy was assessed by comparing NBM data with paired self-monitoring of blood glucose meter readings.

RESULTS

In vitro experiments showed a strong correlation between calculated and actual glucose concentrations. The clinical trial produced a total of 1690 paired glucose values (NBM vs reference). In the paired data set, the reference glucose range was 40-496 mg/dl. No systematic bias was found at any of the glucose levels examined (70, 100, 150, and 200 mg/dl). The mean relative absolute difference was 17.2%, and a Clarke error grid analysis showed that 95.5% of the measurements fall within the clinically acceptable A&B regions (zone A, 69.7%; and zone B, 25.7%).

CONCLUSIONS

This study indicates the potential use of OrSense's NBM device as a noninvasive sensor for continuous blood glucose evaluation. The device was safe and well tolerated.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.

Biocompatibility of an enzyme-based, electrochemical glucose sensor for short-term implantation in the subcutis

BACKGROUND

Continuous glucose measurements provide improved glycemic control and may prevent hypoglycemia and long-term complications of diabetes. One of the most promising techniques is the short-term implantation of electrochemical glucose sensors in subcutis. However, the inflammatory reaction to these sensors may lead to bioinstability of sensor measurements. The purpose of the present investigation was to examine factors contributing to the observed subcutaneous inflammatory reaction to an enzyme-based electrochemical glucose sensor for continuous glucose measurements. The sensor biocompatibility was assessed in vitro and in vivo.

METHODS

A toxicological assessment was performed on sensor materials and leachables, and the endotoxin content of sensors was determined by a Limulus amoebocyte lysate (LAL) test. Moreover, as a consequence of permanent penetration of the skin by the sensor the role of bacterial migration to the tissue was investigated. In vivo biocompatibility was investigated through histological examination of implanted sensor membranes for 3 days in pigs. Additionally, the effect of needle size and type (normal vs. inserter needle) on tissue trauma at sensor insertion was evaluated, and the healing of subcutis was assessed histologically from 3 to 14 days after removal of sensors.

RESULTS

The toxicological assessment and the LAL test showed no concerns in a 3-day implantation scenario, and bacterial migration to the subcutis could not be detected. The histological examination showed that a reduction in needle size reduced the extent of inflammation to very low levels, and that the different sensor membranes showed similar extent and type of inflammation. Additionally, the extent of subcutaneous tissue reaction after removal of sensors declined gradually over time and returned to near-normal levels after 2 weeks.

CONCLUSION

The electrochemical enzyme-based glucose sensor for continuous glucose measurements in subcutis is acceptable from a biocompatibility point of view. Reducing the inserter needle in size reduces the trauma induced at sensor implantation to neglible levels. Furthermore, the tissue reaction to the sensor returns to near-normal 2 weeks after the sensor has been removed following a 3-day implantation period.

Prediction of glucose in whole blood by near-infrared spectroscopy: influence of wavelength region, preprocessing, and hemoglobin concentration

Measurement accuracy for predicting glucose in whole blood was studied based on near-infrared spectroscopy. Optimal wavelength regions, preprocessing, and the influence of hemoglobin were examined using partial least-squares regression. Spectra between 1100 and 2400 nm were measured from 98 whole blood samples. In order to study the influence of hemoglobin, which is the most dominant component in blood, 98 samples were arranged such that glucose and hemoglobin concentrations were distributed in their physiological ranges. Samples were grouped into three depending on hemoglobin level. The results showed that glucose prediction was influenced by hemoglobin concentrations in the calibration model. It was necessary for samples used in the calibration model to represent the entire range of hemoglobin level. The cross-validation errors were the smallest when the wavelength regions of 1390 to 1888 nm and 2044 to 2393 nm were used. However, prediction accuracy was not very dependent on preprocessing methods in this optimal region. The standard error of glucose prediction was 25.5 mgdL and the coefficient of variation in prediction was 11.2%.

A long-wavelength fluorescent glucose biosensor based on bioconjugates of galactose/glucose binding protein and Nile Red derivatives

BACKGROUND

Fluorescent biosensors based on galactose/glucose binding protein (GGBP) and environmentally sensitive derivatives of the phenoxazine dye Nile Red are described. These biosensors are proposed as the sensing platform for a minimally invasive, continuous glucose monitoring system that can be implanted under the skin and read transdermally using an external fluorometer.

METHODS

To construct the biosensors, the thiol-reactive Nile Red derivatives INR and IANR were prepared and conjugated to GGBP proteins possessing cysteine mutations that were designed for optimal site-specific fluorophore attachment. The attachment sites were selected to maximize the local environment change for attached dyes between the bound and unbound conformations of GGBP.

RESULTS

Fluorescence responses at the selected cysteine sites of GGBP upon binding to glucose showed that the conjugates typically yielded fluorescence emission around 640-650 nm with up to 50% changes in fluorescence intensity. Conjugate E149C/A213C/L238S INR GGBP also displayed glucose binding in the human physiological range (K (D) = 7.4 mM).

CONCLUSIONS

The phenoxazine derivatives fluoresced at longer wavelengths (>600 nm) approaching the near-infrared spectral window, where interference from scattering and tissue absorbance are minimal. Ultimately, we expect that monitoring systems based on GGBP and longwavelength dyes will be implanted for up to 6 months and can be used to transmit information through the skin to an external monitor.