Dermal interstitial glucose as an indicator of ambient glycemia

Microneedle-based transdermal detection and sensing devices

Microneedles have been expected for the construction of next-generation biosensors towards personalization, digitization, and intellectualization due to their metrics of minimal invasiveness, high integration, and favorable biocompatibility. Herein, an overview of state-of-the-art microneedle-based detection and sensing systems is presented. First, the designs of microneedle devices based on extraction mechanisms are concluded, corresponding to different geometries and materials of microneedles. Second, the targets of equipment-assisted microneedle detections are summarized, as well as the objective significance, revealing the current performance and potential scenarios of these microneedles. Third, the trend towards highly integrated sensors is elaborated by emphasizing the sensing principles (colorimetric, fluorometric and electronic manner). Finally, the key challenges to be tackled and the perspectives on future development are discussed.

Noninvasive Electromagnetic Wave Sensing of Glucose

Diabetic patients need long-term and frequent glucose monitoring to assist in insulin intake. The current finger-prick devices are painful and costly, which places noninvasive glucose sensors in high demand. In this review paper, we list several advanced electromagnetic (EM)-wave-based technologies for noninvasive glucose measurement, including infrared (IR) spectroscopy, photoacoustic (PA) spectroscopy, Raman spectroscopy, fluorescence, optical coherence tomography (OCT), Terahertz (THz) spectroscopy, and microwave sensing. The development of each method is discussed regarding the fundamental principle, system setup, and experimental results. Despite the promising achievements that have been previously reported, no established product has obtained FDA approval or survived a marketing test. The limitations of, and prospects for, these techniques are presented at the end of this review.

Advances in the Design of Transdermal Microneedles for Diagnostic and Monitoring Applications

Due to their intrinsic advantages over classical hypodermic needles, microneedles have received much attention over the last two decades and will likely soon appear in clinics. Although the vast majority of research is focused on designing microneedles for the painless delivery of drugs, their applications for diagnostic purposes have also provided promising results. In this paper, the main advances in the field of microneedles for diagnostic and patient monitoring purposes are introduced and critically discussed.

A new approach for noninvasive transdermal determination of blood uric acid levels

The aims of this study were to investigate the most effective combination of physical forces from laser, electroporation, and reverse iontophoresis for noninvasive transdermal extraction of uric acid, and to develop a highly sensitive uric acid biosensor (UAB) for quantifying the uric acid extracted. It is believed that the combination of these physical forces has additional benefits for extraction of molecules other than uric acid from human skin. A diffusion cell with porcine skin was used to investigate the most effective combination of these physical forces. UABs coated with ZnO₂ nanoparticles and constructed in an array configuration were developed in this study. The results showed that a combination of laser (0.7 W), electroporation (100 V/cm²), and reverse iontophoresis (0.5 mA/cm²) was the most effective and significantly enhanced transdermal extraction of uric acid. A custom-designed UAB coated with ZnO₂ nanoparticles and constructed in a 1×3 array configuration (UAB-1×3-ZnO₂) demonstrated enough sensitivity (9.4 μA/mM) for quantifying uric acid extracted by the combined physical forces of laser, electroporation, and RI. A good linear relationship (R²=0.894) was demonstrated to exist between the concentration of uric acid (0.2–0.8 mM) inside the diffusion cell and the current response of the UAB-1×3-ZnO₂. In conclusion, a new approach to noninvasive transdermal extraction and quantification of uric acid has been established.

Interstitium versus Blood Equilibrium in Glucose Concentration and its Impact on Subcutaneous Continuous Glucose Monitoring Systems

The relationship between both interstitial and blood glucose remains a debated topic, on which there is still no consensus. The experimental evidence suggests that blood and interstitial fluid glucose levels are correlated by a kinetic equilibrium, which as a consequence has a time and magnitude gradient in glucose concentration between blood and interstitium. Furthermore, this equilibrium can be perturbed by several physiological effects (such as foreign body response, wound-healing effect, etc.), with a consequent reduction of interstitial fluid glucose versus blood glucose correlation. In the present study, the impact of operating in the interstitium on continuous glucose monitoring systems (CGMs) will be discussed in depth, both for the application of CGMs in the management of diabetes and in other critical areas, such as tight glycaemic control in critically ill patients.

Overview on self-monitoring of blood glucose

The self-monitoring of blood glucose (SMBG), traditionally performed by "point-of-care" (POC) devices called portable glucose monitors (PGM) is now considered an integral part of managed care of diabetic patients, especially type 1 diabetics and those on insulin therapy. In patients with type 2 diabetes, SMBG can help to achieve a better glycaemic control, although there is not sufficient evidence to attest that strict monitoring in these patients is associated with an improved outcome. The outcome of several clinical studies, especially in diabetics on insulin therapy, has shown that SMBG plays a key role in preventing complications in the short, medium and long term. According to the current recommendations, SMBG is aimed to achieve and maintain glycaemic control, prevent and identify hypoglycaemia, prevent severe hyperglycaemia, adjust lifestyle changes and establish the need to begin treatment with insulin in gestational diabetes mellitus. However, as clearly highlighted by the American Diabetes Association (ADA) and the National Academy of Clinical Biochemistry (NACB), patients and healthcare personnel should be trained on the appropriate use of the device, as well as on the correct interpretation of data. Moreover, definite analytical targets and appropriate acceptance criteria for performance should be fulfilled before a new device is introduced in the hospital environment, or recommended to the patients. Performance limitations such as hematocrit extremes and analytical interferences should be clearly acknowledged by the operators, before taking test results for granted. The current article aims to review the current indications for SMGB and highlight the most important criteria for the appropriate use of PGMs.

Accuracy of the 5-day FreeStyle Navigator Continuous Glucose Monitoring System: comparison with frequent laboratory reference measurements

OBJECTIVE

The purpose of this study was to compare the accuracy of measurements of glucose in interstitial fluid made with the FreeStyle Navigator Continuous Glucose Monitoring System with Yellow Springs Instrument laboratory reference measurements of venous blood glucose.

RESEARCH DESIGN AND METHODS

Fifty-eight subjects with type 1 diabetes, aged 18-64 years, were enrolled in a multicenter, prospective, single-arm study. Each subject wore two sensors simultaneously, which were calibrated with capillary fingerstick measurements at 10, 12, 24, and 72 h after insertion. Measurements from the FreeStyle Navigator system were collected at 1-min intervals and compared with venous measurements taken once every 15 min for 50 h over the 5-day period of sensor wear in an in-patient clinical research center. Periods of high rates of change of glucose were induced by insulin and glucose challenges.

RESULTS

Comparison of the FreeStyle Navigator measurements with the laboratory reference method (n = 20,362) gave mean and median absolute relative differences (ARDs) of 12.8 and 9.3%, respectively. The percentage in the clinically accurate Clarke error grid A zone was 81.7% and that in the in the benign error B zone was 16.7%. During low rates of change (< +/-1 mg x dl(-1) x min(-1)), the percentage in the A zone was higher (84.9%) and the mean and median ARDs were lower (11.7 and 8.5%, respectively).

CONCLUSIONS

Measurements with the FreeStyle Navigator system were found to be consistent and accurate compared with venous measurements made using a laboratory reference method over 5 days of sensor wear (82.5% in the A zone on day 1 and 80.9% on day 5).

Optical sensor for interstitial pH measurements

An optical fiber sensor for measuring the pH in interstitial fluid is described. Microdialysis is the approach followed for extracting the sample from the subcutaneous adipose tissue. The interstitial fluid drawn flows through a microfluidic circuit formed by a microdialysis catheter in series with a pH glass capillary. The pH indicator (phenol red) is covalently immobilized on the internal wall of the glass capillary. An optoelectronic unit that makes use of LEDs and photodetectors is connected to the sensing capillary by means of optical fibers. Optical fibers are used to connect the interrogating unit to the sensing capillary. A resolution of 0.03 pH units and an accuracy of 0.07 pH units are obtained. Preliminary in vivo tests are carried out in pigs with altered respiratory function.

Development of a highly responsive needle-type glucose sensor using polyimide for a wearable artificial endocrine pancreas

To produce a long-life, stable, miniature glucose sensor for a wearable artificial endocrine pancreas (WAEP), we developed a novel microneedle-type glucose sensor using polyimide, designated the PI sensor (outer diameter, 0.3 mm; length, 16 mm), and investigated its characteristics in vitro and in vivo. In the in vitro study, we tested the sensor in 0.9% NaCl solution with varying glucose concentrations and observed an excellent linear relationship between the sensor output and glucose concentration (range: 0-500 mg/100 ml). In in vivo experiments, the PI sensor was inserted into the abdominal subcutaneous tissue of beagle dogs (n = 5), and interstitial fluid glucose concentrations were monitored after sensor calibration. Simultaneously, blood glucose concentrations were also monitored continuously with another PI sensor placed intravenously. The correlation and time delay between subcutaneous tissue glucose (Y) and blood glucose concentrations (X: 30-350 mg/100 ml) were Y = 1.03X + 7.98 (r = 0.969) and 6.6 +/- 1.2 min, respectively. We applied the new WAEP system/PI sensor and an intravenous insulin infusion algorithm developed previously for glycemic control in diabetic dogs. The use of the WAEP system resulted in excellent glycemic control after an oral glucose challenge of 1.5 g/kg (post-challenge blood glucose levels: 176 +/- 18 mg/100 ml at 65 min and 93 +/- 23 mg/100 ml at 240 min), without any hypoglycemia. Thus, we confirmed that our new PI sensor has excellent sensor characteristics in vitro and in vivo. The new WAEP using this sensor is potentially suitable for clinical application.

A geometrical model of dermal capillary clearance

A new microscopic model is developed to describe the dermal capillary clearance process of skin permeants. The physiological structure is represented in terms of a doubly periodic array of absorbing capillaries. Convection-dominated transport in the blood flow within the capillaries is coupled with interstitial diffusion, the latter process being quantified via a slender-body-theory approach. Convection across the capillary wall and in the interstitial phase is treated as a perturbation which may be added to the diffusive transport. The model accounts for the finite permeability of the capillary wall as well as for the geometry of the capillary array, based on realistic values of physiological parameters. Calculated dermal concentration profiles for permeants having the size and lipophilicity of salicylic acid and glucose illustrate the power and general applicability of the model. Furthermore, validation of the model with published in vivo experimental results pertaining to human skin permeation of hydrocortisone is presented. The model offers the possibility for in-depth theoretical understanding and prediction of subsurface drug distribution in the human skin following topical application, as well as rates of capillary clearance into the systemic circulation. A simpler approach that treats the capillary bed as a homogeneously absorbing zone is also employed. The latter may be used in conjunction with the capillary exchange model to estimate measurable dermal transport and clearance parameters in a straightforward manner.

Real-time glucose sensing using transdermal fluid under continuous vacuum pressure in children with type 1 diabetes

BACKGROUND

This study was designed to evaluate the accuracy and tolerance in children of an experimental device for continuous glucose monitoring. This real-time glucose sensing (RTGS) system measures transdermal fluid glucose through micropores in the stratum corneum that are kept open by continuous vacuum pressure.

DESIGN AND METHODS

A comparison of self-monitored blood glucose values and RTGS values was obtained in 110 children with type 1 diabetes ranging in age from 2 to 18 years. The RTGS system was worn for two periods of 48-56 h each. Children went home and had no restrictions in diet during the data collection period. Physical activity also was not restricted, with the exception of swimming or other water immersion that would interfere with or damage the RTGS.

RESULTS

The procedure for obtaining transdermal fluid was well tolerated, and adequate flow was maintained out to 48 h or more in most study participants. Comparison of RTGS glucose and self-measured glucose paired values (3,064 values) indicated 69% within Clarke Error Grid Zone A and 21% within Grid Zone B when device tracking was maintained. Errors in tracking occurred with displacement of the vacuum device, or damage to the glucose sensor.

CONCLUSIONS

Transdermal fluid glucose measurements using a prototype device system were well tolerated by children with type 1 diabetes and showed good correlation with concomitant capillary glucose blood measurements. Changes in glucose as tracked by the RTGS system appeared accurate. The durability of the prototype system will need improvement.

Minimally invasive extraction of dermal interstitial fluid for glucose monitoring using microneedles

BACKGROUND

Compliance with glucose monitoring by patients with diabetes is poor because of the pain and inconvenience of conventional blood collection using lancets. To improve compliance, and thereby reduce morbidity and mortality associated with poor glucose control, this study sought to develop and test minimally invasive microneedles to extract dermal interstitial fluid (ISF) for glucose monitoring.

METHODS

We used a thermal puller to fabricate individual or multi-needle arrays of glass microneedles with tip radii of 15-40 microm to penetrate 700-1,500 microm deep into the skin of anesthetized hairless rats or conscious, normal, adult, human subjects. After applying a vacuum of 200-500 mm Hg for 2-10 min, we extracted ISF and measured glucose concentration. These measurements were compared with glucose levels in blood collected from the tail vein of rats or finger stick on humans.

RESULTS

Using this procedure, 1-10 microL of ISF was extracted out of holes punctured in the skin using microneedles. Human subjects generally reported the procedure as painless. ISF glucose concentration correlated well with blood levels based on 140 measurements on 15 rats and six measurements on six human subjects, where 95% of rat data and 100% of human data fell within the clinically acceptable A + B region in Clarke Error Grid analysis. A linear calibration factor was needed to correlate ISF and blood glucose concentrations using our standard procedure. Modifying the procedure to prevent ISF evaporation during extraction provided a one-to-one correlation that eliminated the need for calibration. ISF glucose measurements tracked rapidly changing blood glucose levels following insulin injection with a time lag of less than 20 min.

CONCLUSIONS

These results suggest that microneedle devices can be used to extract ISF for painless glucose monitoring.

A novel approach to mitigating the physiological lag between blood and interstitial fluid glucose measurements

BACKGROUND

Lag between blood and interstitial fluid (ISF) glucose levels can contribute significantly to accuracy error in current and anticipated continuous glucose monitoring systems. Mitigating this physiological lag can be an important and useful means for improving the accuracy, and hence the clinical utility, of continuous glucose monitors.

METHODS

In a test of 22 subjects with diabetes in which a glucose excursion was induced through oral ingestion of a glucose load, glucose levels in finger blood and forearm dermal ISF were monitored over a 5-6-h period. ISF was sampled from two types of sites: sites at which local blood perfusion was elevated through modulated pressure application (test ISF), and control sites at which no perfusion elevation technique was employed (control ISF).

RESULTS

Average lag times (mean +/- SD values) between the two ISF samples and finger capillary blood glucose were determined to be 38.3 +/- 11.5 and 2.5 +/- 6.6 min, respectively, for the control and test ISF samples. Modulated pressure application mitigated the ISF physiological error by an average of 95% in this test.

CONCLUSIONS

The methodology presented here of using a pressure modulation technique to create an elevation in blood flow holds promise for significantly mitigating one of the most significant components of accuracy error for continuous monitoring systems.

Assessment of transcapillary glucose exchange in human skeletal muscle and adipose tissue

We studied the kinetics of glucose exchange between plasma and interstitial fluid (ISF) in human skeletal muscle and adipose tissue under fasting conditions. Five normal human subjects received an intravenous [6,6-2H2]glucose infusion in a prime-continuous fashion. During the tracer infusion, the open-flow microperfusion technique was employed to frequently sample ISF from quadriceps muscle and subcutaneous adipose tissue. The tracer glucose kinetics observed in muscle and adipose tissue ISF were found to be well described by a capillary-tissue exchange model. As a measure of transcapillary glucose exchange efficiency, the 95% equilibrium time was calculated from the identified model parameters. This time constant was similar for skeletal muscle and adipose tissue (28.6 +/- 3.2 vs. 26.8 +/- 3.6 min; P = 0.60). Furthermore, we found that the (total) interstitial glucose concentration was significantly lower (P < 0.01) in muscle (3.32 +/- 0.46 mmol/l) and adipose tissue (3.51 +/- 0.17 mmol/l) compared with arterialized plasma levels (5.56 +/- 0.13 mmol/l). Thus the observed gradients and dynamic relationships between plasma and ISF glucose in muscle and adipose tissue provide evidence that transcapillary exchange of glucose is limited in these two tissues under fasting conditions.

Glucose sensors and the alternate site testing-like phenomenon: relationship between rapid blood glucose changes and glucose sensor signals

In the last few years blood glucose meters have been developed allowing glucose measurements in capillary blood samples collected at sites other than the fingertips. The main reason for establishing this so-called alternate site testing (AST) was to sample blood from locations with reduced pain perception. It is well known that capillary blood glucose is closely correlated to systemic (i.e., arterial) glucose levels and that under steady-state conditions, glucose values measured in blood samples collected from alternate sites are virtually identical to those collected from the fingertip. However, during rapid changes in blood glucose levels, glucose concentrations in capillary blood samples from the fingertips can differ considerably in both domains (time and concentration) from those determined in capillary blood from alternate sites (i.e., the so-called AST phenomenon). Such differences can have serious clinical consequences (e.g., risky delays in hypoglycemia detection). There is evidence that all skin sites exhibiting a reduced blood flow (in comparison with the fingertip) within the superficial skin layers are prone to this AST phenomenon. Nearly all glucose sensors having been developed so far or being currently under development measure glucose levels at alternate sites and also in another compartment [e.g., interstitial fluid (ISF)] than blood. So, in principle they might be prone to an AST-like phenomenon (i.e., rapid changes in systemic blood glucose levels may also result in delayed ISF glucose readings). Our knowledge about the impact of an AST-like phenomenon on the performance of glucose monitoring systems is presently very limited. Glucose kinetics in the different compartments during dynamic systemic blood glucose changes have not been fully elucidated yet. If an AST-like phenomenon plays a role with glucose sensors should therefore be studied. Depending on the measurement technology used for the individual type of glucose monitoring system probably this phenomenon has a variable impact on the results obtained.

Glucose sensing in transdermal body fluid collected under continuous vacuum pressure via micropores in the stratum corneum

Application of continuous vacuum pressure on skin perforated with tiny micropores created by a focused beam from a low-cost laser system can result in access to a clear, transdermal body fluid (TDF) for the continuous measurement of glucose in vivo. Two clinical studies were performed to assess the feasibility of this approach. In the first study, 56 diabetic subjects were porated on either the arm or abdomen, and glucose was measured in their TDF using a custom assay system contained in a patch that was affixed to the skin above the poration site. The continuous readings of glucose in TDF were compared with fingerstick blood measured every half-hour over a 2-day period, resulting in 1,167 paired data points that yielded a correlation of 0.8745 with 97.75% of the readings in the Clarke Error Grid A and B zones. In a second study, 187 diabetic and 65 nondiabetic subjects had glucose measurements from their TDF made using a commercially available glucose strip and meter. A total of 4,059 data pairs (discrete TDF and capillary blood) were collected over a 2-day period, resulting in a correlation of 0.946 with 99% of the readings in the Clarke Error Grid A and B zones. These studies indicate that TDF drawn through micropores in the stratum corneum of the skin potentially can provide a lesser invasive and continuous method of measuring glucose in diabetic individuals.

Blood extraction from lancet wounds using vacuum combined with skin stretching

Key factors and practical limits of blood extraction from lancet wounds on body sites other than the finger were determined by testing a large number of conditions. During these tests, the pain associated with lancing alternate body sites was rated as less painful than a fingerstick 98% of the time. Vacuum combined with skin stretching was effective in extracting an adequate volume of blood from the forearm for glucose testing, up to an average of 16 microl in 30 s. The amount of blood extracted increases with the application of heat or vacuum before lancing, the level of vacuum, the depth of lancing, the time of collection, and the amount of skin stretching. Vacuum and skin stretching led to significant increases, up to fivefold in the perfusion of blood in the skin as measured by laser Doppler. Our observations suggest that vacuum combined with skin stretching increases blood extraction at alternate sites by increasing the lancet wound opening, increasing the blood available for extraction by vasodilatation, and reducing the venous return of blood through capillaries.

Comparison of glucose levels in dermal interstitial fluid and finger capillary blood

Alternative methods for self-monitoring of blood glucose have been pursued by many researchers, largely in response to evidence gathered in several long-term studies of patients with diabetes mellitus. These studies suggest that long-term complications of the disease may be mitigated if the disease is intensively managed, a component of which is increased monitoring. Many of the alternative methods utilize interstitial fluid (ISF) as the diagnostic fluid, rather than finger blood. A time lag in the distribution of glucose from blood to the interstitium has been observed by many, with estimates of lag time varying from none to 45 min. Dermal ISF was sampled from diabetic subjects in two tests and compared to finger blood glucose. In the first test, data were collected over time in a manner that allowed a cross-correlation analysis to predict an average lag time. Information from this test was then used as input to a data collection format for a method comparison test of 691 patients with diabetes in which ISF data were collected immediately after the finger blood reference and 15 min after the reference. An average lag time of about 25 min was determined from the cross-correlation analysis, with the correlation error reduced by three-fourths within a 15-min lag time. In the method comparison test, the correlation coefficient between finger blood glucose and ISF glucose improved from 0.923 to 0.951, and the percentage of data in the A zone of the Clarke Error Grid rose from 80.2% to 90.6% for the ISF glucose data collected at no lag and 15-min lag, respectively. Dermal ISF glucose measurement might be a reasonable alternative to blood glucose measurement for patients routinely monitoring ambient glycemia, although more testing in the sensitive hypoglycemic range is needed to clarify what might happen in cases of rapidly changing glucose.

Comparison of glucose concentration in interstitial fluid, and capillary and venous blood during rapid changes in blood glucose levels

The relationship between glucose concentrations in interstitial fluid (ISF) and blood has generated great interest due to its importance in minimally invasive and noninvasive techniques for measuring blood glucose. The relationship between glucose levels in dermal ISF, and capillary and venous blood was studied with the dermal ISF samples obtained using the suction blister technique. The study was conducted with intensely managed diabetics whose blood glucose levels were manipulated so as to induce rapid changes in blood glucose levels. Glucose levels in the three compartments exhibited high correlations both when individual subjects were considered separately and when data from all subjects were combined. No significant time lag during glucose excursions was observed among the ISF, and capillary and venous glucose levels.

Noninvasive glucose measurement by monitoring of scattering coefficient during oral glucose tolerance tests. Non-Invasive Task Force

BACKGROUND

Continuous glucose monitoring by means of optical glucose sensors would allow patients with diabetes to check their metabolic control to their convenience. In an earlier study, we showed that noninvasive glucose monitoring is feasible for rapid changes in blood glucose by means of measuring the scattering coefficient of human skin. In this study, we investigated whether also slower changes in blood glucose, this time induced by an oral glucose load, can also be monitored by this approach.

METHODS

Five healthy subjects and 13 patients with type 2 diabetes have been given a 75-g oral glucose load. Portable noninvasive systems were used to measure the skin tissue scattering coefficient. For this purpose, two optical sensor heads were attached directly to the skin of each volunteer. Light was applied to the skin and the reflected light intensity was registered.

RESULTS

In 8 of 10 measurements, correlation of changes in scattering coefficient with changes in glycemia was acceptable. In 19 of 26 measurements (73%) of patients with type 2 diabetes the observed changes in the scattering coefficient also correlated in acceptable manner. The accordance between the simultaneous measurements of the two sensor heads was acceptable in 13 of 18 volunteers and patients studied. There were virtually no differences in the quality of the measurements between healthy volunteers and patients with diabetes.

CONCLUSIONS

This study shows that also slow changes in blood glucose induced by an oral glucose load can be monitored by registration of scattering coefficient changes. It remains to be elucidated why this has not been possible in all experiments.

Can interstitial glucose assessment replace blood glucose measurements?

Current treatment regiments for individuals depending on exogenous insulin are based on measurements of blood glucose obtained through painful finger sticks. The shift to minimal or noninvasive continuous glucose monitoring primarily involves a shift from blood glucose measurements to devices measuring subcutaneous interstitial fluid (ISF) glucose. As the development of these devices progresses, details of the dynamic relationship between blood glucose and interstitial glucose dynamics need to be firmly established. This is a challenging task insofar as direct measures of ISF glucose are not readily available. The current article investigated the dynamic relationship between plasma and ISF glucose using a model-based approach. A two-compartment model system previously validated on data obtained with the MiniMed Continuous Glucose Monitoring System (CGMS) is reviewed and predictions from the original two-compartment model were confirmed using new data analysis of glucose dynamics in plasma and hindlimb lymph (lymph is derived from ISF) in the anesthetized dog. From these data sets, the time delay between plasma and ISF glucose in dogs was established (5-12 minutes) and a simulation study was performed to estimate the errors introduced if ISF is taken as a surrogate for blood. From the simulation study, the error component resulting from the differences in plasma and ISF glucose was estimated to be < 6% during normal day-to-day use in an individual with diabetes (error component calculated as the standard deviation of the ISF/plasma glucose differences under conditions where the maximal time delay was used). This difference is most likely within the variance between arterial and venous blood glucose. We conclude that the differences between plasma and ISF glucose will not be a significant obstacle in advancing the use of ISF as an alternative to blood glucose measurements.

Analytical Characterization of Electrochemical Biosensor Test Strips for Measurement of Glucose in Low-Volume Interstitial Fluid Samples

Background: Minimally invasive interstitial fluid (ISF) sampling and glucose measurement technologies were integrated into a hand-held device for diabetic glucose monitoring investigations.

Methods: Conventional electrochemical test strip technology (Bayer Glucometer Elite®) was adapted to measure glucose in small (0.5–2.0 μL) samples of ISF. Test strip glucose measurements were performed on a commercial potentiostat and were compared to various reference glucose methodologies (YSI 2300 analyzer, microhexokinase procedure, Bayer Glucometer Elite). Characterizations of the integrated ISF sampling-glucose test strip design included accuracy and precision in various sample media (saline, ISF surrogates, diabetic ISF samples), sample volume dependence, test strip sterilization studies (electron beam, γ irradiation), and diabetic ISF sampling and glucose measurements.

Results: Glucose measurements were free from significant media effects. Sample volume variations (0.6–3.2 μL) revealed only modest dependence of glucose measurement bias on sample volume (−1.5% per microliter). Sterilization treatments had only a minor impact on glucose response and test strip aging and no significant impact on interferent responses of the glucose test strips. Diabetic subject testing under minimum fasting conditions of at least 2 h with integrated ISF sampling and glucose measurement gave low ISF glucose measurement imprecision (CV, 4%) and mean glucose results that were indistinguishable from reference (microhexokinase) ISF glucose measurements and from capillary blood glucose measurements (Glucometer Elite).

Conclusions: Conventional single-use, electrochemical glucose test strip and ISF collection technologies can be readily integrated to provide real-time ISF sampling and glucose measurements for diabetic monitoring applications.

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Subcutaneous glucose predicts plasma glucose independent of insulin: implications for continuous monitoring

The present study investigated the relationship between blood and subcutaneous interstitial fluid (ISF) glucose by employing an amperometric glucose sensor specifically developed for 3-day continuous glucose monitoring. The apparent sensor sensitivity and ISF glucose equilibration delay were estimated on separate days during hyperglycemic clamps in four dogs in which insulin was either suppressed with somatostatin, allowed to change, or increased with an exogenous infusion. A 2-h sensor "settling-in" period was allowed before the clamps. During insulin deficiency, the sensor sensitivity and ISF glucose delay were 0.23 +/- 0.03 nA per mg/dl and 4.4 +/- 0. 8 min. Sensitivity was not affected by increases in endogenous (0.30 +/- 0.04 vs. 0.28 +/- 0.04 nA per mg/dl) or exogenous insulin (0.18 +/- 0.01 vs. 0.16 +/- 0.01 nA per mg/dl) nor was the delay (3.3 +/- 1.2 vs. 5.7 +/- 1.1 and 9.2 +/- 2.6 vs. 12.3 +/- 1.7 min; P > 0.05 for all). Sensor glucose accurately predicted plasma glucose without correcting for delays <10 min (r > 0.9 for all), whereas for longer delays a digital corrective filter was used (r = 0.91 with filter). We conclude that the relationship between blood and ISF glucose is not affected by insulin and that delays in ISF glucose equilibration can be corrected with digital filters.

Advances in Photoacoustic Noninvasive Glucose Testing

We report here on in vitro and in vivo experiments that are intended to explore the feasibility of photoacoustic spectroscopy as a tool for the noninvasive measurement of blood glucose. The in vivo results from oral glucose tests on eight subjects showed good correlation with clinical measurements but indicated that physiological factors and person-to-person variability are important. In vitro measurements showed that the sensitivity of the glucose measurement is unaffected by the presence of common blood analytes but that there can be substantial shifts in baseline values. The results indicate the need for spectroscopic data to develop algorithms for the detection of glucose in the presence of other analytes.

Current status and future prospects of parenteral insulin regimens, strategies and delivery systems for diabetes treatment

A strong relationship between long term metabolic control and low frequency of chronic diabetes complications was shown in the Diabetes Control Complication Trial (DCCT). However, the subcutaneous intensive insulin therapy required to achieve the glycemic goals defined by the DCCT led to an unacceptable frequency of severe hypoglycemia and a significant weight gain. This limits the benefits of this therapy and excludes groups of patients such as young children, the elderly or hypoglycemia prone patients. The intensive therapy and self blood glucose monitoring (SMBG) necessary to limit hypoglycemia represent a heavy burden for the patients and their family. Improvements in parenteral insulin therapy are possible by either modifying subcutaneous insulin characteristics (analogs, adjunction of peptides such as amylin, GLP1, IGF1), or by developing better routes of administration and making SMBG easier, which is a key to intensive insulin therapy success. The ultimate goal remains the development of an automated, glucose controlled device.

Fluorescein kinetics in interstitial fluid harvested from diabetic skin during fluorescein angiography: implications for glucose monitoring

BACKGROUND

Glucose monitoring based on sampling skin interstitial fluid (ISF) is being developed as an alternative to fingerstick blood glucose monitoring. Time delays between rapidly changing levels of glucose in blood and interstitial fluid have been reported in the literature to be between 5 and 20 minutes. This study investigated the time delay between the injection of a small molecular weight fluorescent tracer into the circulation and interstitial fluid.

METHODS

Diabetic subjects undergoing fluorescein angiography were studied. Skin ISF was sampled using a proprietary microporation and harvesting process. ISF was drawn through micropores created in the stratum corneum. After intravenous injection of sodium fluorescein, samples of ISF were drawn from 2 sites for 30 seconds over a period of 20 minutes. Fluorescence levels in ISF were measured with a fluorometer and used to create ISF fluorescein concentration versus time profiles.

RESULTS

The ISF fluorescein versus time profiles were characterized by a rapid rise followed by a slow decay. The time to peak of the ISF fluorescein concentration ranged from 2-4 minutes for the patients studied.

CONCLUSIONS

Intravenous injection of a bolus of low molecular weight fluorescent tracer was used to estimate the time delay between changing glucose levels in blood and ISF. The results indicate that the ISF sampling technology utilized here is capable of tracking rapidly rising levels of blood glucose.

Skin mini-erosion sampling technique: feasibility study with regard to serial glucose measurement

PURPOSE

To describe a dermally non-invasive serial sampling technique and to test its clinical feasibility with regard to glucose measurement.

METHODS

A standardized skin mini-erosion devoid of the epidermal barrier, and covered by an artificial one, was formed by a suctioning technique. Interstitial fluid (IF) was extracted serially by brief application of negative pressure, and its glucose content compared with that in capillary or venous blood samples.

RESULTS

The procedure caused no discomfort. The epidermis regenerated rapidly after experimentation. There were no complications. In non-diabetic subjects (n = 13) the mean of all IF values measured daily for 6 days was 6.2 +/- 0.1 mmol/l (+/-SE). The corresponding capillary blood glucose value was 5.6 +/- 0.1 mmol/l, and the venous glucose value was 5.4 +/- 0.1 mmol/l. The differences between IF glucose values and invasive control values remained within narrow limits throughout. The 2SD limits of agreement for the differences were 1.44 mmol/l (IF vs. capillary blood samples) and 1.76 mmol/l (IF vs venous samples) respectively. The OGTT curves suggested glucose kinetics to be similar in IF and in capillary blood. In diabetic subjects, the mean of IF values determined serially during one day was 15.3 +/- 1.0 mmol/l (range, 6.7-21.8 mmol/l), and the corresponding mean capillary value was 12.0 +/- 0.9 mmol/l (range, 3.3-17.2 mmol/l). The ICC for all paired photometric observations was 0.948.

CONCLUSIONS

The results suggest the new sampling technique to be a feasible approach for clinical and experimental purposes. A functionally integrated sampling patch is entering the clinical testing stage.