Continuous glucose monitoring by means of the microdialysis technique: underlying fundamental aspects

Epidermal Wearable Biosensors for the Continuous Monitoring of Biomarkers of Chronic Disease in Interstitial Fluid

Healthcare technology has allowed individuals to monitor and track various physiological and biological parameters. With the growing trend of the use of the internet of things and big data, wearable biosensors have shown great potential in gaining access to the human body, and providing additional functionality to analyze physiological and biochemical information, which has led to a better personalized and more efficient healthcare. In this review, we summarize the biomarkers in interstitial fluid, introduce and explain the extraction methods for interstitial fluid, and discuss the application of epidermal wearable biosensors for the continuous monitoring of markers in clinical biology. In addition, the current needs, development prospects and challenges are briefly discussed.

Continuous Glucose Monitoring in Dogs and Cats: Application of New Technology to an Old Problem

In recent years, glucose monitoring has been revolutionized by the development of continuous glucose monitoring systems (CGMS), which are wearable non/minimally invasive devices that measure glucose concentration almost continuously for several consecutive d/wk. The Abbott FreeStyle Libre is the CGMS used most commonly. It has adequate clinical accuracy both in dogs and cats, even though the accuracy is lower in the hypoglycemic range. It allows an accurate identification of glycemic excursions occurring throughout the day as well as of glucose variations during consecutive days, enabling the clinician to make a more informed decision about the insulin dose and frequency of administration.

Fluidic enabled bioelectronic implants: opportunities and challenges

Bioelectronic implants are increasingly facilitating novel strategies for clinical diagnosis and treatment. The integration of fluidic technologies into such implants enables new complementary routes for sensing and therapy alongside electrical interaction. Indeed, these two technologies, electrical and fluidic, can work synergistically in a bioelectronics implant towards the fabrication of a complete therapeutic platform. In this perspective article, the leading applications of fluidic enabled bioelectronic implants are highlighted and methods of operation and material choices are discussed. Furthermore, a forward-looking perspective is offered on emerging opportunities as well as critical materials and technological challenges.

Design of the micropump and mass-transfer compartment of a microfluidic system for regular nonenzymatic glucose measurement

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An implantable integrated microfluidic device is designed for regular glucose measurement in diabetic patients.

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The designed device has a sufficiently small size and enjoys the benefits of microdialysis method and nonenzymatic glucose measurement.

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The mass-transfer compartment has the Recovery Factor of 72%, utilizing an array of 24 × 12 hollow microneedles.

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The designed micropump is a piezoelectrically activated diaphragm-type pump which uses two passive flapper valves.

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With a 2 Hz frequency and a 7.5 V input voltage, the micropump provides the flow rate of 1 μL/min.

The aim of this paper is to design and numerically simulate the mass-transfer compartment and piezoelectric micropump of an implantable integrated microfluidic device for regular microdialysis-based nonenzymatic measurement of glucose level in diabetic patients. The device function is based on the process that the piezoelectric micropump pumps the dialysis fluid into the mass-transfer compartment microchannels, where the interstitial fluid (ISF) glucose diffusion into this dialysis fluid gives it a glucose content, then detected and measured in the sensor section. This diffusion takes place through the semipermeable membranes located in the microchannels at the base of the hollow microneedles entering the body skin painlessly. The value of dialysis fluid flow rate (1 μL/min) was chosen so that the best achievable recovery factor can be obtained while the size and time delay of system were being kept at the best minimum possible. In the mass-transfer compartment, the number of microneedles, the dimensions of microchannels and the thickness of membranes were selected so as to achieve the best appropriate recovery factor, minimum possible size as well as considering the fabrication feasibility. Furthermore, in the different parts of micropump, the materials and dimensions were chosen so as to provide the needed flow rate with the best minimum voltage, sufficiently small size and fabrication feasibility.

Skin Patchable Sensor Surveillance for Continuous Glucose Monitoring

Diabetes mellitus is a physiological and metabolic disorder affecting millions of people worldwide, associated with global morbidity, mortality, and financial expenses. Long-term complications can be avoided by frequent, continuous self-monitoring of blood glucose. Therefore, this review summarizes the current state-of-art glycemic control regimes involving measurement approaches and basic concepts. Following an introduction to the significance of continuous glucose sensing, we have tracked the evolution of glucose monitoring devices from minimally invasive to non-invasive methods to present an overview of the spectrum of continuous glucose monitoring (CGM) technologies. The conveniences, accuracy, and cost-effectiveness of the real-time CGM systems (rt-CGMs) are the factors considered for discussion. Transdermal biosensing and drug delivery routes have recently emerged as an innovative approach to substitute hypodermal needles. This work reviews skin-patchable glucose monitoring sensors for the first time, providing specifics of all the major findings in the past 6 years. Skin patch sensors and their progressive form, i.e., microneedle (MN) array sensory and delivery systems, are elaborated, covering self-powered, enzymatic, and non-enzymatic devices. The critical aspects reviewed are material design and assembly techniques focusing on flexibility, sensitivity, selectivity, biocompatibility, and user-end comfort. The review highlights the advantages of patchable MNs' multi-sensor technology designed to maintain precise blood glucose levels and administer diabetes drugs or insulin through a "sense and act" feedback loop. Subsequently, the limitations and potential challenges encountered from the MN array as rt-CGMs are listed. Furthermore, the current statuses of working prototype glucose-responsive "closed-loop" insulin delivery systems are discussed. Finally, the expected future developments and outlooks in clinical applications are discussed.

Open-flow microperfusion combined with mass spectrometry for in vivo liver lipidomic analysis

At present, conventional microdialysis (MD) techniques cannot efficiently sample lipids in vivo, possibly due to the high mass transfer resistance and/or the serious adsorption of lipids onto the semi-permeable membrane of a MD probe. The in vivo monitoring of lipids could be of great significance for the study of disease development and mechanisms. In this work, an open-flow microperfusion (OFM) probe was fabricated, and the conditions for sampling lipids via OFM were optimized. Using OFM, the recovery of lipid standards was improved to more than 34.7%. OFM is used for the in vivo sampling of lipids in mouse liver tissue with fibrosis, and it is then combined with mass spectrometry (MS) to perform lipidomic analysis. 156 kinds of lipids were identified in the dialysate collected via OFM, and it was found that the phospholipid levels, including PC, PE, and SM, were significantly higher in a liver suffering from fibrosis. For the first time, OFM combined with MS to sample and analyze lipids has provided a promising platform for in vivo lipidomic studies.

Accuracy of Blood Glucose Measurement and Blood Glucose Targets

BACKGROUND

To summarize new evidence regarding the methodological aspects of blood glucose control in the intensive care unit (ICU).

METHODS

We reviewed the literature on blood glucose control in the ICU up to August 2019 through Ovid Medline and Pubmed.

RESULTS

Since the publication of the Leuven studies, the benefits of glycemic control have been recognized. However, the methodology of blood glucose control, notably the blood glucose measurement accuracy and the insulin titration protocol, plays an important but underestimated role. This may partially explain the negative results of the large, pragmatic multicenter trials and made everyone realize that tight glycemic control with less-frequent glucose measurements on less accurate blood glucose meters is neither feasible nor advisable in daily practice. Blood gas analyzers remain the gold standard. New generation point-of-care blood glucose meters may be an alternative when using whole blood of critically ill patients in combination with a clinically validated insulin dosing algorithm.

CONCLUSION

When implementing blood glucose management in an ICU one needs to take into account the interaction between aimed glycemic target and blood glucose measurement methodology.

Revolutionizing Therapeutic Drug Monitoring with the Use of Interstitial Fluid and Microneedles Technology

While therapeutic drug monitoring (TDM) that uses blood as the biological matrix is the traditional gold standard, this practice may be impossible, impractical, or unethical for some patient populations (e.g., elderly, pediatric, anemic) and those with fragile veins. In the context of finding an alternative biological matrix for TDM, this manuscript will provide a qualitative review on: (1) the principles of TDM; (2) alternative matrices for TDM; (3) current evidence supporting the use of interstitial fluid (ISF) for TDM in clinical models; (4) the use of microneedle technologies, which is potentially minimally invasive and pain-free, for the collection of ISF; and (5) future directions. The current state of knowledge on the use of ISF for TDM in humans is still limited. A thorough literature review indicates that only a few drug classes have been investigated (i.e., anti-infectives, anticonvulsants, and miscellaneous other agents). Studies have successfully demonstrated techniques for ISF extraction from the skin but have failed to demonstrate commercial feasibility of ISF extraction followed by analysis of its content outside the ISF-collecting microneedle device. In contrast, microneedle-integrated biosensors built to extract ISF and perform the biomolecule analysis on-device, with a key feature of not needing to transfer ISF to a separate instrument, have yielded promising results that need to be validated in pre-clinical and clinical studies. The most promising applications for microneedle-integrated biosensors is continuous monitoring of biomolecules from the skin's ISF. Conducting TDM using ISF is at the stage where its clinical utility should be investigated. Based on the advancements described in the current review, the immediate future direction for this area of research is to establish the suitability of using ISF for TDM in human models for drugs that have been found suitable in pre-clinical experiments.

Optimization of insulin pump therapy based on high order run-to-run control scheme

BACKGROUND AND OBJECTIVES

Continuous subcutaneous insulin infusion (CSII) pump is widely considered a convenience and promising way for type 1 diabetes mellitus (T1DM) subjects, who need exogenous insulin infusion. In the standard insulin pump therapy, there are two modes for insulin infusion: basal and bolus insulin. The basal-bolus therapy should be individualized and optimized in order to keep one subject's blood glucose (BG) level within the normal range; however, the optimization procedure is troublesome and it perturb the patients a lot. Therefore, an automatic adjustment method is needed to reduce the burden of the patients, and run-to-run (R2R) control algorithm can be used to handle this significant task.

METHODS

In this study, two kinds of high order R2R control methods are presented to adjust the basal and bolus insulin simultaneously. For clarity, a second order R2R control algorithm is first derived and studied. Furthermore, considering the differences between weekdays and weekends, a seventh order R2R control algorithm is also proposed and tested.

RESULTS

In order to simulate real situation, the proposed method has been tested with uncertainties on measurement noise, drifts, meal size, meal time and snack. The proposed method can converge even when there are ±60 min random variations in meal timing or ±50% random variations in meal size.

CONCLUSIONS

According to the robustness analysis, one can see that the proposed high order R2R has excellent robustness and could be a promising candidate to optimize insulin pump therapy.

Spectral characteristics of the mutant form GGBP/H152C of D-glucose/D-galactose-binding protein labeled with fluorescent dye BADAN: influence of external factors

The mutant form GGBP/H152C of the D-glucose/D-galactose-binding protein with the solvatochromic dye BADAN linked to cysteine residue Cys 152 can be used as a potential base for a sensitive element of glucose biosensor system. We investigated the influence of various external factors on the physical-chemical properties of GGBP/H152C-BADAN and its complex with glucose. The high affinity (K_d = 8.5 µM) and high binding rate of glucose make GGBP/H152C-BADAN a good candidate to determine the sugar content in biological fluids extracted using transdermal techniques. It was shown that changes in the ionic strength and pH of solution within the physiological range did not have a significant influence on the fluorescent characteristics of GGBP/H152C-BADAN. The mutant form GGBP/H152C has relatively low resistance to denaturation action of GdnHCl and urea. This result emphasizes the need to find more stable proteins for the creation of a sensitive element for a glucose biosensor system.

Microdialysis-coupled enzymatic microreactor for in vivo glucose monitoring in rats

Continuous glucose monitoring (CGM) is an important aid for diabetic patients to optimize glycemic control and to prevent long-term complications. However, current CGM devices need further miniaturization and improved functional performance. We have coupled a previously described microfluidic chip with enzymatic microreactor (EMR) to a microdialysis probe and evaluated the performance of this system for monitoring subcutaneous glucose concentration in rats. Nanoliter volumes of microdialysis sample are efficiently reacted with continuously supplied glucose oxidase (GOx) solution in the EMR. The hydrogen peroxide produced is amperometrically detected at a (polypyrrole (PPy)-protected) thin-film Pt electrode. Subcutaneous glucose concentration was continuously monitored in anesthetized rats in response to intravenous injections of 20% glucose (w/v), 5 U/kg insulin, or saline as a control. In vitro evaluation showed a linear range of 2.1-20.6 mM and a sensitivity of 7.8 ± 1.0 nA/mM (n = 6). The physical lag time between microdialysis and the analytical signal was approximately 18 min. The baseline concentration of blood glucose was 10.2 ± 2.3 mM. After administering glucose to the rats, glucose levels increased by about 2 mM to 12.1 ± 2.3 mM in blood and 11.9 ± 1.5 mM in subcutaneous interstitial fluid (ISF). After insulin administration, glucose levels decreased by about 8 mM relative to baseline to 2.1 ± 0.6 mM in blood and 2.1 ± 0.9 mM in ISF. A microfluidic device with integrated chaotic mixer and EMR has been successfully combined with subcutaneous microdialysis to continuously monitor glucose in rats. This proof-of-principle demonstrates the feasibility of improved miniaturization in CGM based on microfluidics.

Reduction of 8-iso-prostaglandin F2α in the first week after Roux-en-Y gastric bypass surgery

Obesity is associated with increased markers of oxidative stress. We examined whether oxidative stress is reduced within the first week after Roux-en-Y gastric bypass (RYGB) surgery and could be related to changes in adipose tissue depots. The reactive oxygen species (ROS) marker 8-iso-prostaglandin F2α (8-iso-PGF2α) and activity of antioxidant glutathione peroxidases (GPX) in plasma were compared before and ~1 week after RYGB. The effects of RYGB on subcutaneous adipose tissue and interstitial fluid 8-iso-PGF2α levels and subcutaneous adipose tissue expression of GPX-3 were also assessed. Levels of 8-iso-PGF2α in subcutaneous and visceral adipose tissue were determined. Plasma 8-iso-PGF2α levels decreased (122 ± 75 to 56 ± 15 pg/ml, P = 0.001) and GPX activity increased (84 ± 18 to 108 ± 25 nmol/min/ml, P = 0.003) in the first week post-RYGB. RYGB also resulted in reductions of 8-iso-PGF2α in subcutaneous adipose tissue (1,742 ± 931 to 1,132 ± 420 pg/g fat, P = 0.046) and interstitial fluid (348 ± 118 to 221 ± 83 pg/ml, P = 0.046) that were comparable to plasma (26-33%, P = 0.74). Adipose GPX-3 expression was increased (6.7 ± 4.7-fold, P = 0.004) in the first postoperative week. The improvements in oxidative stress occurred with minimal weight loss (2.4 ± 3.4%, P = 0.031) and elevations in plasma interleukin-6 (18.0 ± 46.8 to 28.0 ± 58.9 pg/ml, P = 0.004). Subcutaneous and visceral adipose tissues express comparable 8-iso-PGF2α levels (1,204 ± 470 and 1,331 ± 264 pg/g fat, respectively; P = 0.34). These data suggest that RYGB affects adipose tissue leading to the restoration of adipose redox balance within the first postoperative week and that plasma 8-iso-PGF2α is primarily derived from subcutaneous adipose tissue.

Delays in minimally invasive continuous glucose monitoring devices: a review of current technology

Through the use of enzymatic sensors-inserted subcutaneously in the abdomen or ex vivo by means of microdialysis fluid extraction-real-time minimally invasive continuous glucose monitoring (CGM) devices estimate blood glucose by measuring a patient's interstitial fluid (ISF) glucose concentration. Signals acquired from the interstitial space are subsequently calibrated with capillary blood glucose samples, a method that has raised certain questions regarding the effects of physiological time lags and of the duration of processing delays built into these devices. The time delay between a blood glucose reading and the value displayed by a continuous glucose monitor consists of the sum of the time lag between ISF and plasma glucose, in addition to the inherent electrochemical sensor delay due to the reaction process and any front-end signal processing delays required to produce smooth traces. Presented is a review of commercially available, minimally invasive continuous glucose monitors with manufacturer reported device delays. The data acquisition process for the Medtronic MiniMed (Northridge, CA) continuous glucose monitoring system-CGMS Gold-and the Guardian RT monitor is described with associated delays incurred for each processing step. Filter responses for each algorithm are examined using in vitro hypoglycemic and hyperglycemic clamps, as well as with an analysis of fast glucose excursions from a typical meal response. Results demonstrate that the digital filters used by each algorithm do not cause adverse effects to fast physiologic glucose excursions, although nonphysiologic signal characteristics can produce greater delays.

A novel adaptive basal therapy based on the value and rate of change of blood glucose

BACKGROUND

Modern insulin pump therapy for type 1 diabetes mellitus offers the freedom to program several basal profiles that may accommodate diurnal ariability in insulin sensitivity and activity level. However, these basal profiles do not change even if a pending hypoglycemic or hyperglycemic event is foreseen. New insulin pumps could receive a direct feed of glucose values from a continuous glucose monitoring (CGM) system and could enable dynamic basal adaptation to improve glycemic control.

METHOD

The proposed method is a two-step procedure. After the design of an initial basal profile, an adaptation of the basal rate is suggested as a gain multiplier based on the current CGM glucose value and its rate of change (ROC). Taking the glucose value and its ROC as axes, a two-dimensional plane is divided into a nine-zone mosaic, where each zone is given a predefined basal multiplier; for example, a basal multiplier of zero indicates a recommendation to shut off the pump.

RESULTS

The proposed therapy was evaluated on 20 in silico subjects (ten adults and ten adolescents) in the Food and Drug Administration-approved UVa/Padova simulator. Compared with conventional basal therapy, the proposed basal adjustment improved the percentage of glucose levels that stayed in the range of 60-180 mg/dl for all 20 subjects. In addition, the adaptive basal therapy reduced the average blood glucose index values.

CONCLUSIONS

The proposed therapy provides the flexibility to account for insulin sensitivity variations that may result from stress and/or physical activities. Because of its simplicity, the proposed method could be embedded in a chip in a future artificial pancreatic beta cell or used in a "smart" insulin pump.

Closed-loop control of artificial pancreatic Beta -cell in type 1 diabetes mellitus using model predictive iterative learning control

A novel combination of iterative learning control (ILC) and model predictive control (MPC), referred to here as model predictive iterative learning control (MPILC), is proposed for glycemic control in type 1 diabetes mellitus. MPILC exploits two key factors: frequent glucose readings made possible by continuous glucose monitoring technology; and the repetitive nature of glucose-meal-insulin dynamics with a 24-h cycle. The proposed algorithm can learn from an individual's lifestyle, allowing the control performance to be improved from day to day. After less than 10 days, the blood glucose concentrations can be kept within a range of 90-170 mg/dL. Generally, control performance under MPILC is better than that under MPC. The proposed methodology is robust to random variations in meal timings within +/-60 min or meal amounts within +/-75% of the nominal value, which validates MPILC's superior robustness compared to run-to-run control. Moreover, to further improve the algorithm's robustness, an automatic scheme for setpoint update that ensures safe convergence is proposed. Furthermore, the proposed method does not require user intervention; hence, the algorithm should be of particular interest for glycemic control in children and adolescents.

Noninvasive ultrasonic glucose sensing with large pigs (approximately 200 pounds) using a lightweight cymbal transducer array and biosensors

BACKGROUND

To prevent complications in diabetes, the proper management of blood glucose levels is essential. Since conventional glucose meters require pricking fingers or other areas of the skin, a noninvasive method for monitoring blood glucose levels is desired. Using a lightweight cymbal transducer array, this study was conducted to noninvasively determine the glucose levels of pigs having a similar size to humans.

METHOD

In vivo experiments using eight pigs (approximately 200 pounds) were performed in five groups. A cymbal array with four biosensors was attached to the axillary area of the pig. The array was operated at 20 kHz at special peak-temporal peak intensity (I(sptp)) equal to 50 or 100 mW/cm(2) for 5, 10, or 20 minutes. After the ultrasound exposure, glucose concentrations of the interstitial fluid were determined using biosensors. For comparison, glucose levels of blood samples collected from the ear vein were measured by a commercial glucose meter.

RESULT

In comparison, glucose levels determined by a cymbal array and biosensor system were close to those measured by a glucose meter. After a 20-minute ultrasound exposure at I(sptp) = 100 mW/cm(2), the average glucose level determined by the ultrasound system was 175 +/- 7 mg/dl, which is close to 166 +/- 5 mg/dl measured by the glucose meter.

CONCLUSION

Results indicate the feasibility of using a cymbal array for noninvasive glucose sensing on pigs having a similar size to humans. Further studies on the ultrasound conditions, such as frequency, intensity, and exposure time, will be continued for effective glucose sensing.

A pilot study to evaluate a novel subcutaneous continuous glucose monitoring system in healthy Beagle dogs

Serial blood glucose measurements are currently regarded as the 'gold standard' for evaluating glycaemic control of canine diabetic patients. The aim of this study was to investigate a subcutaneous continuous glucose monitoring system based on microdialysis. Analyses were performed by taking interstitial glucose samples from two different anatomical regions (interscapular region [IR], thoracic region [TR]) in six healthy Beagle dogs during induced hypoglycaemia and hyperglycaemia, the feeding period and a period of stable glucose values. For comparison, plasma glucose concentration was measured simultaneously by a hexokinase-based automated chemistry analyser. The mean absolute differences ranged from 11.7 mg/dL (SD 23.5 mg/dL, TR) to 5.3 mg/dL (SD 19.2 mg/dL, IR) with a correlation of r=0.43-0.92 (P<0.01). Sensitivity for the detection of hypoglycaemia was 85.0% TR and 93.3% IR, and the specificity was 99.5% TR and 99.7% IR, respectively. In a modified Clarke error grid analysis, 99.3% TR and 99.7% IR of the values fell into zone A and B. Collapse of the microdialysis fibre was a recurrent problem although this was easily detectable by fluctuations in system pressure. The microdialysis system reflects physiological as well as induced variations in blood glucose in a valid manner.

A physiological model to evaluate drug kinetics in patients with hemorrhagic shock followed by fluid resuscitation. Application to amoxicillin-clavulanate

PURPOSE

To build a physiologically based pharmacokinetic model describing drug kinetics in interstitial fluid in case of hemorrhagic shock, and to propose a simple method to determine the subset of influential parameters that may be estimated with the data at hand.

METHODS

The model, which accounts for alterations of regional blood flows and body water distribution, was fitted to amoxicillin and clavulanate kinetic data, assessed in 12 trauma patients with hemorrhagic shock by comparison with 12 healthy volunteers. The predictions were the free concentrations of amoxicillin and clavulanate in 14 organs.

RESULTS

In all tissues of trauma patients, the rate of distribution was lower, but the steady-state level was higher than those in healthy participants. Blood volume was reduced by 25% and blood flow in organs other than lung, brain, and heart were reduced by 18%. Compared with healthy subjects, the time that free amoxicillin concentration remained above 8 mg/L in the interstitial fluid of trauma patients was higher in blood and muscles, and lower in the tendon compartment.

CONCLUSIONS

The results and predictions were consistent with the knowledge in this field. The model may be useful to optimize clinical trial designs and drug dosing regimens.

Comparing the performance of the optical glucose assay based on glucose binding protein with high-performance anion-exchange chromatography with pulsed electrochemical detection: efforts to design a low-cost point-of-care glucose sensor

BACKGROUND

The glucose binding protein (GBP) is one of many soluble binding proteins found in the periplasmic space of gram-negative bacteria. These proteins are responsible for chemotactic responses and active transport of chemical species across the membrane. Upon ligand binding, binding proteins undergo a large conformational change, which is the basis for converting these proteins into optical biosensors.

METHODS

The GBP biosensor was prepared by attaching a polarity-sensitive fluorescent probe to a single cysteine mutation at a site on the protein that is allosterically responsive to glucose binding. The fluorescence response of the resulting sensor was validated against high-performance anion-exchange chromatography (HPAEC) with pulsed electrochemical detection. Finally, a simple fluorescence reader was built using a lifetime-assisted ratiometric technique.

RESULTS

The GBP assay has a linear range of quantification of 0.100-2.00 microM and a sensitivity of 0.164 microM(-1) under the specified experimental conditions. The comparison between GBP and HPAEC readings for nine blind samples indicates that there is no statistical difference between the analytical results of the two methods at the 95% confidence level. Although the methods of fluorescence detection are based on different principles, the response of the homemade device to glucose concentrations was comparable to the response of the larger and more expensive tabletop fluorescence spectrophotometer.

CONCLUSIONS

A glucose binding protein labeled with a polarity-sensitive probe can be used for measuring micromolar amounts of glucose. Using a lifetime-assisted ratiometric technique, a low-cost GBP-based micromolar glucose monitor could be built.

Subcutaneous adipose tissue exerts proinflammatory cytokines after minimal trauma in humans

Inflammatory cytokines released from adipose tissue play an important role in different pathological processes. In the present study, we investigated the inflammatory cytokine response of human subcutaneous adipose tissue (SAT) by applying the open-flow microperfusion technique. Four standard 18-gauge microperfusion catheters were inserted into periumbilical SAT of eight healthy male volunteers [29 +/- 3 yr, BMI 24.3 +/- 1.9 (mean +/- SD)]. SAT probe effluents were collected at 60-min intervals for 8 h after catheter insertion. Different perfusion fluids were used to measure the local effect of insulin and/or glucose on the cytokine response. SAT probe effluents were analyzed for IL-1beta, IL-6, CXCL8 (IL-8), and TNF-alpha. SAT concentrations of IL-1beta increased 100-fold from 1.0 +/- 0.2 pg/ml (mean +/- SE) to 101.5 +/- 23.2 pg/ml (P < 0.001) after 8 h. A 130-fold increase was observed for CXCL8, from 49 +/- 29 to 6,554 +/- 1,713 pg/ml (P < 0.001). Furthermore, a 20-fold increase of IL-6 was observed within the first 5 h (from 159 +/- 123 to 3,554 +/- 394 pg/ml; P < 0.001), and a significant decline to 2,154 +/- 216 pg/ml (P < 0.01) was seen thereafter. Finally, TNF-alpha increased from 1.4 +/- 0.6 to 2.5 +/- 0.5 pg/ml (P < 0.05) in hour 2 and remained stable thereafter. Local administration of insulin exerted a stimulatory effect on the inflammatory response of IL-6. In conclusion, SAT exerts a highly reproducible and consistent proinflammatory cytokine response after minimally invasive trauma caused by the insertion of a catheter in humans.

[Sub-cutaneous closed-loop: continuous glucose measurement and external insulin pump: hope in treatment of type 1 diabetes]

The development of an artificial pancreas allowing a continuous insulin infusion according to glucose measurement is closed to be an ideal device for type 1 diabetic patients and for the diabetologits. The development of miniaturized external pumps infusing short acting analogues with pharmacokinetic profile closer to physiology, and the availability of accurate continuous glucose sensors has led to the development of closed-loop subcutaneous system. The feasibility of this solution as been proven at a small scale but remains to be confirmed in a home setting. Intermediate solutions, such as semi-automatic systems might be immediately valuable.

The need for continuous blood glucose monitoring in the intensive care unit

Insulin-based regimens decrease morbidity and mortality among critically ill patients by way of keeping glucose at tight control. Utilizing these regimens involves multiple measurements of glucose by way of finger pricking or through indwelling vascular catheters in order to adjust insulin doses. The limitations and risks of these methods of glucose monitoring are related to potential erroneous measurements, increased risk of infection, and a significant excess workload. An automated blood glucose monitoring device for glucose monitoring of critically ill patients is needed to improve patient care while avoiding the disadvantages of currently used glucose monitoring methodologies.

On-chip microdialysis system with flow-through glucose sensing capabilities

BACKGROUND

Microdialysis is a sampling technique based on controlling the mass transfer rate of different-sized molecules across a semipermeable membrane. Because the dialysis process has minimal effects on the surrounding fluid, it is viewed as a tool for continuous monitoring of human metabolites. In diabetes treatment, microdialysis probes have been used as sampling systems coupled to a glucose biosensor but may struggle to obtain high recoveries of analytes, as the sampling housing, probes, and glucose sensors are fabricated as separate pieces and then assembled, resulting in a large dead volume, which limits sensing frequency. An in situ combination of a miniaturized microdialysis probe with an integrated glucose sensor could help solve some of these problems.

METHOD

The system was fabricated by bonding a 6-mum-thick polycarbonate track-etch membrane with 100-nm-diameter pores onto microfluidic channels with the electrochemical glucose sensing electrodes patterned within the microchannels.

RESULTS

In vitro experiments demonstrating glucose microdialysis with continuous sensing were conducted. The permeability of glucose to the polycarbonate membrane with a 100-nm-diameter pore size was obtained to be 5.44 mum/s. Glucose recovery of 99% was observed using this microdialysis system at a perfusion flow rate of 0.5 microl/min. Experiments monitoring fluctuating glucose concentrations in the time domain at 99% recovery were also performed. The lag time was measured to be 210 seconds with 45 seconds contributed by mass transfer limitations and the rest from dead volume within the experimental setup.

CONCLUSION

The electrochemical sensing component was able to continuously track concentration changes in the reservoir. This system is expected to have the proper sensitivity to track physiologically relevant concentration changes of glucose with a lag time of less than 1 minute and minimal amplitude reduction for continuous glucose monitoring for diabetes treatment.

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.

Continuous glucose monitoring and external insulin pump: towards a subcutaneous closed loop

The development of an artificial pancreas for the treatment of type 1 diabetes is a highly desired endeavour for type 1 diabetic patients, physicians, scientists and engineers. The development of the first miniaturized external pumps in the 70s and the pharmacokinetic properties of short acting insulin analogues, closer to physiology, have raised hopes for the elaboration of such a device. Recent technological progress in the development of continuous glucose sensors, have improved the reliability and accuracy of these devices. This has led to the development of prototypes of closed-loop system based on the combination of a continuous monitor, a control algorithm, and an insulin pump. This review focuses on the SC-SC approach, employing subcutaneous glucose monitoring and subcutaneous insulin delivery. The feasibility of this solution has been proven at a small scale, but remains to be confirmed in the home setting. Intermediate solutions, such as semi-automatic systems, might be immediately valuable.

Clinical evaluation of alternative-site glucose measurements in patients after major cardiac surgery

OBJECTIVE

Tight glycemic control improves outcome in critically ill patients but requires frequent glucose measurements. Subcutaneous adipose tissue (SAT) has been characterized as promising for glucose monitoring in diabetes, but it remains unknown whether it can also be used as an alternative site in critically ill patients. The present study was performed to clinically evaluate the relation of glucose in SAT compared with arterial blood in patients after major cardiac surgery.

RESEARCH DESIGN AND METHODS

Forty critically ill patients were investigated at two clinical centers after major cardiac surgery. Arterial blood and SAT microdialysis samples were taken in hourly intervals for a period of up to 48 h. The glucose concentration in dialysate was calibrated using a two-step approach, first using the ionic reference technique to calculate the SAT glucose concentration (SATg) and second using a one-point calibration procedure to obtain a glucose profile comparable to SAT-derived blood glucose (BgSAT). Clinical validation of the data was performed by introducing data analysis based on an insulin titration algorithm.

RESULTS

Correlation between dialysate glucose and blood glucose (median 0.80 [interquartile range 0.68-0.88]) was significantly improved using the ionic reference calibration technique (SATg vs.blood glucose 0.90 [0.83-0.94]; P < 0.001). Clinical evaluation of the data indicated that 96.1% of glucose readings from SAT would allow acceptable treatment according to a well-established insulin titration protocol.

CONCLUSIONS

The results indicate good correlation between SATg and blood glucose in patients after major cardiac surgery. Clinical evaluation of the data suggests that with minor limitations, glucose from SAT can be used to establish tight glycemic control in this patient group.

Continuous glucose monitoring and closed-loop systems

BACKGROUND

The last two decades have witnessed unprecedented technological progress in the development of continuous glucose sensors, resulting in the first generation of commercial glucose monitors. This has fuelled the development of prototypes of a closed-loop system based on the combination of a continuous monitor, a control algorithm, and an insulin pump.

METHOD

A review of electromechanical closed-loop approaches is presented. This is followed by a review of existing prototypes and associated glucose sensors. A literature review was undertaken from 1960 to 2004.

RESULTS

Two main approaches exist. The extracorporeal s.c.-s.c. approach employs subcutaneous glucose monitoring and subcutaneous insulin delivery. The implantable i.v.-i.p. approach adopts intravenous sampling and intraperitoneal insulin delivery. Feasibility of both solutions has been demonstrated in small-scale laboratory studies using either the classical proportional-integral-derivative controller or a model predictive controller. Performance in the home setting has yet to be demonstrated.

CONCLUSIONS

The glucose monitor remains the main limiting factor in the development of a commercially viable closed-loop system, as presently available monitors fail to demonstrate satisfactory characteristics in terms of reliability and/or accuracy. Regulatory issues are the second limiting factor. Closed-loop systems are likely to be used first by health-care professionals in controlled environments such as intensive care units.

Components of an integrated microfluidic device for continuous glucose monitoring with responsive insulin delivery

Miniaturized medical diagnostic and treatment devices are currently being developed. Microneedles and miniaturized microdialysis systems are particularly well suited to impact diabetes treatment for continuous glucose monitoring and feedback-controlled insulin delivery. Microneedles are an attractive advanced drug delivery system used to mechanically penetrate the skin and inject insulin intradermally where it is rapidly absorbed by the capillary bed into the bloodstream. The real advantage of microneedle-enhanced drug delivery lies in the fact that drug is actively injected into a patient so the dosage may be varied with time to allow complex drug delivery profiles. The delivery is independent of the drug composition and merely relies on the subsequent drug absorption into the bloodstream. A miniaturized microdialysis probe for continuous glucose sensing has also been designed. Microdialysis is based upon controlling the mass transfer rate of glucose diffusing across a semipermeable membrane into a dialysis fluid while excluding larger molecules such as proteins. Polymer microdialysis membranes are integrated with microfluidic systems. Because of the high surface area to fluid volume ratio of miniaturized fluid channels, faster recovery of glucose to increase glucose sensing frequency is expected. This work highlights recent advances made in the design and fabrication of microneedles to make them more biocompatible and more fracture resistant in order to effectively enter the biomedical market. In addition, the design of a miniaturized microdialysis system for increased glucose sampling frequency is presented. The sensing and infusion technologies may be combined into a miniaturized "artificial pancreas" for minimally invasive feedback-controlled insulin delivery.

Biocompatibility of an electrochemical sensor for continuous glucose monitoring in subcutaneous tissue

BACKGROUND

The continuous monitoring of glucose allows for tighter control of the glucose concentration and thus may prevent hyper- and hypoglycemia as well as long-term complications of diabetes. While most current systems depend on the transport of fluid to a glucose sensor outside the body, we investigate the possibility of implanting a reagent-based sensor directly into the skin. In this manuscript, the biocompatibility of an electrochemical sensor for continuous glucose monitoring was assessed in vitro and in vivo.

METHODS

Cytotoxicity was investigated in vitro using agar diffusion testing. In vivo biocompatibility was assessed by means of histomorphological examination of the surrounding tissue 10 days after sensor implantation in rats.

RESULTS

The grade of cytotoxicity of the individual sensor components in vitro was between none and mild based on agar diffusion testing. The complete sensor also showed no cytotoxic effects when coated with the co-polymer MPC (2-methacryloyloxyethyl phosphorylcholine, Lipidure CM 5206, NOF Corp., Tokyo, Japan) and when assessed under working conditions, i.e., when a bias voltage was applied to the sensor. Additionally, the hydrogen peroxide-which is inherently generated by the enzymatic glucose detection process using glucose oxidase (GOD)-is likely to have been sufficiently decomposed under these working conditions. Finally, no toxic leachable substances were found during the cytotoxicity testing of sensors and its extracts in vitro. In the in vivo experiments, the strongest foreign body reaction (FBR) was found near the GOD-electrode using a sensor without MPC coating and without a porous membrane. Covering the sensor with MPC, a porous membrane, or both led to a gradual decrease of the FBR down to the level of the negative control.

CONCLUSIONS

The electrochemical, reagent-based sensor with MPC coating and/or a porous membrane is suitable for continuous monitoring of glucose from a biocompatibility standpoint.

Physiological Modeling for Indirect Evaluation of Drug Tissular Pharmacokinetics under Non-Steady-State Conditions: An Example of Antimicrobial Prophylaxis During Liver Surgery

Cefazolin, a time-dependent first-generation cephalosporin with non-linear binding to albumin, is widely recommended for antimicrobial prophylaxis during liver surgery to decrease the incidence of postoperative wound infections. The recommended protocol (2 g IV at anesthesia induction followed by 1 g 4 h later) is expected to maintain the free cefazolin concentration in exposed intratissular fluids above its minimal inhibitory concentration (MIC) for potentially encountered microorganisms, from skin incision to skin closure. Since this dosing protocol fails to take into account either of patients status (total body weight and renal function) or of surgical and anesthetic consequences (variations of cardiac output and regional blood flows, progressive decrease of plasma albumin concentration) on cefazolin tissular pharmacokinetics, a physiological modeling study was conducted to investigate protocol suitability for liver surgery in six populations: obese (body mass index >34), renal insufficiency (GFR = 10, 30 or 50 ml min(-1)) and high intraoperative blood loss (three times that usually observed during this surgery) and none of these features referred to as controls. A previously validated physiologically based pharmacokinetic (PB-PK) model for cefazolin in humans was used and then further adapted to simulate obese or renal insufficiency patients as well as the consequences of general anesthesia and liver surgery on cefazolin pharmacokinetics. Clinical data required for simulation (intraoperative kinetics of percent expired isoflurane and plasma albumin concentration, mean intraoperative blood loss) were obtained from 10 patients who underwent right hepatectomy in our institution. Using a fixed MIC of 2 microg ml(-1) against potentially encountered bacteria, it was concluded that the recommended dosing schedule was suitable in all tested populations, including obese patients, although prolongation of the interval between injections appeared advisable for renal insufficiency patients. Furthermore, when a MIC of 3 microg ml(-1) was considered, the recommended cefazolin-dosing regimen failed to maintain sufficient free cefazolin concentrations in the interstitial fluids during surgery in all tested populations except renal insufficiency patients (GFR < 50 ml min(-1)).

Enregistrement de la glycémie en continu : quelles retombées ?

Continuous glucose monitoring is a recent technique which appears to be highly useful in diabetic patients, particularly in children and adolescents. Its potential interest to study unstable diabetes, to detect asymptomatic and nocturnal hypoglycaemias, to adapt and/or to adjust insulin treatment, etc. has lead to several recent clinical studies, conducted in various conditions. The different techniques, the studies conducted in children and adult diabetic patients, as well as the main current indications for continuous glucose monitoring are described in this article.