Performance of subcutaneously implanted glucose sensors for continuous monitoring

The impact of digital healthcare and teledentistry on dentistry in the 21st Century: a survey of Hungarian dentists

BACKGROUND

The era of digitalization has arrived in the field of dentistry. Teledentistry (TD), the use of digital solutions in dentistry, is already used in practice; however, only some possibilities are considered. During the COVID-19 pandemic, remote patient monitoring and patient communication had to be solved with TD, thus causing a rapid spread of new tools. In addition to digital workflows, patient communication, AI, and online forums are also available.

METHODS

An online self-administered survey was developed for the study. The Hungarian Medical Chamber contacted potential respondents in a newsletter or e-mail. The Evasys survey system was used. The weighting procedure was executed for gender, age group, and type of settlement. A digital dental index variable was created and built with a linear regression model as a dependent variable. Explanatory variables are advantages, disadvantages, what would be necessary, experienced needs from the patients, and age.

RESULTS

A total of 171 dentists completed the survey. The best-known digital technologies are online conferences (96.5%), E-prescriptions (94.6%), and digital impressions (86.0%). Unawareness is the highest in the field of artificial intelligence in dentistry (50.5%), store-forward solutions (43.5%), and real-time solutions (41.8%). The digital dental index is 14.24 (standard deviation (SD) = 5.5), with a high power of the model.

CONCLUSION

Hungarian dentists need to be made aware of all the possibilities of TD. In addition to digital workflows, store-forward and real-time solutions can be beneficial to substitute face-to-face visits. TD can be used effectively to monitor oral cavity changes and develop dental confidence and proper oral care habits. Our survey suggests that it is necessary and inevitable to integrate TD into both graduate and postgraduate education, which may form the basis of primary health care in the next decade.

Comparison of continuous glucose monitoring to reference standard oral glucose tolerance test for the detection of dysglycemia in cystic Fibrosis: A systematic review

Aims

Increasing evidence for benefit of early detection of cystic fibrosis related diabetes (CFRD) coupled with limitations of current diagnostic investigations has led to interest and utilisation of continuous glucose monitoring (CGM). We conducted a systematic review to assess current evidence on CGM compared to reference standard oral glucose tolerance test for the detection of dysglycemia in people with cystic fibrosis without confirmed diabetes.

Methods

MEDLINE, Embase, CENTRAL, Evidence-Based Medicine Reviews, grey literature and six relevant journals were searched for studies published after year 2000. Studies reporting contemporaneous CGM metrics and oral glucose tolerance test results were included. Outcomes on oral glucose tolerance tests were categorised into a) normal, b) abnormal (indeterminate and impaired) or c) diabetic as defined by American Diabetes Association criteria. CGM outcomes were defined as hyperglycemia (≥1 peak sensor glucose ≥ 200 mg/dL), dysglycemia (≥1 peak sensor glucose ≥ 140–199 mg/dL) or normoglycemia (all sensor glucose peaks < 140 mg/dL). CGM hyperglycemia in people with normal or abnormal glucose tolerances was used to define an arbitrary CGM-diagnosis of diabetes. The Quality Assessment of Diagnostic Accuracy Studies tool was used to assess risk of bias. Primary outcome was relative risk of an arbitrary CGM-diagnosis of diabetes compared to the oral glucose tolerance test.

Results

We identified 1277 publications, of which 19 studies were eligible comprising total of 416 individuals with contemporaneous CGM and oral glucose tolerance test results. Relative risk of an arbitrary CGM-diagnosis of diabetes compared to oral glucose tolerance test was 2.92. Studies analysed were highly heterogenous, prone to bias and inadequately assessed longitudinal associations between CGM and relevant disease-specific sequela.

Conclusions

A single reading > 200 mg/dL on CGM is not appropriate for the diagnosis of CFRD. Prospective studies correlating CGM metrics to disease-specific outcomes are needed to determine appropriate cut-points.

„Time in range“ (TIR) vs. Glykohämoglobin Typ A1c (HbA1c): was zählt für unsere Patienten?

„Continuous glucose monitoring systems“ (CGM-Systeme) und CGM-basierte Metrik gewannen in den letzten 10 Jahren massiv an Bedeutung. Dennoch ist der HbA_1c nach wie vor der meistverwendete und international anerkannte Marker zur Beurteilung der glykämischen Kontrolle. Ebenso stellt er in klinischen Studien immer noch den wichtigsten Surrogatparameter zur Beurteilung klinischer Outcomes dar. Die Verwendung der Zeit im Zielbereich („time in range“ [TIR]) hat im Vergleich zum HbA_1c den Vorteil, dass Hypoglykämien und Glukosevariabilität besser dargestellt werden. Durch Nutzung der TIR kann man auch individuelle Zielbereiche definieren, beispielsweise bei Schwangeren oder multimorbiden Personen. Auch gibt es erste Hinweise, dass klinische Studienergebnisse anhand von TIR und anderen CGM-basierten Metriken bewertet werden können, auch wenn hierzu noch Langzeit- und Endpunktstudien fehlen. Einen wesentlichen Vorteil zeigt die TIR bei der Prädiktion diabetesassoziierter Komplikationen. So kann, basierend auf Änderungen beim erreichten Zielbereich, nicht nur das Auftreten neuropathischer, mikro- oder makrovaskulärer Komplikationen vorhergesagt werden, sondern auch das relative Risiko deren Manifestation. Die Nutzung von CGM im Allgemeinen und das Erreichen der TIR-Ziele spielen auch für Menschen mit Diabetes mellitus und deren Einschätzung ihrer Lebensqualität eine immer größere Rolle.

Semi-Implantable Bioelectronics

Developing techniques to effectively and real-time monitor and regulate the interior environment of biological objects is significantly important for many biomedical engineering and scientific applications, including drug delivery, electrophysiological recording and regulation of intracellular activities. Semi-implantable bioelectronics is currently a hot spot in biomedical engineering research area, because it not only meets the increasing technical demands for precise detection or regulation of biological activities, but also provides a desirable platform for externally incorporating complex functionalities and electronic integration. Although there is less definition and summary to distinguish it from the well-reviewed non-invasive bioelectronics and fully implantable bioelectronics, semi-implantable bioelectronics have emerged as highly unique technology to boost the development of biochips and smart wearable device. Here, we reviewed the recent progress in this field and raised the concept of "Semi-implantable bioelectronics", summarizing the principle and strategies of semi-implantable device for cell applications and in vivo applications, discussing the typical methodologies to access to intracellular environment or in vivo environment, biosafety aspects and typical applications. This review is meaningful for understanding in-depth the design principles, materials fabrication techniques, device integration processes, cell/tissue penetration methodologies, biosafety aspects, and applications strategies that are essential to the development of future minimally invasive bioelectronics.

Continuous Glucose Monitoring Devices: Past, Present, and Future Focus on the History and Evolution of Technological Innovation

The concept of implantable glucose sensors has been promulgated for more than 40 years. It is now accepted that continuous glucose monitoring (CGM) increases quality of life by allowing informed diabetes management decisions as a result of more optimized glucose control. The focus of this article is to provide a brief overview of the CGM market history, emerging technologies, and the foreseeable challenges for the next CGM generations as well as proposing possible solutions in an effort to advance the next generation of implantable sensor.

Dexamethasone-Enhanced Microdialysis and Penetration Injury

Microdialysis probes, electrochemical microsensors, and neural prosthetics are often used for in vivo monitoring, but these are invasive devices that are implanted directly into brain tissue. Although the selectivity, sensitivity, and temporal resolution of these devices have been characterized in detail, less attention has been paid to the impact of the trauma they inflict on the tissue or the effect of any such trauma on the outcome of the measurements they are used to perform. Factors affecting brain tissue reaction to the implanted devices include: the mechanical trauma during insertion, the foreign body response, implantation method, and physical properties of the device (size, shape, and surface characteristics. Modulation of the immune response is an important step toward making these devices with reliable long-term performance. Local release of anti-inflammatory agents such as dexamethasone (DEX) are often used to mitigate the foreign body response. In this article microdialysis is used to locally deliver DEX to the surrounding brain tissue. This work discusses the immune response resulting from microdialysis probe implantation. We briefly review the principles of microdialysis and the applications of DEX with microdialysis in (i) neuronal devices, (ii) dopamine and fast scan cyclic voltammetry, (iii) the attenuation of microglial cells, (iv) macrophage polarization states, and (v) spreading depolarizations. The difficulties and complexities in these applications are herein discussed.

A self-cleaning, mechanically robust membrane for minimizing the foreign body reaction: towards extending the lifetime of sub-Q glucose biosensors

Long-term, subcutaneously implanted continuous glucose biosensors have the potential to improve diabetes management and reduce associated complications. However, the innate foreign body reaction (FBR) both alters the local glucose concentrations in the surrounding tissues and compromises glucose diffusion to the biosensor due to the recruitment of high-metabolizing inflammatory cells and the formation of a dense, collagenous fibrous capsule. Minimizing the FBR has mainly focused on "passively antifouling" materials that reduce initial cellular attachment, including poly(ethylene glycol) (PEG). Instead, the membrane reported herein utilizes an "actively antifouling" or "self-cleaning" mechanism to inhibit cellular attachment through continuous, cyclic deswelling/reswelling in response to normal temperature fluctuations of the subcutaneous tissue. This thermoresponsive double network (DN) membrane is based on N-isopropylacrylamide (NIPAAm) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (75:25 and 100:0 NIPAAm:AMPS in the 1st and 2nd networks, respectively; "DN-25%"). The extent of the FBR reaction of a subcutaneously implanted DN-25% cylindrical membrane was evaluated in rodents in parallel with a PEG-diacrylate (PEG-DA) hydrogel as an established benchmark biocompatible control. Notably, the DN-25% implants were more than 25× stronger and tougher than the PEG-DA implants while maintaining a modulus near that of subcutaneous tissue. From examining the FBR at 7, 30 and 90 days after implantation, the thermoresponsive DN-25% implants demonstrated a rapid healing response and a minimal fibrous capsule (~20-25 µm), similar to the PEG-DA implants. Thus, the dynamic self-cleaning mechanism of the DN-25% membranes represents a new approach to limit the FBR while achieving the durability necessary for long-term implantable glucose biosensors.

Low-Fouling Characteristics of Ultrathin Zwitterionic Cysteine SAMs

Surface fouling remains an exigent issue for many biological implants. Unwanted solutes adsorb to reduce device efficiency and hasten degradation while increasing the risks of microbial colonization and adverse inflammatory response. To address unwanted fouling in modern implants in vivo, surface modification with antifouling polymers has become indispensable. Recently, zwitterionic self-assembled monolayers, which contain two or more charged functional groups but are electrostatically neutral and form highly hydrated surfaces, have been the focus of many antifouling coatings. Reports using various compositions of zwitterionic polymer brushes have demonstrated ultralow fouling in the ng/cm² range. These coatings, however, are thick and can hinder the target application of biological devices. Here, we report an ultrathin (8.52 Å) antifouling self-assembled monolayer composed of cysteine that is amenable to facile fabrication. The antifouling characteristics of the zwitterionic surfaces were evaluated against bovine serum albumin, fibrinogen, and human blood in real time using quartz crystal microbalance and surface plasmon resonance imaging. Compared to untreated gold surfaces, the ultrathin cysteine coating reduced the adsorption of bovine serum albumin by 95% (43 ng/cm² adsorbed) after 3 h and 90% reduction after 24 h. Similarly, the cysteine self-assembled monolayer reduced the adsorption of fibrinogen as well as human blood by >90%. The surfaces were further characterized using scanning electron microscopy: protein-enhanced adsorption and cellular adsorption in human blood was found on untreated surfaces but not on the cysteine SAM-protected surfaces. These findings suggest that surfaces can be functionalized with an ultrathin layer of cysteine to resist the adsorption of key proteins, with performance comparable to zwitterionic polymer brushes. As such, cysteine surface coatings are a promising methodology to improve the long-term utility of biological devices.

Reduction of measurement noise in a continuous glucose monitor by coating the sensor with a zwitterionic polymer

Continuous glucose monitors (CGMs), used by patients with diabetes mellitus, can autonomously track fluctuations in blood glucose over time. However, the signal produced by CGMs during the initial recording period following sensor implantation contains substantial noise, requiring frequent recalibration via fingerprick tests. Here, we show that coating the sensor with a zwitterionic polymer, found via a combinatorial-chemistry approach, significantly reduces signal noise and improves CGM performance. We evaluated the polymer-coated sensors in mice as well as in healthy and diabetic non-human primates, and show that the sensors accurately record glucose levels without the need for recalibration. We also show that the polymer-coated sensors significantly abrogated immune responses to the sensor, as indicated by histology, fluorescent whole-body imaging of inflammation-associated protease activity, and gene expression of inflammation markers. The polymer coating may allow CGMs to become standalone measuring devices.

Sustained effect of glucagon on body weight and blood glucose: Assessed by continuous glucose monitoring in diabetic rats

Insulin is a vital part of diabetes treatment, whereas glucagon is primarily used to treat insulin-induced hypoglycemia. However, glucagon is suggested to have a central role in the regulation of body weight, which would be beneficial for diabetic patients. Since the glucagon effect on blood glucose is known to be transient, it is relevant to investigate the pharmacodynamics of glucagon after repeated dosing. In the present study, we used telemetry to continuously measure blood glucose in streptozotocin induced diabetic Sprague-Dawley rats. This allowed for a more detailed analysis of glucose regulation compared to intermittent blood sampling. In particular, we evaluated the blood glucose-lowering effect of different insulin doses alone, and in combination with a long acting glucagon analog (LAG). We showed how the effect of the LAG accumulated and persisted over time. Furthermore, we found that addition of the LAG decreased body weight without affecting food intake. In a subsequent study, we focused on the glucagon effect on body weight and food intake during equal glycemic control. In order to obtain comparable maximum blood glucose lowering effect to insulin alone, the insulin dose had to be increased four times in combination with 1 nmol/kg of the LAG. In this set-up the LAG prevented further increase in body weight despite the four times higher insulin-dose. However, the body composition was changed. The insulin group increased both lean and fat mass, whereas the group receiving four times insulin in combination with the LAG only significantly increased the fat mass. No differences were observed in food intake, suggesting a direct effect on energy expenditure by glucagon. Surprisingly, we observed decreased levels of FGF21 in plasma compared to insulin treatment alone. With the combination of insulin and the LAG the blood glucose-lowering effect of insulin was prolonged, which could potentially be beneficial in diabetes treatment.

Nitric Oxide Release for Improving Performance of Implantable Chemical Sensors - A Review

Over the last three decades, there has been extensive interest in developing in vivo chemical sensors that can provide real-time measurements of blood gases (oxygen, carbon dioxide, and pH), glucose/lactate, and potentially other critical care analytes in the blood of hospitalized patients. However, clot formation with intravascular sensors and foreign body response toward sensors implanted subcutaneously can cause inaccurate analytical results. Further, the risk of bacterial infection from any sensor implanted in the human body is another major concern. To solve these issues, the release of an endogenous gas molecule, nitric oxide (NO), from the surface of such sensors has been investigated owing to NO's ability to inhibit platelet activation/adhesion, foreign body response and bacterial growth. This paper summarizes the importance of NO's therapeutic potential for this application and reviews the publications to date that report on the analytical performance of NO release sensors in laboratory testing and/or during in vivo testing.

Point-of-Care Glucose and Ketone Monitoring

Early and rapid identification of hypo- and hyperglycemia as well as ketosis is essential for the practicing veterinarian as these conditions can be life threatening and require emergent treatment. Point-of-care testing for both glucose and ketone is available for clinical use and it is important for the veterinarian to understand the limitations and potential sources of error with these tests. This article discusses the devices used to monitor blood glucose including portable blood glucose meters, point-of-care blood gas analyzers and continuous glucose monitoring systems. Ketone monitoring options discussed include the nitroprusside reagent test strips and the 3-β-hydroxybutyrate ketone meter.

Local release of masitinib alters in vivo implantable continuous glucose sensor performance

Continuous glucose monitoring (CGM) sensors are often advocated as a clinical solution to improve long-term glycemic control in the context of diabetes. Subcutaneous sensor inflammatory response, fouling and fibrous encapsulation resulting from the host foreign body response (FBR) reduce sensor sensitivity to glucose, eventually resulting in sensor performance compromise and device failure. Several combination device strategies load CGM sensors with drug payloads that release locally to tissue sites to mitigate FBR-mediated sensor failure. In this study, the mast cell-targeting tyrosine kinase inhibitor, masitinib, was released from degradable polymer microspheres delivered from the surfaces of FDA-approved human commercial CGM needle-type implanted sensors in a rodent subcutaneous test bed. By targeting the mast cell c-Kit receptor and inhibiting mast cell activation and degranulation, local masitinib penetration around the CGM to several hundred microns sought to reduce sensor fibrosis to extend CGM functional lifetimes in subcutaneous sites. Drug-releasing and control CGM implants were compared in murine percutaneous implant sites for 21 days using direct-wire continuous glucose reporting. Drug-releasing implants exhibited no significant difference in CGM fibrosis at implant sites but showed relatively stable continuous sensor responses over the study period compared to blank microsphere control CGM implants.

Predicting Plasma Glucose From Interstitial Glucose Observations Using Bayesian Methods

One way of constructing a control algorithm for an artificial pancreas is to identify a model capable of predicting plasma glucose (PG) from interstitial glucose (IG) observations. Stochastic differential equations (SDEs) make it possible to account both for the unknown influence of the continuous glucose monitor (CGM) and for unknown physiological influences. Combined with prior knowledge about the measurement devices, this approach can be used to obtain a robust predictive model. A stochastic-differential-equation-based gray box (SDE-GB) model is formulated on the basis of an identifiable physiological model of the glucoregulatory system for type 1 diabetes mellitus (T1DM) patients. A Bayesian method is used to estimate robust parameters from clinical data. The models are then used to predict PG from IG observations from 2 separate study occasions on the same patient. First, all statistically significant diffusion terms of the model are identified using likelihood ratio tests, yielding inclusion of [Formula: see text], [Formula: see text], and [Formula: see text]. Second, estimates using maximum likelihood are obtained, but prediction capability is poor. Finally a Bayesian method is implemented. Using this method the identified models are able to predict PG using only IG observations. These predictions are assessed visually. We are also able to validate these estimates on a separate data set from the same patient. This study shows that SDE-GBs and a Bayesian method can be used to identify a reliable model for prediction of PG using IG observations obtained with a CGM. The model could eventually be used in an artificial pancreas.

Cell based metabolic barriers to glucose diffusion: macrophages and continuous glucose monitoring

It is assumed that MQ are central to glucose sensor bio-fouling and therefore have a major negative impact on continuous glucose monitoring (CGM) performance in vivo. However to our knowledge there is no data in the literature to directly support or refute this assumption. Since glucose and oxygen (O2) are key to glucose sensor function in vivo, understanding and controlling glucose and O2 metabolic activity of MQ is likely key to successful glucose sensor performance. We hypothesized that the accumulation of MQ at the glucose sensor-tissue interface will act as "Cell Based Metabolic Barriers" (CBMB) to glucose diffusing from the interstitial tissue compartment to the implanted glucose sensor and as such creating an artificially low sensor output, thereby compromising sensor function and CGM. Our studies demonstrated that 1) direct injections of MQ at in vivo sensor implantation sites dramatically decreased sensor output (measured in nA), 2) addition of MQ to glucose sensors in vitro resulted in a rapid and dramatic fall in sensor output and 3) lymphocytes did not affect sensor function in vitro or in vivo. These data support our hypothesis that MQ can act as metabolic barriers to glucose and O2 diffusion in vivo and in vitro.

Modulation of the foreign body response to implanted sensor models through device-based delivery of the tyrosine kinase inhibitor, masitinib

The host foreign body response (FBR) adversely effects the performance of numerous implanted biomaterials especially biosensors, including clinically popular glucose-monitoring sensors. Reactive formation of a fibrous capsule around implanted sensors hinders the transport of essential analytes to the sensor from the surrounding tissue, resulting in loss of glucose response sensitivity and eventual sensor failure. Several strategies have sought to mitigate the foreign body response's effects on CGM sensors through the use of local delivery of pharmaceuticals and biomolecules with limited success. This study describes release of a tyrosine kinase inhibitor - masitinib - from the sensor implant to target tissue resident mast cells as key mediators of the FBR. Model implants are coated with a composite polymer hydrophilic matrix that rapidly dissolves upon tissue implantation to deposit slower-degrading polymer microparticles containing masitinib. Matrix dissolution limits coating interference with sensor function while establishing a local controlled-release delivery depot formulation to alter implant tissue pharmacology and addressing the FBR. Drug efficacy was evaluated in a murine subcutaneous pocket implant model. Drug release extends to more than 30 days in vitro. The resulting FBR in vivo, evaluated by implant capsule thickness and inflammatory cell densities at 14, 21, and 28 days, displays statistically significant reduction in capsule thickness around masitinib-releasing implant sites compared to control implant sites.

Novel molecular and nanosensors for in vivo sensing

In vivo sensors are an emerging field with the potential to revolutionize our understanding of basic biology and our treatment of disease. In this review, we highlight recent advances in the fields of in vivo electrochemical, optical, and magnetic resonance biosensors with a focus on recent developments that have been validated in rodent models or human subjects. In addition, we discuss major challenges in the development and translation of in vivo biosensors and present potential solutions to these problems. The field of nanotechnology, in particular, has recently been instrumental in driving the field of in vivo sensors forward. We conclude with a discussion of emerging paradigms and techniques for the development of future biosensors.

Continuous glucose monitoring: 40 years, what we've learned and what's next

After 40 years of research and development, today continuous glucose monitoring (CGM) is demonstrating the benefit it provides for millions with diabetes. To provide in vivo accuracy, new permselective membranes and mediated systems have been developed to prevent enzyme saturation and to minimize interference signals. Early in vivo implanted sensor research clearly showed that the foreign body response was a more difficult issue to overcome. Understanding the biological interface and circumventing the inflammatory response continue to drive development of a CGM sensor with accuracy and reliability performance suitable in a closed-loop artificial pancreas. Along with biocompatible polymer development, other complimentary algorithm and data analysis techniques have improved the performance of commercial systems significantly. For example, the mean average relative difference of Dexcom's CGM system improved from 26 to 14% and its use-life was extended from 3 to 7 d. Significant gains in usability, including size, flexibility, insertion, calibration, and data interface, have been incorporated into new generations of commercial CGM systems. Besides Medtronic, Dexcom, and Abbott, other major players are also investing in CGM. Becton Dickinson is conducting clinical trials with an optical galactose glucose binding system. Development of fully implanted sensor systems fulfills the desire for a discreet, reliable CGM system. Research continues to find innovative ways to help make living with diabetes easier and more normal, and new segments are being pursued (intensive care unit, surgery, behavior modification) in which CGM is being utilized.

Biofouling of polymer hydrogel materials and its effect on diffusion and enzyme-based luminescent glucose sensor functional characteristics

BACKGROUND

Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However, biofouling and host response make in vivo sensor performance difficult to predict. We investigated changes in glucose diffusivity and sensor response of optical enzymatic glucose sensors due to biological exposure.

METHOD

Three hydrogel materials, poly(2-hydroxyethyl methacrylate) (pHEMA), poly(acrylamide) (pAM), and poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (p(HEMA-co-AM)), were tested for glucose diffusivity before and after exposure to serum or implantation in rats for 1 month. Luminescent sensors based on these materials were measured to compare the response to glucose before and after serum exposure.

RESULTS

Glucose diffusivity through the pHEMA [(8.1 ± 0.38) × 10(-8) cm(2)/s] slabs was much lower than diffusivity through pAM [(2.7 ± 0.15) × 10(-6) cm(2)/s] and p(HEMA-co-AM) [(2.5 ± 0.08) × 10(-6)]. As expected from these differences, sensor response was highly dependent on material type. The pHEMA sensors had a maximum sensitivity of 2.5%/(mg/dl) and an analytical range of 4.2-356 mg/dl, while the p(HEMA-co-AM) sensors had a higher sensitivity [14.9%/(mg/dl)] and a narrower analytical range (17.6-70.5 mg/dl). After serum exposure, the pHEMA sensors were unaffected, whereas the p(HEMA-co-AM) sensors exhibited significantly decreased sensitivity and increased analytical range.

CONCLUSIONS

Decreases in glucose diffusivity in the polymers resulting from in vitro serum exposure and residence in vivo were shown to be similar, suggesting that serum incubation was a reasonable approximation of in vivo fouling. While biofouling is expected to affect the response of flux-based sensors, we have shown that this depended on the type of sensor and matrix used. Therefore, proper design and materials selection may minimize response alterations occurring upon implantation.

Characterization of an early passage Merkel cell polyomavirus-positive Merkel cell carcinoma cell line, MS-1, and its growth in NOD scid gamma mice

Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high mortality rate. The majority of MCC (70-80%) harbor clonally integrated Merkel cell polyomavirus (MCV) in the tumor genome and express viral T antigen oncoproteins. The characterization of an early passage MCV-positive MCC cell line MS-1 is described, and its cellular, immunohistochemical, and virological features to MCV-negative (UISO, MCC13, and MCC26) and MCV-positive cell lines (MKL-1 and MKL-2) were compared. The MS-1 cellular genome harbors integrated MCV, which preserves an identical viral sequence from its parental tumor. Neither VP2 gene transcripts nor VP1 protein are detectable in MS-1 or other MCV-positive MCC cell lines tested. Mapping of viral and cellular integration sites in MS-1 and MCC tumor samples demonstrates no consistent viral or cellular gene integration locus. All MCV-positive cell lines show cytokeratin 20 positivity and grow in suspension. When injected subcutaneously into NOD scid gamma (NSG) mice, MS-1 forms a discrete macroscopic tumor. Immunophenotypic analysis of the MS-1 cell line and xenografts in mice show identical profiles to the parental tumor biopsy. Hence, MS-1 is an early passage cell line that provides a useful in vitro model to characterize MCV-positive MCC.

Multicolor saccharide-sensing chips based on boronic acid-containing thin films showing stepwise release and binding of dyes

A novel saccharide sensor that shows a distinct color change resembling a "traffic signal" was developed. By copolymerizing a boronic acid monomer and an amine monomer on a glass plate, a boronic acid-containing thin film was obtained. After adsorbing anionic blue and yellow dyes, the thin film was immersed in aqueous saccharide solutions containing a cationic red dye. As saccharide concentration increased, the thin film changed its color from green to red via yellow. Origin of the distinct color change was attributed to a stepwise release and binding of dyes.

"Smart tattoo" glucose biosensors and effect of coencapsulated anti-inflammatory agents

BACKGROUND

Minimally invasive glucose biosensors with increased functional longevity form one of the most promising techniques for continuous glucose monitoring. In the present study, we developed a novel nanoengineered microsphere formulation comprising alginate microsphere glucose sensors and anti-inflammatory-drug-loaded alginate microspheres.

METHODS

The formulation was prepared and characterized for size, shape, in vitro drug release, biocompatibility, and in vivo acceptability. Glucose oxidase (GOx)- and Apo-GOx-based glucose sensors were prepared and characterized. Sensing was performed both in distilled water and simulated interstitial body fluid. Layer-by-layer self-assembly techniques were used for preventing drug and sensing chemistry release. Finally, in vivo studies, involving histopathologic examination of subcutaneous tissue surrounding the implanted sensors using Sprague-Dawley rats, were performed to test the suppression of inflammation and fibrosis associated with glucose sensor implantation.

RESULTS

The drug formulation showed 100% drug release with in 30 days with zero-order release kinetics. The GOx-based sensors showed good enzyme retention and enzyme activity over a period of 1 month. Apo-GOx-based visible and near-infrared sensors showed good sensitivity and analytical response range of 0-50 mM glucose, with linear range up to 12 mM glucose concentration. In vitro cell line studies proved biocompatibility of the material used. Finally, both anti-inflammatory drugs were successful in controlling the implant-tissue interface by suppressing inflammation at the implant site.

CONCLUSION

The incorporation of anti-inflammatory drug with glucose biosensors shows promise in improving sensor biocompatibility, thereby suggesting potential application of alginate microspheres as "smart tattoo" glucose sensors with increased functional longevity.

The Correlation and Accuracy of Glucose Levels between Interstitial Fluid and Venous Plasma by Continuous Glucose Monitoring System

Background

Clinical experience with the continuous glucose monitoring systems (CGMS) is limited in Korea. The objective of this study is to evaluate the accuracy of the CGMS and the correlation between interstitial fluid and venous plasma glucose level in Korean healthy male subjects.

Methods

Thirty-two subjects were served with glucose solution contained same amount of test food's carbohydrate and test foods after separate overnight fasts. CGMS was performed over 3 days during hopitalization for each subjects. Venous plasma glucose measurements were carried out during 4 hours (0, 0.25, 0.5, 0.75, 1, 2, 4 hours) just before and after glucose solution and test food load. The performance of the CGMS was evaluated by comparing its readings to those obtained at the same time by the hexokinase method using the auto biochemistry machine (Hitachi 7600-110). Also, correlations between glucose recorded with CGMS and venous plasma glucose value were examined.

Results

CGMS slightly underestimated the glucose value as compared with the venous plasma glucose level (16.3 ± 22.2 mg/dL). Correlation between CGMS and venous plasma glucose values throughout sensor lifetime is 0.73 (regression analysis: slope = 1.08, intercept = 8.38 mg/dL). Sensor sensitivity can deteriorate over time, with correlations between venous blood glucose and CGMS values dropping from 0.77 during 1st day to 0.65 during 2nd and 3rd day.

Conclusion

The accuracy of data provided by CGMS may be less than expected. CGMS sensor sensitivity is decreased with the passage of time. But, from this study, CGMS can be used for glucose variability tendency monitoring conveniently to the Korean.

Accuracy of a continuous glucose monitoring system in dogs and cats with diabetic ketoacidosis

OBJECTIVE

(1) To determine the ability of a continuous interstitial glucose monitoring system (CGMS) to accurately estimate blood glucose (BG) in dogs and cats with diabetic ketoacidosis. (2) To determine the effect of perfusion, hydration, body condition score, severity of ketosis, and frequency of calibration on the accuracy of the CGMS.

DESIGN

Prospective study.

SETTING

University Teaching Hospital.

ANIMALS

Thirteen dogs and 11 cats diagnosed with diabetic ketoacidosis were enrolled in the study within 24 hours of presentation.

INTERVENTIONS

Once BG dropped below 22.2 mmol/L (400 mg/dL), a sterile flexible glucose sensor was placed aseptically in the interstitial space and attached to the continuous glucose monitoring device for estimation of the interstitial glucose every 5 minutes.

MEASUREMENTS AND MAIN RESULTS

BG measurements were taken with a portable BG meter every 2-4 hours at the discretion of the primary clinician and compared with CGMS glucose measurements. The CGMS estimates of BG and BG measured on the glucometer were strongly associated regardless of calibration frequency (calibration every 8 h: r=0.86, P<0.001; calibration every 12 h: r=0.85, P<0.001). Evaluation of this data using both the Clarke and Consensus error grids showed that 96.7% and 99% of the CGMS readings, respectively, were deemed clinically acceptable (Zones A and B errors). Interpatient variability in the accuracy of the CGMS glucose measurements was found but was not associated with body condition, perfusion, or degree of ketosis. A weak association between hydration status of the patient as assessed with the visual analog scale and absolute percent error (Spearman's rank correlation, rho=-0.079, 95% CI=-0.15 to -0.01, P=0.03) was found, with the device being more accurate in the more hydrated patients.

CONCLUSIONS

The CGMS provides clinically accurate estimates of BG in patients with diabetic ketoacidosis.

Technologies for continuous glucose monitoring: current problems and future promises

Devices for continuous glucose monitoring (CGM) are currently a major focus of research in the area of diabetes management. It is envisioned that such devices will have the ability to alert a diabetes patient (or the parent or medical care giver of a diabetes patient) of impending hypoglycemic/hyperglycemic events and thereby enable the patient to avoid extreme hypoglycemic/hyperglycemic excursions as well as minimize deviations outside the normal glucose range, thus preventing both life-threatening events and the debilitating complications associated with diabetes. It is anticipated that CGM devices will utilize constant feedback of analytical information from a glucose sensor to activate an insulin delivery pump, thereby ultimately realizing the concept of an artificial pancreas. Depending on whether the CGM device penetrates/breaks the skin and/or the sample is measured extracorporeally, these devices can be categorized as totally invasive, minimally invasive, and noninvasive. In addition, CGM devices are further classified according to the transduction mechanisms used for glucose sensing (i.e., electrochemical, optical, and piezoelectric). However, at present, most of these technologies are plagued by a variety of issues that affect their accuracy and long-term performance. This article presents a critical comparison of existing CGM technologies, highlighting critical issues of device accuracy, foreign body response, calibration, and miniaturization. An outlook on future developments with an emphasis on long-term reliability and performance is also presented.

Emerging synergy between nanotechnology and implantable biosensors: a review

The development of implantable biosensors for continuous monitoring of metabolites is an area of sustained scientific and technological interests. On the other hand, nanotechnology, a discipline which deals with the properties of materials at the nanoscale, is developing as a potent tool to enhance the performance of these biosensors. This article reviews the current state of implantable biosensors, highlighting the synergy between nanotechnology and sensor performance. Emphasis is placed on the electrochemical method of detection in light of its widespread usage and substantial nanotechnology based improvements in various aspects of electrochemical biosensor performance. Finally, issues regarding toxicity and biocompatibility of nanomaterials, along with future prospects for the application of nanotechnology in implantable biosensors, are discussed.

Enhanced glucose sensor linearity using poly(vinyl alcohol) hydrogels

BACKGROUND

High linearities, sensitivities, and low oxygen dependence constitute prime requisites for electrochemical glucose sensors. However, for implantable sensors the need to control tissue inflammation requires the use outer membranes that permit inward analyte diffusion while continuously releasing anti-inflammatory drugs and other tissue response-modifying (TRM) agents. We have shown previously that while outer membranes based on layer-by-layer (LBL) assembly enhance linearity, poly(vinyl alcohol)(PVA) hydrogels loaded with TRM-containing microspheres enable a significant reduction in tissue inflammation. This article discusses amperometric performance of glucose sensors coated with stacked LBL/PVA hydrogel outer membranes.

METHODS

Sensors were fabricated by immobilizing glucose oxidase enzyme on a 50-microm platinum wire followed by deposition of stacked LBL/PVA hydrogel outer membranes. The sensor response to various glucose concentrations was determined by applying 0.7 V vs an Ag/AgCl reference electrode in phosphate-buffered saline (37 degrees C). Michaelis-Menten analysis was performed to quantify sensor performance in terms of linearity (K(m,glu)(app)) and oxygen dependence (K(m,O(2))(app)/[Glucose]).

RESULTS

When overlaid onto LBL-assembled outer membranes, PVA hydrogels improved sensor linearity by 60% from 10 to 16 mM of glucose and resulted in a twofold decrease in oxygen dependence.

CONCLUSIONS

Enhancement in the performance of a PVA-coated sensor is attributed to the oxygen-storing capability of PVA hydrogel due to the formation of hydrophobic domains during its freezing and thawing employed to physical cross-link the PVA. Such membranes with the capability to release TRMs continuously while storing oxygen constitute a major improvement over current outer membrane technologies.

New modalities in the treatment of pregnancies complicated by diabetes: drugs and devices

The development of drugs and devices in the treatment of pregnancies complicated by diabetes is in constant forward progression to compensate for pancreatic beta cell insufficiency. Maternal hyperglycemia during pregnancy is of particular interest due to the severe consequences that surface when a fetus is in development. The drugs that are currently recommended for use during pregnancy include rapid-acting insulin analogs lispro and aspart for meal-related bolus insulin and intermediate-acting NPH for basal insulin. Oral anti-diabetic agents are not recommended for use during pregnancy. Better control may be achieved with the incorporation of real-time glucose sensors and new insulin pumps with hopes of improving pregnancy outcome.

In silico preclinical trials: a proof of concept in closed-loop control of type 1 diabetes

Arguably, a minimally invasive system using subcutaneous (s.c.) continuous glucose monitoring (CGM) and s.c. insulin delivery via insulin pump would be a most feasible step to closed-loop control in type 1 diabetes mellitus (T1DM). Consequently, diabetes technology is focusing on developing an artificial pancreas using control algorithms to link CGM with s.c. insulin delivery. The future development of the artificial pancreas will be greatly accelerated by employing mathematical modeling and computer simulation. Realistic computer simulation is capable of providing invaluable information about the safety and the limitations of closed-loop control algorithms, guiding clinical studies, and out-ruling ineffective control scenarios in a cost-effective manner. Thus computer simulation testing of closed-loop control algorithms is regarded as a prerequisite to clinical trials of the artificial pancreas. In this paper, we present a system for in silico testing of control algorithms that has three principal components: (1) a large cohort of n=300 simulated "subjects" (n=100 adults, 100 adolescents, and 100 children) based on real individuals' data and spanning the observed variability of key metabolic parameters in the general population of people with T1DM; (2) a simulator of CGM sensor errors representative of Freestyle Navigator™, Guardian RT, or Dexcom™ STS™, 7-day sensor; and (3) a simulator of discrete s.c. insulin delivery via OmniPod Insulin Management System or Deltec Cozmo(®) insulin pump. The system has been shown to represent adequate glucose fluctuations in T1DM observed during meal challenges, and has been accepted by the Food and Drug Administration as a substitute to animal trials in the preclinical testing of closed-loop control strategies.

A review of the biocompatibility of implantable devices: current challenges to overcome foreign body response

In recent years, a variety of devices (drug-eluting stents, artificial organs, biosensors, catheters, scaffolds for tissue engineering, heart valves, etc.) have been developed for implantation into patients. However, when such devices are implanted into the body, the body can react to these in a number of different ways. These reactions can result in an unexpected risk for patients. Therefore, it is important to assess and optimize the biocompatibility of implantable devices. To date, numerous strategies have been investigated to overcome body reactions induced by the implantation of devices. This review focuses on the foreign body response and the approaches that have been taken to overcome this. The biological response following device implantation and the methods for biocompatibility evaluation are summarized. Then the risks of implantable devices and the challenges to overcome these problems are introduced. Specifically, the challenges used to overcome the functional loss of glucose sensors, restenosis after stent implantation, and calcification induced by implantable devices are discussed.

A review of the development of a vehicle for localized and controlled drug delivery for implantable biosensors

A major obstacle to the development of implantable biosensors is the foreign body response (FBR) that results from tissue trauma during implantation and the continuous presence of the implant in the body. The in vivo stability and functionality of biosensors are compromised by damage to sensor components and decreased analyte transport to the sensor. This paper summarizes research undertaken by our group since 2001 to control the FBR toward implanted sensors. Localized and sustained delivery of the anti-inflammatory drug, dexamethasone, and the angiogenic growth factor, vascular endothelial growth factor (VEGF), was utilized to inhibit inflammation as well as fibrosis and provide a stable tissue-device interface without producing systemic adverse effects. The drug-loaded polylactic-co-glycolic acid (PLGA) microspheres were embedded in a polyvinyl alcohol (PVA) hydrogel composite to fabricate a drug-eluting, permeable external coating for implantable devices. The composites were fabricated using the freeze-thaw cycle method and had mechanical properties similar to soft body tissue. Dexamethasone-loaded microsphere/hydrogel composites were able to provide anti-inflammatory protection, preventing the FBR. Moreover, concurrent release of dexamethasone with VEGF induced neoangiogenesis in addition to providing anti-inflammatory protection. Sustained release of dexamethasone is required for the entire sensor lifetime, as a delayed inflammatory response developed after depletion of the drug from the composites. These studies have shown the potential of PLGA microsphere/PVA hydrogel-based composites as drug-eluting external coatings for implantable biosensors.

Analysis, modeling, and simulation of the accuracy of continuous glucose sensors

BACKGROUND

Continuous glucose monitors (CGMs) collect a detailed time series of consecutive observations of the underlying process of glucose fluctuations. To some extent, however, the high temporal resolution of the data is accompanied by increased probability of error in any single data point. Due to both physiological and technical reasons, the structure of these errors is complex and their analysis is not straightforward. In this article, we describe some of the methods needed to obtain a description of the sensor error that is detailed enough for simulation.

METHODS

Data were provided by Abbott Diabetes Care and included two data sets collected by the FreeStyle Navigator(™) CGM: The first set consisted of 1032 time series of glucose readings from 136 patients with type 1 diabetes and parallel time series of reference blood glucose (BG) collected via self-monitoring at irregular intervals. The average duration of a time series was 5 days; the total number of sensor-reference data pairs was approximately 20,600. The second data set consisted of 56 time series of glucose readings from 28 patients with type 1 diabetes and a parallel time series of reference BG measured via the YSI 2300 Stat Plus(™) analyzer every 15 minutes. The average duration of a time series was 2 days; the total number of sensor-reference data pairs was approximately 7000.

RESULTS

THREE SETS OF RESULTS ARE DISCUSSED: analysis of sensor errors with respect to the BG rate of change, mathematical modeling of sensor error patterns and distribution, and computer simulation of sensor errors: SENSOR ERRORS DEPEND NONLINEARLY ON THE BG RATE OF CHANGE: Errors tend to be positive (high readings) when the BG rate of change is negative and negative (low readings) when the BG rate of change is positive, which is indicative of an underlying time delay. In addition, the sensor noise is non-white (non-Gaussian) and the consecutive sensor errors are highly interdependent.Thus, the modeling of sensor errors is based on a diffusion model of blood-to-interstitial glucose transport, which accounts for the time delay, and a time-series approach, which includes autoregressive moving average (ARMA) noise to account for the interdependence of consecutive sensor errors.Based on modeling, we have developed a computer simulator of sensor errors that includes both generic and sensor-specific error components. A χ(2) test showed that no significant difference exists between the observed and the simulated distribution of sensor errors and the distribution of errors of the FreeStyle Navigator (p > .46).

CONCLUSIONS

CGM accuracy was modeled via diffusion and additive ARMA noise, which allowed for designing a computer simulator of sensor errors. The simulator, a component of a larger simulation platform approved by the Food and Drug Administration in January 2008 for pre-clinical testing of closed-loop strategies, has been successfully applied to in silico testing of closed-loop control algorithms, resulting in an investigational device exemption for closed-loop trials at the University of Virginia.

Study of the effects of tissue reactions on the function of implanted glucose sensors

The relationship between tissue reactions to a subcutaneously implanted glucose sensor and the function of the sensor was evaluated over a period of 4-weeks using tubular, porous polyvinyl alcohol (PVA) sponges implanted subcutaneously in rats. The PVA sponges were used as scaffolds in which the foreign body response could develop. Coil-type glucose sensors were then placed in the center of the PVA sponges and tested on day 3, and weekly thereafter. In the first approach, the sensors were placed in the sponges still implanted in the rats and tested. In vivo glucose sensor sensitivity peaked at day 7 and steadily decreased until day 35. In the second approach, the sensors were placed in the explanted sponges and then tested. This test showed no sensor function after day 7, indicating that functional blood vessels are critical in maintaining any function whatsoever. In both cases the sensors themselves were never implanted to eliminate any potential in vivo degradation of the sensors that could have affected the outcome of this study. Sensors were then tested in absence of sponges and found to be working properly with no change from preimplantation sensitivity. Once sensor testing was concluded, the PVA sponge/tissue samples were prepared for quantitative histological analysis. It was determined that the increase in collagen deposition within the sponge correlated with the decrease in sensor sensitivity. It was also observed that natural angiogenesis (peak at day 14) did not overcome the barrier to glucose diffusion created by the fibrous capsule.

Nitric oxide-releasing/generating polymers for the development of implantable chemical sensors with enhanced biocompatibility

The development of reliable in vivo chemical sensors for real-time clinical monitoring of blood gases, electrolytes, glucose, etc. in critically ill and diabetic patients remains a great challenge owing to inherent biocompatibility problems that can cause errant analytical results upon sensor implantation (e.g., cell adhesion, thrombosis, inflammation). Nitric oxide (NO) is a well-known inhibitor of platelet activation and adhesion, and also a potent inhibitor of smooth muscle cell proliferation. In addition, NO mediates inflammatory response and promotes angiogenesis. Polymers that release or generate NO at their surfaces have been shown to exhibit greatly enhanced thromboresistance in vivo when in contact with flowing blood, as well as reduce inflammatory response when placed subcutaneously, and thus have the potential to improve the biocompatibility of implanted chemical sensors. Locally elevated NO levels at the surface of implanted devices can be achieved by using polymers that incorporate NO donor species that can decompose and release NO spontaneously when in contact with physiological fluids, or NO-generating polymers that possess an immobilized catalyst that decompose endogenous S-nitrosothiols to generate NO in situ. The potential use of such NO-releasing/generating materials for preparing in vivo sensors implanted either intravascularly or subcutaneously, is examined in this review.

Determination of human blood glucose levels using microchip electrophoresis

A high-performance monitoring system for human blood glucose levels was developed using microchip electrophoresis with a plastic chip. The combination of reductive amination as glucose labeling with fluorescent 2-aminoacridone (AMAC) and glucose-borate complex formation realized the highly selective detection of glucose even in a complex matrix such as a blood sample. The migration time of a single peak, observed on an electropherogram of AMAC-labeled plasma, closely resembled that of glucose standard solution. The treatment of plasma with hexokinase or glucokinase for glucose phosphorylation resulted in a peak shift from approximately 145 to 70 s, corresponding to glucose and glucose-6-phosphate, respectively. A double-logarithm plot revealed a linear relationship between glucose concentration and fluorescence intensity in the range of 1-300 microM of glucose (r(2) = 0.9963; p <0.01), and the detection limit was 0.92 microM. Furthermore, blood glucose concentrations estimated from the standard curves of three subjects were compared with results obtained by conventional colorimetric analysis using glucose dehydrogenase. Good correlation was observed between methods according to simple linear regression analysis (p <0.05). The reproducibility of the assay was about 6.3-9.1% (RSD) and the within-days and between-days reproducibility were 1.6-8.4 and 5.2-7.2%, respectively. This system enables us to determine blood glucose with high sensitivity and accuracy, and will be applicable to clinical diagnosis.

Use of new technologies for monitoring and treating diabetes in pregnancy

During the last century, there were two breakthroughs in diabetes management and monitoring that changed the course of treatment: the discovery of insulin and the progress in the understanding of glucose monitoring. As technology evolved, glucose monitoring and insulin administration can now be achieved in a continuous fashion. In this review, the authors focus on the utility of new technologies in the management and monitoring of diabetes in pregnancy.

An investigation of long-term performance of minimally invasive glucose biosensors

OBJECTIVE

The long-term performance stability of minimally invasive glucose biosensors was evaluated in vitro and in vivo.

METHODS

Coil-type glucose biosensors were constructed using an epoxy-polyurethane membrane. Seven sensors were continuously polarized for 12 weeks in a 5 mM glucose-phosphate-buffered saline (PBS) solution, and the sensor sensitivities were tested weekly. Glucose biosensors (n = 44) were also subcutaneously implanted in rats, and the in vivo sensitivities were determined for up to 4 weeks. Histological analysis was performed on the tissue surrounding the sensors.

RESULTS

During a period of 12 weeks, the normalized sensitivity (S/S(0)) of the sensors tested in vitro first increased from 1.10 +/- 0.13 (week 1) to 2.30 +/- 0.90 at week 6 and then decreased to 1.07 +/- 0.24 at week 12 (n = 7). After 6 weeks, the sensors showed a much more significant response to acetaminophen. With continuous polarization in 5 mM glucose-PBS, the sensor functioned for at least 3 months, or about a half of the observed lifetime of sensors stored in the solution with occasional sensitivity measurements (e.g., tested twice each month). For the 15 implanted sensors that lasted for at least 28 days, the average sensitivities values were 4.4 +/- 2.0 (S(0), in vitro), 3.5 +/- 1.3 (day 7, in vivo), 3.3 +/- 1.1 (day 14), 3.6 +/- 1.4 (day 21), and 2.9 +/- 2.2 nA/mM (day 28). Histological analysis showed that the implanted sensors were covered by a 200-800-mu-thick fibrous capsule after 1 week. Blood vessels were found in the fibrous tissue from day 7 through day 34. In addition, the background current that was observed during in vivo sensor testing could be successfully eliminated by using an enzyme-free sensor.

CONCLUSION

This study confirms that coil-type glucose biosensors based on an epoxy-polyurethane membrane can perform stably in vitro for months and in vivo for weeks.

Modeling of Calibration Effectiveness and Blood-to-Interstitial Glucose Dynamics as Potential Confounders of the Accuracy of Continuous Glucose Sensors during Hyperinsulinemic Clamp

BACKGROUND

Models of the dynamics of interstitial fluid-based continuous glucose sensors imply a variable sensor deviation from reference blood glucose (BG), depending on both sensor calibration procedure and BG dynamics. These effects could have a significant effect on the cross-interpretation of nonidentical accuracy studies.

METHODS

Hyperinsulinemic euglycemic and hypoglycemic clamps were performed on 39 subjects with type 1 diabetes wearing the Medtronic Continuous Glucose Monitoring System®. Sensor calibration and interstitial glucose (IG) dynamics were modeled and analyzed as potential confounders of sensor deviation from reference BG.

RESULTS

The mean absolute deviation (MAD) of sensor data was 20.9 mg/dl during euglycemia and 24.5 mg/dl during descent into and recovery from hypoglycemia. Computer-generated recalibration reduced MAD to 10.6 and 14.6 mg/dl, respectively. Modeling of IG dynamics reduced the MAD further to 10.0 and 10.4 mg/dl (using idiosyncratic parameters) or to 10.6 and 11.5 mg/dl (using model parameters common for all subjects), respectively.

CONCLUSIONS

The sensor MAD from reference is strongly influenced by the choice of calibration points. Thus, cross-experiment comparisons of sensor accuracy are likely to be heavily dependent on the employed calibration procedures. Demanding calibration points substantially differing in value was found to improve calibration effectiveness. Simulation using existing IG models and population parameters reduced the bias resulting from BG-IG dynamics.

In vitro, in vivo and post explantation testing of glucose-detecting biosensors: current methods and recommendations

To date, there have been a number of cases where glucose sensors have performed well over long periods of implantation; however, it remains difficult to predict whether a given sensor will perform reliably, will exhibit gradual degradation of performance, or will fail outright soon after implantation. Typically, the literature emphasizes the sensor that performed well, while only briefly (if at all) mentioning the failed devices. This leaves open the question of whether current sensor designs are adequate for the hostile in vivo environment, and whether these sensors have been assessed by the proper regimen of testing protocols. This paper reviews the current in vitro and in vivo testing procedures used to evaluate the functionality and biocompatibility of implantable glucose sensors. An overview of the standards and regulatory bodies that govern biomaterials and end product device testing precedes a discussion of up-to-date invasive and non-invasive technologies for diabetes management. Analysis of current in vitro, in vivo, and then post explantation testing is presented. Given the underlying assumption that the success of the sensor in vitro foreshadows the long-term reliability of the sensor in the human body, the relative merits of these testing methods are evaluated with respect to how representative they are of human models.

Layer-by-layer assembled semipermeable membrane for amperometric glucose sensors

BACKGROUND

The performance of implantable glucose sensors is closely related to the behavior of the outer membrane. Such membranes govern the diffusion characteristics of glucose and, correspondingly, the sensitivity of the sensors. This manuscript discusses the selection of various membrane materials and their effect on the device response.

METHODS

Sensors were fabricated utilizing a 50-microm platinum wire followed by immobilization of the glucose oxidase (GO(X)) enzyme. Sequential adsorption of various ionic species via a layer-by-layer process created devices coated with bilayers of humic acids/ferric cations (HAs/Fe(3+)), humic acids/poly(diallyldimethylammonium chloride) (HAs/PDDA), and poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDDA). The in vitro amperometric response of the sensors was determined at 0.7 V vs an Ag/AgCl reference electrode in phosphate-buffered saline (37 degrees C) for various glucose concentrations. The diffusion coefficients of glucose and hydrogen peroxide (H(2)O(2)) through these membranes were calculated and analyzed.

RESULTS

Outer membranes based on the sequential deposition of bilayers of HAs/Fe(3+), HAs/PDDA, and PSS/PDDA were grown successfully on immobilized layers of GO(X). The amperometric response and reversibility upon changing the in vitro concentration of glucose were investigated.

CONCLUSIONS

Through alteration of the number of bilayers of the outer membrane, it was possible to modulate the diffusion of glucose toward the sensor as a result of its flux-limiting characteristics. Semipermeable membranes based on five HAs/Fe(3+) bilayers exhibited a superior behavior with a minimum hysterisis response to glucose cycling and a lesser current saturation at hyperglycemic glucose concentrations because of a more balanced inward diffusion of glucose and outward diffusion of H(2)O(2).

Controlling acute inflammation with fast releasing dexamethasone-PLGA microsphere/pva hydrogel composites for implantable devices

BACKGROUND

Continuous release of dexamethasone from PLGA microsphere/PVA hydrogel composites has been shown to suppress the inflammatory tissue reaction in response to subcutaneously implanted foreign material for a period of one month. The scope of the present work is to investigate whether suppressing the initial acute inflammatory phase with fast releasing dexamethasone-PLGA microsphere/PVA composites (that release the drug over a period of one week) would prevent the development of a foreign body reaction in response to implantation in the subcutaneous tissue using a rat model.

METHODS

Dexamethasone loaded PLGA microspheres were prepared using the solvent evaporation method. In vitro release from microspheres was analyzed using USP apparatus 4 in phosphate buffered saline (PBS) at 37 degrees C. Composites were fabricated in 18G needles by freeze-thaw cycling the PVA/microsphere dispersion. The composites were implanted in the subcutaneous tissue of anesthetized rats. The pharmacodynamic effect was evaluated by histological examination of the tissue surrounding the composites at pre-determined time points.

RESULTS

In vitro release studies showed that most of the drug entrapped in the microspheres was released within one week. At days 3 and 8, these fast releasing dexamethasone containing composites suppressed the acute phase of inflammation but did not prevent the development of an inflammatory reaction after dexamethasone was completely released from the composites. By day 30, chronic inflammation and fibrosis were observed in the tissue surrounding the drug-containing composites. On days 3 and 8, the number of inflammatory cells in the vicinity of the dexamethasone containing composites was similar to that in normal tissue. However, the number of inflammatory cells was higher in drug-containing composites as compared to drug-free composites by day 30. This was due to the inflammation being in a more advanced stage in drug-free composites where a granulomatous reaction had already developed.

CONCLUSION

Fast release of dexamethasone from PLGA/PVA composites did not provide long-term protection against the foreign body reaction in response to implantation. It would appear that a sustained delivery of anti-inflammatory agents such as dexamethasone is necessary to suppress inflammation throughout the implant life-time.