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

Distributed sensor and actuator networks for closed-loop bioelectronic medicine

Designing implantable bioelectronic systems that continuously monitor physiological functions and simultaneously provide personalized therapeutic solutions for patients remains a persistent challenge across many applications ranging from neural systems to bioelectronic organs. Closed-loop systems typically consist of three functional blocks, namely, sensors, signal processors and actuators. An effective system, that can provide the necessary therapeutics, tailored to individual physiological factors requires a distributed network of sensors and actuators. While significant progress has been made, closed-loop systems still face many challenges before they can truly be considered as long-term solutions for many diseases. In this review, we consider three important criteria where materials play a critical role to enable implantable closed-loop systems: Specificity, Biocompatibility and Connectivity. We look at the progress made in each of these fields with respect to a specific application and outline the challenges in creating bioelectronic technologies for the future.

Blood-Compatible Surfaces with Phosphorylcholine-Based Polymers for Cardiovascular Medical Devices

For the acquisition of blood-compatible materials, various hydrophilic polymers for surface modification have been examined. Among them, polymers with a representative phospholipid polar group, the phosphorylcholine (PC) group, are a successful example. These polymers were designed from inspiration of the cell membrane surface and provide protein adsorption resistance even following contact with plasma. This important property is based on the unique hydration state of water molecules surrounding hydrated polymer; in other words, water molecules weakly interact with the polymers and maintain their favorable cluster structure through hydrogen bonding. These polymers are not only hydrophilic, but also electrically neutral, important characteristics which make hydrogen bonding with water molecules less likely to occur and avoid hydrophobic interactions. Phosphorylcholine groups and other zwitterionic structures are significant as hydrophilic functional groups meeting these important requirements. In this review, blood compatibility of a polymer having a PC group is introduced in relation to its hydration structure, followed by a description of the applications of this polymer to cardiovascular medical devices.

Basement Membrane-Based Glucose Sensor Coatings Enhance Continuous Glucose Monitoring in Vivo

BACKGROUND

Implantable glucose sensors demonstrate a rapid decline in function that is likely due to biofouling of the sensor. Previous efforts directed at overcoming this issue has generally focused on the use of synthetic polymer coatings, with little apparent effect in vivo, clearly a novel approach is required. We believe that the key to extending sensor life span in vivo is the development of biocompatible basement membrane (BM) based bio-hydrogels as coatings for glucose sensors.

METHOD

BM based bio-hydrogel sensor coatings were developed using purified BM preparations (ie, Cultrex from Trevigen Inc). Modified Abbott sensors were coated with Cultrex BM extracts. Sensor performance was evaluated for the impact of these coatings in vitro and in vivo in a continuous glucose monitoring (CGM) mouse model. In vivo sensor function was assessed over a 28-day time period expressed as mean absolute relative difference (MARD) values. Tissue reactivity of both Cultrex coated and uncoated glucose sensors was evaluated at 7, 14, 21 and 28 days post-sensor implantation with standard histological techniques.

RESULTS

The data demonstrate that Cultrex-based sensor coatings had no effect on glucose sensor function in vitro. In vivo glucose sensor performance was enhanced following BM coating as determined by MARD analysis, particularly in weeks 2 and 3. In vivo studies also demonstrated that Cultrex coatings significantly decreased sensor-induced tissue reactions at the sensor implantation sites.

CONCLUSION

Basement-membrane-based sensor coatings enhance glucose sensor function in vivo, by minimizing or preventing sensor-induced tissues reactions.

Preclinical Performance Evaluation of Percutaneous Glucose Biosensors: Experimental Considerations and Recommendations

The utility of continuous glucose monitoring devices remains limited by an obstinate foreign body response (FBR) that degrades the analytical performance of the in vivo sensor. A number of novel materials that resist or delay the FBR have been proposed as outer, tissue-contacting glucose sensor membranes as a strategy to improve sensor accuracy. Traditionally, researchers have examined the ability of a material to minimize the host response by assessing adsorbed cell morphology and tissue histology. However, these techniques do not adequately predict in vivo glucose sensor function, necessitating sensor performance evaluation in a relevant animal model prior to human testing. Herein, the effects of critical experimental parameters, including the animal model and data processing methods, on the reliability and usefulness of preclinical sensor performance data are considered.

Development of FRET biosensors for mammalian and plant systems

Genetically encoded biosensors are increasingly used in visualising signalling processes in different organisms. Sensors based on green fluorescent protein technology are providing a great opportunity for using Förster resonance energy transfer (FRET) as a tool that allows for monitoring dynamic processes in living cells. The development of these FRET biosensors requires careful selection of fluorophores, substrates and recognition domains. In this review, we will discuss recent developments, strategies to create and optimise FRET biosensors and applications of FRET-based biosensors for use in the two major eukaryotic kingdoms and elaborate on different methods for FRET detection.

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

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

A MEMS differential viscometric sensor for affinity glucose detection in continuous glucose monitoring

Micromachined viscometric affinity glucose sensors have been previously demonstrated using vibrational cantilever and diaphragm. These devices featured a single glucose detection module that determines glucose concentrations through viscosity changes of glucose-sensitive polymer solutions. However, fluctuations in temperature and other environmental parameters might potentially affect the stability and reliability of these devices, creating complexity in their applications in subcutaneously implanted continuous glucose monitoring (CGM). To address these issues, we present a MEMS differential sensor that can effectively reject environmental disturbances while allowing accurate glucose detection. The sensor consists of two magnetically driven vibrating diaphragms situated inside microchambers filled with a boronic-acid based glucose-sensing solution and a reference solution insensitive to glucose. Glucose concentrations can be accurately determined by characteristics of the diaphragm vibration through differential capacitive detection. Our in-vitro and preliminary in-vivo experimental data demonstrate the potential of this sensor for highly stable subcutaneous CGM applications.

Multiplexed microneedle-based biosensor array for characterization of metabolic acidosis

The development of a microneedle-based biosensor array for multiplexed in situ detection of exercise-induced metabolic acidosis, tumor microenvironment, and other variations in tissue chemistry is described. Simultaneous and selective amperometric detection of pH, glucose, and lactate over a range of physiologically relevant concentrations in complex media is demonstrated. Furthermore, materials modified with a cell-resistant (Lipidure(®)) coating were shown to inhibit macrophage adhesion; no signs of coating delamination were noted over a 48-h period.

New-generation diabetes management: glucose sensor-augmented insulin pump therapy

Diabetes is one of the most common chronic disorders with an increasing incidence worldwide. Technologic advances in the field of diabetes have provided new tools for clinicians to manage this challenging disease. For example, the development of continuous subcutaneous insulin infusion systems have allowed for refinement in the delivery of insulin, while continuous glucose monitors provide patients and clinicians with a better understanding of the minute to minute glucose variability, leading to the titration of insulin delivery based on this variability when applicable. Merging of these devices has resulted in sensor-augmented insulin pump therapy, which became a major building block upon which the artificial pancreas (closed-loop systems) can be developed. This article summarizes the evolution of sensor-augmented insulin pump therapy until present day and its future applications in new-generation diabetes management.

Continuous intra-arterial blood pH monitoring in rabbits with acid-base disorders

The acid-base balance of arterial blood is important for the clinical management of seriously ill patients, especially patients with acute lung injury or acute respiratory distress syndrome. We developed a novel fluorosensor for continuous blood pH monitoring and evaluated its performance both in vitro and in vivo in rabbits with acid-base disorders. The pH sensor is made of N-allyl-4-piperazinyl-1, 8-napthalimide and 2-hydroxyethyl methacrylate, which were bonded at the distal end of the optical fiber. The fluorescence intensity increased as the pH decreased with good reproducibility, selectivity and linearity in the pH range of 6-8. The pH measurement precision was 0.03 ± 0.03 pH units with a bias of -0.02 ± 0.04 (n = 105) and -0.00 ± 0.05 pH units (n=189) in rabbits with metabolic and respiratory acid-base orders, respectively. The optical pH sensor can accurately measure pH fluctuations with a fast response and is a promising candidate for continuous in-line measurements of blood pH in critical care patients.

Continuous glucose monitoring by means of mid-infrared transmission laser spectroscopy in vitro

The continuous surveillance of glucose concentration reduces short-term risks and long-term complications for people with diabetes mellitus, a disorder of glucose metabolism. As a first step towards the continuous monitoring of glucose, reagent-free transmission spectroscopy in the mid-infrared region has been carried out in vitro using a quantum cascade laser and an optical silver halide fiber. A 30 μm gap in the fiber allowed for transmission spectroscopy of aqueous glucose solutions at a wavelength of 9.69 μm, which is specific to a molecular vibration of glucose. A noise-equivalent concentration as low as 4 mg/dL was achieved at an average power of 1.8 mW and an integration time of 50 s. This is among the most precise of glucose measurements using mid-infrared spectroscopy. Even with the very low average laser power of 0.07 mW the sensitivity of previous results (using a fiber optical evanescent field analysis) has been improved upon by almost one order of magnitude. Finally, the impact of potentially interfering substances such as other carbohydrates was analyzed.

Effect of subcutaneous glucose sensor implantation on skin mRNA expression in pigs

BACKGROUND

Tight glycemic control has the potential to reduce long- and short-term effects of diabetes mellitus. New and improved glucose sensors for short-term implantation in the subcutis offer an alternative to the classical self-monitored blood glucose concentration in the management of diabetes. The use of glucose sensors has an advantage over the latter due to its capacity to obtain continuous glucose measurements. However, instability of in vivo glucose sensor measurements has been reported, and this bioinstability is likely to be influenced by the inflammatory reaction to the implanted sensor. Gene expression analysis is now performed in an existing porcine model of subcutaneous glucose sensor implantation to investigate the time course of inflammation from a new perspective.

METHODS

Tissue surrounding glucose sensors was sampled to different time points (2 h, 24 h, 3 days, and 7 days) after implantation in the subcutis of pigs. From the tissue RNA was extracted, cDNA was synthesized, and real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed for the quantification of immunoregulatory genes.

RESULTS

Genes coding for adhesion molecules, chemokines, cytokines, CD markers, and antigen presentation molecules were differentially expressed over time. Most of the investigated genes were significantly up-regulated 24 h and 7 days after implantation.

CONCLUSIONS

The present study demonstrated that the technology for targeted multiple-gene expression by real-time RT-PCR is useful in the evaluation of the immune response to subcutaneously implanted glucose sensors and that the expression levels also seemed to correspond to the histomorphological observations over time.

A tale of two compartments: interstitial versus blood glucose monitoring

Self-monitoring of blood glucose was described as one of the most important advancements in diabetes management since the invention of insulin in 1920. Recent advances in glucose sensor technology for measuring interstitial glucose concentrations have challenged the dominance of glucose meters in diabetes management, while raising questions about the relationships between interstitial and blood glucose levels. This article will review the differences between interstitial and blood glucose and some of the challenges in measuring interstitial glucose levels accurately.

A review of the foreign-body response to subcutaneously-implanted devices: the role of macrophages and cytokines in biofouling and fibrosis

The biological response to implanted biomaterials in mammals is a complex series of events that involves many biochemical pathways. Shortly after implantation, fibrinogen and other proteins bind to the device surface, a process known as biofouling. Macrophages then bind to receptors on the proteins, join into multinucleated giant cells, and release transforming growth factor beta and other inflammatory cytokines. In response to these signals, quiescent fibroblasts are transformed into myofibroblasts, which synthesize procollagen via activation of Smad mediators. The procollagen becomes crosslinked after secretion into the extracellular space. Mature crosslinked collagen and other extracellular matrix proteins gradually contribute to formation of a hypocellular dense fibrous capsule that becomes impermeable or hypopermeable to many compounds. Porous substrates and angiogenic growth factors can stimulate formation of microvessels, which to some extent can maintain analyte delivery to implanted sensors. However, stimulation by vascular endothelial growth factor alone may lead to formation of leaky, thin-walled, immature vessels. Other growth factors are most probably needed to act upon these immature structures to create more robust vessels.During implantation of foreign bodies, the foreign-body response is difficult to overcome, and thousands of biomaterials have been tested. Biomimicry (i.e., creating membranes whose chemical structure mimics natural cellular compounds) may diminish the response, but as of this writing, it has not been possible to create a stealth material that circumvents the ability of the mammalian surveillance systems to distinguish foreign from self.

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

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

Tissue response to subcutaneous implantation of glucose-oxidase-based glucose sensors in rats

Considerable progress in improved control of disturbed glucose metabolism can be expected by continuous glucose monitoring. The aim of the study was to evaluate in male Sprague-Dawley rats tissue response to implantation of a new amperometric glucose-oxidase-based glucose sensor (NTS) compared to a commercially available sensor system CGMS of MiniMed. Both sensors were tested under working conditions over a period of 3 days. Using NTS, glucose in interstitial fluid reflected glucose in arterial blood even in rapidly changing hyper- and hypoglycaemia whereas the CGMS did not detect the experimentally induced glucose changes adequately. Gene expression profiling was performed using Affymetrix chips. Acute phase response to injury by sensor application for a short time is indicated by down regulation of the increase in mRNA of proteases e.g. metallothionein-1alpha and matrix metalloprotease-3 at day 3. Improvement of anabolic situation is suggested by decrease in mRNA of insulin-like growth factor binding protein whereas increase of heme oxygenase and hypoxia-inducible factor may be related to defense mechanisms. Changes of mRNA together with slight fibrous capsule formation suggest good histocompatibility. Comparability of the patterns of changed mRNA in tissue surrounding SCGM with and without operating voltage as shown in dendrogram indicates no contribution of hydrogen peroxide to worsening biocompatibility. Beside established histological investigations of foreign body reaction weeks or months after implantation, gene expression profiling provides additional information to biocompatibility already early after implantation.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Continuous glucose monitoring in interstitial fluid using glucose oxidase-based sensor compared to established blood glucose measurement in rats

Glucose monitoring is of importance for success of complex therapeutic interventions in diabetic patients. Its impact on treatment and glycemic control is demonstrated in large clinical trials. Up to eight blood glucose measurements per day are recommended. Notwithstanding, a substantial number of diabetic patients cannot or will not monitor their blood glucose appropriately. Considerable progress in control of disturbed metabolism in diabetic patients can be expected by continuous glucose monitoring. The aim of the study was to evaluate the performance of a new amperometric glucose oxidase-based glucose sensor in vitro and in vivo after subcutaneous implantation into rats. For in vitro testing current output of sensors was measured by exposure to increasing and decreasing glucose concentrations up to 472 mg dL(-1) over a time period of 7 days. After subcutaneous implantation of sensors into interscapular region of male rats glucose in interstitial fluid was evaluated and compared to glucose in arterial blood up to 7 days. Hyper- and hypoglycaemia were induced by intravenous application of glucose and insulin, respectively. Current of each implanted sensor was converted into glucose concentration using the first blood glucose measurement only. A change of current with glucose of 0.35 nA mg(-1)dL(-1) indicates high sensitivity of the sensor in vitro. The response time (90% of steady state) was calculated by approximately 60s. Test strips for blood glucose measurement as reference for sensor readings was found as an appropriate and rapidly available method in rats by comparison with established hexokinase method in an automated lab analyzer with limits of agreement of +32.8 and -25.7 mg dL(-1) in Bland-Altman analysis. In normo- and hypoglycaemic range sensor readings in interstitial fluid correlated well with blood glucose measurements whereas hyperglycaemia was not reflected by the sensor completely when blood glucose was changing rapidly. The data given characterize a sensor with high sensitivity, long term stability and short response time. A single calibration of the sensor is required only in measurement periods up to 7 days. The findings demonstrate that the sensor is a highly promising candidate for assessment in humans.

Biocompatibility of electrochemical glucose sensors implanted in the subcutis of pigs

BACKGROUND

Lowering blood glucose concentration slows or prevents the development of complications in diabetes. One of the tools to control glucose levels is continuous glucose measurements. A promising technique involves measurements from glucose sensors implanted directly in skin/subcutis. However, in vivo bioinstability and drift in sensor signals have been reported after implantation, suggestively caused by the infiltration of inflammatory cells and adhesion of proteins to sensor membranes. The aim of this study was to evaluate the in vivo biocompatibility of two electrochemical glucose sensors after implantation in the skin of pigs.

METHODS

In vivo biocompatibility of in-house fabricated electrochemical glucose sensors and a commercially available continuous glucose monitoring system (CGMS, Medtronic MiniMed, Northridge, CA) implanted 1 h, 2 h, 24 h, 3 days, or 7 days was examined by histological and immunohistochemical techniques.

RESULTS

The extent of inflammation increased significantly as a function of time. The inflammation ranged from an acute focal fibrinous/suppurative dermatitis to a chronic fibrinous and granulating foreign body dermatitis 7 days after implantation. Immunohistochemical stainings showed that heterophilic granulocytes, macrophages, and fibrinogen/fibrinogen fragments D and E were consistent findings. Infiltration of CD3epsilon-positive T-cells was primarily confined to day 7 of implantation. In addition, the pro-inflammatory cytokines interleukin-1 and tumor necrosis factor-alpha played a role in the reaction to sensors.

CONCLUSION

The reported in vivo bioinstability of sensors is likely to be caused by protein and cellular biofouling on the sensor membrane. Furthermore, the consistent finding of fibrinogen and fibrinogen fragments D and E at the sensor-tissue interface seems to play an important role in the pathogenesis as it possibly maintains the inflammation by promoting the recruitment of inflammatory cells to the implantation site.

Continuous glucose monitoring by means of fiber-based, mid-infrared laser spectroscopy

The application of mid-infrared laser spectroscopy to the reagent-free quantification of the concentration of glucose was investigated using cryogenically cooled lead salt lasers or, alternatively, quantum cascade lasers operating at room temperature. The concentration of glucose in aqueous solutions was quantified by means of fiber-based attenuated total reflection (ATR) spectroscopy (fiber-optical evanescent field analysis, FEFA) as well as fiber-based transmission spectroscopy. Both methods have the potential to be utilized by small fiber sensors, which can be inserted into the subcutaneous tissue in order to continuously measure the local concentration of glucose. In our in vitro experiments, noise-equivalent concentrations as low as 10 mg/dL were achieved. The mid-term stability of the measurement schemes was investigated by means of Allan variance analysis. Based on the research presented in this manuscript, an all-room-temperature measurement scheme using quantum cascade lasers, miniaturized fiber-optic sensors, and pyroelectric detectors appears well suited for the continuous monitoring of glucose concentrations at physiological levels.