With the rising prevalence of diabetes, effective means of successful management of blood glucose levels are increasingly important. To improve on the ease of measurements, new technology is being developed to enable less invasive measurements. Some recent efforts have focused on the development of optical microscale glucose sensing systems based on the encapsulation of glucose oxidase within microspheres coated with polyelectrolyte multilayer nanofilms. In such sensors, a phosphorescent oxygen indicator is also co-encapsulated with the enzyme inside so that when glucose is present, glucose oxidase within the sensor reduces the local oxygen levels, causing a corresponding change in the luminescence intensity of the sensors. To test the aforementioned factors, a two-substrate, 2D FEM model of microscale optical glucose sensors in the dermis was developed. The model was used to predict the response time and sensitivity of glucose sensors with varying number and spacing of particles distributed in the dermis and varying physiological characteristics of the surrounding tissue; specifically, capillary density, blood vessel location relative to sensor, and glucose and oxygen consumption in tissue. Simulations were conducted to determine the magnitude of the change in the response time of sensors. Because the steady-state oxygen concentration within the sensors for a given blood glucose level determines the signal output, steady-state concentration of oxygen within sensors and the surrounding tissue for the entire physiological glucose range was evaluated. The utility of the model to predict the performance and efficacy of the sensors in the event of a host response to the foreign body implant was also evaluated. Simulations were performed to evaluate changes in sensor response and sensitivity in the occurrence of inflammation and progression of fibrous encapsulation of various thickness and density. The results from these simulations have provided knowledge on the impact of physiological factors that can potentially degrade sensor function in vivo. Our results indicate that upon the occurrence of a host response, sensitivity is reduced while range is extended. Furthermore, using the model we have been able to determine which conditions in vivo improve response time, sensitivity, and the linear response range for these sensors.

<p>As an increasingly prevalent chronic disease, diabetes represents one of the fastest growing health burdens to both the developed and developing world. In an effort to improve the management and treatment of diabetes, implantable sensors that continuously monitor glucose levels have become popular alternatives to patient-administered finger prick measurements of blood glucose. However, following implantation, the performance of these implants suffers from inaccurate and erratic readings that compromise their useful lives. As a result, implantable glucose sensors remain limited as a platform for the reliable management of diabetes. While the interaction between the sensor and its surrounding tissue has been posited as a culprit for erroneous in vivo sensor performance, there remains little evidence to support that theory.</p><p>This dissertation describes the effects that implant-associated tissue reactions have on implantable sensor function. Since tissue response to an implant changes over time, the overall effect of these tissue reactions is broken into two temporal phases: (1) the phase of weeks to months following implantation when a mature foreign body capsule is present around the sensor and (2) the phase of days to weeks immediately following sensor implantation when a provisional matrix of proteins and inflammatory cells envelops the sensor.</p><p>Late stage sensor responses to implantation are marked by both an attenuated sensor signal and a significant time lag relative to blood glucose readings. For this later stage of sensor response, a computational model of glucose transport through the interstitial space and foreign body capsule was derived and implemented. Utilizing physiologically relevant parameters, the model was used to mechanistically study how each constituent part of the capsular tissue could affect sensor response with respect to signal attenuation and lag. Each parameter was then analyzed using logarithmic sensitivity analysis to study the effects of different transport variables on both lag and attenuation. Results identified capsule thickness as the strongest determinant of sensor time lag, while subcutaneous vessel density and capsule porosity had the largest effects on attenuation of the sensor signal.</p><p>For the phase of early stage tissue response, human whole blood was used as a simple ex vivo experimental system. The impacts of protein accumulation at the sensor surface (biofouling effects) and cellular consumption of glucose in both the biofouling layer and in the bulk (metabolic effects) on sensor response were assessed. Medtronic Minimed SofSensor glucose sensors were incubated in whole blood, plasma diluted whole blood, and cell-free platelet poor plasma (PPP) to analyze the effects of different blood constituents on sensor function. Experimental conditions were then simulated using MATLAB to predict the relative impacts of biofouling and metabolic effects on the observed sensor responses. It was found that the physical barrier to glucose transport presented by protein biofouling did not hinder glucose movement to the sensor surface. Instead, glucose consumption by inflammatory cells was identified as the major culprit for generating poor sensor performance immediately following implantation.</p><p>Lastly, a novel, biomimetic construct was designed to mimic the in vivo 3D cellular setting around the sensor for the focused in vitro investigation of early stage effects of implantation on glucose sensor performance. Results with this construct demonstrate similar trends in sensor signal decline to the ex vivo cases described above, suggesting this construct could be used as an in vitro platform for assessing implantable glucose sensor performance.</p><p>In total, it may be concluded from this dissertation that instead of sensors "failing" in vivo, as is often reported, that different physiological factors mediate long term sensor function by altering the environment around the implant. For times immediately following implantation, sensor signals are mediated by the presence of inflammatory macrophages adhered on the surface. However, at longer times post-implantation, sensor signals are mediated not by the consumptive capacity of macrophages, but instead by the subcutaneous vessel density surrounding the sensor as well as the porosity and thickness of the foreign body capsule itself. Taken in concert, the results of this dissertation provide a temporal framework for outlining the effects of tissue response on sensor performance, hopefully informing more biocompatible glucose sensor designs in the future.</p> / Dissertation

The electrochemical enzymatic electrodes dominate the world market for blood glucose monitoring devices for controlling, as well as reducing the detrimental effects of diabetes. However, the enzymatic electrodes exhibit constraints restricting their reliance on the enzyme’s activity which can be influenced by the external, and the environmental factors such as temperature, pH, and humidity etc. However, the greater thickness of the enzyme layer hinders the performance of the glucose biosensors resulting in signal dampening or loss. In addition, the selectivity of the electrodes is affected by the interferents present in blood. Moreover, the invasive nature of the electrodes is a major problem considering the patient’s perspective. In contrast, recent research activities demonstrated that the electrochemical non-enzymatic electrodes possess huge potential for inexpensive and highly sensitive glucose monitoring devices, yet these electrodes are invasive in nature. Therefore, the purpose of this research was to fabricate electrochemical non-enzymatic non-invasive electrodes for sweat glucose monitoring devices. A very simple low-cost fabrication technique has been shown to make the facile, flexible, and inexpensive electrodes to detect sugar in sweat bio-analyte for a non-invasive glucose monitoring system using the native stable Cu oxides (CuNOx), Cu2O, layers grown on 35 µm thin Cu foils keeping under ambient conditions (25℃- and 760-mm Hg) for more than 2 years so that the oxide layers are full-grown, and fully stable. Moreover, the foils also annealed at various temperatures such as 160, 230, and 280℃ with new temperature profile for reducing the required time of growing stable oxides and producing oxides with larger crystallized structures with higher surface – to – volume ratio. The X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) results supported that at 280℃ annealing temperature the surface, mostly, transformed into highly electrocatalytic CuO with larger grain sizes, crystallized structures, and the uniform layer of ~ 140 nm. The electrochemical characterization, and sensing performance of the electrodes have been done by cyclic voltammetry (CV), one of the excellent and well accepted electrochemical methods, with the 3 – electrode configuration of the potentiostat. The CuNOx sensors of having ~10 nm layer of stable Cu2O exhibited a sensitivity of 603.42 μA mM−1 cm−2, a linear range beyond the desired limit of 7.00 mM with excellent linearity (R2 = 0.9983) and a low limit of detection of 94.21 μM. In contrast, the new annealing profile has. the CuNOx sensors annealed at 280 ºC using new temperature profile provided twin calibration curves of linear ranges of 0.05 – 1.00 mM and 1.00 – 7.00 mM, that applicable for sweat and blood glucose sensing, respectively, and exhibited a sensitivity of 1795 μA mM−1 cm−2, a linear range up to the desired limit of 1.00 mM for sweat glucose sensing with excellent linearity (R2 = 0.9844), and a lower limit of detection of 135.39 μM. In addition, it has been shown that the peak electro-oxidation current of glucose sensing is linearly related with the squire root of the annealing temperature, √T. This can help to figure out the required applied annealing temperature for getting desired peak electro-oxidation current of glucose in a human health monitoring system. / Dissertation / Doctor of Philosophy (PhD)

<p>Inflammation and the formation of an avascular fibrous capsule have been identified as the key factors controlling the wound healing associated failure of implantable glucose sensors. Our aim is to guide advantageous tissue remodeling around implanted sensor leads by the temporal release of dexamethasone (Dex), a potent anti-inflammatory agent, in combination with the presentation of a stable textured surface. </p><p>First, Dex-releasing polyurethane porous coatings of controlled pore size and thickness were fabricated using salt-leaching/gas-foaming technique. Porosity, pore size, thickness, drug release kinetics, drug loading amount, and drug bioactivity were evaluated. In vitro sensor functionality test were performed to determine if Dex-releasing porous coatings interfered with sensor performance (increased signal attenuation and/or response times) compared to bare sensors. Drug release from coatings monitored over two weeks presented an initial fast release followed by a slower release. Total release from coatings was highly dependent on initial drug loading amount. Functional in vitro testing of glucose sensors deployed with porous coatings against glucose standards demonstrated that highly porous coatings minimally affected signal strength and response rate. Bioactivity of the released drug was determined by monitoring Dex-mediated, dose-dependent apoptosis of human peripheral blood derived monocytes in culture. </p><p>The tissue modifying effects of Dex-releasing porous coatings were accessed by fully implanting Tygon® tubing in the subcutaneous space of healthy and diabetic rats. Based on encouraging results from these studies, we deployed Dex-releasing porous coatings from the tips of functional sensors in both diabetic and healthy rats. We evaluated if the tissue modifying effects translated into accurate, maintainable and reliable sensor signals in the long-term. Sensor functionality was accessed by continuously monitoring glucose levels and performing acute glucose challenges at specified time points. </p><p>Sensors treated with porous Dex-releasing coatings showed diminished inflammation and enhanced vascularization of the tissue surrounding the implants in healthy rats. Functional sensors with Dex-releasing porous coatings showed enhanced sensor sensitivity over a 21-day period when compared to controls. Enhanced sensor sensitivity was accompanied with an increase in sensor signal lag and MARD score. These results indicated that Dex-loaded porous coatings were able to elicit a favorable tissue response, and that such tissue microenvironment could be conducive towards extending the performance window of glucose sensors in vivo.</p><p>The diabetic pilot animal study showed differences in wound healing patters between healthy and diabetic subjects. Diabetic rats showed lower levels of inflammation and vascularization of the tissue surrounding implants when compared to their healthy counterparts. Also, functional sensors treated with Dex-releasing porous coatings did not show enhanced sensor sensitivity over a 21-day period. Moreover, increased in sensor signal lag and MARD scores were present in porous coated sensors regardless of Dex-loading when compared to bare implants. These results suggest that the altered wound healing patterns presented in diabetic tissues may lead to premature sensor failure when compared to sensors implanted in healthy rats.</p> / Dissertation

The boronic acid functional group is known to bind compounds with the diol group tightly and reversibly in aqueous environment and has been used as a recognition moiety for the design of carbohydrate sensors. The first chapter of the dissertation studies the synthesis and substitution effect on the affinity and selectivity of a known boronic acid-based glucose sensor. In such a sensor design effort, the availability of a signaling event, whether it is fluorescence or UV, is crucial. The second chapter studies the detailed mechanism on how a well-known fluorescent boronic acid compound changes fluorescent properties upon binding. A new mechanism has been established which corrected a decade old mistake. In the third chapter, a series of boronic acid-based sensors were designed and synthesized for sialic acid, which is part of tetrasaccharide found on many cell surface carbohydrates. Such sialic acid sensors could be very useful for the development of new type of anti-influenza therapy. The fourth is on the design and synthesis novel and selective inhibitors for phosphodiesterase 4 (PDE4), which are potential anti-asthma agents.

No description available.

This PhD research relates to the design, fabrication, characterization, and optimization of on-chip electrochemical microelectrode arrays (MEAs) for measurement of transient concentrations and gradients, focusing on fast transients and physiological applications. In particular, this work presents the determination of kinetic mechanisms taking place at an active interface (either physiological or non-physiological) in contact with a liquid phase using the MEA device to simultaneously estimate the concentration and gradient of the analyte of interest at the surface of the active interface. The design approach of the MEA device and the corresponding measurement methodology to acquire reliable concentration information is discussed. The ability of the MEA device to measure fast (i.e., in sub-second time scale) transient gradients is demonstrated experimentally using a controllable diffusion-reaction system which mimics the consumption of hydrogen peroxide by a 2D cell culture. The proposed MEA device and measurement methodology meet effectively most of the requirements for physiological applications and as a demonstration of this, two physiological applications are presented. In one application, the MEA device was tailored to measure the hydrogen peroxide uptake rate of human astrocytes and glioblastoma multiforme cells in 2D cell culture as a function of hydrogen peroxide concentration at the cell surface; the results allowed to quantitatively determine the uptake kinetics mechanisms which are well-described by linear and Michaelis-Menten expressions, in agreement with the literature. In the other application, further customization of the MEA device was realized to study the glucose uptake kinetics of human bronchial epithelial and small cell lung cancer cells, these latter with and without DDX5 gene knockdown; the results allowed to distinguish mechanistic differences in the glucose uptake kinetics among the three cell lines. These results were complemented with measurements of glycolytic and respiration rates to obtain a bigger picture of the glucose metabolism of the three cell lines. Finally, additional applications, both physiological and non-physiological, are proposed for the developed MEA device.