Diabetes is a chronic metabolic disorder whereby the body loses its ability to maintain normal glucose levels. Despite of development of implantable glucose sensors in long periods, none of the biosensors are capable of continuously monitoring glucose levels during long-term implantation reliably. Progressive loss of sensor function occurs due in part to biofouling and to the consequences of a foreign body response such as inflammation, fibrosis, and loss of vasculature. In order to improve the function and lifetime of implantable glucose sensors, a new 3D porous and bio-stable collagen scaffold has been developed to improve the biocompatibility of implantable glucose sensors. The novel collagen scaffold was crosslinked using nordihydroguaiaretic acid (NDGA) to enhance biostability. NDGA-treated collagen scaffolds were stable without any physical deformation in the subcutaneous tissue of rats for 4 weeks. The scaffold application does not impair the function of our sensor. The effect of the scaffolds on sensor function and biocompatibility was examined during long-term in vitro and in vivo experiments and compared with control bare sensors. The sensitivity of the short sensors was greater than the sensitivity of long sensors presumably due to less micro-motions in the sub-cutis of the rats. The NDGA-crosslinked scaffolds induced much less inflammation and retained their physical structure in contrast to the glutaraldehyde (GA)-crosslinked scaffolds. We also have developed a new dexamethasone (Dex, anti-inflammatory drug)-loaded poly(lactic-co-glycolic acid) (PLGA) microspheres/porous collagen scaffold composite for implantable glucose sensors. The composite system showed a much slower and sustained drug release than the standard microspheres. The composite system was also shown to not significantly affect the function of the sensors. The sensitivity of the sensors with the composite system in vivo remained higher than for sensors without the composites (no scaffold, scaffold without microspheres). Histology showed that the inflammatory response to the Dex-loaded composite was much lower than for the control scaffold. The Dex-loaded composite system might be useful to reduce inflammation to glucose sensors and therefore extend their function and lifetime.

Recent biosensors using glucose oxidase enzyme to detect glucose (“blood sugar”) were made with intrinsic conducting polymers such as poly pyrrole (PPY) to mediate the reaction. PPY coated electrodes were difficult to employ via eletropolymerization because PPY is only soluble in solvents potentially damaging to enzymes. Nano crystalline cellulose – poly pyrrole (NCC-PPY) colloid circumvents this by forming natural, enzyme compatible, and hydrophilic films mechanically bound to electrodes using easy-to-disperse colloids. NCC-PPY was studied as mediator to investigate use in biosensor applications. Using NCC-PPY film casted on microfabricated interdigitated electrodes, a glucose biosensor with sensitivity factor of 20 was achieved. NCC-PPY showed enhanced catalysis with no enzyme inactivation and a total current of 2mA. Enhanced sensitivity was attributed to resistance changes of doped PPY, redox mediation, and compatibility of cellulose with enzyme.

Recent biosensors using glucose oxidase enzyme to detect glucose (“blood sugar”) were made with intrinsic conducting polymers such as poly pyrrole (PPY) to mediate the reaction. PPY coated electrodes were difficult to employ via eletropolymerization because PPY is only soluble in solvents potentially damaging to enzymes. Nano crystalline cellulose – poly pyrrole (NCC-PPY) colloid circumvents this by forming natural, enzyme compatible, and hydrophilic films mechanically bound to electrodes using easy-to-disperse colloids. NCC-PPY was studied as mediator to investigate use in biosensor applications. Using NCC-PPY film casted on microfabricated interdigitated electrodes, a glucose biosensor with sensitivity factor of 20 was achieved. NCC-PPY showed enhanced catalysis with no enzyme inactivation and a total current of 2mA. Enhanced sensitivity was attributed to resistance changes of doped PPY, redox mediation, and compatibility of cellulose with enzyme.

<p>Efforts to create a reliable, long–term implantable glucose sensor have been stymied by the effects of the foreign body response and wound healing that introduce delayed response times as well as unpredictable sensor performance. Loss of vascularization from fibrotic encapsulation around implanted sensors is purported as a key contributor to sensor failure, as glucose and oxygen transport to the sensor becomes impeded. Improving sensor performance by increasing angiogenesis and/or reducing capsule thickness using tissue-modifying textured coatings is attractive because texturing is not dependent upon a depletable drug reservoir. A significant range of materials and pore sizes are capable of promoting angiogenesis and reducing capsule thickness, provided pores have open-architecture with dimensions sufficiently large enough to allow inflammatory cell infiltration. </p><p><br></p><p>Poly–L–lactic acid was gas foamed/salt leached with ammonium bicarbonate to produce porous coatings for Medtronic MiniMed SOF–sensor glucose sensors. Coating properties included 30μm pore diameters, 90% porosity, and 50μm wall thickness. Cytotoxicity, degradation, and sensor response time studies were performed to ensure the porous coatings were non–toxic and negligibly retarded glucose diffusion prior to <italic>in vivo</italic> testing. Histology was used to evaluate angiogenesis and collagen deposition adjacent to porous coated and bare (i.e. smooth, uncoated) non–functional sensor strips after three weeks in the rat dorsal subcutis. Functional Medtronic glucose sensors, with and without porous coatings, were percutaneously implanted in the rat dorsum to assess if the angiogenic–inducing properties observed around the non–functional porous coated sensor strips translated into stable, non–attenuated sensor signals over two and three weeks. MiniLink<super>TM </super>transmitters were attached to the rats, permitting continuous glucose monitoring. Vessel counts and collagen deposition adjacent to sensors were determined from histological analysis. A one–sided dorsal window model was developed to further evaluate the interplay between vascularization and sensor performance Sensors were inserted beneath the windows, allowing visualization of microvascular changes adjacent to sensor surfaces, with simultaneous evaluation of how vascular changes impacted interstitial glucose monitoring. </p><p><br></p><p>Porous coating did have angiogenic–inducing effects on the surrounding tissue. When fully implanted in the rat dorsum, sensor strips with porous coatings induced three–fold more vessels within 100μm<super>2</super> of the sensor strip surface after three weeks and two-fold more cumulative vessel lengths within 1mm<super>2</super> after two weeks, compared to bare surfaces. In contrast, when percutaneously implanted in the rat dorsum, porous coated and bare sensors were equally highly vascularized, with two–fold more vessels than fully implanted bare sensors. </p><p><br></p><p>Despite increased angiogenesis adjacent to percutaneous sensors, sensor signal attenuation occurred over 14 days, suggesting that angiogenesis plays a secondary role in maintaining sensor function. Percutaneously implanted porous coated sensors had greater reductions in baseline current (20 to 50+%) over two weeks than bare sensors (10 to 30%). Mechanical stresses imposed by percutaneous tethering may override the beneficial effects of porous coatings. Furthermore, integration of the porous coating with the surrounding tissue may have increased tissue tearing at the porous coating–tissue, increasing inflammation and collagen deposition resulting in greater signal attenuation compared with bare sensors. Future investigations of the role mechanical irritation has on wound healing around percutaneous glucose sensors are warranted.</p> / Dissertation

Luminescent microspheres encapsulating glucose oxidase and an oxygensensitive lumophore have recently been reported as potential implantable sensors for in vivo glucose monitoring. However, there are two main issues that must be addressed for enzymatic systems such as these to realize the goal of minimally-invasive glucose monitoring. The first issue is related to the short response range of such sensors, less than 200 mg/dL, which must be extended to cover the full physiological range (0-600 mg/dL) of glucose possible for diabetics. The second issue is concerning the short operating lifetime of these systems due to enzyme degradation (less than 7 days). Two approaches were considered for increasing the range of the sensor response; nanofilm coatings and particle porosity. In the first approach, microparticle sensors were coated with layer-by-layer deposited thin nanofilms to increase the response range. It was observed that, a precise control on the response range of such sensors can be achieved by manipulating different characteristics (e.g., thickness, deposition condition, and the outermost capping layer) of the nanofilms. However, even with 15 bilayers of poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) nanofilm, limited range was achieved (less than 200 mg/dL). By performing extrapolation on the data obtained for the experimentally-determined response range versus the number of PAH/PSS bilayers, it was predicted that a nanofilm coating comprising of more than 60 PAH/PSS bilayers will be needed to achieve a linear response up to 600 mg/dL. Using modeling, it was realized that a more effective method for achieving a linear response up to 600 mg/dL is to employ microparticles with higher porosity. Sensors were prepared from highly porous silica microparticles (diameter = 7 mu m, porosity = 0.6) and their experimental response was determined. Not surprisingly, the experimentally determined response range of such sensors was found to be higher than 600 mg/dL. To improve the longevity of these sensors, two approaches were employed; incorporation of catalase and increasing the loading of glucose oxidase. Catalase was incorporated into microparticles, which protects the enzyme from peroxide-mediated deactivation, and thus improves the stability of such sensors. Sensors incorporating catalase were found to ~5 times more stable than the GOx-only sensors. It was theoretically predicted, that by maximizing the loading of glucose oxidase within the microparticles, the longevity of such sensors can be substantially improved. Based on this understanding, sensors were fabricated using highly porous microparticles; response range did not vary even after one month of continuous operation under normal physiological conditions. Modeling predicts that 1 mM of glucose oxidase and 1 mM of catalase would extend the operating lifetime to more than 90 days.

A continuous glucose sensor employing radio frequency (RF) signals is presented using the biocompatible material Silicon Carbide (SiC). Unlike biosensors that require direct contact with interstitial fluids to trigger chemical reactions to operate, this biocompatible SiC sensor does not require a direct interface. The sensing mechanism for this SiC sensor is based upon a shift in resonant frequency, as a function of change in glucose levels, which electrically manifests itself as a change in blood permittivity and conductivity. For in vivo applications the antenna sensor needs to operate inside the body environment, and it has been found that the best operational location of this biocompatible SiC sensor is within fatty tissue in close proximity to blood vessels. To test glucose levels, measurements using synthetic body fluid (SBF), which is electrically equivalent to blood plasma, were performed. Changes in sensor performance to varying glucose levels were measured and a shift in resonant frequency to lower values observed with increasing glucose level. In vitro sensor performance demonstrated that the sensor showed a dose dependent response to glucose concentration from 120 mg/dl to 530 mg/dl. A shift of 40 MHz was observed corresponding to a 97 kHz shift per 1 mg/dl change in blood glucose. Similarly the blood glucose levels were measured in pig blood using the same SiC based antenna sensor. The dependence of glucose concentration on resonance frequency observed with pig blood followed the same trend as the bloodviii mimicking experiment discussed earlier. The sensor performance was linear with the frequency shift being a direct function of glucose concentration. An in vivo experiment for foreign body response to subcutaneously-implanted antenna has been conducted using a pig/swine animal model. Tissue histology analysis showed that all-SiC antenna and poly ethylene glycol (PEG) coated Ti/Au antenna did not have any inflammatory immune response for 30 days. However, some inflammatory signs were found on bare Ti/Au antenna. The histological tissue analysis on a-SiC coated and single crystal 3C-SiC samples did not show any significant inflammatory response.

abstract: Glucose sensors have had many paradigm shifts, beginning with using urine, to point of care blood, now being approved for implant. This review covers various aspects of the sensors, ranging from the types of surface chemistry, and electron transduction. All the way to the algorithms, and filters used to alter and understand the signal being transduced. Focus is given to Dr. Hellerâ’s work using redox mediators, as well as Dr. Sode in his advances for direct electron transfer. Simple process of designing sensors are described, as well as the possible errors that may come with glucose sensor use. Finally, a small window into the future trends of glucose sensors is described both from a device view point, as well as organic viewpoint. Using this history the initial point of care sensor for insulin published through LaBelle’s lab is reevaluated critically. In addition, the modeling of the possibility of continuously measuring insulin is researched. To better understand the design for a continuous glucose sensor, the basic kinetic model is set up, and ran through a design of experiments to then optimized what the binding kinetics for an ideal insulin molecular recognition element would be. In addition, the phenomena of two electrochemical impedance spectroscopy peaks is analyzed, and two theories are suggests, and demonstrated to a modest level. / Dissertation/Thesis / Masters Thesis Biomedical Engineering 2018

No description available.

Zinc oxide (ZnO) is a well-known II-VI semiconductor material that has gained renewed interest in the past decade due to the developments of growth technologies and the availability of high-quality ZnO bulk single crystals. Owing to a wide direct band gap (3.37 eV), large exciton binding energy (60 meV), and high electron mobility (440 cm2 V-1 s-1), ZnO has been used for applications including actuators, optoelectronics, and sensors. ZnO nanoparticles can be synthesized in a broad variety of morphologies, such as nanotetrapods, nanotubes, and nanowires. Among these nanostructures, the tetrapods have attracted significant attention due to their unique morphology consisting of four legs connected together in a tetrahedral symmetry. Recently, it has been reported that nano-microstructured ZnO tetrapods (ZnO-Ts) can be synthesized by flame transport synthesis (FTS) in a rapid and up-scalable approach. Compared to conventional ZnO nanoparticles, the nano-microstructured ZnO-Ts can reduce cellular uptake, while still exhibiting specific nanomaterial properties due to the nanoscale tips. Moreover, the anisotropic ZnO-Ts have the advantages of multiple electron transfer paths, chemical stability, and biocompatibility, which make the ZnO-Ts promising candidates for biomolecule sensing applications. This work herein reports a systematical study on the structural, optical and electrochemical properties of the ZnO-Ts, which were synthesized by FTS using precursor Zn microparticles. The morphology of the ZnO-Ts was confirmed by scanning electron microscopy (SEM) as joint structures of four single crystalline legs, of which the diameter of each leg is 0.7-2.2 μm in average from the tip to the stem. The ZnO-Ts were dispersed in glucose solutions to study the photoluminescence as well as photocatalytic activity in a mimicked biological environment. The photoluminescence (PL) intensity in the ultraviolet (UV) region decreased with linear dependence on the glucose concentration up to 4 mM. The ZnO-Ts were also attached with glucose oxidase (GOx) and over coated with Nafion® to form the active media for electrochemical glucose sensing. The active layers were confirmed by Fourier transform infrared spectroscopy (FT-IR). Furthermore, the current response of the active layers to glucose was studied by cyclic voltammetry (CV) in various glucose concentration conditions. Stable current response to glucose was detected with linear dependence on the glucose concentration up to 12 mM, which confirms the potential of ZnO-Ts for biomolecule sensing applications.

Diabetes is a disease that causes many complications in human normal function. This disease represents the sixth-leading cause of death in USA. Prevention of diabetes-related complications can be accomplished through tight control of glucose levels in blood. In the last decades many different glucose sensors have been developed, however, none of them are really non invasive. Herein, we present the study of the application of gold and silver nanoparticles with different shapes and aspect ratios to detect glucose traces in human fluids such as tears and sweat. This is to our knowledge the first truly non invasive glucose optical sensor, with extraordinary limit of detection and selectivity. The best proven nanoparticles for this application were gold nanospheres. Gold nanospheres were synthesized using chloroauric acid tri-hydrated (HAuCl4.3H2O) in solution, in the presence of glucose and ammonia hydroxide. The higher the glucose concentration, the higher the number of nanoparticles generated, thus the higher the extinction efficiency of the solution. The linear dependence of the extinction efficiency of the gold nanoparticles solution with glucose concentration makes of this new sensor suitable for direct applications in biomedical sensing. Our approach is based on the well known Tollens test.