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.
The development of implantable biosensors for continuous monitoring of metabolites is an area of sustained scientific and technological interest. 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.
Abbreviations: (AP) acetaminophen, (BSA) bovine serum albumin, (FAD) flavin adenine dinucleotide, (GO x ) glucose oxidase, (PBS) phosphatebuffered saline, (PPh) polyphenol, (PU) polyurethane, (PVA) polyvinyl alcohol Keywords: bienzymatic, biosensor lag time, drug delivery coatings, foreign body response, implantable glucose sensors, outer membranes, selectivity Development of electrochemical sensors for continuous glucose monitoring is currently hindered by a variety of problems associated with low selectivity, low sensitivity, narrow linearities, delayed response times, hysteresis, biofouling, and tissue inflammation. We present an optimized sensor architecture based on layer stratification, which provides solutions that help address the aforementioned issues. Method:The working electrode of the electrochemical glucose sensors is sequentially coated with five layers containing: (1) electropolymerized polyphenol (PPh), (2) glutaraldehyde-immobilized glucose oxidase (GO x ) enzyme, (3) dip-coated polyurethane (PU), (4) glutaraldehyde-immobilized catalase enzyme, and (5) a physically cross linked polyvinyl alcohol (PVA) hydrogel membrane. The response of these sensors to glucose and electroactive interference agents (i.e., acetaminophen) was investigated following application of the various layers. Sensor hysteresis (i.e., the difference in current for a particular glucose concentration during ascending and descending cycles after 200 s) was also investigated. Results:The inner PPh membrane improved sensor selectivity via elimination of electrochemical interferences, while the third PU layer afforded high linearity by decreasing the glucose-to-O 2 ratio. The fourth catalase layer improved sensor response time and eliminated hysteresis through active withdrawal of GO x -generated H 2 O 2 from the inner sensory compartments. The outer PVA hydrogel provided mechanical support and a continuous pathway for diffusion of various participating species while acting as a host matrix for drug-eluting microspheres. Conclusions:Optimal sensor performance has been achieved through a five-layer stratification, where each coating layer works complementarily with the others. The versatility of the sensor design together with the ease of fabrication renders it a powerful tool for continuous glucose monitoring.
The performance of an implantable glucose sensor is strongly dependent on the ability of their outer membrane to govern the diffusion of the various participating species. In this contribution, using a series of layer by layer (LBL) assembled outer membranes, the role of outwards of H 2 O 2 diffusion through the outer membrane of glucose sensors has been correlated to sensor sensitivity. Glucose sensors with highly permeable Humic Acids/Ferric cations (HAs/Fe 3+ ) outer membranes displayed a combination of lower sensitivities and better linearities when compared with sensors coated with lesser permeable outer membranes (namely HAs/poly (diallyldimethylammonium chloride) (PDDA) and poly (styrene sulfonate) (PSS)/PDDA). On the basis of a comprehensive evaluation of the oxygen dependence of through these membranes, it these sensors in conjunction with the permeability of H 2 O 2 was concluded that the outer diffusion of H 2 O 2 is crucial to attain optimized sensor performance. This finding has important implications to the design of various bio-sensing elements employing perm-selective membranes.
Highly sensitive, long-term stable and reusable microfluidics electrodes have been fabricated and evaluated using H 2 O 2 and hydroquinone as model analytes. These electrodes composed of a 300 nm Pt-black layer situated on a 5 μm thick electrodeposited Au layer, provide effective protection against electrooxidation of an underlying chromium adhesion layer. Using repeated cyclic voltammetric (CV) sweeps in flowing buffer solution, highly sensitive Pt-black working electrodes were realized with five-(four-) decade linear dynamic range for H 2 O 2 (hydroquinone) and low detection limit of 10 nM for H 2 O 2 and 100 nM for hydroquinone. Moreover, high sensitivity for H 2 O 2 was demonstrated at low (0.3 V vs. Ag/AgCl) oxidation potentials, together with long-term stability and reusability for at least 30 days. Microfluidic flow was employed for desorption and reactivation of the nominally planar Pt-black electrodes. Such electrocatalytic surface architecture should be appropriate for long-term electrochemical detection of various molecules and biomolecules as well as in reusable immunoassay configurations.
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