Theoretical analysis of the performance of glucose sensors with layer-by-layer assembled outer membranes

Numerical Modeling and Investigation of Amperometric Biosensors with Perforated Membranes

The present paper aims to investigate the influence of perforated membrane geometry on the performance of biosensors. For this purpose, a 2-D axisymmetric model of an amperometric biosensor is analyzed. The governing equations describing the reaction-diffusion equations containing a nonlinear term related to the Michaelis–Menten kinetics of the enzymatic reaction are introduced. The partial differential governing equations, along with the boundary conditions, are first non-dimensionalized by using appropriate dimensionless variables and then solved in a non-uniform unstructured grid by employing the Galerkin Finite Element Method. To examine the impact of the hole-geometry of the perforated membrane, seven different geometries—including cylindrical, upward circular cone, downward circular cone, upward paraboloid, downward paraboloid, upward concave paraboloid, and downward concave paraboloid—are studied. Moreover, the effects of the perforation level of the perforated membrane, the filling level of the enzyme on the transient and steady-state current of the biosensor, and the half-time response are presented. The results of the simulations show that the transient and steady-state current of the biosensor are affected by the geometry dramatically. Thus, the sensitivity of the biosensor can be influenced by different hole-geometries. The minimum and maximum output current can be obtained from the cylindrical and upward concave paraboloid holes. On the other hand, the least half-time response of the biosensor can be obtained in the cylindrical geometry.

A Detailed Model of Electroenzymatic Glutamate Biosensors To Aid in Sensor Optimization and in Applications in Vivo

Simulations conducted with a detailed model of glutamate biosensor performance describe the observed sensor performance well, illustrate the limits of sensor performance, and suggest a path toward sensor optimization. Glutamate is the most important excitatory neurotransmitter in the brain, and electroenzymatic sensors have emerged as a useful tool for the monitoring of glutamate signaling in vivo. However, the utility of these sensors currently is limited by their sensitivity and response time. A mathematical model of a typical glutamate biosensor consisting of a Pt electrode coated with a permselective polymer film and a top layer of cross-linked glutamate oxidase has been constructed in terms of differential material balances on glutamate, H₂O₂, and O₂ in one spatial dimension. Simulations suggest that reducing thicknesses of the permselective polymer and enzyme layers can increase sensitivity ∼6-fold and reduce response time ∼7-fold, and thereby improve resolution of transient glutamate signals. At currently employed enzyme layer thicknesses, both intrinsic enzyme kinetics and enzyme deactivation likely are masked by mass transfer. However, O₂-dependence studies show essentially no reduction in signal at the lowest anticipated O₂ concentrations for expected glutamate concentrations in the brain and that O₂ transport limitations in vitro are anticipated only at glutamate concentrations in the mM range. Finally, the limitations of current biosensors in monitoring glutamate transients is simulated and used to illustrate the need for optimized biosensors to report glutamate signaling accurately on a subsecond time scale. This work demonstrates how a detailed model can be used to guide optimization of electroenzymatic sensors similar to that for glutamate and to ensure appropriate interpretation of data gathered using such biosensors.

Enzyme Biosensors for Biomedical Applications: Strategies for Safeguarding Analytical Performances in Biological Fluids

Enzyme-based chemical biosensors are based on biological recognition. In order to operate, the enzymes must be available to catalyze a specific biochemical reaction and be stable under the normal operating conditions of the biosensor. Design of biosensors is based on knowledge about the target analyte, as well as the complexity of the matrix in which the analyte has to be quantified. This article reviews the problems resulting from the interaction of enzyme-based amperometric biosensors with complex biological matrices containing the target analyte(s). One of the most challenging disadvantages of amperometric enzyme-based biosensor detection is signal reduction from fouling agents and interference from chemicals present in the sample matrix. This article, therefore, investigates the principles of functioning of enzymatic biosensors, their analytical performance over time and the strategies used to optimize their performance. Moreover, the composition of biological fluids as a function of their interaction with biosensing will be presented.

Multiple tissue response modifiers to promote angiogenesis and prevent the foreign body reaction around subcutaneous implants

Dexamethasone-releasing PLGA poly(lactic-co-glycolic acid) microsphere/PVA (polyvinyl alcohol) hydrogel composite coatings have been shown to prevent the foreign body reaction (FBR) to subcutaneous implants in small and large animal models. Such coatings were developed to extend the lifetime of implantable biosensors. However, long-term exposure of tissue to low levels of dexamethasone results in a reduction in blood vessel density due to the anti-angiogenic effect of dexamethasone. This mild effect, while not threatening to the subject's health, may interfere with analyte detection and the sensor response time over the long-term. The present work is focused on the development of coatings that deliver combinations of three tissue response modifiers (TRMs): dexamethasone, VEGF (vascular endothelial growth factor) and PDGF (platelet derived growth factor). Dexamethasone, VEGF and PDGF prevent the FBR, increase angiogenesis and promote blood vessel maturation (which increases blood flow), respectively. To minimize any potential interference among these three TRMs (for example, PDGF increases fibrosis), the relative doses of dexamethasone, VEGF and PDGF were adjusted. It was determined that: a) all three TRMs are required for maximum promotion of angiogenesis, blood vessel maturation and prevention of the FBR; b) VEGF has to be administered at higher doses than PDGF; c) an increase in dexamethasone dosing must be accompanied by a proportional increase in growth factor dosing; and d) modification of the TRM ratio can achieve a constant capillary density throughout the implantation period which is important for applications such as biosensors to maintain sensitivity and a stable sensor baseline. Moreover, an osmosis-driven process for encapsulation of proteins in PLGA microspheres that showed low burst release was developed.

Computational modeling of mediator oxidation by oxygen in an amperometric glucose biosensor

In this paper, an amperometric glucose biosensor is modeled numerically. The model is based on non-stationary reaction-diffusion type equations. The model consists of four layers. An enzyme layer lies directly on a working electrode surface. The enzyme layer is attached to an electrode by a polyvinyl alcohol (PVA) coated terylene membrane. This membrane is modeled as a PVA layer and a terylene layer, which have different diffusivities. The fourth layer of the model is the diffusion layer, which is modeled using the Nernst approach. The system of partial differential equations is solved numerically using the finite difference technique. The operation of the biosensor was analyzed computationally with special emphasis on the biosensor response sensitivity to oxygen when the experiment was carried out in aerobic conditions. Particularly, numerical experiments show that the overall biosensor response sensitivity to oxygen is insignificant. The simulation results qualitatively explain and confirm the experimentally observed biosensor behavior.