Hierarchical Structure of Gold and Carbon Electrode for Bilirubin Oxidase-Biocathode.

Biofuel cells (BFCs) with enzymatic electrocatalysts have attracted significant attention, especially as power sources for wearable and implantable devices; however, the applications of BFCs are limited owing to the limited O2 supply. This can be addressed by using air-diffusion-type bilirubin oxidase (BOD) cathodes, and thus the further development of the hierarchical structure of porous electrodes with highly effective specific surface areas is critical. In this study, a porous layer of gold is deposited over magnesium-oxide-templated carbon (MgOC) to form BOD-based biocathodes for the oxygen reduction reaction (ORR). Porous gold structures are constructed via electrochemical deposition of gold via dynamic hydrogen bubble templating (DHBT). Hydrogen bubbles used as a template and controlled by the Coulomb number yield a porous gold structure during the electrochemical deposition process. The current density of the ORR catalyzed by BOD without a redox mediator on the gold-modified MgOC electrode was 1.3 times higher than that of the ORR on the MgOC electrode. Furthermore, the gold-deposited electrodes were modified with aromatic thiols containing negatively charged functional groups to improve the orientation of BOD on the electrode surface to facilitate efficient electron transfer at the heterogeneous surface, thereby achieving an ORR current of 12 mA cm-2 at pH 5 and 25 °C. These results suggest that DHBT is an efficient method for the fabrication of nanostructured electrodes that promote direct electron transfer with oxidoreductase enzymes.

Insight into continuous glucose monitoring: from medical basics to commercialized devices.

According to the latest statistics, more than 537 million people around the world struggle with diabetes and its adverse consequences. As well as acute risks of hypo- or hyper- glycemia, long-term vascular complications may occur, including coronary heart disease or stroke, as well as diabetic nephropathy leading to end-stage disease, neuropathy or retinopathy. Therefore, there is an urgent need to improve diabetes management to reduce the risk of complications but also to improve patient's quality life. The impact of continuous glucose monitoring (CGM) is well recognized, in this regard. The current review aims at introducing the basic principles of glucose sensing, including electrochemical and optical detection, summarizing CGM technology, its requirements, advantages, and disadvantages. The role of CGM systems in the clinical diagnostics/personal testing, difficulties in their utilization, and recommendations are also discussed. In the end, challenges and prospects in future CGM systems are discussed and non-invasive, wearable glucose biosensors are introduced. Though the scope of this review is CGMs and provides information about medical issues and analytical principles, consideration of broader use will be critical in future if the right systems are to be selected for effective diabetes management.

A disposable enzymatic biofuel cell for glucose sensing via short-circuit current.

It is essential to construct a biofuel cell-based sensor and develop an effective strategy to detect glucose without any potentiostat circuitry in order to create a simple and miniaturized device. In this report, an enzymatic biofuel cell (EBFC) is fabricated by the facile design of an anode and cathode on a screen-printed carbon electrode (SPCE). To construct the anode, thionine and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) are covalently immobilized via a crosslinker to make a cross-linked redox network. As a cathode, the Pt-free oxygen reduction carbon catalyst is employed alternative to the commonly used bilirubin oxidase. We proposed the importance of EBFC-based sensors through the connection of anode and cathode; they can identify a short-circuit current by means of applied zero external voltage, thereby capable of glucose detection without under the operation of the potentiostat. The result shows that the EBFC-based sensor could be able to detect based on a short-circuit current with a wide range of glucose concentrations from 0.28 to 30 mM. Further, an EBFC is employed as a one-compartment model energy harvester with a maximum power density of (36 ± 3) µW cm- 2 in sample volume 5 µL. In addition, the constructed EBFC-based sensor demonstrates that the physiological range of ascorbic acid and uric acid shows no significant effect on the short-circuit current generation. Moreover, this EBFC can be used as a sensor in artificial plasma without losing its performance and thereby used as a disposable test strip in real blood sample analysis.

Flexible doctor blade-coated abiotic cathodes for implantable glucose/oxygen biofuel cells.

Implantable devices powered by batteries have been used for sixty years. In recent devices, lithium-based batteries are the most widely used power source. However, lithium batteries have many disadvantages in terms of safety, reliability, and longevity and require regular monitoring and substitution. Implantable glucose biofuel cells (BFCs) are increasingly seen as a potential future technology for replacing lithium-based batteries because they do not require surgical replacement after 8-10 years and have a theoretically unlimited lifetime thanks to the continued recovery of glucose and oxygen present in the human body. This paper shows the fabrication of flexible implantable abiotic cathodes, based on a nitrogen/iron-doped graphene catalyst, for glucose/oxygen biofuel cell application. An ink, based on nitrogen-iron doped graphene as the abiotic catalyst and chitosan as a binder, was prepared and coated on a flexible teflonated gas diffusion layer using doctor blade coating. The characterization of the biocathode shows an open potential circuit corresponding to the potential of the abiotic catalyst and a high oxygen reduction current density of up to 66 µA cm-2 under physiological conditions. Those cathodes remain stable for up to two years with a current density loss of only 25%. The flexible abiotic electrode cytotoxicity was evaluated by cell culture experiments showing living cells' high tolerance on the biocathode surface. This work demonstrates that this abiotic catalyst can be a promising alternative for the development of implantable glucose BFCs due to its stability and its cytocompatibility.

Investigation of GOx Stability in a Chitosan Matrix: Applications for Enzymatic Electrodes.

In this study, we designed a new biosensing membrane for the development of an electrochemical glucose biosensor. To proceed, we used a chitosan-based hydrogel that entraps glucose oxidase enzyme (GOx), and we crosslinked the whole matrix using glutaraldehyde, which is known for its quick and reactive crosslinking behavior. Then, the stability of the designed biosensors was investigated over time, according to different storage conditions (in PBS solution at temperatures of 4 °C and 37 °C and in the presence or absence of glucose). In some specific conditions, we found that our biosensor is capable of maintaining its stability for more than six months of storage. We also included catalase to protect the biosensing membranes from the enzymatic reaction by-products (e.g., hydrogen peroxide). This design protects the biocatalytic activity of GOx and enhances the lifetime of the biosensor.

Electrochemical Biosensing of Glucose Based on the Enzymatic Reduction of Glucose.

In this work, the enzyme aldehyde reductase, also known as aldose reductase, was synthesized and cloned from a human gene. Spectrophotometric measurements show that in presence of the nicotinamide adenine dinucleotide phosphate cofactor (NADPH), the aldehyde reductase catalyzed the reduction of glucose to sorbitol. Electrochemical measurements performed on an electrodeposited poly(methylene green)-modified gold electrode showed that in the presence of the enzyme aldehyde reductase, the electrocatalytic oxidation current of NADPH decreased drastically after the addition of glucose. These results demonstrate that aldehyde reductase is an enzyme that allows the construction of an efficient electrochemical glucose biosensor based on glucose reduction.

New Microfluidic System for Electrochemical Impedance Spectroscopy Assessment of Cell Culture Performance: Design and Development of New Electrode Material.

Electrochemical impedance spectroscopy (EIS) is widely accepted as an effective and non-destructive method to assess cell health during cell-culture. However, there is a lack of compact devices compatible with microfluidic integration and microscopy that could provide the real-time and non-invasive monitoring of cell-cultures using EIS. In this paper, we reported the design and characterization of a modular EIS testing system based on a patented technology. This device was fabricated using easily processable methodologies including screen-printing of the impedance electrodes and molding or micromachining of the cell culture chamber with an easy assembly procedure. Accordingly, to obtain processable, biocompatible and sterilizable electrode materials that lower the impact of interfacial impedance on TEER (Transepithelial electrical resistance) measurements, and to enable concomitant microscopy observations, we optimized the formulation of the electrode inks and the design of the EIS electrodes, respectively. First, electrode materials were based on carbon biocompatible inks enriched with IrOx particles to obtain low interfacial impedance electrodes approaching the performances of classical non-biocompatible Ag/AgCl second-species electrodes. Secondly, we proposed three original electrode designs, which were compared to classical disk electrodes that were optically compatible with microscopy. We assessed the impact of the electrode design on the response of the impedance sensor using COMSOL Multiphysics. Finally, the performance of the impedance spectroscopy devices was assessed in vitro using human airway epithelial cell cultures.

Chitosan-based enzyme ink for screen-printed bioanodes.

In this study, magnesium oxide (MgO)-templated mesoporous carbon (MgOC) and chitosan cross-linked with genipin (chitosan-genipin) were considered bio-composite inks for screen-printed bioanodes. The fabrication processes were optimized using rheological and structural data, and a bioanode ink containing glucose oxidase (GOx) and 1,2-naphthoquinone (1,2-NQ) was successfully developed. The optimal bioanode-ink contained MgOC pre-treated by washing to achieve a hydrophilic and neutral surface, which helped maintain enzyme activity and resulted in a highly porous electrode structure, which is essential for the accessibility of glucose to GOx. A bioanode fabricated using this ink showed a linear response current dependency up to 8 mM glucose with a sensitivity of 25.83 µA cm-2 mM-1. Combined with a conventional biocathode, an electromotive force of 0.54 V and a maximal power density of 96 µW cm-2 were achieved. These results show that this bio-composite ink can be used to replace the multi-step process of printing with conventional ink followed by drop-casting enzyme and mediator with a one-step printing process.

The biocompatibility of biofuel cells operating inside the body.

In 1968 Wolfson et al. published the concept for producing energy inside the body using catalytic electrodes exposed to the body fluid as an electrolyte and utilising naturally occurring fuels such as glucose. Since then, the technology has advanced to enhance the levels of power using enzymes immobilised within three-dimensional bioelectrodes that are nanostructured. Current research in the field of enzymatic fuel cells is directed toward applying electrochemical and nanostructural expertise to increase the energy density, to increase the power density, to increase the operational stability, and to increase the voltage output. Nonetheless, biocompatibility remains the major challenge for increasing the life-time for implanted enzymatic biofuel cells. Here, we discuss the current issues for biocompatibility and suggest directions to enhance the design of biofuel cells so as to increase the life-time of implantation whilst maintaining sufficient performance to provide power for implanted medical devices.

Mediated electrochemical oxidation of glucose via poly(methylene green) grafted on the carbon surface catalyzed by flavin adenine dinucleotide-dependent glucose dehydrogenase.

Electrochemically polymerized phenothiazines (thionine, methylene green, methylene blue, and toluidine blue) on carbon electrodes were investigated as electron transfer mediators of glucose oxidation by flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) for biosensor and biofuel cell applications. Among the tested polyphenothiazines grafted on a glassy carbon electrode, clear redox-mediating activity was observed for poly(methylene green), and the catalytic oxidation current depended on the concentrations of glucose and enzymes and the amount of polymer deposited on the electrode surface. The poly(methylene green)-grafted porous carbon electrodes showed 3 mA cm-2 of glucose oxidation current catalyzed by FAD-GDH.

A PANI supported lipid bilayer that contains NhaA transporter proteins provides a basis for a biomimetic biocapacitor.

We designed a supported lipid bilayer (SLB) biomimetic membrane system that comprised polyaniline (PANI) to support a lipid bilayer membrane that incorporated Na+/H+ transporter proteins (NhaA) to give the system the capability of controllable electrogenic ion transport. The high turnover rate of NhaA (∼105 per min) provides the basis for this PANI-SLB-NhaA system to be a high-speed rechargeable biocapacitor that functions as a low-energy-consuming fast switch for biological engineering applications.

Challenges for successful implantation of biofuel cells.

There is a growing interest in the design and engineering of operational biofuel cells that can be implanted. This review highlights the recent progress in the electrochemistry of biofuel cell technologies, but with a particular emphasis on the medical and physiological aspects that impact the biocompatibility of biofuel cells operating inside a living body. We discuss the challenge of supplying power to implantable medical devices, with regard to the limitations of lithium battery technology and why implantable biofuel cells can be a promising alternative to provide the levels of power required for medical devices. In addition to the challenge of designing a biofuel cell that provides a stable level of sufficient power, the review highlights the biocompatibility and biofouling problems of implanting a biofuel cell that have a major impact on the availability of the substrates inside body that provide fuel for the biofuel cell. These physiological challenges and associated ethical considerations are essential to consider for biofuel cells that are designed to be implanted for long-term operation inside a living animal and eventually to human clinical applications.

[Shades of grays for implanting an enzymatic biofuel cell]. / La biopile enzymatique à glucose/oxygène - Quelques nuances de Grays .

An implanted biofuel cell (IBFC) is a novel device that provides the means to create electricity from glucose and oxygen, using an original architecture for the IBFC that provides efficient work inside a living organism. In the future these IBFCs will be required to power implanted devices to assist failing physiological functions in humans. The active ingredients of such IBFCs are glucose oxidase at the anode and laccase at the cathode. These enzymes are entrapped in a 3D network of conductive and insulated materials. This publication solves the issue of the sterilization of such a complex device, using gamma irradiation. A 12kGy dose was sufficient to show absence of implant infection in all the implantations performed. We also prove in vitro functioning of both bioelectrodes with a high dose of 42kGy.

Chitosan improves stability of carbon nanotube biocathodes for glucose biofuel cells.

We demonstrate a novel combined chitosan-carbon-nanotube-enzyme biocathode with a greatly enhanced and stable long-term current density of -0.19 mA mL(-1). The fibrous microstructure of the electrode improves the performance of the biocathode by creating a protective microenvironment, preventing the loss of the electrocatalytic activity of the enzyme, and providing good oxygen diffusion.

High power enzymatic biofuel cell based on naphthoquinone-mediated oxidation of glucose by glucose oxidase in a carbon nanotube 3D matrix.

We report the design of a novel glucose/O2 biofuel cell (GBFC) integrating carbon nanotube-based 3D bioelectrodes and using naphthoquinone-mediated oxidation of glucose by glucose oxidase and direct oxygen reduction by laccase. The GBFCs exhibit high open circuit voltages of 0.76 V, high current densities of 4.47 mA cm(-2), and maximum power output of 1.54 mW cm(-2), 1.92 mW mL(-1) and 2.67 mW g(-1). The GBFC is able to constantly deliver 0.56 mW h cm(-2) under discharge at 0.5 V, showing among the best in vitro performances for a GBFC. Using a charge pump, the GBFC finally powered a Light Emitting Diode (LED), demonstrating its ability to amplify micro watts to power mW-demanding electronic devices.

Mediatorless high-power glucose biofuel cells based on compressed carbon nanotube-enzyme electrodes.

Enzymatic fuel cells use enzymes to produce energy from bioavailable substrates. However, such biofuel cells are limited by the difficult electrical wiring of enzymes to the electrode. Here we show the efficient wiring of enzymes in a conductive pure carbon nanotube matrix for the fabrication of a glucose biofuel cell (GBFC). Glucose oxidase and laccase were respectively incorporated in carbon nanotube disks by mechanical compression. The characterization of each bioelectrode shows an open circuit potential corresponding to the redox potential of the respective enzymes, and high current densities for glucose oxidation and oxygen reduction. The mediatorless GBFC delivers a high power density up to 1.3 mW cm(-2) and an open circuit voltage of 0.95 V. Moreover, the GBFC remains stable for 1 month and delivers 1 mW cm(-2) power density under physiological conditions (5×10(-3) mol l(-1) glucose, pH 7). To date, these values are the best performances obtained for a GBFC.