Fuel cell implantable

Possibilities for Enzymes in Implantable Fuel Cells There is significant and increasing demand for power supplies for implantable medical devices, including continuous glucose monitors for diabetic patients, thermal sensors for... 

Fuel cells based on unmediated electrocatalysis by heme-containing sugar dehydrogenases have not yet been tested in biological fluids, but may be useful for implantable applications, as they avoid the need for toxic or expensive mediators and have minimal design constraints. Realistically, the lifetime of biofuel cells is still insufficient for biomedical applications requiring surgical installation. 

Biocatalytic fuel cells using isolated redox enzymes were first investigated in 1964 [4], These fuel cells represent a more realistic opportunity for provision of implantable power, given the exquisite selectivity of enzyme catalysts, their activity under physiological conditions, and the relative ease of immobilization of isolated enzymes,... 

While direct electron transfer to laccases may help elucidate the mechanism of action of these enzymes it is unlikely that this process will supply sufficient power for a viable implantable biocatalytic fuel cell, because of difficulties associated with the correct orientation of the laccase and the two-dimensional nature of the biocatalytic layer on the surface. However, a recent attempt to immobilize laccase in a carbon dispersion, to provide electrodes with correctly oriented laccase for direct electron transfer, and a higher density of electrode material shows promise [53],... 

In this section, the enzymes, and associated substrates, used as biocatalysts in anodes are presented. For the development of biocatalytic anodes, there is a wide range of fuels available for use as substrates, such as alcohols, lactate, hydrogen, fructose, sucrose, all of which can be oxidized by biocatalysts. The fuel that is the most widely considered, however, in the context of an implantable biocatalytic fuel cell is glucose. We shall focus our attention on this fuel, but will mention briefly research on the use of some other fuels in biocatalytic anodes. 

While this anode is not useful in the context of implantable fuel cells, it is of interest because methanol is an attractive anodic fuel due to its availability and ease of transport and storage. The oxidation of one equivalent of methanol requires the reduction of three equivalents of NAD+ to NADH. As the NADH cofactor itself is not a useful redox mediator, a benzylviologen/diaphorase redox cycle, with a redox potential of 0.55 V vs SCE at pH 7, was used to regenerate NAD+ for use by the dehydrogenases, as depicted in Fig. 12.10. 

A remaining crucial technological milestone to pass for an implanted device remains the stability of the biocatalytic fuel cell, which should be expressed in months or years rather than days or weeks. Recent reports on the use of BOD biocatalytic electrodes in serum have, for example, highlighted instabilities associated with the presence of 02, urate or metal ions [99, 100], and enzyme deactivation in its oxidized state [101]. Strategies to be considered include the use of new biocatalysts with improved thermal properties, or stability towards interferences and inhibitors, the use of nanostructured electrode surfaces and chemical coupling of films to such surfaces, to improve film stability, and the design of redox mediator libraries tailored towards both mediation and immobilization. 

In contrast, stability is a key aspect of any practical fuel cell, and biofuel cells must have lifetimes ranging from months to years to justify implanted, highly distributed, or consumer portable applications. Such stability is often difficult to achieve in redox enzymes, although introduction of thermophilic species and the use of mutagenic techniques might provide future... 

Classes 1 and 2 are closely related The reactants available for implantable power, such as blood-borne glucose, lactate, or oxygen, are ambient in that environment. These two classes are distinct, however, in that an ambient-fueled cell need not be implanted and utilizes plant- or waste-derived fuels, whereas the implantable cell utilizes animal-derived fuels. Class 3 is unique because it competes with well-established conventional fuel cell technology. 

Conventional fuel cell systems provide the designer with greater control over operating conditions as compared to the implantable and ambient-fuel categories. For example, the pH of the system can be adjusted well above or below neutral, and the opportunity exists to eradicate all poison species from the system. As previously mentioned, the realm of... 

For example, the small scale of the device was intended as a demonstration of architecture suitable for implanted applications. Mano et al. demonstrated a miniature fuel cell with bilirubin oxidase at the cathode catalyst that is more active at pH 7 and tolerates higher halide concentrations than does laccase. Additionally, the long-side-chain poly-(vinylpyridine)—Os(dialkyl-bis-imidazole)3 redox polymer discussed above was employed to both lower the anode potential and, via the long side chains, enhance electron transport from the biocatalyst. The cell achieved a current density of 830 at 0.52 V... 

Ion implantation has also been used for the creation of novel catalytically active materials. Ruthenium oxide is used as an electrode for chlorine production because of its superior corrosion resistance. Platinum was implanted in ruthenium oxide and the performance of the catalyst tested with respect to the oxidation of formic acid and methanol (fuel cell reactions) (131). The implantation of platinum produced of which a catalytically active electrode, the performance of which is superior to both pure and smooth platinum. It also has good long-term stability. The most interesting finding, however, is the complete inactivity of the electrode for the methanol oxidation. 

Based on the high specificity of enzymatic reactions enzymatic fuel cells can be constructed compartmentless, i.e., without a physical separation of the anodic and the cathodic compartments. This allows miniaturization of the devices, e.g., for biomedical (implantable) devices and -> biosensors [iii]. 

An enzymatic glucose/02 fuel cell which was implanted in a living plant was presented by Heller and coworkers [147[. 

Mediated enzyme electrodes were also realized on combined microscale and nanoscale supports [300]. Bioelectrocatalytic hydrogels have also been realized by co-assembling electron-conducting metallopolypeptides with bifunctional building blocks [301]. More recently, redox-modified polymers have been employed to build biofuel cells [25, 70, 302, 303]. In 2003, an enzymatic glucose/02 fuel cell which was implanted in a living plant was introduced [147]. 

The aim of this type of devices is to further extend the operational lifetime of the implanted medical devices to match the life time of the patient. To that end, fuel cell technology will have to be involved. 

Automobiles cost more than 100 times the price of the battery, but the battery permits easy, reliable starting, allowing it to be a convenient mode of transportation for normal lifestyles. Indeed, batteries provide the only efficient small-scale devices for the storage of instantly available electrical energy. Dependent on batteries are all automobiles all telephone circuits most modem watches, calculators, and standby power sources most modem weapons systems for propulsion (torpedoes, for example), communication, and guidance systems space exploration (which also uses fuel cells) and implanted heart pacers. 

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