Biomedical Engineering Reference
In-Depth Information
CO
2
3 NADH
Anode
ADH
O
HCOH
Redox
Mediator
ox
6e
AldDH
O
Enzyme
Redox
Mediator
red
HCH
ADH
CH
3
OH
3 NAD
FIGURE 12.10
Oxidation of methanol to CO
2
, catalyzed by NAD
-dependent alcohol (ADH), alde-
hyde (AldDH), and formate (FDH) dehydrogenase, with regeneration of NAD
via redox mediation to dia-
phorase. (From [91], with permission from Elsevier.)
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 trans-
port 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 use-
ful 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.
Simon
et al.
[92] investigated a biocatalytic anode based on lactate oxidation by lac-
tate dehydrogenase (LDH). The anodic current is generated by the oxidation of NADH
(produced by NAD
and substrate) while LDH catalyzes the electro-oxidation of lac-
tate into pyruvate. As previously mentioned, the oxidation of NADH at bare electrodes
requires a large overpotential, so these authors used poly(aniline) fi lms doped with
polyanions to catalyze NADH oxidation. Subsequent research by this group focused
on targeting mutants of LDH that are amenable to immobilization on the polyaniline
surface [93].
12.6 BIOCATALYTIC FUEL CELLS
12.6.1 Physiological conditions
In this section, recent progress on assembled biocatalytic fuel cells is reviewed. One
long-term goal of biocatalytic fuel cell research is the development of a low power-
ing device implanted in the human body and extracting electrical energy through the
oxidation of a biocatalytic fuel (e.g. glucose) and reduction of a bio-available oxidant
(e.g. O
2
). Therefore it is important to keep in mind the basic physiological conditions
such as, among others, concentration of substrates or inhibitors, fl uid pH, temperature
and velocity, which will eventually become the immediate working environment of the
implanted biocatalytic fuel cell. Although the list focuses on blood parameters, it is not
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