Biomedical Engineering Reference
In-Depth Information
peroxide, in the case of PPy-glucose oxidase sensor for glucose. 35 PPy can be easily synthesized
in a variety of sizes and geometries. This material property was exploited by a number of inves-
tigators seeking to improve the function of the existing sensors or create new sensor paradigms
through scaling the sensor down to micro- or nanodimensions. For example, a change in PPy sensor
geometry, namely, the polymerization of PPy in the pores of a track-etched membrane led to the
construction of more effi cient glucose sensors with prolonged lifetime. 36,37 Sensor arrays can be
constructed by polymerizing PPy on electrode arrays fabricated with conventional photolitho-
graphic techniques. 38
PPy has also been used as a conductive coating to improve the sensor biocompatibility both in
vivo and in vitro . This coating is particularly important for sensors used in the study of neuronal
electrical and chemical activity. It was demonstrated that PPy in its oxidized state is compatible
with neural cell growth in vitro by allowing effi cient adsorption of extracellular matrix molecules
such as fi bronectin. Interestingly, when an electric potential was applied to switch the polymer into
a reduced neutral state, cell extension and DNA synthesis were inhibited without affecting the cell
viability, providing potential means to reversibly control cell geometry in culture. 39 Neuroblastoma
cells were successfully cultured on current-carrying PPy for several days, demonstrating the abil-
ity of PPy to deliver long-term electrical stimulation to the cells without affecting their viability. 40
The same authors found that PPy implanted into rat muscle elicited only a minimal infl ammatory
response after 4 weeks in vivo . It was further found that the peripheral nerve cell line PC-12 dis-
played signifi cant increase in neurite growth when electrically stimulated through PPy fi lm acting
as culture substrate. 41 These fi ndings raised the possibility that PPy might have potential applica-
tion as a coating for implantable synthetic guidance channels for peripheral nerve repair. PPy
doped with polystyrene sulfonate (PSS) or NaDBS was implanted into cerebral cortex of the rat
and displayed good biocompatibility (low gliosis compared with Tefl on) and envelopment of the
implant by neurons and glial cells. 42 Multichannel neural probes are used to stimulate and record
electrical impulses from the brain. PSS was electropolymerized on the active sites of the probe
to reduce the electrical impedance, and high-quality signals were recorded from the cerebellum
of the guinea pig. 43
Electrochemical methods for the detection of neurotransmitters, such as dopamine and
norepinephrine, can provide valuable information about the function of mammalian central ner-
vous system and are used both in vivo and in vitro . 44,45 Typically, such detection schemes utilize a
carbon microelectrode to sample neurotransmitter release in different regions of the brain or a brain
slice with or without external stimulation. The concentration of catecholamine neurotransmitters,
dopamine and norepinephrine, is measured by applying a potential at the electrode and measuring
the anodic current that results from oxidation of the catecholamine molecules. Ideally, detection
is carried out in real time to record the variation in concentration due to release and subsequent
uptake of neurotransmitters. A variety of electrochemical techniques such as cyclic voltammetry
and differential pulse voltammetry (DPV) are used to provide detection specifi city by discrimi-
nating among electrochemical behavior of a variety of oxidizable molecules in the extracellular
fl uid. However, such voltammetric methods require scanning through a range of different voltages
to record the current and take more time than simple constant potential amperometry. To enable
the use of these techniques for measuring real-time changes in neurotransmitter concentration,
fast electrode response is required. 46,47 It was found that by decreasing the size of the electrodes,
improved mass transport and decreased double-layer capacitance can be achieved, resulting in
an increase in signal-to-noise ratio and a better time response. 48,49 In this work, PPy-coated gold
nanoelectrodes fabricated by using alumina membrane template synthesis have been demonstrated.
The small size of the nanoelectrodes allowed rapid time response and detection of nanomolar
concentrations of dopamine. Overoxidized PPy (o-PPY) coating was applied to the nanoelectrodes
to improve selectivity against a physiologically relevant background concentration of ascorbic acid
(AA). These sensors have potential applications for in vitro neurotransmitter studies, particularly
 
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