Biomedical Engineering Reference
In-Depth Information
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
Film
0.2
0.1
Cylinder
0
0.0
0.2
0.4
0.6
0.8
1
Dimensionless time
(
Dt/l
2
, Dt / a
2
)
FIGURE 13.27
Diffusion-controlled current plotted versus dimensionless time
Dt
/
l
2
in a planar geometry
and
Dt
/
a
2
in a cylinder.
The normalized currents for both planar and cylindrical geometries are plotted in Figure 13.27.
It can be seen from the plot that for equal characteristic dimensions,
l
=
a
, the current in a cylinder
decays faster than in the plane due to the more concentrated diffusion fl ux lines in the cylinder. Fur-
thermore, the characteristic current decay time, signifying the completion of the doping or undop-
ing process in the polymer, is directly proportional to the square of the characteristic dimension
(thickness in a fi lm and radius in a cylinder). Therefore, PPy devices with small nanoscale dimen-
sions have a signifi cantly smaller diffusion contribution to the amount of time it takes to completely
oxidize or reduce the polymer matrix than macro- and even microscale actuators. This conclusion
is supported by experimental evidence as reported above with the characteristic time of the elec-
trochemical reactions in nanowires with
r
=
100 nm below 0.5 s. Unfortunately, the quantitative
analysis of the diffusion component of PPy time response is impeded by the fact that
D
, the diffu-
sion coeffi cient, is not constant but changes during the course of redox processes in the polymer
(reported values in LiClO
4
electrolyte range from 10
−
8
to 10
−
9
cm
2
/s).
31
However, understanding
how the relationship between diffusion and polymer device geometry affects the time response
should prove useful for the design of PPy electrochemical devices.
Since the linear expansion of PPy nanowires is only
∼
3%, a useful nanoactuator can be made by
forming a bilayer structure involving a single nanowire attached to a parallel layer or a nanowire of
a different material. Bilayer structures are commonly used to amplify a small change in volume to
a large angular movement by generating a stress gradient across the interface between the polymer
and the secondary material. For example, PPy-gold microbilayers have been utilized to convert
0.5-3% of linear motion into 180° or more of angular motion.
5
An analogous structure can poten-
tially be produced at nanoscale dimensions, namely, a bilayer structure amplifying the motion of a
single PPy nanowire and capable of performing tasks such as nanoparticle manipulation and cilia-
like motion.
13.5 POLYPYRROLE BIOSENSORS
Conducting polymers, including PPy, have long been recognized as suitable materials for the
construction of biosensors (reviewed in Refs. 32-34). In particular, the ability to embed and
immobilize enzymes within the polymer matrix provides an electrical readout of the reaction
catalyzed by the enzyme through electrochemical detection of reaction products such as hydrogen
