Polypyrrole Nano- and Microsensors and Actuators for Biomedical Applications - Biomaterials Fabrication and Processing - page 368

13.4.2.3 Real-Time Optical Microscopy of Nanowire Actuation........................ 385
13.4.2.4 Discussion of Optical Microscopy and Cyclic
Voltammetry Data.................................................................................. 386
13.4.3 Polypyrrole Nanowire Morphology ....................................................................... 387
13.4.4 Time Response of Isolated Nanowires................................................................... 389
13.4.4.1 Actuation Speed in High-Density Nanowires........................................ 389
13.4.4.2 Fabrication of Isolated (Low-Density) Nanowires................................. 389
13.4.4.3 Time Response....................................................................................... 391
13.4.4.4 Theory and Discussion .......................................................................... 391
13.5 Polypyrrole Biosensors ........................................................................................................ 393
References ...................................................................................................................................... 398
13.1 INTRODUCTION
The conducting polymer polypyrrole (PPy) possesses many interesting properties that make it an
attractive material for a variety of applications. In particular, this polymer undergoes reversible
redox reactions in electrolyte with applied voltage that allow control over many material properties
such as conductivity, ion exchange, hydrophobicity, and even the material dimensions. The control
of ion exchange properties of the polymer is important in the design of selective sensors, while
voltage control over material geometry makes PPy a particularly suitable material for the design of
actuators. Furthermore, the controllable ion exchange and volume change are material properties
of PPy and can be utilized at any dimension ranging from macroscale, or devices with the size of
several centimeters, to nanoscale, or devices with the size of several tens of nanometers. This fl ex-
ibility allows the design of a great variety of PPy devices, including “artifi cial muscles” intended
for medical prosthetics, microactuators for lab-on-a-chip integrated systems and microrobotics, and
electrochemical sensors for various biological molecules.
In this chapter, the focus is on the devices fabricated from PPy doped with dodecylben-
zenesulfonate (DBS), which are capable of operating in aqueous environments ranging from sea
water to blood plasma. DBS-doped PPy has long-term stability in water solutions and operates
by uptake or expulsion of small positive ions during voltage-controlled reduction or oxidation,
respectively, from or to the surrounding electrolyte. This property makes the PPy(DBS) devices
particularly suitable for biomedical applications in which the environment consists of an electri-
cally conducting aqueous solution containing a number of small positive ions such as Na + , K + ,
and Ca 2 + . Therefore, a PPy actuator device that operates on the principle of volume change due to
the movement of sodium ions can use the biological solution such as extracellular fl uid or blood
plasma as a supporting electrolyte without the need for device encapsulation.
Several micro- and nanosized devices based on the properties of PPy have been developed by
a number of laboratories for biomedical applications. Devices described in this chapter include
microvalve designs based on a PPy volume change for use in microfl uidics and drug delivery, a
microrobotic device that uses PPy-gold bilayers as actuators, PPy nanowires capable of reversible
length change with the applied voltage, PPy-coated nanosensor for detection of catecholamine
neurotransmitters, which displays nanomolar sensitivity and improved time response, and PPy
coatings for cell growth substrates and microelectrodes used in neuroscience.
Section 13.2 begins with a description of PPy electrochemistry and principles of PPy syn-
thesis and reversible volume change that occurs during electrochemical cycling of the polymer.
Section 13.3 describes how this volume change has been harnessed by several laboratories to
design and fabricate microdevices for biological applications. In Section 13.4, the steps toward
the fabrication and characterization of PPy nanoactuators are reported, and Section 13.5 reviews
PPy microscale biosensors and the use of PPy in tissue and cell cultures.