Biomedical Engineering Reference
In-Depth Information
onto silicon wafers with polymer thickness even down to 1 µm [85-89]. Many interesting applica-
tions of this kind of actuators have been described, including microgrippers [85,89], gates for “cell
clinics” [85,89], self-assembling boxes [90,91], microrobots [85,89,92], and positioning microhinges
[88]. Recently, different techniques such as ink-jet printing, soft lithography and deposition via
controlled-volume or pneumatic microsyringes have been demonstrated as well. Ink-jet printing
is a simple and fairly economical technique consisting of a drop-by-drop deposition of a polymer,
previously dissolved in a volatile solvent, by using a printing head [93-95]. Soft lithography is a
methodology derived from photolithography and includes microinjection molding in capillaries
and microcontact printing [96-98]. Microinjection molding uses microfabricated stamps made of
poly(dimethylsiloxane). The elastomeric stamps are fi lled up with a polymer solution and the excess
of solvent is evaporated so that the polymer fi lling the microchannels assumes a specifi ed geometry.
The realized microstructure is then removed from the mold via lift-off [99]. The use of microsy-
ringes as extruders mounted on micropositioning systems enables the deposition of polymers in
two- and three-dimensional structures [100]. According to the principle of extrusion, two types
of systems can be recognized: (1) those with pneumatic microsyringes, where the solution fl ow is
enabled and regulated by compressed air and (2) those with volumetric microsyringes, driven by
the controlled movement of a piston. All these systems have been used to fabricate benders. Some
examples can be found in Refs. 101, 102.
The actuation technology based on conducting polymers has opened interesting perspectives and
its important applications in the biomedical fi eld are currently being studied [46]. Microfabricated
benders to be used as gates for the so-called “cell clinics” represent an interesting example. They
consist of a microcavity that can be closed with a lid activated by conducting polymer microbenders,
working as active hinges, as shown in Figure 16.9. The microcavity can be equipped with sensors
to study a single cell.
A second type of relevant area of application consists of controlled releases of biologically
active agents by means of external electrical stimuli. As an example, very recently nanotubes of
poly(3,4-ethylenedioxythiophene) (PEDOT) were fabricated and demonstrated to be useful for such
a purpose (Figure 16.10) [103]. In particular, PEDOT nanotubular structures were fabricated by fi rst
producing, by electrospinning, nanofi bers of biodegradable poly(l-lactide) (PLLA) or poly(lactide-
co -glycolide) (PLGA); these fi bers were used as templates for a following electrochemical deposi-
tion of the conducting polymers around the nanofi bers. Then, the fi ber templates were removed or
allowed to degrade slowly.
As another example, a potential application of conducting polymer actuators deserves to be
mentioned. It consists of the possible development of steerable catheters or endoscopes. They are
Lid
Vial
Cell
Microfabricated conducting polymer
bender working as an active hinge
FIGURE 16.9 Cell clinic: Schematic drawings ( left and center ) and picture of a 100 µm × 100 µm sample
( right ). (Adapted from Smela, E., Adv. Mater ., 15, 481-494, 2003; Jager, E.W.H., Smela, E., Inganäs, O.,
Science , 290, 1540-1545, 2000. With permission.)
 
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