Biomedical Engineering Reference
In-Depth Information
detection of glucose in the bloodstream. For this purpose, such systems are typically conceived
according to the reaction of glucose in the blood with the enzyme glucose oxidase, which can be
immobilized within the delivery polymer. In fact, this reaction causes a lowering of the pH, which
induces a swelling of the polymer system with a related release of insulin. Examples of materials
studied for such a purpose include copolymers containing
N
,
N
-dimethylaminoethyl methacrylate
[24] or polyacrylamide [25]. Systems working on shrinking gels, rather than swelling, consisting of
poly(methacrylic acid-
g
-poly(ethylene glycol)) copolymers, have been reported as well [26].
As shown by the examples mentioned above, the considerable promising results achieved so
far are very encouraging. Nevertheless, it is necessary to stress here that further investigations and
developments are still required in order to achieve reliable and safe clinical uses of such types of
materials and systems.
16.4 IONIC POLYMER-METAL COMPOSITES
The IPMC are used to fabricate actuators capable of large deformations driven by low applied volt-
ages [27,28]. They consist of polymer networks (such as Nafi on, produced by Du Pont de Nemours,
Wilmington, Delaware having in their molecular chain ionizable groups that can be dissociated
in various polar solvents, showing a resulting net charge. These net charges of the network macro-
molecules are called polyions. They are electrically compensated by the presence of mobile coun-
terions within the network. When equilibrated with aqueous solutions, the membranes are swollen
and they contain a certain amount of water. Swelling equilibrium results from a balance between
the elastic recovery force of the polymeric matrix and the water affi nity to the fi xed ion exchanging
sites and the moving counterions. The water content depends not only on the hydrophilic proper-
ties of the ionic species inside the membranes, but also on the electrolyte concentration of the
external solution. Such an ability of the membrane to swell in water can be controlled in an electric
fi eld due to the ionic nature of the membrane. For this purpose, two electrodes are placed in close
proximity of the membrane and a low voltage (typically of the order of 1 V), below the threshold
for electrolysis, is applied. As a result, electrophoretic migrations (due to the imposed electric
fi eld) of the mobile ions within the solution and through the macromolecular network can cause the
network itself to be deformed accordingly [27-41]. In fact, the shifting of ions of the same polar-
ity within the network results in both electrostatic interactions with the fi xed charges of opposite
polarity (contained in the side groups of the polymer chains) and transport of solvent molecules.
Both these factors concur to produce a stress gradient between the opposite sides of the membrane.
In particular, local expansions and collapses (swelling and deswelling) occur on the two sides of
the membrane depending on the polarity of the nearby electrode. Accordingly, these phenomena
determine a macroscopic bending of the structure. Figure 16.6 shows a schematic drawing of this
electrochemomechanical activation.
The basic principle above described is exploited to use IPMC structures as actuators. The
applied external voltage makes the structure bending toward the anode. By reversing the polarities
of the electrodes, a bending toward the opposite direction is achieved. An increase of the volt-
age level causes a larger bending. When an alternate voltage is applied, the membrane undergoes
movements like a swing. Of course, the displacement depends not only on the voltage magnitude,
but also on the frequency (lower frequencies lead to higher displacements according to the device
bandwidth) [27-41].
A typical material used to fabricate IPMC actuators consists of fi lms of Nafi on
®
(Du Pont de
Nemours, Wilmington, Delaware an ion exchange membrane. Platinum electrodes are deposited
on both sides of a fi lm. The thickness of the actuator is typically around 0.2 mm. To maintain the
actuation capability, the fi lm needs to be kept moist. Structure and properties of Nafi on membranes
have been subjected to numerous investigations. One of the interesting properties of this mate-
rial is its ability of absorbing large amounts of polar solvents, that is, water. Platinum ions, which
are dispersed throughout the hydrophilic regions of the polymer, are subsequently reduced to the
