Biomedical Engineering Reference
In-Depth Information
signifi cant electrically induced strains. In particular, a dielectric elastomer actuator consists of a
thin layer of an insulating rubber-like material sandwiched between two electrodes that must be
compliant (deformable). The electrodes, which can be made for instance of carbon loaded elastomer
or carbon grease, are electrically charged by the application of a high potential difference (typically
of the order of kV). As a result, the activation of the material by the imposed electric fi eld (usually of
the order of 10-100 V/µm) causes a deformation at constant volume consisting of a squeezing of the
dimension included between the electrodes and related expansions along the orthogonal directions
[106-109]. Such a deformation is mainly due to the so-called Maxwell stress, arising from the elec-
trostatic interactions among the free charges on electrodes. Although this kind of stress acts in any
kind of dielectric material subjected to an applied electric fi eld, for this technology the corresponding
deformations are greatly emphasized by both the compliance of the electrodes and the softness of the
dielectric polymer. These key features basically distinguish actuating devices made of dielectric elas-
tomers from those based on different electric-fi eld-driven dielectrics, such as a piezoelectric material
[110] or an electrostrictive material [111]. In comparison with the latter, dielectric elastomers share a
strain and stress dependence on the square of the electric fi eld, but are capable of signifi cantly larger
deformations for comparable fi eld strengths. This is due to a different activation mechanism and a
lower elastic modulus. However, the latter feature reduces, as a counterpart, the achievable stresses.
Acrylic and silicone rubbers are the most signifi cant types of dielectric elastomers used for
actuation. Such kinds of polymers comprehend representative materials, which can be very com-
pliant, being able of showing the highest actuating deformations among all EAP [107]. High-level
actuation capabilities have been reported for certain types of acrylic polymers: thickness strains up
to 60-70% at 400 V/µm, area strains up to 200% at 200 V/µm and corresponding stresses of some
MPa [107]. Such performances are enabled by low elastic moduli and high dielectric strengths
(dielectric breakdown can occur at electric fi elds up to about 500 V/µm). The highest active per-
formances were achieved by prestretching the material: this operation was demonstrated to increase
the dielectric strength, permitting the application of higher electric fi elds [107]. Besides acrylates,
silicones (mainly polydimethylsiloxanes) offer attracting characteristics: they are easily processable
(by spin-coating, casting, etc.) and permit the realization of rubber-like dielectrics with suitable
elastic properties, arising from the fl exibility of the material molecular chains. Certain silicone
elastomers have been actuated with electric fi elds up to 100-350 V/µm, enabling thickness strains
up to 40-50% and area strains up to 100%, with related stresses of 0.3-0.4 MPa [107].
The principle of electromechanical transduction in dielectric elastomers has been exploited
by implementing devices with different types of geometries and structures. Examples include pla-
nar actuators [106-109], diaphragms [106], benders [106], linear extending devices, such as tubes
[106,112] and rolls [106,113,114], linear contractile devices such as stacks [115], helical [116] and
folded structures [117], etc.
Owing to the excellent fi gures of merit shown by several dielectric elastomers (very high actua-
tion strains, considerable stresses, very fast response, high effi ciency, stability, reliability, and durabil-
ity), this class of EAP is nowadays considered as one of the most outstanding for polymer actuation.
Nevertheless, a major drawback affects this performing technology: the high driving electric fi elds
needed imply the use of the mentioned very high voltages (although at low currents). This is a sig-
nifi cant disadvantage for several types of applications, especially in the biomedical area. In order to
solve or at least reduce such a drawback, research efforts are today focused on the development of new
elastomers with superior electromechanical properties, particularly by means of an increase of the
dielectric permittivity [118]. However, the absence of signifi cant contributions in this direction so far
(due to both the challenging nature of the problem and the “youth” of this technology, launched at the
end of the 1990s) prevents the adoption of dielectric elastomer actuation for devices that must work
in contact with body tissues. Accordingly, different types of applications are emerging. Within the
biomedical area, an example concerns the use of dielectric elastomer actuators for dynamic orthotic
systems to be used for rehabilitation. In such a context, Figure 16.13 shows a prototype sample of a
hand splint for rehabilitation of fi ngers, which is currently being developed in our laboratory.
