Biomedical Engineering Reference
In-Depth Information
1.2.15
M
S
M
(MSM) M
AGNETIC
HAPE
EMORY
ATERIALS
Magnetically controlled shape memory material is a new way to produce motion
and force. The MSM mechanism was suggested by Ullakko (1996) and O'Handley
(1998) and was demonstrated for a Ni-Mn-Ga alloy as early as 1996. The magnetic
shape memory effect has demonstrated that certain shape memory materials that are
also ferromagnetic can show very large dimensional changes (6%) under the appli-
cation of a magnetic field. These strains occur within the low-temperature (marten-
sitic) phase.
Ferromagnetic shape memory (FSM) materials are a new class of active mate-
rials that combine the properties of ferromagnetism with those of a diffusionless,
reversible, martensitic transformation. Materials such as SMA Ni2MnGa, which
has a cubic Heusler structure in the high-temperature austenitic phase and under-
goes a cubic-to-tetragonal martensitic transformation, clearly exhibit MSM effects.
The FSM effect refers to the reversible field-induced austenite-to-martensite trans-
formation or the rearrangement of martensitic variants by an applied magnetic
field, leading to an overall change of shape. Typically, contractile/expansive defor-
mation of the order of 6% is routinely observed in these materials. See also Lavrov
et al. (2002).
1.2.16
G
M
M
(GMM
)
IANT
AGNETOSTRICTIVE
ATERIALS
S
Magnetostrictive materials allow interchange of mechanical and magnetic energies
that, for example, produce strains in magnetic field. These strains are called mag-
netostriction
. The most advanced magnetostrictive materials (called giant magne-
tostrictive materials), such as commercially available TERFENOL-D, among other
materials, exhibit
λ
= 1000 ppm (or about 0.1% strain) in H = 80 kA/m. They are
alloys composed of iron (Fe), dysprosium (Dy), and terbium (Tb): TbxDy1-xFey.
λ
1.3
A BRIEF HISTORY OF ELECTROMOTIVE POLYMERS
Kuhn and Kunzle (Kuhn et al., 1948) of Basle University in Switzerland and
Katchalsky (1949) of Weizmann Institute of Science in Israel were among the first
scientists to discover the shape change of ionized polymers such as a polyacid or
polybase when stimulated by various pH solutions. We call this class of ionic
polymers pH-activated ionic polymer gels (or simply pH muscles).
A mechanically stressed foil of polyacrylic acid containing glycerol and H
SO
2
4
was heated to make a contractile filament. Ionization of the resulting polymeric acid
caused change of molecular shape in aqueous solution. Stretch of the polymer
molecules was attributed to electrostatic repulsive forces between carboxylic ions
present in solution. The contraction of the polymer molecule was attributed to the
Brownian motion resulting from neutralization of charged groups. In their experi-
ment, Kuhn and Kunzle and Katchalsky used copolymer of metacrylic acid with
divinyl benzene. They used an intermittent wash cycle with distilled water to remove
salt formed between application of acid and alkali. They proposed that equilibrium
swelling of the polymeric acid gels was caused by:
