Page 51 - Handbook of Biomechatronics

Actuator Technologies 45

Fig. 6 McKibben muscles. (Reproduced with permission from Daerden, F., Lefeber, D.,
2002. Pneumatic artificial muscles: actuators for robotics and automation. Eur. J. Mech.
Environ. Eng. 47 (1), 11–21.)

3.1.3 Shape Memory Alloys
The term “smart materials” is often used to describe materials with inherent
transduction behavior. These materials all change shape in response to
applied energy, with different mechanisms governing the properties of each
material: SMAs change shape when exposed to temperature or magnetic
field changes, piezoelectric materials deform in response to an electric field
(and vice versa), and magnetostrictive materials deform in response to mag-
netization (and vice versa). These transduction behaviors enable smart mate-
rials to be used for both actuation and sensing—sometimes simultaneously.
Each class of materials has different advantages and disadvantages—and
sometimes, combinations of materials provide the best blend of features.
SMAs earned their name from their ability to “remember” an original
shape: when in a deformed state, they respond to thermal or magnetic stim-
uli by returning to their original shape. This shape memory effect is possible
because SMAs have two stable solid phases with different crystal structures.
The phase transformation is stimulated by temperature changes, which
are typically achieved by applying electrical current (certain types of alloys
also respond to magnetic fields). The phase transformation occurs even in
the presence of heavy loading, which makes SMAs good candidates for
actuators. For two excellent reviews of SMAs, see Mohd Jani et al.
(2014) and (2017).
The advantages of SMAs include their high power-to-weight ratio,
noiseless operation, biocompatibility, and the ability to form nearly any
shape. Because of their inherent material properties, SMAs can be formed
into three-dimensional actuators with unique shapes such as helical springs
(Figs. 7 and 8 show two possible forms). The most common SMA, Nitinol,
is often used in medical devices such as stents, catheters, and surgical tools.