Page 353 - Control Theory in Biomedical Engineering

Tunable stiffness using negative Poisson's ratio 319

Changing magnetic
field intensity

Pull

Catheter MR fluids container -

Magnetically responsive Thermally responsive
variable stiffness variable stiffness
(A) (B)
Fig. 1 Two typical mechanisms for stiffness tuning: (A) Variable stiffness principle with
magnetically responsive material in action; (B) Construction of a phase change variable
stiffness material with microchannels filled with a low melting point.

properties, which can be exploited for use in variable stiffness (Chen et al.,
2007; Liu et al., 2001; Wu et al., 2010; Zhou, 2003). The advantage of using
such fluids is that the orientation of particles can be changed on the order of
milliseconds, thus allowing for a quick way to achieve stiffness tunability
(Lindler and Wereley, 1999). There is a wide range of uses for such tunable
stiffness fluids, and their engineering applications have been explored in var-
ious studies (Ditaranto, 1964; Nakamura et al., 2002; Parthasarathy and
Klingenberg, 1996; Wei et al., 2007; Yeh and Shih, 2005). There is also lit-
erature available that details the process of preparing such ER/MR fluids and
the integration of these fluids with devices as a variable stiffness module
(Behbahani and Tan, 2017; Carlson and Jolly, 2000; Hao, 2002; Parthasar-
athy and Klingenberg, 1996; See et al., 2001; Tangboriboon et al., 2009;
Varga et al., 2006), and the integration of this fluid into a device to be used
as a variable stiffness module. However, the methods used to prepare ER/
MR fluids involve complex setups and multiple treatment processes. Addi-
tionally, these methods require highly controlled environments, where even
the slightest impurities can render the fluids unusable. Furthermore, integra-
tion of the ER/MR fluid into a modular device requires complex fabrica-
tion methods. Lastly, the application of an electric or a magnetic field
typically requires a bulky setup.

2.2 Phase change materials

Phase-changing materials comprise another field of tunable stiffness mate-
rials. The stiffness of the material is altered by either atmospheric or direct
joule heating, which leads to a reversible transition from solid to liquid phase