Page 354 - Control Theory in Biomedical Engineering

320 Control theory in biomedical engineering

(Fig. 1B) (Taghavi et al., 2018). Many experiments have been conducted
with the thermal activation properties of nonconductive shape memory
polymers (SMP) (Balasubramanian et al., 2014; Clark et al., 2010; McKnight
et al., 2010; Shan et al., 2013), coiled fibers (Haines et al., 2014), thermo-
plastics (including wax soaked) (Cheng et al., 2014; McEvoy and Correll,
2015), and low-melting point alloys (Schubert and Floreano, 2013; Shan
et al., 2013; Taghavi et al., 2018) through the means of external heating ele-
ments or through self-joule heating. Based on the material properties and
stiffness-varying mechanisms, these materials can be classified into either
phase change materials (melting) or glass transition behavior (softening).
The process of thermal activation is accelerated when the composite is
embedded with an external heating element, which can remain conductive
even under high levels of deformation. This is possible using microfluidic
channels of liquid-phase metals, which can also be sewn into fabrics to create
thin and modular variable stiffness structures (Taghavi et al., 2018). While
this method of phase change materials seems attractive, there are challenges
associated with it, especially in the context of surgical robotics. Using such
microfluidic channels and the need for high heat inside the human body
introduces sealing and insulation problems, requiring one to develop a
means for effective separation of the thermally responsive material. Methods
of heat dissipation must also be included, which increases the bulkiness of the
device (Shan et al., 2015). Furthermore, phase change materials have low
bandwidth and need relatively larger amounts of time to input and output
thermal energy into the system (Taghavi et al., 2018). Thus, they cannot be
used for fast-changing applications.

2.3 Jamming methods
Another class of tunable stiffness device involves jamming flexible actuation
methods using friction or negative pressure, resulting in a change from a
compliant to a rigid state. Jamming is broadly classified into particle/granular
jamming and layer jamming methods. In granular jamming, many tiny solid
grains/particles act as “fragile matter.” In the absence of external stress, the
bulk matter is free to move, and the granule system can act as a fluid-like
compliant structure. Upon application of a vacuum, the granules jam
together and transition into a solid-like state, thereby increasing its stiffness
( Jiang et al., 2014). There already exists numerous applications of granular
jamming, especially in the context of industrial robotic arms and manipula-
tors, among others (Brown et al., 2010; Loeve et al., 2010). However, these