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Biomimetics of Muscle Design 51

The rules governing muscle adaptation are complex and far from being resolved (Russell et al.,
2000). Regulatory pathways are triggered by growth signals (mechanical, hormonal), resulting in
gene transcription followed by translation and assembly of the proteins into the contractile
architecture (Russell et al., 2000). Several myogenic regulatory factors are involved in the remod-
eling of muscle, they are triggered by multiple signals and they can activate or inhibit each other’s
action (Brooks and Faulkner, 2000). Teasing out the exact relationships is experimentally difficult
and time consuming. As a consequence, our understanding of the adaptation laws at the molecular
level is still fragmentary. Modeling approaches might be helpful in understanding the intricate
relationships (Jacobs and Meijer, 1999)

2.6 BIOMIMETICS OF MUSCLE DESIGN

It is unlikely and probably undesirable that future polymer actuators will use the exact working
principles as the contractile mechanism of biological muscle. Consequently, current research
focuses on the design of polymer actuators that mimic the functionality of muscle based on
alternative working principles (Bar-Cohen, 2001b; Kornbluh et al., 2001; Meijer et al., 2003). It
is argued in this chapter that it might be useful to look at the design principles that enable the variety
in muscle function. Unlike current EAP actuators, muscle design is modular. Muscle function is
achieved by concerted action of thousands of functional units called sarcomeres. It has been shown
that muscle function is shaped by sarcomere design and arrangement. Hence, an evaluation of the
benefits of sarcomeric design in relation to synthetic muscle design may be useful.
Robustness is an important requirement for an actuator. It is crucial that an actuator does not
breakdown while functioning, in other words it needs to avoid mechanical failure. Biological
materials are remarkably tough, meaning that it requires a lot of energy to break them. They
achieve this by using energy release mechanisms that help to avoid crack propagation. As a
consequence, small failures do not become catastrophic (Gordon, 1976). Although there is little
data on the fracture mechanics of muscle, it can be argued that the sarcomere design of muscle
helps to avoid small injuries that may make the muscle nonfunctional. It is well known that
muscle injury in response to tensile stresses results in local disruptions of sarcomeres. These
lesions are local and do not seem to propagate through the muscle. Morgan (1990) provided an
explanation for these lesions and their functional consequences in what is now known as the
‘popping sarcomere’ theory. He proposed that sarcomeres that are subjected to high tensile stress
undergo rapid lengthening that is stopped by the structures responsible for the passive tension of
muscles (titin, external membranes). The popping has three functional consequences: (1) the
rapid lengthening releases some of the energy, (2) the lengthened sarcomere will act as a
spring in series with the remaining sarcomeres and will be able to withstand higher tensile
stresses, and (3) the remaining sarcomeres will shorten somewhat and increase their strength as a
consequence they will be able to withstand higher tensile stresses as well. In other words, under
high tensile stresses individual sarcomeres will be sacrificed to maintain the structural integrity of
the muscle. From experience it is known that some EAP actuators break very easily under tensile
stresses, it could be argued that a modular design might help to increase the robustness of these
actuators.
The modular design of muscle also facilitates the remodeling and repair of the muscle. The self-
healing properties of muscle emerge from the integration of muscles into a system that allows
wound healing and continuous turnover via transport of nutrients and removal of waste products. It
is arguably much simpler to grow and repair individual units than having to adapt the entire
structure. Furthermore, it may be argued that the variety in designs is facilitated by the modular
design — just like Lego enables designs only limited by one’s imagination. Until recently,
remodeling and repair was only feasible within the domain of biological materials and systems.
However, recent innovations in material science have resulted in self-repairing polymers (Wool,