Biomedical Engineering Reference
In-Depth Information
development of soft, flexible polymer materials with actuation properties, biological-like locomo-
tion has been made possible (Breazeal and Bar-Cohen, 2003). The benefits are many, provided that
we can identify the principles that constitute the basis for biological-like locomotion. Nature can
serve as a template for future designs, given that the proper questions are asked and the potential
pitfalls are identified. The most important pitfall to consider is the fact that nature does not strive for
optimality. Natural designs are built upon their evolutionary history, which may impose consider-
able constraints. Nature's design process works on a ''good enough'' basis (Vogel, 1998). Direct
copying from Nature is likely to result in suboptimal performances; rather we should strive for
understanding the enabling principles and develop them further to achieve optimal performance
(Full and Meijer, 2001; Meijer et al., 2003).
Biomimetic design requires that engineers and biologists work closely together. To make this
collaboration work, one should understand that both fields have very different approaches as
Vincent (2004) concluded: ''Engineers look at the problem and try to find an answer, biologists
look at the answers and try to find out what the problem was.'' The starting point of any biomimetic
design should be the function to be emulated. For example, for a legged biomimetic robot, one
would like to emulate the spring-mass and pendulum characteristics that are exploited by animals
(Full and Koditschek, 1999). The technological aim here is to build mobile platforms that are
robust, agile, flexible, energy-efficient, self-sustaining, self-repairing, independent movers (no
cables), as well as adaptable to requirements set by the task and the environment. To this aim, it
is insightful to study the solutions that animals have found to meet these requirements (Full and
Meijer, 2001; Meijer et al., 2003). Moving animals exploit various energy-saving mechanisms; they
have a redundant set of actuators, they are soft and flexible, and most important they can adapt and
repair their tissues in response to injury and changing requirements. The key to successful animal
locomotion is the multi-functionality of their muscles.
Primordial biological qualities like adaptation, modularity, robustness are important principles
for R&D of new artificial muscles. They represent the basis for new developments in bionics,
mechatronics, orthotics, and prosthetics that explore the simplicity of a mechanism or material with
the complexity or sophistication of a control system mimicking the biological parts with state-of-
the-art actuators. Biomimetic control, in which adaptation of state-of-the-art actuators and design of
control systems provide new functionalities to current aids for disabled, is an important new field.
Understanding the behavior of the musculoskeletal system will lead to active or semiactive systems
for interaction with the human limbs: spring-based actuator system for a knee-ankle-foot orthosis
(KAFO) mimicking the lacking functionalities of a certain group of muscles during walking, upper
limb orthotics for active treatment of pathological tremor by means of dampers, and ultrasonic
motors compensating a certain disorder.
In recent years, material scientists have developed soft and compliant electroactive polymers
(EAP) that have actuating abilities (Bar-Cohen, 2001a,b; Kornbluh et al., 2001). It has been argued
that these novel technologies will enable the development of artificial muscles and eventually
lead to legged robots that outperform their biological counterparts (Bar-Cohen, 2001a,b; Kornbluh
et al., 2001). Preliminary comparisons between rudimentary EAP actuators and biological
muscles have revealed that their mechanical performance is comparable (Full and Meijer, 2000,
2001; Meijer et al., 2003; Wax and Sands, 1999). Specifically, it has been found that stress, strain,
and power capabilities of the EAP actuators are within or even exceed that of natural muscle
(Meijer et al., 2001, 2003). Despite the resemblance in these performance metrics, none of these
actuators could be called truly ''muscle-like'' for two reasons. First the working principle of EAP
actuators is very different from biological muscle; it will be argued in this chapter that the
uniqueness of muscle as an actuator is partly due to its contractile mechanism. Second, muscles
are complex and dynamic actuators that are capable of tailoring to specific functional demands by
modification of their structure, thus far no human-made actuator possesses this capacity for
remodeling.
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