Engineered Muscle Actuators: Cells and Tissues - Biomimetics: Biologically Inspired Technologies - page 245

objective. The remainder of this chapter will discuss many of the reasons why living muscle is being
given serious consideration for use as a mechanical actuator in hybrid robotic systems, as well as
the many special considerations involved when attempting to employ living actuators in
an engineered biohybrid system. The incorporation of functional living elements into otherwise
synthetic engineered systems is called biomechatronics .

9.2 SYSTEMS ENGINEERING OF LIVING MUSCLE ACTUATORS

Tissue engineering of skeletal muscle could be broadly defined to include any alteration to or
enhancement of the musculature of a living organism. This definition, though interesting, would not
be specific enough to be useful, as it would include the agricultural use of steroids to rapidly
increase the total lean body mass of livestock, the use of resistance training by athletes to induce
hypertrophy, and surgical procedures including transplants and flaps in which preexisting
skeletal muscle is modified and utilized in clinically relevant procedures (including graciloplasty,
cardiomyoplasty, and musculoskeletal reconstructive surgery). Though all of these approaches to
the modification and use of skeletal muscle are of interest, this chapter will only address skeletal
muscle tissue engineering to generate functional muscle tissues actuators.
Successful tissue engineering must include a focus on the organization of large numbers of cells
into higher-order structures that confer emergent properties, which are an important aspect of the
tissue-level function. Thus, the engineering of functional tissues is by definition within the domain
of ''systems engineering.'' These living structures may be known as tissues or organs depending on
the level of anatomical complexity and structural integration. Though all tissue functions arise from
fundamental cellular mechanisms, the organization of tissues and organs confers function that is not
possible to achieve with individual cells or masses of unorganized cells in a scaffold. By analogy, a
pile of bricks does not provide the functionality of a house, nor does a crate full of car parts function
as an automobile. Furthermore, when removed from an organism, muscle tissue in general does not
persist for long periods. Isolated from its proper environment, muscle tissue tends to degenerate
rapidly. The environment that is required to maintain healthy, adult phenotype muscle is highly
complex and incompletely understood, involving many chemical, structural, and mechanical
signals. In order to understand both natural and tissue-engineered skeletal muscle, we must have
a clear working definition of muscle function and understand how the structure of muscle contrib-
utes to the emergence of that function. A major challenge facing the use of muscle tissue as a
practical living actuator is the identification of suitable tissue interfaces to allow the application of
external cues (such as mechanical forces and growth factors) to guide tissue development and to
allow the controlled generation of mechanical power.

9.3 MUSCLE: NATURE'S ACTUATOR

Skeletal muscle accounts for nearly half of the total mass of the average adult human and is unique in
its ability to actively modify its mechanical properties within tens of milliseconds to allow animals to
rapidly react to their environment. Muscle tissues have evolved over the last several billion years as
nature's premier living generators of force, work, and power. The success of muscle tissue actuators
hinges in part upon the very favorable efficiency of biomolecular motors. Biomolecular motors are
the mechanically functional units of muscle cells and tissues, providing motility and mobility for
organs and organisms. Muscle cells (also known as muscle fibers ) serve to self-organize, maintain
and repair, and control the mechanical actions of large arrays of biomolecular motors. The tremen-
dous plasticity of form of muscle actuators is first realized at the level of cells: biomolecular motors
are added in parallel to allow greater force generation, and are added in series to permit more rapid
movements over larger displacements. Damaged biomolecular motors are repaired or replaced by