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Engineered Muscle Actuators 249

upon the muscle tested and the method of evaluation. There are many synthetic actuator systems
with much higher power density, but in these cases often excluded is the external power supply and
related hardware that are required to drive the actuator. Examples include hydraulic and pneumatic
actuators, as well as some types of electro-magnetic actuator systems.

9.5 QUANTITATIVE ASSESSMENT OF THE FUNCTION
OF LIVING MUSCLE ACTUATORS

There are many FoM that have been formulated to quantify the performance of muscle actuators to
allow comparisons between each class of muscle actuator and synthetic actuators. These standard-
ized FoM may be employed when evaluating a new engineered muscle construct or any living
muscle-based actuator system.

9.5.1 Efficiency (Volumetric, Metabolic, Excitatory)

9.5.1.1 Volumetric

Natural muscle tissue is characterized by an extraordinarily efficient packaging of biomolecular
motors. Histologic cross-sections of healthy muscle clearly demonstrate that approximately 95% of
the muscle CSA is comprised of tightly packed filaments of biomolecular motors (the contractile
proteins actin and myosin) in a hexagonal lattice. There is little opportunity for improvement upon
nature with respect to the volumetric efficiency of the packaging of biomolecular motors into
functional macroscopic actuators. Synthetically organized contractile proteins are likely to have an
advantage only in single-molecule or molecular monolayer applications, and are likely to be
extremely disadvantaged when compared with natural muscle, in terms of volumetric efficiency.
Current cultured muscle tissues suffer from low volumetric efficiency in terms of contractile
proteins, typically 5 to 10% of the value of adult phenotype healthy control muscle. Also, muscle
actuators do not require external support machinery to operate in the same way that many synthetic
actuators do. One could reasonably argue that muscle requires many of the other physiologic
systems of the body to operate (pulmonary, cardiovascular, neural, gastro-intestinal, etc.), so
consider the relative masses of the actuators and the external support system. In an adult human,
approximately half of the body mass is muscle tissue. This is supported entirely by the remaining
mass of the body, which comprises all other physiologic systems. Compare this with hydraulic or
pneumatic systems, for example, for which the power generation system often weighs many times
the total mass of all actuators in the system.

9.5.1.2 Metabolic (Chemomechanical Transduction)

The metabolic efficiency is readily mapped into the most commonly defined form of thermodynamic
efficiency: work OUTPUT 7 energy INPUT. In the case of muscle, this would translate simply into
the mechanical work done by the muscle actuator divided by the caloric content of the fuel (e.g.,
glucose) consumed plus the energy required to excite the muscle to contract. Corrections must be
made for the glucose stored within the muscle prior to the measurement, and for this and a number of
other reasons several indirect measures of metabolism are well advised, such as lactate production.
The metabolic efficiency of the muscle actuators will of course be sensitive to many factors,
including the mechanical load, muscle phenotype, fuel source, pH, temperature, diffusion distances
within the tissues, etc. The sensitivity of the actuator to these factors should be considered, in
addition to ‘‘peak’’ or ‘‘optimal’’ efficiency values. For example, certain species of amphibians have
muscles that operate relatively efficiently over large temperature ranges.