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Engineered Muscle Actuators 251

these systems can employ commercially available components that are typically used in muscle
physiology research. For smaller muscles, or muscles at early stages of development, it is in general
necessary to build many of the components. The necessary instrumentation and methods have been
reviewed by Dennis and Kosnik (2002).
In addition to the quantitative performance FoM above, the following additional evaluation
tools can be employed to quantitatively evaluate living muscle actuator systems:

. Functional resilience: Total work output capacity per unit mass of actuator over the functional
lifetime of the actuator (J/kg) lifetime .
. Cellular function and phenotype, as determined by quantitative histology and molecular biology.
This will include the presence of known adult isoforms of myosin heavy chain, mitochondrial
density, prevalence of central nuclei in myotubes and muscle fibers, cross-sectional density of
contractile protein lattice, and indications of cellular necrosis or apoptosis.
. Failure mode analysis and the demonstrated efficacy of countermeasures that have been engineered
into the living actuator system.

9.6 PRACTICAL CONSIDERATIONS FOR THE
USE OF LIVING MUSCLE ACTUATORS

When considering the use of living muscle in an engineered system, it is important to take into
account a number of factors that are generally not significant challenges in traditional mechatronic
system design.

9.6.1 Fuel Sources

Muscle ex vivo can operate on a range of fuel sources that are inexpensive and readily available.
Ultimately, biomechatronic designers envision the ability of a robot to eat while it travels, much
like a fish or a horse. It will be necessary to experimentally evaluate each fuel source for use with
muscle powered actuators, since each fuel source has practical limitations and advantages. These
include specific energy (kJ/kg), solubility, thermal lability, chemical stability, toxicity, transmem-
brane transport ability in the absence of systemic metabolic modulatory hormones, and second-
order effects such as undesired chemical reactivity. The two principal fuel groups utilized by living
muscle are fatty acids and sugars. Evaluation criteria should include quantitative comparisons of
muscle actuator efficiency and contractility.

9.6.2 Failure Modes

Based upon our experience with each class of living muscle actuators, the following modes of
failure have been identified. For each failure mode, the muscle actuator class(es) that are subject to
the failure mode is identified, the theoretical basis for the failure is addressed with supporting
experimental verification (if available), and corrective actions are proposed that could be imple-
mented in a biomechatronic system.

9.6.2.1 Septic Degradation of Tissue Structure

This mode of failure affects all types of muscle actuators and at room temperature typically results
in rapid functional deterioration within 24 h in the absence of countermeasures. Barrier asepsis is
probably not practical in the ultimate field applications of living muscle actuators. Chemical
countermeasures using broad-spectrum antibiotic or antimycotic formulations in the culture
media are effective (Dennis et al., 2000, 2001). These are commercially available for tissue and