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244 Biomimetics: Biologically Inspired Technologies

9.9.1 Vascular Tissue Interface...................................................................................................... 256
9.9.2 Strategies for Engineering Functional Vascularized Muscle Tissue.................................... 256
9.9.2.1 Recellularization of an Acellular Muscle Construct............................................ 257
9.9.2.2 Coculture Systems ................................................................................................ 257
9.9.2.3 Induced Microvessel Sprouting............................................................................ 257
9.9.3 Engineered Tissue Interface: Tendon ................................................................................... 258
9.9.4 Nerve–Muscle Interfaces ...................................................................................................... 258
9.9.5 Tissue–Synthetic Interfaces .................................................................................................. 258
9.10 Muscle Bioreactor Design for the Identification, Control, and Maintenance of Muscle Tissue ...... 259
9.11 Case Study in Biomechatronics: A Muscle Actuated Swimming Robot .......................................... 260
9.12 Concluding Remarks .......................................................................................................................... 262
Further Reading .............................................................................................................................................. 262
References....................................................................................................................................................... 264
Websites.......................................................................................................................................................... 266

9.1 INTRODUCTION

Muscle tissue mechanical actuators have evolved over millions of years within animals as nature’s
premier living generators of force, work, and power. The unparalleled efficiency and plasticity of
form of living muscles arise from the properties of biomolecular motors. Muscle cells serve to self-
organize, maintain and repair, and control the mechanical actions of large arrays of biomolecular
motors. Muscle tissues provide the chemomechanical interface between muscle cells and the
environment. It is at the tissue level that muscle becomes a practical, responsive, and robust
actuator because of the presence of the critical tissue interfaces: the neuromuscular interface, the
myotendinous junction (MTJ), and the vascular bed. Tendon tissue is an extension of the muscle
extracellular matrix (ECM) and muscle tendon junctions at the end of each muscle fiber.
The mechanical structures that make up this transition from muscle to tendon are critical for the
transduction of force, work, and power between muscle tissue and the external environment.
Systematic derangements of these structures at any level result in severe and sometimes lethal
disease resulting from the impairment of the contractility of skeletal muscle and the increased
susceptibility to contraction-induced injury.
A detailed description of the biology of muscle development and morphology is beyond the
scope of this chapter. The interested reader is referred to the definitive text on this subject: Myology
(Volume I, Chapters 1, 2, 3, and 4, Engel and Armstrong, eds., 1994, McGraw-Hill).
Embedded within the genetic code of naturally occurring muscles lies the potential to build
mechanical actuators that are adaptable, self-healing smart materials (with integrated sensors for
position, force, and velocity) from the submillimeter to meter size scale in the form of tubes, rods,
sheets, hollow spheres, cones, and many other physical configurations. The key to engineering
efficient, robust, and practical muscle actuators lies in understanding the mechanisms by which to
control muscle phenotype, that is, the size, shape, fiber type, and architecture of the muscle tissue
itself. The environmental signals that control muscle phenotype are mediated by the tissue
interfaces, and thus it is critical to understand, and to ultimately engineer, adequate tissue interfaces
for muscle actuators.
There are four basic approaches (classes) to engineering functional living muscle actuators: the
use of whole surgically explanted muscles, recellularized muscle within a muscle-derived ECM,
scaffold-based engineered muscle, and self-organized muscle tissue engineered culture. When
considering muscle tissue as a functional element in an engineered system it is important to
formulate well-defined quantitative Figures of Merit (FoM). It is also important to note that at
the time of this writing, practical living muscle actuators are an as-yet unachieved research