This chapter reviews the so-called "single-molecule" approach to the design of artificial muscles. More specifically, it will focus on the development of artificial muscles that are capable of function from the bulk (macroscale) to the single-molecule (nanoscale) level. Such materials can only exist if each building block is encoded with all the information necessary to perform an expansionâcontraction (or related) cycle (single-molecule approach). Thus, a distinction between intrinsic and bulk properties must first be addressed. Furthermore, the systems reviewed herein are limited to well-defined organic molecules or macromolecules that have some analogy to biological systems. For the sake of clarity, this section uses the term engine to include muscles and all other related forms of biological devices that can transduce one form of energy to mechanical energy. An excellent review on biological engines has recently appeared in the literature, and describes not only muscles, but also biological springs and ratchets [Mahadevan et al, 2000]. All such engines have synthetic counterparts at the molecular level, and are described herein. It should be noted that carbon nanotubes, another class of well-defined actuators, are discussed elsewhere in this book.
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