Molecular motors in materials science
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oduction Molecular motors utilize chemical fuel or light to do mechanical work in a cyclic process, and therefore can serve as key components of active materials.1–5 This article introduces the body of materials science research inspired by this, and provides our perspective on the promises of and challenges for materials incorporating molecular motors. Nature provides the proof of principle for the transformative potential of such materials, in the form of muscle tissue, for instance.6 Muscle hierarchically integrates myosin motor proteins and actin filaments into large-scale arrays that multiply the force and displacement generated by each individual motor protein7 (Figure 1a–b).8–10 The evolutionary process has solved complex engineering problems related to, for example, the provision of fuel, the control of motor activity, and the maintenance of function.11 The reward for these engineering accomplishments is maybe best appreciated by a thought experiment where we subtract muscle from the natural world. Animal life from the smallest insect12 to the largest whale13 (Figure 1c) would come to a standstill, because muscle drives the circulation of blood, enables motion, assists digestion, and tunes sensory systems. Notably, skeletal muscle has a “unit cell,” the sarcomere, consisting of a bundle of myosin motors (the “thick filament”) connecting two actin filaments (the “thin filaments”) capable
of active contraction, but only passive extension. A hexagonal arrangement of hundreds of thick and thin filaments spaced about 50 nm apart forms the sarcomere with a length between 1.6 µm (contracted) and 2.2 µm (relaxed).14 The sarcomere can be considered as the fundamental microstructural unit, which is then integrated into multinucleated cells, fibers, and fiber bundles (“fascicles”) as larger structural units capable of exerting contractile stresses on the order of 100 kPa and achieve strain rates on the order of 10 s–1.15–17 Muscle can thus be seen as a soft, active metamaterial—a repeating structure of identical subunits with the ability to reversibly contract against force in one dimension. In contrast to the linear actuation of muscle, engineered motors often first generate rotary motion, which is subsequently converted into linear motion by a system of gears or hydraulics.18 In nature, rotary motion at the molecular scale exists (e.g., in the F0F1-ATPase19 or the bacterial flagellar motor).20 However, rotors are difficult to couple together, so it is hard—but not impossible as we discuss later—to assemble a force-producing material from rotary subunits.21 Contractile materials can be assembled without molecular motors, with piezoelectric ceramics being one widely used example.22 An obvious question is what is the added benefit of employing molecular motors to create active materials? In an engineering world increasingly dominated by electricity,
Henry Hess, Columbia University, USA; [email protected] Parag Katira, San Diego State University, USA; [email protected] Ingmar H. Riedel-Kruse, Stanford University, USA; i
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