Rolled-up helical nanobelts: from fabrication to swimming microrobots
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Rolled-up helical nanobelts: from fabrication to swimming microrobots Li Zhang and Bradley J. Nelson Institute of Robotics and Intelligent Systems, ETH Zurich, Tannenstrasse 3, Zurich, CH 8092, Switzerland ABSTRACT We present recent developments in rolled-up helical nanobelts in which helical structures are fabricated by the self-scrolling technique. Nanorobotic manipulation results show that these structures are highly flexible and mechanically stable. Inspired by the helical-shaped flagella of motile bacteria, such as E. coli, artificial bacterial flagella (ABFs) are a new type of swimming microrobot. Experimental investigation shows that the motion, force, and torque generated by an ABF can be precisely controlled using a low-strength, rotating magnetic field. These miniaturized helical swimming microrobots can be used as magnetically driven wireless manipulators for manipulation of microobjects in fluid and for target drug delivery. INTRODUCTION Microfabrication of helical structures is difficult for lithography-based techniques due to lithography’s inherent 2-D patterning. Previously, a number of nanohelix fabrication methods have been developed based on “bottom-up” approaches [1-3]. Recentla, a strategy that combines “top-down” and “bottom-up” approaches for fabricating 3D micro-/nanostructures has been introduced [4-5]. This method is based on the coiling of strained 2D thin films to form 3D structures after the films detach from the substrate by selective etching, a type of self-assembly. Diverse 3D micro-/nanostructures have been achieved, such as tubes [4-6], helices [4, 7-8], micro-origami [9-11] and wrinkles [12-13]. It was found that rolled-up helical nanobelts can be designed with a specific geometrical shape, i.e., their diameter, chirality, pitch, helicity angle and length can be precisely controlled [14-15]. The as-fabricated helical nanobelts are highly flexible and retain a strong “memory” of their original shape [16-20]. Because of their interesting morphology and mechanical and electromagnetic properties, potential applications of these helical nanobelts include force sensors, chemical and biological sensors, inductors, and actuators. Inspired by the natural bacterial flagellum [21-23], we have developed helical swimming microrobots driven by a rotating magnetic field [24-26]. These miniaturized devices can be used as wireless manipulators for medical and biological applications in fluid environments, such as cell manipulation and removal of tissue. Due to its large surface to volume ratio, surface functionalized ABFs have the potential to sense and transmit inter- or intracellular information, and to perform targeted drug delivery. EXPERIMENT The fabrication of rolled-up helical nanobelts is illustrated schematically in Fig. 1. Bilayer or multi-layer thin films are deposited on a single crystal wafer, such as Si or GaAs, after the deposition of a sacrificial layer. The hetero-films are grown and patterned to a ribbon-like mesa by lithography. The ribbon is then detached from the s
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