Tiny hairlike sensors or cilia play a very important role in detection for many biological species, including humans. This research took inspiration from the packaging and transduction processes of the inner ear's cochlea and cilia to design acoustic sensors. Specifically, this work uses nanowires of magnetostrictive materials as artificial cilia to sense acoustic signals. Anodic aluminum oxide (AAO) templates with hexagonal spacings were fabricated using a two-step anodization process as well as nanoimprint assisted self-assembly and were characterized using atomic force microscopy. Patterned microelectrodes were also fabricated at the backside of several templates using photolithography. Ni, Co, and Galfenol (Fe1-x Gax 0.1≤x≤0.25 at. %) nanowires were fabricated using electrochemical deposition into nanoporous AAO templates where the pores had various geometries and some had large-area ordering as dictated by nanoimprinting. High aspect ratio nanowires with diameters varying from 10 to 200 nm and lengths up to 60 μm were fabricated in arrays and were collectively and individually characterized using scanning electron microscopy. Galfenol thin films, fabricated electrochemically using a Hull cell, were characterized using x-ray diffraction and energy dispersive x-ray spectroscopy to determine the optimum current density for deposition. The magnetic response of millimeter-scale cantilevered beams to dynamic bending loads was also measured and compared to constitutive and free-energy models. A giant magnetoresistive sensor behind the beam measured the magnetic response of mechanical excitation applied to the tip of each rod and validated the models. Potenial applications of these nanowire cilia include sonar arrays, underwater cameras, and medical devices.
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We would like to thank Norris Lindsey, Marilyn Wun-Fogle, and Jim Restorff from NSWC, Carderock Division for stimulating discussions on electrochemistry and magnetostriction. We are also grateful to Etrema Products Inc. of Ames, IA for insight into Galfenol alloys and for donated Galfenol materials, and to Nonvolatile Electronics of Eden Prairie MN for GMR sensors. Finally, we would like to thank the Office of Naval Research for funding through Grant Nos. N000140310953 and N000140410583 and the National Science Foundation for Grant No. CMS0329975. Parts of this work were carried out in the University of Minnesota Nanofabrication Center and Characterization Facility which receive partial support through the NNIN Program.