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Bioinspired Carbon Nanotube Fuzzy Fiber Hair Sensor for Air-Flow Detection Matthew R. Maschmann, Gregory J. Ehlert, Benjamin T. Dickinson, David M. Phillips, Cody W. Ray, Greg W. Reich, and Jeffery W. Baur* Sensory hairs are common throughout the natural world, serving diverse functions in varied environments. For example, crickets utilize finely tuned hair arrays called celia to rapidly detect air-flow disturbances from approaching predators.[1] Locusts use precisely arranged sensory hairs on their heads and directional impulses to navigate in flight.[2] Hairs located on the wings of bats are thought to detect the flow pattern of air during flight for enhanced navigation and aerobatic-like flight control.[3] This rapid detection of small-scale air-flow variations via the hair shaft deflection of a single sensor or as part of distributed arrays contributes to natural fliers having greater flight agility than current engineered systems and is the inspiration for the development of bioinspired flow-sensing systems. Biologically inspired artificial hair sensors (AHS) attempt to mimic the awareness and rapid response to external fluid flows of natural flyers. Engineered AHS devices have taken diverse forms to emulate biology. Most AHS designs are silicon-based and are fabricated using MEMS technology in order to fabricate feature sizes comparable to biological hairs (10–100 μm diameter, 1 mm length). Mechanisms for measuring hair shaft displacement can include piezoresistive[4–12] transduction, capacitive[13–17] detection, and more exotic methods like lipid bilayer current generation.[18] Typical hair shaft materials include polymer,[8–16] metals,[17] or microcantilever semiconducting Dr. M. R. Maschmann, Dr. G. J. Ehlert, Dr. J. W. Baur Air Force Research Laboratory Materials and Manufacturing Directorate AFRL/RX, Wright Patterson Air Force Base OH 45433, USA E-mail: [email protected] Dr. M. R. Maschmann Department of Mechanical and Aerospace Engineering University of Missouri Columbia, MO 65211, USA Dr. D. M. Phillips Universal Technology Corporation Beavercreek, OH 45424, USA Dr. B. T. Dickinson Air Force Research Laboratory Munitions Directorate AFRL/RW, Eglin Air Force Base FL 32542, USA Dr. D. M. Phillips, Dr. C. W. Ray, Dr. G. W. Reich Air Force Research Laboratory Aerospace Systems Directorate AFRL/RQ, Wright Patterson Air Force Base OH 45433, USA
DOI: 10.1002/adma.201305285
3230
wileyonlinelibrary.com
films.[4–7] Many designs enhance the sensing of hair deflection by using extended surfaces in the transduction region[4,5,7,9–17] or through additional deflection of the area (cupula) surrounding the hair,[4,12] adding additional complexity and mass to the sensor assembly. The sensitivity for detecting flow using AHS devices is approaching that of biological hairs, with minimal dectable limits on the order of 1 mm s−1 in air[17,19] and 0.1 mm s−1 in water.[20] These sensors demonstrate that it is possible to achieve sensitive biolike flow sensing, but their typical complex fabrication, fragility, large physical footprint, and often unidirectional sensing capability are currently prohibiting their application into integration systems. We introduce a highly-sensitive “hair-plug”-style AHS device for the omnidirectional detection of low-speed air-flow. An assembled device consists of a single CNT-coated microfiber (7–25 μm diameter) embedded in a glass microcapillary, avoiding MEMs fabrication techniques. Device fabrication and assembly followed the procedure found in Figure 1a. Briefly, Au/Pd electrodes were sputtered onto a glass microcapillary using a silicone rubber sheet as a mask to define top and bottom electrical contacts. A single glass microfiber was then inserted into the capillary, and CNTs are selectively grown on to the glass fiber using an injection catalyst chemical vapor deposition (CVD) method, as detailed in the Experimental Section. External stimulation of the hair shaft imparts localized compression of the piezoresistive CNT coating[21,22] against sensing electrodes located on the wall of a microcapillary pore which function as a notional “nerve cell.” The AHS hair plug can easily be intergrated into a membrane or surface through potting into a host materials, including flexible and deformable materials such as silicone rubber (see Supporting Information). The footprint of the integrated device is determined by the outer diameter of the microcapillary, typically between 150–350 μm, as seen in Figure 1c. The sensor is highly durable, capable of withstanding angular deflection exceeding 45 angular degrees in any direction without inflicting sensor damage (see Figure 1d). The responsiveness of the AHS to steady boundary layer air-flow was examined using an optimized Blasius flat plate[23] mounted in an open-test-section low Reynolds number wind tunnel, shown schematically in Figure 2a. The pitch angle was held at 0° for all tests with free stream velocity ranging from 0 to 10 m s−1. The hair-plug sensor was integrated into a polycarbonate disc assembly and mounted flush with the test surface 35 cm from the leading edge of the plate. The boundary layer profile over the plate was thoroughly characterized with a boundary layer hotwire anemometer and was found to agree well with laminar flow theory.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 3230–3234
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COMMUNICATION Figure 1. a) Schematic of AHS assembly and b) low magnification SEM image of a CNT-coated glass fiber AHS “hair-plug” sensor. c) Photograph of 150 μm and 350 μm diameter capillary sensors, placed next to a dime for scale. d) Overlaid SEM images of potted in situ hair shaft tip deflection by 1 mm.
The AHS response to the free stream velocity change was a change in resistance, closely resembling the ramps and constant velocity holds of the free stream cycling over nominal velocities ranging from 100 μg), low power (
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Bioinspired Carbon Nanotube Fuzzy Fiber Hair Sensor for Air-Flow Detection Matthew R. Maschmann, Gregory J. Ehlert, Benjamin T. Dickinson, David M. Phillips, Cody W. Ray, Greg W. Reich, and Jeffery W. Baur* Sensory hairs are common throughout the natural world, serving diverse functions in varied environments. For example, crickets utilize finely tuned hair arrays called celia to rapidly detect air-flow disturbances from approaching predators.[1] Locusts use precisely arranged sensory hairs on their heads and directional impulses to navigate in flight.[2] Hairs located on the wings of bats are thought to detect the flow pattern of air during flight for enhanced navigation and aerobatic-like flight control.[3] This rapid detection of small-scale air-flow variations via the hair shaft deflection of a single sensor or as part of distributed arrays contributes to natural fliers having greater flight agility than current engineered systems and is the inspiration for the development of bioinspired flow-sensing systems. Biologically inspired artificial hair sensors (AHS) attempt to mimic the awareness and rapid response to external fluid flows of natural flyers. Engineered AHS devices have taken diverse forms to emulate biology. Most AHS designs are silicon-based and are fabricated using MEMS technology in order to fabricate feature sizes comparable to biological hairs (10–100 μm diameter, 1 mm length). Mechanisms for measuring hair shaft displacement can include piezoresistive[4–12] transduction, capacitive[13–17] detection, and more exotic methods like lipid bilayer current generation.[18] Typical hair shaft materials include polymer,[8–16] metals,[17] or microcantilever semiconducting Dr. M. R. Maschmann, Dr. G. J. Ehlert, Dr. J. W. Baur Air Force Research Laboratory Materials and Manufacturing Directorate AFRL/RX, Wright Patterson Air Force Base OH 45433, USA E-mail: [email protected] Dr. M. R. Maschmann Department of Mechanical and Aerospace Engineering University of Missouri Columbia, MO 65211, USA Dr. D. M. Phillips Universal Technology Corporation Beavercreek, OH 45424, USA Dr. B. T. Dickinson Air Force Research Laboratory Munitions Directorate AFRL/RW, Eglin Air Force Base FL 32542, USA Dr. D. M. Phillips, Dr. C. W. Ray, Dr. G. W. Reich Air Force Research Laboratory Aerospace Systems Directorate AFRL/RQ, Wright Patterson Air Force Base OH 45433, USA
DOI: 10.1002/adma.201305285
3230
wileyonlinelibrary.com
films.[4–7] Many designs enhance the sensing of hair deflection by using extended surfaces in the transduction region[4,5,7,9–17] or through additional deflection of the area (cupula) surrounding the hair,[4,12] adding additional complexity and mass to the sensor assembly. The sensitivity for detecting flow using AHS devices is approaching that of biological hairs, with minimal dectable limits on the order of 1 mm s−1 in air[17,19] and 0.1 mm s−1 in water.[20] These sensors demonstrate that it is possible to achieve sensitive biolike flow sensing, but their typical complex fabrication, fragility, large physical footprint, and often unidirectional sensing capability are currently prohibiting their application into integration systems. We introduce a highly-sensitive “hair-plug”-style AHS device for the omnidirectional detection of low-speed air-flow. An assembled device consists of a single CNT-coated microfiber (7–25 μm diameter) embedded in a glass microcapillary, avoiding MEMs fabrication techniques. Device fabrication and assembly followed the procedure found in Figure 1a. Briefly, Au/Pd electrodes were sputtered onto a glass microcapillary using a silicone rubber sheet as a mask to define top and bottom electrical contacts. A single glass microfiber was then inserted into the capillary, and CNTs are selectively grown on to the glass fiber using an injection catalyst chemical vapor deposition (CVD) method, as detailed in the Experimental Section. External stimulation of the hair shaft imparts localized compression of the piezoresistive CNT coating[21,22] against sensing electrodes located on the wall of a microcapillary pore which function as a notional “nerve cell.” The AHS hair plug can easily be intergrated into a membrane or surface through potting into a host materials, including flexible and deformable materials such as silicone rubber (see Supporting Information). The footprint of the integrated device is determined by the outer diameter of the microcapillary, typically between 150–350 μm, as seen in Figure 1c. The sensor is highly durable, capable of withstanding angular deflection exceeding 45 angular degrees in any direction without inflicting sensor damage (see Figure 1d). The responsiveness of the AHS to steady boundary layer air-flow was examined using an optimized Blasius flat plate[23] mounted in an open-test-section low Reynolds number wind tunnel, shown schematically in Figure 2a. The pitch angle was held at 0° for all tests with free stream velocity ranging from 0 to 10 m s−1. The hair-plug sensor was integrated into a polycarbonate disc assembly and mounted flush with the test surface 35 cm from the leading edge of the plate. The boundary layer profile over the plate was thoroughly characterized with a boundary layer hotwire anemometer and was found to agree well with laminar flow theory.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 3230–3234
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a) Schematic of AHS assembly and b) low magnification SEM image of a CNT-coated glass fiber AHS “hair-plug” sensor. c) Photograph of 150 μm and 350 μm diameter capillary sensors, placed next to a dime for scale. d) Overlaid SEM images of potted in situ hair shaft tip deflection by 1 mm.
The AHS response to the free stream velocity change was a change in resistance, closely resembling the ramps and constant velocity holds of the free stream cycling over nominal velocities ranging from 100 μg), low power (
