Biomimetic Hair Sensor Arrays: from Inspiration to Implementation

In this work, we report on the successful implementation of highly sensitive artificial hair-based flow-sensor arrays for sensing low-frequency air flows. Artificial hair sensors are bio-inspired from crickets’ cercal filiform hairs, one of nature’s best in sensing small air flows. The presented artificial hair sensor arrays aim for sensing performances on a par with the actual insects by means of model-based design optimizations and are fabricated using advanced MEMS technologies. Crickets have a pair of abdominal appendages called cerci, comprising of numerous mechano-receptive hair sensors, which respond even to the slightest air movements (ca. 0.03 mm/s), effectively operating at thermal noise threshold [1]. Figures 1 and 2 show the magnified image of a cricket’s cerci and the schematic representation of an artificial hair sensor, respectively. Each flow sensor has a suspended silicon nitride membrane with Aluminium electrodes with a long SU-8 hair mounted on its top. Air flows induce deflection on the SU-8 hair and consequently tilt the silicon nitride membrane, which is sensed by differential capacitive read-out. By means of model-based optimization [2], the presented sensor arrays feature: (i) hairs designed for higher drag-torque reception and for reduced moment of inertia; (ii) modified spring geometry for small torsional stiffness and (iii) improved capacitive sensing elements. The sensitivity and directionality measurements were done using a loudspeaker as the flow source and a lock-in amplifier was used to measure the amplitude and phase of the capacitive-based sensor response. The fabricated sensor arrays (Figure 3) show a clear figure-ofeight response, indicating a preferred directivity. The sensor arrays also display a remarkable performance with sensitivities to oscillating air flows down to 0.85 mm/s that surpass noise levels even at 1 kHz operational bandwidths (Figure 4). In conclusion, we report significant advancements of our artificial hair sensorarrays and insights for further improvements. References[1] T. Shimozawa, J. Murakami, T. Kumagai, “Cricket wind receptors: thermalnoise for the highest sensitivity known”, Chapter 10 in Sensors and Sensing inBiology and Engineering, ed. Barth, Hamphry and Coombs, Springer, Vienna,2003. [2] G.J.M. Krijnen, A. Floris, M. Dijkstra, T.S.J. Lammerink, R. Wiegerink,“Biomimetic micromechanical adaptive flow sensor arrays”, SPIE Microtechnologiesfor the New Millennium, Gran Canaria, 2007, pp. 6592-6616 Figure 1: Crickets’ cerci containing numerousmechano-receptive filiform hairs. [Courtesy:G. Jeronimides, University of Reading, UK]Figure 2: Schematic representation of anartificial flow sensor with differentialcapacitive flow sensing upon flow-drivenmembrane tilts. Figure 3: SEM images of the fabricatedartificial hair sensor arrays.Figure 4: Sensitivity measurements at threedifferent frequencies, with the horizontal linesindicating the RMS noise level for differentbandwidths.