This paper reports on recent developments to increase the performance of biomimetic flow-sensor arrays by means of several technological advancements in the fabrication procedures and corresponding sensor design optimizations. Advancements include fabrication procedures with higher process latitude and geometrical modifications of several parts of the flow sensor. The conclusive measurements in this paper support our sensormodel predictions for a 100-fold increase in acoustic sensitivity (down to oscillating flow amplitudes in the order of 1 mm⋅s-1) translating to substantially higher capacitive outputs in comparison to our first-generation biomimetic flow-sensor arrays.

In this paper we report on the latest developments in biomimetic flow-sensors based on the flow sensitive mechanosensors of crickets. Crickets have one form of acoustic sensing evolved in the form of mechanoreceptive sensory hairs. These filiform hairs are highly perceptive to low-frequency sound with energy sensitivities close to thermal threshold. Arrays of artificial hair sensors have been fabricated using a surface micromachining technology to form suspended silicon nitride membranes and double-layer SU-8 processing to form 1 mm long hairs. Previously, we have shown that these hairs are sensitive to low-frequency sound, using a laser vibrometer setup to detect the movements of the nitride membranes. We have now realized readout electronics to detect the movements capacitively, using electrodes integrated on the membranes.

This paper presents the results of active adaptation of sensor sensitivity. By applying a DC-bias voltage to the sensing electrodes of a cricket inspired artificial hair sensor the effective spring stiffness can be adapted resulting in a reduced resonance frequency and increased sensitivity. An array of flow sensors was actuated using electrical and acoustical signals at different values of the DC-bias voltage. Characterization was done using a scanning laser vibrometer. Both resonance frequency versus applied DC-bias voltage and deflection-amplitude versus DC-bias voltage behave well in accordance to theory and show that adaptation by DC-biasing can be used for frequency focusing and increasing sensitivity.

In this paper, we report, to the best of our knowledge [1,2], the most sensitive artificial hair-based flowsensor arrays operating in air, to date. Artificial hair sensors are bio-inspired from the cerci of crickets, one of nature's best in sensing small air flows. The presented hair sensor arrays aim to realize comparable sensitivity to nature by means of model-based design optimizations and were fabricated using advanced MEMS technologies. The presented artificial hair sensor arrays display a clear figure-of-eight response and show remarkable sensitivities to oscillating air flows down to 0.85 mm/s surpassing noise levels at 1 kHz operational bandwidths.

Crickets and other arthropods are evolved with numerous flow-sensitive hairs on their body. These sensory hairs have garnered interest among scientists resulting in the development of bio-inspired artificial hair-shaped flow sensors. Flow-sensitive hairs are arranged in dense arrays, both in natural and bio-inspired cases. Do the hair-sensors which occur in closely-packed settings affect each other's performance by so-called viscous coupling? Answering this question is key to the optimal arrangement of hair-sensors for future applications. In this work viscous coupling is investigated from two angles. First, what does the existence of many hairs at close mutual distance mean for the flow profiles? How is the air-flow around a hair changed by it's neighbours proximity? Secondly, in what way do the incurred differences in air-flow profile alter the drag-torque on the hairs and their subsequent rotations? The first question is attacked both from a theoretical approach as well as by experimental investigations using particle image velocimetry to observe air flow profiles around regular arrays of millimeter sized micro-machined pillar structures. Both approaches confirm significant reductions in flow-velocity for high density hair arrays in dependence of air-flow frequency. For the second set of questions we used dedicated micro-fabricated chips consisting of artificial hair-sensors to controllably and reliably investigate viscous coupling effects between hairsensors. The experimental results confirm the presence of coupling effects (including secondary) between hairsensors when placed at inter-hair distances of less than 10 hair diameters (d). Moreover, these results give a thorough insight into viscous coupling effects. Insight which can be used equally well to further our understanding of the biological implications of high density arrays as well as have a better base for the design of biomimetic artificial hair-sensor arrays where spatial resolution needs to be balanced by sufficiently mutually decoupled hair-sensor responses †.