Sensing fluctuating airflow with spider silk Jian Zhoua,1 and Ronald N. Milesa,1,2 aDepartment of Mechanical Engineering, Binghamton University, Binghamton, NY 13902 Edited by John G. Hildebrand, University of Arizona, Tucson, AZ, and approved September 18, 2017 (received for review June 15, 2017) The ultimate aim of flow sensing is to represent the perturbations bandwidth. Other designs seek to avoid resonances to maximize of the medium perfectly. Hundreds of millions of years of their bandwidth at the expense of sensitivity. Here we show that evolution resulted in hair-based flow sensors in terrestrial arthro- nanodimensional spider silk can overcome these adverse costs by pods that stand out among the most sensitive biological sensors following the airflow with maximum physical efficiency (Vsilk/Vair known, even better than photoreceptors which can detect a single ∼ 1) over a frequency range from infrasound to ultrasound − − photon (10 18–10 19 J) of visible light. These tiny sensory hairs can (1 Hz–50 kHz), despite the low viscosity and low density of air. move with a velocity close to that of the surrounding air at fre- The performance closely resembles that of an ideal resonant quencies near their mechanical resonance, despite the low viscos- sensor but without the usual bandwidth limitation. This finding ity and low density of air. No man-made technology to date provides a design that exceeds the performance of existing hair- demonstrates comparable efficiency. Here we show that nanodi- based flow sensors. mensional spider silk captures fluctuating airflow with maximum Results and Discussion physical efficiency (Vsilk/Vair ∼ 1) from 1 Hz to 50 kHz, providing an effective means for miniaturized flow sensing. Our mathematical To intuitively illustrate the transverse motion of spider silk due model shows excellent agreement with experimental results for to fluctuating airflow in the direction perpendicular to its long silk with various diameters: 500 nm, 1.6 μm, and 3 μm. When a axis, we record sound from the silk motion. The complex air- fiber is sufficiently thin, it can move with the medium flow per- borne acoustic signal used here contains low-frequency (100– fectly due to the domination of forces applied to it by the medium 700 Hz) wing beat of insects and high-frequency (2–10 kHz) song over those associated with its mechanical properties. These results of birds. Fig. 1 shows a schematic of our experimental setup and suggest that the aerodynamic property of silk can provide an air- measured airborne motion of spider silk. We collect spider SCIENCES borne acoustic signal to a spider directly, in addition to the well- = A dragline silk with diameter d 500 nm (Fig. S1 ) from a female APPLIED PHYSICAL known substrate-borne information. By modifying a spider silk to spiderling, Araneus diadematus (body length of the spider is be conductive and transducing its motion using electromagnetic about 3 mm). Fig. 1A shows a spider hanging on its web. The induction, we demonstrate a miniature, directional, broadband, experimental setup is schematically shown in Fig. 1B. A strand of passive, low-cost approach to detect airflow with full fidelity over spider silk (length L = 8 mm) is supported at its two ends slackly, a frequency bandwidth that easily spans the full range of human and placed perpendicularly to the flow field. The airflow field is hearing, as well as that of many other mammals. prepared by playing sound using loudspeakers. A plane sound wave is generated at the location of the spider silk by placing the spider silk | airborne motion | flow sensing | acoustics | loudspeakers far away (3 m) from the silk in our anechoic nanodimensional fiber chamber. The silk motion is measured using a laser vibrometer (Polytec OFV-534). Detailed descriptions of the experimental iniaturized flow sensing with high spatial and temporal setups and procedures can be found in SI Methods and Fig. S2. Mresolution is crucial for numerous applications, such as The top image of Fig. 1C shows the airborne signal measured high-resolution flow mapping (1), controlled microfluidic sys- – tems (2), unmanned microaerial vehicles (3 5), boundary-layer Significance flow measurement (6), low-frequency sound-source localization (7), and directional hearing aids (8). It has important socioeco- We find nanodimensional spider silk captures airflow with nomic impacts involved with defense and civilian tasks, bio- maximum physical efficiency over an extremely wide fre- medical and healthcare, energy saving and noise reduction of quency range from infrasound to ultrasound. The aerodynamic aircraft, natural and man-made hazard monitoring and warning, – property of spider silk provides the sensitivity of an ideal res- etc (1 10). Traditional flow-sensing approaches such as laser onator but without the usual bandwidth limitation. This pro- Doppler velocimetry, particle image velocimetry, and hot-wire vides an effective means for miniaturized flow sensing, anemometry have demonstrated significant success in certain surpassing the frequency response of hair-based flow sensors applications. However, their applicability in a small space is of- of animals, which has been pursued in past decades. The re- ten limited by their large size, high power consumption, limited sults are significant because they elucidate the highly re- bandwidth, high interaction with medium flow, and/or complex sponsive nature of materials such as spider silk. This setups. There are many examples of sensory hairs in nature that bioinspired approach will be valuable to various disciplines sense fluctuating flow by deflecting in a direction perpendicular which have been pursuing miniaturized flow measurement to their long axis due to forces applied by the surrounding me- and control in various mediums (air, gas, liquid) and situations dium (11–16). The simple, efficient, and tiny natural hair-based (from steady flow to highly fluctuating flow). flow sensors provide an inspiration to address these difficulties. Miniature artificial flow sensors based on various transduction Author contributions: J.Z. and R.N.M. designed research, performed research, analyzed approaches have been created that are inspired by natural hairs data, and wrote the paper. (9, 10, 17–21). Unfortunately, their motion relative to that of the The authors declare no conflict of interest. surrounding flow is far less than that of natural hairs, signifi- This article is a PNAS Direct Submission. cantly limiting their performance (9, 10). Published under the PNAS license. An ideal sensor should represent the measured quantity with 1J.Z. and R.N.M. contributed equally to this work. full fidelity. All dynamic mechanical sensors have resonances, a 2To whom correspondence should be addressed. Email: [email protected]. fact which is exploited in some sensor designs to achieve suffi- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. cient sensitivity. This comes with the cost of limiting their 1073/pnas.1710559114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1710559114 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 A C 10 5 Frequency (kHz) 0 0 0.5 1 1.5 2 Time (s) B Speaker 10 Spider silk 5 Frequency (kHz) 0 Laser vibrometer 0 0.5 1 1.5 2 Time (s) Fig. 1. Airborne motion of spider silk. (A) A spider hanging on its web. (B) Schematic diagram showing the motion measurement of spider silk. A loose spider silk (length = 8 mm, diameter = 500 nm) is placed perpendicularly to the flow field in our anechoic chamber. The flow field is generated by speakers. The motion of the silk strand at the middle point is measured using a laser vibrometer. A more detailed description of the setup is provided in SI Methods and Fig. S2.(C) Measured time-domain signals and spectrograms. The airborne signal (top image) is measured by a pressure microphone and the silk motion (bottom image) is measured by the laser vibrometer. The acoustic signal contains low-frequency (100–700 Hz) wing beat of insects and high-frequency (2–10 kHz) song of birds. The silk motion clearly captures the airborne signal. using a probe microphone (B&K type 4182) and the silk motion the fluctuating airflow, and V is the velocity amplitude). We use shown in the bottom image is measured by the laser vibrometer. a long (L = 3.8 cm) and loose spider silk strand to avoid possible As shown in the waveform and spectrogram, the motion of the nonlinear stretching when the deflection is relatively large at very silk (bottom image) clearly captures the broadband acoustic low frequencies. The measured silk velocity relative to the air signals (top image) with high fidelity. More detailed laser signals particle velocity at the middle of the silk strand is presented in can be found in Movie S1, which contains a time-domain trace, Fig. 2A. It shows that the nanodimensional spider silk can follow frequency-dependent spectrogram, and audio. the airflow with maximum physical efficiency (Vhair/Vair ∼ 1) in While the geometric forms (cobweb, orb web, and single the measured frequency range from 1 Hz to 50 kHz. Fig. 2B strand), size, and tension of the spider silk shape the ultimate shows 50-ms-long time-domain data of the silk motion due to a time and frequency responses, this intrinsic aerodynamic prop- 10,000-Hz airflow. The velocity and displacement amplitude of erty of silk to represent the motion of the medium suggests that it the flow field are 0.83 mm/s and 13.2 nm, respectively. This can provide the acoustic information propagated through air to shows that the silk motion accurately tracks the air velocity at the spiders. This may allow them to detect and discriminate potential initial transient as well as when the motion becomes periodic in nearby prey and predators (22, 23), which is different from the the steady state.