The Nanophone: Sensing Sound with Nanoscale Spider Silk
Abstract: Hundreds of millions of years of evolution have resulted in hair-based flow sensors in terrestrial arthropods that stand out among the most sensitive biological sensors known. These tiny sensory hairs can move with a velocity close to that of the surrounding air at frequencies near their mechanical resonance, in spite of the low viscosity and low density of air. No manmade technology to date demonstrates comparable efficiency.
Here we show that nanoscale spider silk captures fluctuating airflow with maximum physical efficiency (Vsilk / Vair≈1) from 1 Hz to 50 kHz, providing an effective means for miniaturized flow sensing. Our mathematical model shows excellent agreement with experimental results for silk with various diameters: 500 nm, 1.6 µm, 3 µm. When a fiber is sufficiently thin, it can move with the medium flow perfectly because of the domination of forces applied to it by the medium over those associated with its mechanical properties. While traditional dynamic sensors trade sensitivity for bandwidth — or vice versa — the proposed approach enables the sensitivity of an ideal vibrational sensor without succumbing to the usual bandwidth limitations. By modifying a spider silk to be conductive and transducing its motion by using electromagnetic induction, we demonstrate a miniature, directional, broadband, passive, low-cost approach to detect fluctuating airflow with almost full fidelity over a frequency bandwidth that easily spans the full range of human hearing, as well as that of other mammals, birds, amphibians, and reptiles.