Towards Gas Sensors for Aquatic Chemical Sensing Inspired By the Star-Nosed Mole
Authors: Lee Alexander Bo , Biagioni Morgan , Tanade Cyrus , Watson John Joseph , Seleb Benjamin , Spencer Thomas Lee , Hu David L 
Published: February 2020

Aquatic chemical sensors, as any other surface submerged in water, are subject to biofouling, by bacteria and algae that quickly form biofilms on the submerged surface [1]. In the case of sensors, accumulated biofilms skew the chemical measurements until the sensors are cleaned and recalibrated. A number of mechanical and chemical solutions have been proposed to scrape off or kill the biofilms, but they can be sensor-specific, costly, or have adverse effects on the environment [2]. The process of biofouling happens over the span of days, making long-term chemical measurements labor-intensive and oftentimes unfeasible.
The star-nosed mole, with its bizarre nose geometry, presents a way to circumvent the issue of biofouling. These semi-aquatic moles exhale and inhale bubbles rapidly underwater. When the bubbles contact a target of interest, molecules are collected on the bubble surface and brought back into the nose. In this fashion, its olfactory receptors never contact the water and the mole can use its terrestrial nose to track prey at the bottom of swamps [3]. The appendages that make up the star on their nose stabilize the bubbles, allowing them to consistently blow larger bubbles without the bubble pinching off and floating away [4].
We develop an underwater electronic nose, inspired by the star-nosed mole’s underwater sniffing. A motor pushes and pulls on a syringe to form and contract a 0.5 mL bubble underwater at 1 Hz. A 3D printed star, inspired by the mole’s nose geometry, stabilizes the bubble and prevents pinch-off [4]. The 6 fins of the star, separated by 30-degree angles, are slanted downward at a 35-degree angle to optimally stabilize the largest volume bubble. The air from the bubble is pushed and pulled past an array of metal oxide sensors. As a proof of concept, this underwater e-nose is used in an on-going experiment to detect the presence or absence of sparkler fragments at the bottom of a water sample. In the event of the bubble pinching off, inhaled water shorts out the sensor array. At the same time, if the volume of dead space between the star and the sensor array is greater than the volume of the bubble, tests show that the e-nose will be incapable of detecting changes in the underwater environment. Thus, stabilizing the bubble during the sniff is key in designing this underwater e-nose. This prototype e-nose marks the expansion of machine olfaction to underwater applications.
Figure 1. Underwater bubble sniffing can allow gas sensors to be used underwater. A) The star-nosed mole sniffs bubbles to pick up scents underwater. Photo courtesy of K. Catania. B) Mimicking this behavior could allow gas sensors to detect chemicals without directly exposing the sensor to its environment. C) A motor and syringe sniff underwater across a metal oxide sensor to form an underwater e-nose.
[1] G. Xu, W. Shen, and X. Wang, “Applications of Wireless Sensor Networks in Marine Environment Monitoring: A Survey,” Sensors, vol. 14, no. 9, pp. 16932–16954, Sep. 2014.
[2] L. Delauney, C. Compère, and M. Lehaitre, “Biofouling protection for marine environmental sensors,” Ocean Sci, vol. 6, no. 2, pp. 503–511, May 2010.
[3] K. C. Catania, “Olfaction: Underwater ‘sniffing’ by semi-aquatic mammals,” Nature, vol. 444, no. 7122, p. 1024, Dec. 2006.
[4] A. B. Lee and D. L. Hu, “Bubble stabilization by the star-nosed mole,” Phys. Rev. Fluids, vol. 3, no. 12, 2018.

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