Researchers have found a new use for a 3,000-year-old African musical instrument: detecting toxic substances and counterfeit medications. The sensor, based on the mbira (pronounced “em-bir’-uh”) is inexpensive and easy to operate, allowing its use in developing regions, the researchers say. They report their results in ACS Omega.
The mbira, also known as the kalimba or thumb piano, consists of metal tines of different sizes and lengths attached to a wooden sounding box. Musicians pluck the tines to create musical notes. Longer or larger tines produce low notes, whereas shorter or smaller tines create high notes. In addition to length and size, the density of the tine material influences the sound’s frequency. William Grover and colleagues wondered if they could take advantage of this property to develop a sensor that could distinguish liquids by their density. Existing devices for this purpose are expensive or require great skill to make, calibrate and use. Grover wanted to develop a sensor that requires only an mbira and a smartphone.
The researchers assembled a simple mbira that contained a single hollow, U-shaped tine. When the tine was filled only with air, the mbira produced the note of G#, and when it was filled with water, the pitch dropped to an F#. To measure frequency changes undetectable by the human ear, Grover and colleagues developed free, web-based software that allows users to upload mbira sound recordings from their smartphones. The software determines the frequency of the note, which can then be entered into an equation to calculate the density of the liquid. The team showed that their sensor could distinguish between sodium chloride solutions of different densities. To show the potential of the sensor to detect counterfeit medication, the team analyzed six samples of the same cold medicine with different lots and expiration dates. All six samples produced nearly identical mbira frequencies, indicating that they were all authentic. In addition, the sensor clearly distinguished a toxic substance (diethylene glycol) from a non-toxic one (glycerol) with similar properties. The researchers note that the mbira sensor can be built from scrap materials, does not require calibration when comparing two substances and is easy to use.
The authors acknowledge funding from the National Science Foundation and the Bill and Melinda Gates Foundation’s Grand Challenges Explorations program.
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Heran C. Bhakta et al. Musical Instruments As Sensors, ACS Omega (2018). DOI: 10.1021/acsomega.8b01673