Researchers Develop Ultrasensitive Acoustic Sensor

11-02-2021 | By Robin Mitchell

Recently researchers from Wayne State University have developed an electrochemical sensor that can be used to detect heartbeats and record breathing. What are electrochemical sensors, what features does the new sensor provide, and how can it help the medical field?

What are electrochemical sensors?

Before we can understand what electrochemical sensors are we first need to understand the term “electrochemical” means. Electrochemical means that electricity (a flow of electric charge), occurs as a result of a chemical reaction, or a chemical reaction as a result of an electrical current.

One common example of an electrochemical reaction is the electrolysis of water when electricity is passed through it. When an electrical charge is placed across a body of water using electrodes, the water is separated into hydrogen and oxygen ions which then travel towards the oppositely charged electrode (i.e. oxygen ions are attracted to the anode while hydrogen is attracted to the cathode). 

The electrolysis reaction of water is reversible and the combination of hydrogen and oxygen can create an electric current. This is how HHO fuel cells operate, but constructing these requires advanced membranes that can filter our hydrogen ions from water.

An electrochemical sensor is a sensor that takes advantage of chemical reactions that generate electrical current. One common example of such sensors is gas sensors that utilise tin oxide as a resistive element. When heated, volatile gases react with the tin oxide which adjusts its conductivity, and thus the sensor can detect traces of volatile compounds in the air.

Researchers Develop Electrochemical Acoustic Sensor

Acoustic sensors vary in build, but often take advantage of the mechanical energy transferred from a sound wave to a mechanical element. For example, piezo microphones directly convert incoming sound waves into electricity using the piezo effect. Magnetic coil microphones utilise a coil of copper that vibrates in a magnetic field as sound waves hit a diaphragm, the net result is the induction of current.

Recently, a team of researchers from Wayne State University and Arizona State University demonstrated a wearable acoustic sensor that utilises an electrochemical reaction. Furthermore, the researchers demonstrated the sensor's capabilities by creating a medical device that can listen to heartbeats and breathing.

The new sensor utilises a commonly used electrochemical compound mix, Iodine and Triiodide (I/I3) which generates current when agitated. This chemical mix is commonly found in applications such as seismometers that need to detect small vibrations. To be able to detect breathing and heartbeats, the device has a small cavity that is then connected to the patient’s chest with the use of a 500um thin EcoFlex diaphragm. The sensor's electrical side is very similar to electret microphones; a 10kΩ feedback resistor and 8.5nF capacitor. 

Currently, the device has only been tested on a single patient, but the study results show that the sensor can clearly detect both heartbeats and breathing. To isolate the two signals from each other, two different filters are used. Breathing is determined using a low-pass filter that can detect the slow changes resulting from breathing. A high-frequency filter is used to listen to heartbeats which produce a more audible thump.

How can such a sensor help with medical scenarios?

The first factor to consider with the device is the ability to listen to breathing and heartbeats. While other methods can be used to determine heart rate (such as a colour change in a finger with a bright LED), breathing is harder to determine. 

Detecting the simple action of breathing in and out does not tell you anything regarding the quality of the breath. When doctors use a stethoscope to listen to a patients breathing, they often try to listen to moving fluid and air channels that may be wheezy. A listening device that can listen to breathing may be able to detect such problems without the need for a doctor. Furthermore, the use of simpler sensors that can be easily self-administered provides patients with the ability to isolate from other patients and doctors during times of pandemics.

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By Robin Mitchell

Robin Mitchell is an electronic engineer who has been involved in electronics since the age of 13. After completing a BEng at the University of Warwick, Robin moved into the field of online content creation, developing articles, news pieces, and projects aimed at professionals and makers alike. Currently, Robin runs a small electronics business, MitchElectronics, which produces educational kits and resources.