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Hearing is one of the five senses that humans may have. The human ear plays a key role in converting physical vibration into electrical signals and then into nerve impulses. The functional principle to the human ear is based on sound transducing and conducting mechanisms. The sound conducting mechanism includes the outer ear and middle ear and the sound transducing mechanism includes an inner ear/cochlea. The function of the outer ear is to transmit sound to a tympanic membrane.
Sound is transmitted to the inner ear through the bones of the middle ear called the malleus, incus, and stapes. The sound is then passed to perilymph that fills the cochlea through an ossicular chain to be converted into a nerve impulse. The impulse is then transmitted to the brain to perceive the sound.
In 2005 White R.D et al (2005) conducted an experiment to create micromachined cochlear waveguides using isotropic and orthotropic membranes, given the importance of the cochlea in transducing vibrations into nerve impulses. In the experiment, the waveguides that mimic mammalian cochlea were microfabricated. The experimental results revealed that the models exhibit acoustically excited traveling ﬂuid-structure waves, which have phase accumulations in the range of 1.5 to 3 radians at maximum response.
The team also concluded that the orthotropy ratio obtained is not enough to achieve sharp filtering, found in computational models and animal experiments. In addition, they identified that the generation of non-physiological standing wave patterns can be avoided using high viscosity ﬂuids. Thus, it has been proved that a micromachined device acts as a distinct model for acoustic sensing.
Application of Electronic Sound Sensors to Measure Arterial Sound
Currently, several cardiovascular assisted devices have been developed for extending the lifetime of individuals with cardiac disease and these have a sensor for detecting mechanical disorder. However, with the use of the devices, there is a chance of malrotation due to thrombogenesis which leads to fatal damage in the patient's heart. Hence, Tanishiro H et al (2004) proposed Korotkoff sounds, which are capable of measuring blood pressure using a rotary left ventricular assist device (LVAD).
The team measured Korotkoff sound produced during the circulation of rotary LVAD and arterial sound from an occluded brachial artery. They also studied the arterial sound of various blood pressure conditions using a circulatory simulator and created a relationship between arterial sound characteristics and abnormal pressure conditions. As a result, the team concluded that malrotation can be identified by changing the arterial sound.
Measuring Respiratory Acoustic Signals
Pasterkamp H et al (1993) estimated the characteristics of four contact sensors and three air-coupled sensors based on the recording of lung sounds. The sounds were filtered at a frequency range of 100 to 2000 Hz and sampled using calibrated airflow. The spectra of the sounds were analyzed with the help of Fourier techniques and compared with the background noise recorded at zero flow. Maximum signal:noise ratios between air-coupled and contact sensors were observed alongside yielding identical figures for parameters of the spectrum.
In addition, the air-coupled devices were found to have a low signal:noise ratio/sensitivity under high frequency conditions. Therefore, it is evident from this study that respiratory acoustic signals can be measured by taking an average of the spectra of lung sounds for different breaths, followed by a comparison of the measurements with sounds at zero flow.
Acoustic sensing is carried out based on the principle that the characteristics of a surface acoustic wave (SAW) passing along the surface of a SAW device changes due to mechanical or electrical changes in the environment. A research study carried out by Cole M et al (2010) suggested the use of a biological sensor to monitor cell viability and the response of the cells to specific biological molecules. The sensor used in the experiment was a shear horizontal SAW sensor coated with HEK293 cells and a micro-fluidic chamber that facilitates in-situ culturing of cells using micro-stereolithography.
Research by The University of New Mexico Sciences Center and Sandia National Laboratories revealed the possibility of using a portable surface acoustic wave biosensor for the detection of deadly and medically relevant bacteria and viruses such as HIV, anthrax, and influenza. This research is a clear example of how electronic sound sensors that were once inspired by the anatomical and functional principle of the human ear have evolved into a more intricate field of biomedical application of foreign pathogens as is described in the following video:
Surface Acoustic Wave Biosensor for Detecting Pathogens at the Point of Care
Acoustic sensors have been widely used for biomedical applications. One of the best examples is the cell-based acoustic sensor developed by Cole M et al (2010). This sensor consists of a low-loss SH-SAW biosensor and a cell monitoring and measurement system that facilitates real-time optical imaging. The sensor allows cells to be grown on its surface using a customized immobilization protocol. The experiment has proved that the sensors are susceptible to cellular properties like viscoelasticity and ionic conductivity. It has been observed that the acoustic biosensors are highly sensitive to transfected cells and avoid interfering signals.
Further research to develop more efficient acoustic sensors is ongoing and they deal mainly with the immobilization of cells having binding sites for ligands on the biosensors and the acoustical analysis of cell response to biological molecules like activators, hormones, drugs, toxins and inhibitors in real-time.
Sources and Further Reading
- White R.D, Grosh K. Microengineered hydromechanical cochlear model. Proceedings of the National Academy of Sciences. 2005;102:1296–1301.
- Cole M, Gardner J.W, Pathak S, Rácz Z, Challiss R.A.J. Cell-based acoustic sensors for biomedical applications. From Proceeding (680) Biomedical Engineering. 2010; Volume 1,2.
- Winquist F, Krantz-Rülcker C, Lundström I. Electronic tongues and combination of artificial senses. Wiley. 2003;11(1).
- Tanishiro H, Funakubo A, Fukui Y. Arterial sound based noninvasive malrotation detection of rotary LVAD. ASAIO J. 2004;50(4):306–10.
- Pasterkamp H, Kraman SS, DeFrain PD, Wodicka GR. Measurement of respiratory acoustical signals. Comparison of sensors. Chest. 1993.104(5):1518–25.
This article was updated on 14th February, 2020.