Piezoelectric Materials Applications for Cardiovascular Devices

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Artificial Heart Valve Sensor

Physicians often rely on sounds to be able to assess the functionality of heart valves. The normal heart sound can be typically described as “lub-dub”, and corresponds to the heart valves closing. The “lub” sound is usually softer and last longer than the “dub” sound, and it is associated with the closing of the atrio-ventricular (AV) valves at the beginning of systole [9]. The “dub” sound corresponds to the closing of the semilunar valves (aortic and pulmonary valves) [9]. The sound is shorter and loader during this period due to the greater rigidity of the semilunar valves. The following video shows what these sounds are like [10]:

By listening to the heart sounds the physician is capable of assessing the conditions of the heart valves. Variations from the normal “lub-dub” can indicate the presence of heart valve stenosis or insufficiency [9].

Piezoelectric based sensors are being designed by Hall et al., for the purpose of converting heart sounds into a visual and quantitative tool [11]. This particular piezoelectric crystal displays a sensitivity of 10 mV/dyne force and 40 mV/micron displacement, capacitance of 20 nF, and frequency response of 0.02-2 kHz [11].  However, one major problem is that the heart sound recording via this method is overshadowed by unwanted background noise. The authors attribute this issue to various sources, such as: contact between skin and stethoscope, breathing, and talking during the procedure [11].

Moreover, an approach to reduce this noise was investigated by this group. They utilized the method of “wavelet denoising”, which relies on the wavelet transform rather than the Fourier transform [11]. This method is used due to the similarity between the wavelets and the time-domain shape of the heart sounds. Fortunately, as shown in the figure below, this technique shows that the noise can be greatly reduced by applying a particular threshold [11].

Heart Sounds

A new approach by Lanning S. and Shandas R. uses piezoelectric sensors mounted on artificial heart valves to analyze their functionality [12]. The sensor is a noninvasive method used to detect variations normal heart valve vibrations. The loading of thrombi or irregular vibrations will create a heart signal which can indicate forthcoming malfunction. The figure below shows several diagrams of the piezoelectric sensors mounted on tww different heart valve models [12]:

Sensor Schematic

Results of this research [12] showed that the frequency content of the sounds generated by the mechanical valve closing (first model) contain peaks between 100Hz-10kHz, while the closing sound of the bioprosthetic valve (second model) contain a lower frequency range. In addition, the sensors were able to detect leaflet stiffening; showing and increase in the frequency peak from 474+15Hz to 1154+12Hz. Finally, thrombus formation was also successfully measured through this method; showing a decreased in the total low frequency range as the thrombus formation increased.