Hermetically-Encapsulated Precision Accelerometer Contact Microphones for Wearable Medical Devices
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Gupta, Pranav
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Abstract
Cardiopulmonary diseases are considered the leading cause of mortality globally, and the interdependence of cardiac and pulmonary health makes continuous monitoring of vital health parameters essential for an accurate and timely diagnosis. Mechano-acoustic signals emanating from the heart and lungs contain valuable information about the cardiopulmonary system and use of unobtrusive wearable sensors capable of monitoring these signals longitudinally can potentially detect early pathological signatures and assist in titrating care accordingly.
This dissertation focuses on implementing a wearable, hermetically-sealed high-precision vibration sensor that combines the characteristics of an inertial device - an accelerometer and an auditory sensor – a microphone – to acquire wideband mechano-acoustic physiological signals, and enable simultaneous monitoring of multiple health factors associated with the cardiopulmonary system including heart and respiratory rate, heart sounds, lung sounds, and body motion and position of an individual. The MEMS device - an Accelerometer Contact Microphone (ACM) - utilizes a hermetically-encapsulated nano-gap transduction technology to achieve very high sensitivity (micro-g resolution) in a wide bandwidth (DC-10kHz), while achieving a high dynamic range of up to 16-g. The sensor only responds to vibrations from its contact surface while being insensitive to airborne acoustic emissions, making it suitable for use in wearable applications.
The ACM is implemented as an out-of-plane accelerometer exhibiting a wide operational bandwidth of over 10 kHz, while maintaining a low noise performance of less than 10 µgHz−1/2 . To realize the sensor, a dedicated fabrication process (i.e. HARPSS+) is used in which the transduction gap size of 250 nm is achieved by a thermally grown sacrificial oxide layer. The sensor is hermetically encapsulated into miniature dies of 2.8 mm x 2.8 mm size, at low vacuum levels (1 − 10 Torr), enabling the wide operational bandwidth along with making it resistant to environmental damage. Multi-axis ACM devices are also developed to capture the mechano-acoustic signal along all axes.
A wireless, wearable module to house the fabricated ACM devices is developed that can seamlessly integrate with a user without hindering their activities. The module is capable of transmitting the recorded data over Bluetooth or store it locally on an SD card. A long battery life, small form factor and easy-to-use interface further enhance the functionality of the module.
The performance of the developed sensors in recording physical biomarkers, such as heart and lungs sounds, is validated by comparing the recorded signals with state-of-art digital stethoscopes. The two sensors exhibit a strong correlation of 70% similarity in the time domain signals, validating the use of ACM in recording cardiopulmonary mechano-acoustic signals. The sensors are used to obtain health factors of six control subjects with varying body mass index, and the signals are processed to further extract parameters such as heart rate, respiratory rate and breathing patterns. The accuracy of the system in detection of these vital signs is evaluated by comparing it against a medical-grade ECG system. The results indicate a high correlation r2 = 0.98, verifying the use of the ACM for vital parameter monitoring in place of ECG electrodes without compromising on signal strength, quality, and accuracy, while providing additional benefits of auscultatory ability simultaneously.
Feasibility of the sensor in clinical applications is studied by deploying the ACM to capture the weak mechano-acoustic signals such as pathological heart sounds in patients with preexisting cardiopulmonary conditions. Abnormal ventricular gallop (S3 heart sound) is captured by the ACM on patients diagnosed with congestive heart failure. Further, shallow breathing patterns are also observed indicating labored breathing. Wheezing lung sounds are recorded in patients suffering from chronic obstructive pulmonary disorder. Bronchial breath sounds and high respiratory rates are also recorded from patients with pneumonia.
This inexpensive wearable sensing technology presented in this work has the potential to enhance remote healthcare delivery and improve the quality of life and outcome in patients with chronic diseases and reduce overall health care costs.
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2021-04-28
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Dissertation