Beyond the Stethoscope: Wearables for Recording Mechano-Acoustic Signatures of the Heart and Lungs

Beyond the Stethoscope: Wearables for Recording Mechano-Acoustic Signatures of the Heart and Lungs

Congestive heart failure (CHF) is a progressive condition wherein the heart is unable to adequately pump blood to meet the metabolic demands of the body, is a leading cause of hospitalization and mortality, and substantially degrades the quality of life of affected individuals. An effective strategy to improve survival rate is to detect early physiological changes, such as abnormal heart sounds, associated with CHF and implement treatments proactively to slow its progression. However, early CHF is highly underdiagnosed and general screening of the population remains a major challenge.

In this paper, our research team focuses on tackling this issue by developing a micro-sensor for capturing the body’s mechano-acoustic signals in a wide frequency range of DC-12 kHz and enable longitudinal study and monitoring of the cardiopulmonary system. The 2 mm × 2 mm encapsulated microsensor chip can record a wide range of vibrations on human skin, ranging from very low frequency (below 1 Hz) movements associated with the chest wall and body position to high frequency acoustic signals (up to 12 kHz) emanating from the heart and lungs. The sensor are fabricated using a unique and high-precision fabrication technique, the high aspect-ratio combined poly- and single-crystal silicon micromachining technology (HARPSS), to achieve an ultrasensitive, wideband capacitive vibration transducer. The performance of these hermetically-encapsulated sensors is not compromised by body sweat and environmental effects. Moreover, the sensor only responds to vibrations on the surface and is not susceptible to airborne acoustic noise in the environment.

Hermetically-sealed sensor with nanogaps for cardiopulmonary health monitoring
Hermetically-sealed sensor with nanogaps for cardiopulmonary health monitoring Conceptual representation of encapsulated sensor positioned on the on the chest wall (blue circle) to simultaneously monitor heart rate, heart sounds, respiratory rate, breath sounds along with body motion and position. The exploded view displays the fabricated microsensor (2 mm × 2 mm × 1 mm).

We apply the sensor to demonstrate that heart rate, heart sounds, respiratory rate, lung sounds, and body motion of an individual can be simultaneously recorded in a continuous and unobtrusive manner using a single integrated sensor. Proof-of-concept studies were conducted on healthy control subjects as well as patients with preexisting conditions to monitor physiological and pathological mechano-acoustic signals emanating from the heart and lungs.

Development of such high precision sensors with a small footprint will enable us to replace bulky stethoscopes with ergonomic wearable auscultation systems in the future, that can precisely measure cardiopulmonary signals, and hence open new gateways in telemedicine and remote health monitoring. Such an integrated solution will significantly reduce fabrication cost of wearable technology, making it more accessible and affordable to the masses.

Recording cardiopulmonary vibrations
Recording cardiopulmonary vibrations a. Time domain plot of measured SCG signal. Peaks corresponding to occurrence of closing of mitral valve (MC), opening of aortic valve (AO), closing of aortic valve (AC) and opening of mitral valve (MO) are indicated. b. Recorded waveforms of two cardiac cycles showcasing sensitivity to the two major cardiac sounds (S1 and S2). Time intervals of inter-beat, systole and diastole are specified. c. Sensor output signal representing motion of the chest wall during deep-breathing respiratory cycles. Time intervals of inhalation and exhalation are identified for computation of respiratory rate. d. High frequency lung sounds of inhalation and exhalation as recorded by the vibration microsensor.