Flexible Textile Sensors-Based Smart T-Shirt for Respiratory Monitoring: Design, Development, and Preliminary Validation

Abstract

Respiratory rate (fR) monitoring through wearable devices is crucial in several scenarios, providing insights into well-being and sports performance while minimizing interference with daily activities. Strain sensors embedded into garments stand out but require thorough investigation for optimal deployment. Optimal sensor positioning is often overlooked, and when addressed, the quality of the respiratory signal is neglected. Additionally, sensor metrological characterization after sensor integration is often omitted. In this study, we present the design, development, and feasibility assessment of a smart t-shirt embedded with two flexible sensors for fR monitoring. Guided by a motion capture system, optimal sensor design and position on the chest wall were defined, considering both signal magnitude and quality. The sensors were developed, embedded into the wearable system, and metrologically characterized, demonstrating a remarkable response to both static (sensitivity 9.4 Ω⋅%-1 and 9.1 Ω⋅%-1 for sensor A and sensor B, respectively) and cyclic loads (min. hysteresis span 20.4% at 36 bpm obtained for sensor A). The feasibility of the wearable system was assessed on healthy volunteers both under static and dynamic conditions (such as running, walking, and climbing stairs). A mean absolute error of 0.32 bpm was obtained by averaging all subjects and tests using the combination of the two sensors. This value was lower than that obtained using both sensor A (0.53 bpm) and sensor B (0.78 bpm) individually. Our study highlights the importance of signal amplitude and quality in optimal sensor placement evaluation, as well as the characterization of the embedded sensors for metrological assessment.

Keywords: breathing monitoring; conductive textiles; flexible sensors; wearable device.

Figure 1

(a) Illustration depicting the spatial distribution of 89 photo-reflective markers on the chest wall, segmented into the abdomen (depicted in violet) and rib cage (depicted in blue). The diagram also outlines the connections between pairs of markers; (b) Representation of a relative displacement signal (i.e., x) between two markers over time, with the identification of maximum (end of inhalation, depicted as green triangles) and minimum (end of exhalation, depicted as blue triangles) peaks.

Figure 2

Representation of the connections among markers placed on both the anterior and posterior rib cage. The color map corresponds to $U_{avg}^{norm}$ values, representing the combination of deformation and normalized SNR values.

Figure 3

Schematization of the main steps carried out for the realization the two piezoresistive textile sensors and the integration into the wearable system by means of a polymer matrix.

Figure 4

(A) The upper panels represent the output of the flexible sensors integrated into the wearable system (sensor A and sensor B) following the application of a quasi-static tensile load up to a maximum strain equal to 10% of the initial length of each sensor; the lower panels represent the plot of the tensile load applied to the sensors as a function of their deformation. (B) Representation of the hysteresis cycles obtained by applying cyclic loads to the two flexible sensors simulating different frequencies: 6 bpm, 12 bpm, 24 bpm, and 36 bpm.

Figure 5

(A) Experimental setup consisting of the two flexible sensors embedded into the wearable system and a PLA case placed on the back of the wearable system, which contains both the signal conditioning board for resistance-voltage conversion and the board for data acquisition and storage. Simultaneously, the subject wears the BH system used to collect the reference respiratory wave. (B) Experimental protocol consisting of both static and dynamic phases. An example of the signals collected with the two flexible sensors (sensor A in magenta and sensor B in blue) integrated into the wearable system and with the reference system (depicted in black) during the standing phase is given in the panels below.

Figure 6

In the upper panels, bar charts are displayed for each subject. The color representation is as follows: orange corresponds to the $f_R$ values extracted from the signal collected with sensor A, green represents those extracted from sensor B, and blue illustrates values obtained from BH throughout all phases of the experimental protocol. In the lower panel, the mean values for all subjects, along with their standard deviations, are depicted across all phases of the experimental protocol. qb: quiet breathing; t: tachypnea; WS: wearable system (summed signal).

Figure 7

In the upper panels, bar charts are displayed for each subject. The color representation is as follows: purple corresponds to the $f_R$ values extracted from the summed signal (referred to as WS) and blue illustrates values obtained from BH throughout all phases of the experimental protocol. In the lower panel, the mean values for all subjects, along with their standard deviations, are depicted across all phases of the experimental protocol. qb: quiet breathing; t: tachypnea; WS: wearable system (summed signal).