Page 14 - Read Online

Romano et al. Soft Sci 2024;4:31 https://dx.doi.org/10.20517/ss.2024.24 Page 11 of 15

The volunteer followed an experimental protocol that included different breathing rates and depths. All
protocol steps were performed to investigate the functionality of the pressure sensors at different respiratory
frequencies within the physiological range. As shown in Figure 4B, the data collected with the proposed
sensor allow the identification of slow breathing, and fast breathing phases. The apnea phases are also
identifiable in the signals, shown by an almost constant signal throughout the apnea duration. End-
inspiratory and end-expiratory apnea can be distinguished by their different mean values. This feature could
be crucial for devices studying apnea, not found in many respiratory monitoring devices . The minimum
[57]
thickness achieved in this real-life application is 6.1 mm.

The second proof-of-concept experiment measured tapping, useful in numerous applications including the
[58]
evaluation of muscle control or the assessment of patient recovery after a stroke . Movements are typically
[59]
monitored in amplitude, rhythm, and speed to assess disorders such as muscle tremor and rigidity .
We measured the pressure exerted on the sensor by the impact of the finger at different speeds and
intensities. Specifically, our sensor was tested under three conditions: no contact, soft touch, and hard touch
[Figure 4C]. Figure 4D shows the output voltage in soft touch, hard touch and no touch phases, easily
distinguishable. Then, the sensor was evaluated during four additional tasks: deep tapping with maximum
force, slow tapping, fast tapping, and tapping as fast as possible. Figure 4E shows the sensor output at
different tapping frequencies. Regardless of the tapping frequency, the sensor always returns to its initial
value in the absence of pressure, showing good repeatability. The minimum thickness achieved in this real-
life application is 5.6 mm.

The results from these tests demonstrate the sensors’ ability to follow both slow (e.g., touch and breath
during apnea and quiet breathing) and rapid phenomena (e.g., tapping or breathing during tachypnoea).

CONCLUSIONS
This study introduces a novel approach to designing and fabricating soft pressure sensors with tunable
stiffness, offering adaptability across various applications. We developed and characterized sensors capable
of detecting medium and low-pressure ranges. Our sensor employs a magnetic sensing mechanism, where
deformation of a soft polymeric medium induces changes in magnetic field intensity, allowing for
measurement of applied pressure.

Our findings reveal significant variations in sensor performance based on shape and material composition.
Sensitivity values range from 0.4 × 10 to 5.1 × 10 kPa across different configurations. Moreover, the
-1
-2
-2
sensors exhibit a wide range of maximum detectable pressure values, up to 77.4 kPa, depending on the
stiffness of the material and the sensor’s shape. Experimental tests demonstrated the sensor’s robust
performance under both static and dynamic loading conditions. The sensors also show a promising
recovery time of 0.4 s, making it suitable for real-time pressure monitoring. Table 3 compares our sensor
with similar sensor technologies.

We tested the sensors in two different application fields to demonstrate their versatility. One sensor was
tested for respiratory monitoring during normal breathing and high-frequency breathing (tachypnoea). It
demonstrated the sensor’s ability to follow rapid and minimal changes well. An important feature of our
sensors is their monoaxial design, which primarily detects magnet movements perpendicular to the Hall
sensor (z-axis). This design is expected to reduce susceptibility to motion artifacts during activities such as
walking, where magnet movements predominantly occur in the xy-plane.