Page 156 - Read Online

Page 4 of 9 Guess et al. Soft Sci 2023;3:23 https://dx.doi.org/10.20517/ss.2023.17

Table 1. Comparison of flexible SCG sensors
Sensor
Reference Material Transmission Battery-free Sensitivity
type
This work Capacitive strain sensor Copper LC resonance coupling Yes 2 MHz per 1% ε
[7] Piezoelectric strain sensor PVDF Wired No 0.4 mV per με
[13] Piezoelectric strain sensor PVDF nanofiber Wired No 10 V per Pa
[18] Piezoelectric strain sensor PVDF NFC Yes Not reported
[14] Fiber-optic strain sensor Acrylate fiber Wired No 0.045 nm per mε
[22] Resistive strain sensor Porous graphene Wired No 96 Ω per 1% ε

NFC: near-field communication; PVDF: Polyvinylidene fluoride; ε: strain.

Figure 1. Overview of a wireless soft sensor system for cardiovascular health monitoring. (A) A soft capacitive sensor mounted on the
chest for wireless detection of data; (B) Photos of a fabricated thin-film soft sensor showing flexibility and stretchability; (C) Schematic
of the inductive coupling measurement method. The soft wearable sensor can measure seismocardiography, pulse, and heart rate. SCG:
Seismocardiography.

8 μm. This is important since the spacing between the interdigitated electrodes of the capacitive strain
sensor determines the sensitivity . As the sensor is stretched, these serpentine structures expand to allow a
[23]
large difference in spacing between the fingers. In addition, the serpentine pattern changes between the coil
and the sensor to enable strain at the interface while minimizing stress on the copper [Figure 2C]. To show
the reliability of the sensor, it was stretched and unstretched for 100 cycles. Figure 2D shows repeatable
capacitance measurements with negligible hysteresis as a function of strain. The sensor also shows great
stability and reliability; the capacitances stay constant for 100 cycles [Figure 2E]. The sensor shows virtually
no degradation over the cycles, as the amplitude of the capacitance of the 100th cycle shows the same
amplitude of the 1st cycle (0.362 pF). To measure the relationship between the S parameter and the strain,
11
an experiment was conducted where continuous frequency sweeps were conducted at varying sensor
strains. Figure 2F shows a consistent resonant shift based on the strain. This was important to the design of