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Guess et al. Soft Sci 2023;3:23 https://dx.doi.org/10.20517/ss.2023.17 Page 5 of 9

Figure 2. (A) Schematic of the sensor layers; (B) Photograph of the sensor of human skin with a close-up of the fingers; (C) Photograph
of the sensor mechanism with and without strain; (D) Sensor capacitance as a function of strain; (E) Capacitance changes of the sensor
during 100 stretching cycles; (F) Resonant frequency sweeps at different strains. The resonant frequency increases with increasing
sensor strain; (G) Resonant frequency at low strain values; (H) Resonant frequency at high strain values.

the device as the S parameter is the measurement we use for capturing the SCG signal. The sensor shows
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an excellent linear resonant frequency response in low strains [Figure 2G]. This is especially important for
SCG measurement as the skin strain is on the micrometers scale. The sensor showed repeatable
measurements at each strain (n = 4), indicating low hysteresis and high signal repeatability. Even at higher
strains, the response is still predictable using a square root curve fit [Figure 2H]. The sensor still behaves
predictably at strains much higher than 7%, which is the highest strain under normal conditions .
[24]

The resonance of the LC circuit is measured using an external coil antenna and vector network analyzer.
Figure 3 summarizes the inductive coupling method, system setup, and experimental results. Figure 3A and
B demonstrates how the sensor is coupled with a vector network analyzer and a data acquisition system. As
the capacitance of the sensor decreases, the resonance frequency increases, as described in Supplementary
Note 1. To measure the SCG signal, the S parameter is measured at a stimulus frequency slightly lower
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than the resonance frequency of the unstretched device. This is to ensure that the polarity of the signal stays
consistent throughout the measurement since the direction of the S parameter with respect to a changing
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resonant frequency will depend on whether the stimulus frequency is higher than or lower than the
resonant frequency. When the stimulus frequency is lower than the resonant frequency, the S parameter of
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the stimulus frequency will increase as the resonance frequency increases. This method allows for fast
measurement of the SCG signal. Figure 3C shows the S value at the resonant frequency. As expected, the
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signal quality decreases as the distance between the coils increases [Supplementary Figure 2]. The coupling