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

Sylgard) was spin-coated at 800 rpm for 60 s. Copper foil (6 μm, BR0214, MSE Supplies LLC) was then laid
across the slide. PI (PI2545, HD MicroSystems) was spin-coated onto the copper foil at 800 rpm for 60 s and
cured on a hot plate at 240 °C for 60 min. Afterward, the copper and PI were transferred to the glass slide
coated with elastomer. The sensor was then patterned on a laser micro-machining system (Femtosecond
Laser Micro-Machining System, OPTEC). A bridge made of commercial high gauge enameled magnet wire
was soldered across the pads connecting the interior of the coil with the end of the capacitive sensor.
Elastomer was spin-coated across the device at 500 rpm for 30 s, and the device outline was cut out using
laser micromachining. Finally, a thin coat of high tack silicone gel (A-4717, Factor 2) was added to one side
of the sensor for adhesion to the skin. The assembly steps are shown in Supplementary Figure 1.

Wireless transmission
The capacitive sensor and coil form an LC circuit with a varying capacitive value. When the capacitance of
the sensor decreases, the resonant frequency of the circuit increases. A vector network analyzer (Tektronix
TTR506A) was connected to a spiral receiving antenna. A custom MATLAB program was used to record
the S parameter at a stimulus frequency close to the resonant frequency. The resonant frequency was
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determined by locating the frequency at the minimum S value in the range of 10-60 MHz. The recording
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stimulus frequency was then determined by subtracting 0.1 MHz from the resonant frequency.

Validation
Commercial Ag/AgCl electrodes across Lead I were used for the reading of the ECG, and a low-power
commercial accelerometer (ADXL355, Analog Devices, Inc.) was used for the reference SCG.

RESULTS AND DISCUSSION
We developed a wireless, ultrathin, and soft sensor with no rigid components such as batteries and IC chips,
demonstrating a seamless integration with the human skin. When mounted on the chest, this device can
measure high-fidelity strain changes due to cardiac vibrations. Figure 1A shows the mounting location of
the wearable sensor when placed on the lower chest. Figure 1B captures a series of photos showing a very
small form factor and mechanical compliance of the thin-film soft sensor. This device, encapsulated with
elastomeric membranes, shows superior stretchability and flexibility with multi-modal bending, pulling, and
twisting. Figure 1C shows a schematic of the measurement system of the device and its capabilities in
passive wireless sensing using antennas. The device comprises a capacitive strain sensor and an inductive
coil, forming an LC oscillator circuit [20,21] . This passive circuit offers wireless detection of the measured
capacitive signals without rigid circuits and batteries. When mounted on the skin, this sensor is inductively
coupled with an external antenna to record strain changes caused by cardiac vibrations. Overall, the soft
sensor can measure SCG, pulse, and heart rate data. Table 1 captures the unique advantages of the
developed sensor when compared to the prior work showing different types of sensors.

Figure 2A shows the layer structure of the developed chip-less, battery-less sensor. The thickness of the
entire device is 471 μm. The fabrication of the sensor only requires the patterning of one layer. Using a
femtosecond laser, the sensor is fabricated by micromachining a 6-μm copper foil backed by PI. Laser
micromachining was chosen as the fabrication process since it represents a promising alternative to
traditional cleanroom lithography processes. To avoid direct contact with the human skin, the sensor is
encapsulated in a low-durometer silicone elastomer, providing conformal contact with the skin. An
additional layer of high tack silicone gel was added between the device and the skin to maximize
[2]
conformality . It was also important for the elastomer to be as thin as possible to maximize conformality to
the skin. Since copper is not stretchable, the fingers are supported by connections with serpentine structures
[Figure 2B]. The laser micromachining process allowed interdigitated electrode spacings to be as small as