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Page 2 of 9 Guess et al. Soft Sci 2023;3:23 https://dx.doi.org/10.20517/ss.2023.17

INTRODUCTION
Cardiovascular diseases (CVD) account for over 19 million deaths annually in the United States . Early
[1]
detection and diagnosis of this broad group of diseases are imperative in treatment. The clinical standard for
detecting cardiovascular abnormalities is the electrocardiogram (ECG) . However, ECG alone cannot
[2]
[3]
capture a holistic view of heart function . Therefore, numerous other cardiovascular signals have offered
crucial information to assist in the detection of CVD . One of these signals is the seismocardiogram (SCG).
[4]
[3]
SCG is traditionally measured using an accelerometer to capture the local vibrations of the heart . While
ECG measures myocardial conduction, SCG measures myocardial contractions and can capture the timing
of fiducial points, such as mitral valve openings (MO) and closings and atrial valve openings and closings,
[3]
which ECG cannot capture . SCG is typically measured using integrated circuit (IC) accelerometers .
[3,5]
However, these accelerometer systems are rigid and bulky, making them more uncomfortable to wear for
[6]
long periods. In addition, accelerometers are also prone to the global inertia of the body . Although thin
and flexible electronics are now becoming a widespread research field, exploring soft sensors for SCG is
relatively unexplored. Soft and flexible sensors can offer solutions to these problems [7-12] . Liu et al.
demonstrated a flexible serpentine interconnecting device over a rigid printed circuit board . However, the
[5]
fabrication was still largely dependent on cleanroom processes and photolithography.

Ha et al. first reported a stretchable piezoelectric strain sensor based on polyvinylidene fluoride (PVDF) that
could stretch 110% . Nayeem et al. also reported a PVDF-based mechanoacoustic sensor with high
[7]
[14]
sensitivity . Additionally, Lo Presti et al. reported an SCG sensor using optical strain sensors . All of these
[13]
are improvements on the traditional MEMS accelerometers, yet they still require additional components for
transmission. Adding the seamless wireless capability to these thin sensors is essential for ambulatory
monitoring and patient comfort. Standard accelerometer-based SCG sensors have communicated data using
[15]
[17]
[16]
Bluetooth , WiFi , and Zigbee . On the other hand, thin-film SCG sensors have only been measured
wirelessly using active near-field communication (NFC) components . Additionally, these sensors are still
[18]
limited by rigid circuit components and power consumption limits . All the current SCG sensors are either
[18]
limited by low-throughput fabrication processes or bulky hardware for transmission. Thus, in this work, we
aim to combine the comfort of thin film SCG sensors, a rapid one-step fabrication method, and passive
wireless capability.
Here, we propose a batteryless, wireless, chipless, and passive soft SCG sensor that can be measured using
inductive coupling. The sensor and coil are incorporated in a single layer of copper, backed by polyimide
(PI), eliminating the need for cleanroom processes or other time-consuming methods. The thin sensor is
encapsulated with elastomer and attached to the skin using a low-modulus silicone gel, allowing the device
to attach to the human skin with limited restrictions conformally. Furthermore, the passive sensor does not
require any batteries or IC components, allowing the entire sensor to be flexible, stretchable, and nearly
invisible to the user. To validate the sensor, both an SCG measured with a commercial accelerometer and a
single-lead ECG were used to compare the fiducial points. The fiducial points, including mitral valve closing
[19]
(MC), isovolumic movement (IM), aortic valve opening (AO), aortic valve closing (AC), and MO ,
showed a strong association between the reported device and the commercial accelerometer.

EXPERIMENTAL
Assembly of passive sensors
First, a glass slide was spin-coated with silicone elastomer (Ecoflex 00-30, Smooth-On, Inc.) at 500 rpm for
30 s and cured in an oven at 60 °C for 20 min. On a separate glass slide, polydimethylsiloxane (Sylgard 184,