The soft sternal patch was fabricated using MEMS metallization in the inorganic cleanroom. A polyimide (PI) layer was spun coat on a PDMS coated Si wafer, then a Cu bottom plane, PI encapsulant, Cu top plane, and final PI layer were patterned via photolithography. The resultant circuit was transferred to an elastomeric substrate (EcoFlex 00-30) via a water-soluble tape. Components were then soldered onto the circuit. The final device is shown on the human body in Fig. 1(a) and (b). Firmware was developed to integrate with a high-resolution accelerometer (ADXL-355), photoplethysmography (PPG) sensor (MAX30105) and electrocardiography amplifier (MAX-30003). An android app was developed to receive data from the peripheral, plot it in real time, and save it to a CSV file. The functional block diagram of the device operation is shown in Fig 1(c). In addition, Fig(d) shows a closeup view of the device bending on a human finger, demonstrating the soft mechanics optimized for use on the human body, and Fig 1(e) shows the nanomembrane ECG electrodes on the device bottom view, which were fabricated in a similar MEMS process with ultrathin (<1µm) Au depositions. The acceleration data was collected in simultaneous recordings with clinical grade echocardiography in the m-mode view of the parasternal long axis to determine the optimal signal processing techniques to elucidate cardiac mechanics features from the acceleration. As shown in Fig 1(f), filtering between 4-24Hz yielded SCG waveforms with marked correlations to the opening and closing of the aortic valve in the echocardiogram.
Fig 1. Overview of the Skintronics device with soft, skin like electronics.(a-b) images of the device (a) placed on the sternum and (b) in use during sleep. (c) Diagram of overall device function. (d) Image of device bending on a finger. (e) Image of device with nanomembrane electrodes. Side-to-side comparison of echocardiogram data in the m-mode view of the parasternal long axis and SCG waveforms(g) Magnified view of a single SCG beat. (h) Simultaneous recording of SCG, ECG, and PPG signals measured by an all-in-one, wireless soft sternal patch.
Furthermore, the device mechanics were investigated to determine the ability of the device to capture the SCG signal. In Fig 2(a), an image of the device bending to 180⁰ with a radius of 3mm is shown, and the data quality during 100 rounds of cyclic bending with same parameters is shown in Fig 2(b). Because the data quality is unchanged after each bending round, the device is durable enough for long term use. Next, the device was placed along with a rigid comparison breakout board on a biomimetic elastomer substrate connected to a speaker, as shown in Appendix 1. Data was collected with each device during various frequency sinusoidal excitations with 30mg amplitudes, and the resultant waveforms are shown in Fig 2(c). The signal to noise ratios (SNR) is provided in Fig 2(d), showing the clear advantage of the soft device. In addition, zoomed in waveforms during various excitations are shown in Fig 2(e-f), demonstrating the serious distortion caused by traditional electronics. Finally, the signal spectral contents are provided in Fig 2(g), showing how the spectral peaks are degraded in the rigid comparison. After the device was fully validated, SCG data was collected as described in Fig 1(f) to validate the signal against clinical echocardiography.
Fig 2. Mechanical assessment of the soft sternal patch. a, Image of the device bending to 180⁰ with radius of 3mm. b, Three axis acceleration data plotted during 100 rounds of cyclic bending, demonstrating that the signal is identical at the beginning (bottom left) and end (bottom right) of the trial. c, acceleration data for the soft device compared to a rigid breakout board when excited by a speaker applying tones with stepwise frequency increases through a PDMS skin model. d, SNR comparison for the experiment in c. e, Example modulated sign wave signal captured by the soft device compared to the rigid system. (e-f), Example of signals captured for a (e) pure tone and (f) modulated tone in both devices. g, Spectral contents of the recorded signals for both devices demonstrating a clear increase in signal fidelity in the soft device.