Figure 3. a) Subtle physiological signals monitoring at different parts of human body. b) Monitoring the strain change of the inhalation and exhalation with time by using the as-fabricated optical sensor. The inhalation and exhalation of a volunteer was detected by using the microfiber coupler sensor fixed on the mask. c) Monitoring the pulse signals of a volunteer at different body’s sites: finger, neck, wrist, and ankle. d) Monitoring the weak movement of wrist, where the motion was driven by the far end fingers bending. e) Monitoring the tiny epidermis and muscle movement during speech for phonation recognition.
To demonstrate the fast response, high sensitivity and continuous monitoring characteristics of this flexible sensor, an experiment was designed and conducted to monitor the ant-crawling induced dynamic weak strains. The flexible microfiber coupler sensor was fixed on a stage with stretched form, and an ant with 1.1mg weight could freely craw from left to right on the sensor film surface (Figure 4 a). During the crawling movements, the sensor had captured the ant-crawling induced weak strain signals in real time (Movie 1 in Supporting Information). The recorded parabolic shape curve as shown in Figure 4b indicated that this sensor completely had the ability for ultralight ant movement recognition. However, the non-flat response curve also revealed that the sensitivities were not distributed homogeneously on the sensor film, thereby causing difficulty in quantitative dynamic measurement.
To further explore the dynamic weak strain sensing ability, this flexible microfiber coupler sensor was tested as a non-contact acoustic microphone. As shown in Figure 4c, the sensor was stretched and suspended above a loudspeaker driven by a signal generator. The loudspeaker produced single frequency sinusoidal sound waves from 20 Hz to 20 kHz, with an increment of 10 Hz, 100 Hz and 10 kHz, respectively. At all these acoustic-strain frequency levels, the acoustic pressure level was set, as measured by a calibrated electronic microphone, to be about 96 dBC. As illustrated in Figure 4d, the measured \(CR\) under logarithm function obviously response the acoustic pressure induced strain, where the 3 dB frequency response bandwidth was mainly located within 50 Hz to 3 kHz. And the \(CR\) decayed rapidly at frequencies lower than 50 Hz and higher than 3 kHz, the main reason was that the stiffness of PDMS film greatly affected the frequency response range, especially the high-frequency part. As illustrated in Supporting Information (Figure 1S), the measured CR at the 300 Hz and 3 kHz frequency testing points could be clearly captured and distinguished with nearly undistorted sinusoidal shape. According to amplitude of CR change, it could be deduced that the strain induced by the acoustic wave pressure on the sensor was just about 0.000011%, which was much lower than above measured detection limit 0.001%. Unlike the single frequency sinusoidal acoustic strain applied in the above experiments, the real human voice was a complex signal composed of acoustic waves of different frequencies and intensities, and it changed rapidly with time. To verify the microphone function of this flexible sensor, an audio clips (music “You raise me up”, 15s duration) was applied to the loudspeaker and broadcasted to sensor. Figure 4e represented the demodulated \(CR\)change frequency spectrum under the music audio clip source, which was mainly concentrated in the range from 100 Hz to 3 kHz, and the inset showed the CR change time-varying waveform. This demodulated CR change time-varying waveform was converted into an audio file (Audio 1 in Supporting Information). Listening to this demodulated audio file, the music could be clearly recognized with little noise, which showed the fidelity of flexible microphone application.
The broadband response of this sensor also provided a new mechanism for multi-parameter sensing based on the frequency division multiplex technology. As shown in Figure 4g, the flexible sensor was fixed and attached to the skin of the wrist, so the arterial pulse signals from the attached wrist and the human voice signals broadcasted from the loudspeaker could apply simultaneously on the sensor. Arterial pulse signals mainly located within the frequency range about 0.3-2 Hz, and the human voice signals were in the range from 100 Hz to 3 kHz, so there two signals could be monitored and demodulated simultaneously by this flexible sensor (Figure 4f). As shown in the Figure 4g, the record signals were enveloping curves of low frequency pulse modulated with high frequency sound. Employing frequency domain filter algorithms, the pulse (75 times per minute) and the voice signals were separately filtered and displayed. Audio 2 in Supporting Information showed that the restored sound was similar with the original audio, which further validated the high fidelity of the sensor.