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.