Introduction
Flexible devices exhibit excellent foldable and wearable performances,
have important applications in the fields of physiological
monitoring\citet{Takei_2014},\citet{Liu_2017},soft
robotics\citet{Majidi_2014},\citet{Kim_2013},\citet{Wu_2017},bioelectronics\citet{Kim_2018},\citet{Chen_2020},flexible
display\citet{Sekitani_2009},\cite{Chen_2013},\citet{Kim_2011},and smart
clothing\citet{Choi_2019},\citet{Yang_2019} .Flexible strain sensors that can
measure the environmental strains and mechanical deformations for
real-time in situ during the bending movements, have been the essential
flexible devices in human health monitoring and other fields. In order
to track the extremely weak strain signals in cardiovascular
diseases\citet{Fang_2021}, blood
flow\citet{Gu_2020}, skin tissue
healing\citet{Tang_2021}, and acoustic
vibration\citet{Yan_2022}, improving the sensitivity and
detection limit of the flexible strain sensor has become an urgent
affair. These advanced sensors will capture the change details of human
physiological indicators more easily and provide more effective data for
drug development, clinical diagnosis, and disease
treatment\citet{Li_2018},\citet{Zhao_2020},\cite{Li_2021},\citet{McNerney_2020}.
The electronic flexible sensors achieve the outstanding performances for
identifying the extremely weak strain sensing signals by measuring the
variations of capacitance\citet{Xu_2019},\citet{Mannsfeld_2010},resistance\citet{Jason_2017},\citet{Kim_2018a},piezoelectricity\citet{Zhao_2018},\citet{Zhu_2019}, and
triboelectricity\cite{Ha_2018},\citet{Wu_2018}. For example, Gao et
al.\citet{Gao_2022} embedded carbon nanotubes into new
polyurethane fibers, which was highly sensitive to mechanical signals
such as micro strain and vibration, and the detection limit was
0.0075%. Zhao et al.\citet{Zhao_2021}have achieved the
detection limit of 0.001% by using MXene/CNT to construct the
microcrack structure. However, parasitic effects, insufficient
insulation, and electromagnetic disturbances restrict the practical
applications of electronic sensors, to some
extent\citet{Roriz_2014},\citet{Wang_2013}.
Optical sensors, especially fiber-optic sensors, provide a promising
alternative to electronic sensors due to their distinct advantages of
high sensitivity, high precision, fast response, inherent electric
safety and electromagnetic interference immunity. At present, flexible
optical fiber strain sensors mainly realize the measurement based on
optical power detection scheme, which could be classified into three
types, mechanoluminescent effect, optical loss effect and optical
interference effect. The mechanoluminescent effect type is to generate
and convert mechanical stimuli related strain into fluorescence signals
based on intelligent mechanoluminescent materials.
Liang\citet{Liang_2021}proposed a self-powered stretchable
strain sensor based on the integration of mechanoluminescent phosphors
with an elastomer optical fiber, and demonstrated a relative high
detection limit (10%) due to the high threshold pressure of ZnS:Cu
mechanoluminescent materials. The second optical loss type is to measure
the changes of transmitted light intensity as the fiber is deformed by
strain, where the sensitivity is mainly determined by the compromise
between attenuation coefficient and transmission characteristics of
optical sensing fiber. Zhu et al.\citet{Zhu_2021}designed a
self-assembled wavy optical microfiber strain sensor by detecting the
micro bending loss of the microfibers, and achieved a detection limit of
0.5%. Pan et al.\citet{Pan_2020} proposed a pre-bent
microfiber sensor for controlling the transition of the radiation modes
into guided modes in the bending microfiber and achieved a detection
limit of 0.25%. Guo et al.\citet{Guo_2019} proposed a
flexible polymer fiber strain sensor based on the localized surface
plasmon resonance absorption and scattering effect of gold
nanoparticles, which showed a detection limit of 0.09%. The third
optical interference type is to detect the optical power variation based
on strain-induced optical interference effect. Peng et
al.\citet{Li_2018a} proposed a hybrid plasmonic microfiber
knot resonator that weak strain would shift resonant wavelength
resulting in output light power change, and achieved a detection limit
of 0.01%. It is worth emphasizing that the interference effect in
traditional silica optical fiber has been proved to measure strain with
a sensitivity limit at sub-pε level,\citet{Gagliardi_2010}which
offers the great enormous potentialities for improving the sensitivity
and detection limit of flexible optical fiber strain sensors.
In this paper, we propose a flexible optical fiber strain sensor with an
ultralow detection limit of 0.001% strain. The fabricated microfiber
coupler is encapsulated into PDMS films to develop flexible strain
sensors. The strong coupling effect of this microfiber coupler sensor is
highly sensitive to the strain-induced coupling length change, so the
coupling ratio (CR) between the two output fibers has been adopted to
realize weak strain demodulation. The proposed CR demodulation can
effectively resist the interference of hybrid variables, such as optical
loss and input light intensity fluctuation. Based on these advantages,
this flexible strain sensor exhibits ultrahigh strain sensitivity (gauge
factor, GF=900), low detection limit of 0.001%, ultrafast response time
(<0.167 ms), and superior durability and stability
(>10 000 cycles). The optical loss parameter has been
adopted as the second measurand with CR simultaneously, so the strain
sensing range has been expanded from 0.034% to 0.45%, which greatly
break the sensing range limit within a quarter cycle of strain-CR cosine
curve. These superior sensing properties make this flexible microfiber
coupler sensor feasible for wearable photonics healthcare applications,
such as demonstrations of monitoring ultra-weak strain signals arising
from a human’s arterial pulse, detecting the dynamic impacts associated
with crawling of an ant, and restoring the sound signals caused by weak
vibrations of human voice. In addition, the simultaneous monitoring of
arterial pulse signals and human voice have been achieved based on
frequency division multiplexing demodulation technique. The advantages
of high sensitivity and fast response of this proposed flexible
microfiber coupler sensor shows great potential for the human-machine
interfaces, artificial intelligence and soft robotics.
Experimental Section
The central part of our flexible sensor is the microfiber coupler, where
the slight deformations of the coupler induced by the external stimuli
would cause the apparent variations of CR. The microfiber coupler was
fabricated by tapering and fusing two standard telecommunication
single-mode optical fibers (SMF-28, Corning) together at the same time
using a method known as the flame brushing
technique\citet{Brambilla_2004} as shown in Figure 1 a. An
ideal microfiber coupler with a diameter of several hundred nanometers
to several micrometers and a uniform waist with a length of several
millimeters to several centimeters could be fabricated by carefully
controlling the heating temperature, flame flow, sweeping of
micro-heater, and moving of the translational stages. The optical loss
of our fabricated microfiber coupler was measured as low as about 0.05
dB. The second step was to encapsulate the microfiber coupler into a
flexible sandwich structure for improving its stability. The
encapsulation layer was selected as PDMS film owing to the virtues of
high flexibility, high transparency and biocompatibility. The PDMS
precursor (mixing ratio of base polymer and curing agent) was poured
into a homemade model groove (2 cm × 5 cm size with 100 µm thickness)
and formed through thermal treatment at 80 ℃ for 30 min to obtain a flat
and smooth film as the bottom layer. The prepared bottom PDMS film was
transferred to the other deeper groove mold (2 cm × 5 cm size with 250
µm thickness) for the assembling process. After that, the fabricated
microfiber coupler was transferred onto the bottom PDMS film in the
deeper groove mold, and it could firmly adhere to the PDMS film due to
van der Waals forces. Finally, some PDMS precursor was uniformly
dispersed in the deeper groove mold to cover the microfiber coupler with
a thickness of 150 µm, and the layered flexible microfiber coupler
sensor was fully assembled and realized after 30 min of curing at 80 ℃.
After encapsulation, a highly transparent sensing film with a thickness
of about 250 µm was demonstrated to manipulate in a twisted state
(Figure 1c) and a bent state (Figure 1d,e) which showed the outstanding
structural flexibility. Figure 1b,e revealed that this encapsulated
microfiber coupler kept a good light guiding ability even under giant
deformations. This encapsulated microfiber coupler was stuck on the
wrist by Van der Waals force with good contact which showed that the
sensor had good wearability, as shown in Figure 1f. This prepared
flexible sensor with skin-like mechanical characteristics and excellent
optical transparency enabled it to be a powerful tool for wearable
application.