Figure 2. Operation and characterization of the nanofiber-based
pressure-sensor unit (NFPSU): a) Working principle of the NFPSU. b)
Simulated optical-transmission distribution of 630-nm-wavelength light
guided along an 800-nm-diameter optical nanofiber with bending radius 20
μm. c) SEM image of optical nanofiber. d) Micrograph of a U-shaped
optical nanofiber guiding 633-nm light. e) NFPSU’s wavelength-dependent
transmittance response to pressure in the range 0-28.7 kPa. Inset:
Response at 630 nm. f) 6000 repeated cycles of stability test. g)
Response to a pressure signal with a frequency of 800 Hz.
Figure 2a illustrates the working principle of the proposed
NFPSU. When skin deformations relating to finger movements occur, the
pressure applied to the bottom of the attached soft liquid sac changes.
According to Pascal’s principle, the pressure applied to any point
within an incompressible liquid can be transmitted to every point of the
liquid in real time.[33] Therefore, the pressure
applied to the bottom of the soft-liquid-sac base is transmitted to the
top surface, causing the contact filmy optical-nanofiber sensor to
deform. In this way, the external mechanical signals related to finger
movements captured at any point of the soft liquid sac base are
transmitted to the optical-nanofiber sensor with high fidelity,
eliminating the impact of position drift on the sensing signals. As
shown in Figure 2b , when the optical nanofiber is slightly bent
under pressure, the well-confined symmetric mode of an 800-nm-diameter
fiber at the input port evolves into an asymmetric profile with clear
optical leakage, making it highly sensitive to mechanical stimuli.
In this study, the optical nanofiber was fabricated by heating and
stretching a standard silica single-mode fiber
(SMF).[34] The as-fabricated optical nanofiber
showed excellent flexibility, significantly exceeding the performance of
standard silica or polymer optical fibers. For example, the bending
radius could be made less than 10 µm, as shown in Figure 2c.Owing to its smooth surface and geometric uniformity (Figure 2c), the
as-fabricated fiber offered a transmission greater than
99%[34] and a tensile strength higher than that
of spider silk.[35]
Figure 2d shows a U-shaped nanofiber guiding 633-nm-wavelength
laser light. The bright red light along the fiber indicates the presence
of an evanescent field outside the optical nanofiber. Generally, with
the decrease of the fiber diameter, the fractional power of the light
outside the optical fiber increases exponentially and a stronger
evanescent field results in a higher sensitivity. However, a thinner
optical nanofiber loses mechanical stability when it is manipulated to
form a U shape structure. In this work, an 800-nm-diameter optical
nanofiber was chosen for the trade-off between high sensitivity and
mechanical stability. For high compactness, the curved end of the
U-shaped optical-nanofiber sensor was intentionally positioned slightly
over the edge of the liquid sac to ensure that deformation would occur
in the sensitive area of the nanofiber.
To investigate the sensor’s pressure response, we used a mechanical
testing system (Figure S1 ). Figure 2e shows
the wavelength-dependent transmittance response to applied static
pressure in the range 0-28.7 kPa. In terms of the pressure sensitivity S
= ΔT/ΔP, where ΔT is the change of transmittance and ΔP is the change of
pressure, the sensor achieves a sensitivity of -0.03
kPa-1 at 630 nm (Figure 2e, inset ). With
increasing wavelength, the transmittance of the nanofiber decreases and
the sensitivity increases as a result of the increasing fractional power
of the evanescent field.[17]
The NFPSU demonstrated high durability under a pressure of 28.7 kPa at a
frequency of 0.55 Hz (Fig. 2f ): after 6000 cycles, the sensor
performance showed little change. In addition, a sinusoidal mechanical
signal with a high frequency of 800 Hz was applied to evaluate the
temporal frequency response of the NFPSU. The stable performance shown
in Figure 2g demonstrates the sensor’s ability to obtain
mechanical signals related to finger movements in real time.