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Zhang et al. Soft Sci. 2025, 5, 17 https://dx.doi.org/10.20517/ss.2024.68 Page 5 of 13

Due to the flexibility and breathability, the designed pressure sensors can be integrated into an airbag pillow
for long-term snoring monitoring and sleeping head posture recognition with skin-attaching comfort
[Figure 1D].

Characterization and measurements of the sensor
Figure 2A shows the photograph of the prepared MXene/dust-free paper with black color, indicating the
successful incorporation of MXene on the paper substrate. SEM images further reveal that the MXene sheets
form a tight and wrinkled coating layer around the fibers in the paper. Element mapping images present
that the major elements of C, O, and Ti are uniformly distributed, indicating the uniform coating of MXene
[Figure 2B]. Figure 2C clearly shows the photograph of the printed interdigital silver electrodes on the dust-
free paper. SEM images further reveal that the solid silver paste is deposited on the fabric framework to
form the continuous electrode from a cross-sectional view [Figure 2C]. To make a complete sensor, the size
of the sensitive layer is 12 mm × 12 mm, which is large enough to cover the effective interdigital electrode
led region of 10 mm × 10 mm, and the designed interdigital electrodes [Supplementary Figure 1] are printed
on a dust-free paper with a size of 20 mm × 15 mm as the substrate. The thickness of the sensitive layer is
around 0.25 mm and the overall thickness of the assembled sensor unit is around 0.8 mm. Figure 2D shows
that the sensor unit can be bent as one integrated unit due to the use of a paper fabric framework as
substrate, and its layered structure can be seen from the SEM cross-sectional view with element mapping
[Supplementary Figure 2]. It is noted that the gap between the sensitive layer and the electrode layer
facilitates loose contact between the two, leading to a small initial current and then a large relative current
change for a large sensitivity. The edges of the sensitive layer and the electrode layer are bonded with
adhesive glue, so the assembled sensor is an integrated unit. It will not delaminate during practical bending
usage.

After being coated with MXene, the dust-free paper shows a moderate conductivity of 16 S·m to serve as a
-1
sensitive layer. In addition, its mechanical property is also enhanced due to the bonding effect between the
MXene and the paper fibers [Supplementary Figure 3]. Its resistance is measured to be changed a little
(within 2.4%) after being bent with a bending radius of 2 mm 1,000 times [Figure 2E], indicating its
consistently stable resistance for frequently flexible usage. To demonstrate the breathability of the paper-
based sensor, the sensor and different comparing sample films are used to cover a beaker filled with water at
the same room temperature with 44% humidity for days and the remaining water content is measured
[Figure 2F]. The opened beaker has the largest water-losing rate, which is the most air and moisture-free-
moving case. The pristine dust-free paper has the second largest water-losing rate, indicating the
breathability of the paper due to its intrinsic fabric pores. After decorating MXene, the dust-free paper still
offers a similar large water-losing rate as the pristine one, indicating that good breathability is well reserved.
In contrast, the beaker covered with impermeable polyimide (PI) and thermoplastic polyurethane (TPU)
films (widely used conventional polymer substrates for developing flexible electronics) offers almost no
water loss. Due to the breathability of the paper-based sensor, the skin covered by it for ten days remains the
healthy status while the one covered by the impermeable PDMS film becomes itching with redness
[Figure 2G]. The influence of environmental temperature on the sensor is evaluated, and it is found that the
resistance keeps a very slow increase as the temperature increases from 20 to 45 °C [Figure 2H], indicating
the sensor is capable of delivering relatively stable sensing performance without significant drifting when
used in the body temperature range. In addition, the environmental humidity also has little impact on the
sensor [Supplementary Figure 4].

Sensing performance of the sensor
The sensor adopts the piezoresistive pressure sensing mechanism for a simplified device structure. Different
from conventional piezoresistive sensors with two electrodes sandwiching one sensitive layer, the sensor