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Zhang et al. Soft Sci. 2025, 5, 17 https://dx.doi.org/10.20517/ss.2024.68 Page 3 of 13

of concentrated hydrochloric acid was added to 2.5 mL of deionized water, and then 0.8 g of lithium
fluoride was added, followed by stirring the mixture at 400 rpm for 5 min at room temperature. Afterward,
0.5 g of Ti AlC was added with stirring at 500 rpm at 47 °C for 24 h, and then deionized water was added,
2
3
followed by centrifuging the mixture seven to eight times. Finally, the mixture dispense was treated by
ultrasonic and centrifugation at 3,000 rpm for 20 min, and the supernatant was MXene.

Preparation of the sensor
The prepared MXene was dropped in deionized water, followed by stirring for 20 min and ultrasonic for
5 min to obtain the well-dispersed diluted MXene solution. Then the prepared MXene solution was poured
into a shallow container, and a piece of dust-free paper was laid flat on the surface of the solution. Because
of the hydrophilic nature of the paper fibers, the paper would adsorb the water into its fabric network and
sink with immersion in the solution. MXene can be firmly attached on the surface of fibers due to the
interaction of functional groups, forming a MXene coating layer on the fabric framework. The dip-coating
process lasts for 10 s each time, followed by vacuum drying at 40 °C to further anchor the coating of MXene
on fibers. Multiple cycles of dip-coating and vacuum drying can be applied to coat more MXene on the
dust-free paper to finally obtain the piezoresistive layer. The interdigital electrodes were made by screen
printing silver paste on another piece of dust-free paper to obtain the electrode layer. The screen plate has a
mesh size of 250, a wire diameter of 40 μm, and a screen thickness of 68 μm. The piezoresistive layer and the
electrode layer were compressed together with sides taped to obtain the sensor unit.

Characterization and measurements
The morphology of the prepared fabric samples was characterized by a high-resolution field-emission
scanning electron microscopy (HRFE SEM, JEOL 7610F, Japan Electronics Co., Ltd.), with energy
dispersive spectroscopy (EDS) for the elemental distribution analysis. The sensor was placed on a flat
heating table with adjustable temperature (2020, Shenzhen Hualianchuang Electronic Tools Co., Ltd) to test
the effect of temperature on it. The associated resistance was measured with a high-precision LCR meter
(6300, Guwei Electronic Industry Co., Ltd.). A pressure load is applied by a high-precision universal testing
machine (HTS-LLY9120B, Guangdong Zhongye Instrument Equipment Co., Ltd.) to test the sensing
performance. For the biocompatibility test on human skin, the sensor and poly(dimethylsiloxane) (PDMS)
film were tightly attached to the skin of the volunteer’s forearm for ten days, and the color and health state
of the covered skin were observed.

RESULTS AND DISCUSSION
Design, fabrication and application of the sensor
For simplicity of the device configuration, the sensor adopts a concise two-layered structure by compressing
an MXene network-based piezoresistive layer on top of an interdigital silver electrode layer, both of which
are constructed on the dust-free paper substrates. Being an emerging two-dimensional (2D) material with
excellent electrical conductivity and high specific surface area, MXene has been proven to be an effective
sensitive material with nano-/micro-structures for improved sensing performance for flexible sensors [21,22] .
MXene is obtained by the hydrochloric acid-etching method in an aqueous solution [Figure 1A], and the
scanning electronic microscopy (SEM) image shows the typical accordion-like layer structure of Ti C T
2 x
3
powder [Figure 1B], which facilitates the formation of MXene network in the paper fabric. The hydrophilic
groups in the aqueous solution of MXene interact with the hydrophilic groups in the paper fibers through
the hydrogen bonding and capillary action effects, prompting the deposition of MXene flakes on the surface
of the paper fibers and the formation of a stable conductive network through functional groups bonding.
After being picked up and vacuum-dried, the dust-free paper decorated with MXene network can be used as
the piezoresistive layer [Figure 1C, bottom line]. The interdigital silver electrodes are printed on another
piece of dust-free paper using the screen-printing technique [Figure 1C, upper line]. In the end, the