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Page 2 of 13 Zhang et al. Soft Sci. 2025, 5, 17 https://dx.doi.org/10.20517/ss.2024.68

INTRODUCTION
It is known that sleep accounts for one-third of a person’s life and the quality of sleep plays a crucial role in
restoring the person’s health status. For patients with sleeping diseases such as suffocation during snoring,
convenient and efficient sleep monitoring ways are necessary for warning of potential risks. However,
traditional sleep monitoring tools such as polysomnography (PSG) and portable devices must be used with
complex wires connected to bulky equipment, severely limiting their popularized application in daily life. In
comparison, wearable sensors capable of acquiring physiological signals and posture information have
[1]
become an alternative way for sleep monitoring . However, they need to be tightly worn on the human
body, which still interferes with the quiet sleeping experience. Therefore, it is of significance to develop an
effective approach to monitoring the sleep status in an unrestricted way, and the intelligent pillow
integrated with flexible sensors that can monitor the health status of the sleeper through their contact with
the sleeper’s head would meet such need.

Flexible pressure sensors , as key components of wearable electronics, have attracted great attention for
[2,3]
[6,7]
their applications in the fields of human health monitoring , electronic skin , and human-machine
[4,5]
[9]
intelligence . Among various pressure sensing mechanisms such as piezoresistive , capacitive [10,11] , and
[8]
piezoelectric ones, capable of converting pressure stimuli into responding resistive signals, have been
[12]
widely studied because of their simple device configuration, easy signal acquisition, and low cost. Though
progress has been made, piezoresistive pressure sensors with high sensitivity, wide detection range, fast
response, and stable reliability are still under pursuit. Recently, the microstructuring strategy has been
proven to be an effective way to improve sensitivity, and the typical microstructures include a pyramid [13,14] ,
a dome [15,16] , and so on. However, typically expensive equipment and complex microfabrication processes
are needed to construct such regular microstructures, which limits their mass production and vast
application. Therefore, it is still a challenge to fabricate flexible performance-enhanced pressure sensors in a
facile and economical way . Meanwhile, current flexible sensors are designed and fabricated on
[17]
impermeable polymer film substrates, which would cause itching of the skin after a long time covering of
the skin [18-20] . Therefore, for flexible sensors to be integrated into pillows that will contact the sleeper’s head
during the whole night, breathability becomes a highly required merit for a comfortable sleeping experience.

In this work, we report the design and fabrication of a flexible pressure sensor by constructing both the
decorated MXene piezoresistive network and the printed interdigital silver electrodes on a dust-free paper
platform. The intrinsic fabric of the paper provides good permeability for comfortable skin-attaching, and
-1
the microstructured MXene on fabric offers high sensing performance with high sensitivity (16.7 kPa < 20
kPa), a wide detection range (~100 kPa), and fast response time (~50 ms). As a proof of concept, the
developed sensors integrated into an airbag pillow for snoring monitoring and sleeping head posture
recognition are well demonstrated. The proposed paper-based sensor, along with the intelligent airbag
pillow, provides a promising approach for sleep monitoring in a comfortable unrestricted way.

EXPERIMENTAL
Materials
Dust-free paper (Saige, Kunshan, China) was used as the fabric substrate for MXene sheet coating and silver
electrode printing. Silver paste (Mifang, Shanghai, China) was used to print the interdigital electrodes.
Conductive tape (Deyi, Guangzhou, China) was used to connect sensors and devices.

Preparation of MXene
The hydrochloric acid etching method was used to prepare MXene, in which the Al layer elements in the
MAX phase were selectively etched by dilute hydrochloric acid. The typical procedure is as follows: 7.5 mL