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Page 4 of 13 Zhao et al. Soft Sci. 2025, 5, 10 https://dx.doi.org/10.20517/ss.2024.61

USA) to track resistance changes. The surface topography of the strain sensor was analyzed using a field-
emission scanning electron microscope (SEM, JEOL, Japan) with an accelerating voltage of 10 kilovolts
(kV). The impedance data was obtained using a LCR meter (HTOKI-IM3536, Hioki E.E. corporation,
Japan). Human motion and health-related data were gathered using a portable digital measurement device
(01RC, Hangzhou LinkZill Technology Co., Ltd, China). ECG monitoring was achieved using a miniature
digital ECG development board (WDECG), which integrates ECG signal acquisition, processing, and
wireless transmission. A sleep monitoring system was developed to enable real-time measurement of both
neck posture while sleeping and the associated ECG signals.

RESULTS AND DISCUSSION
Structure and sensing properties of strain sensor
Figure 1A depicts the manufacturing process of the graphene-CNT-TPU/fabric strain sensor. Figure 1B
displays the final sensor, notable for its compactness, rendering it appropriate for a range of human
monitoring applications. The fibrous structure of the elastic knit fabric [Figure 1C] and the smoothness of
its fiber surface attest to the fabric’s superior elasticity and structural stability. The surface morphology of
the HT paper [Figure 1D] reveals a clearly defined porous structure, which enables the conductive ink to
partially infiltrate the HT paper, thereby strengthening ink adhesion during printing. Supplementary Figure
1 illustrates the process of attaching transfer paper to the modified fabric using a heat press, culminating in
the modified fabric as depicted in Supplementary Figure 1B. Supplementary Figure 2 demonstrates the
performance characteristics of the graphene-CNT-TPU ink. When the ink is applied to the HT paper-
modified fabric, it creates a composite film on the fabric’s surface, as depicted in Figure 1E, displaying a
markedly uneven and porous structure. As seen in Figure 1F and Supplementary Figure 3, TPU
encapsulates the graphene surface in an irregular pattern, harboring numerous CNTs. The bridging CNTs
link disparate graphene sheets, significantly enhancing the conductivity network and boosting the film’s
conductivity and sensing stability. Supplementary Figure 4A (i) and (ii) shows the dual-layer structure of
the HT paper and its adherence to the fabric. Supplementary Figure 4B (i) indicates that the boundary
between the composite film and the HT paper coating is indistinct. The sensor’s cross-section substantiates
the non-uniformity of the composite film surface [Supplementary Figure 4B (ii)].

To investigate the impact of HT paper and various elastic fabrics on the strain sensing range and sensitivity
of graphene-CNTs-TPU composite films, three types of sensors were fabricated using the same conductive
ink: one on ultra-elastic fabric with HT paper decoration (Sensor I), one on a cleanroom wiper with HT
paper decoration (Sensor II), and one on untreated ultra-elastic fabric (Sensor III). Figure 2A illustrates the
impact of HT paper and various fabrics on sensor performance. In comparison to Sensors I and III, the
fabric adorned with HT paper significantly enhances the strain-sensing range of the graphene-CNTs-TPU
composite film over that of the untreated fabric. Notably, Sensor III possesses only approximately 8% of the
maximum sensing range. Given that when conductive ink is directly printed onto the fabric, the majority
adheres to the fiber’s outer surface, with only a minor portion infiltrating the interior via the parallel gaps
between fibers, and the fiber junctions remain largely ink-free. During the stretching of the fabric, micro-
cracks form within the composite film when subjected to a force aligned with the fiber deformation
direction, causing a significant rise in the resistance of Sensor III. Should the conductive network be entirely
compromised, Sensor III forfeits its sensing capabilities entirely. When contrasting Sensor I with Sensor II,
it is evident that opting for a fabric with greater stretchability substantially improves the strain-sensing
range of the composite film. Given that the fibers of the cleanroom wiper are less dense than those of the
ultra-elastic fabric, the gap expansion between the cleanroom wiper fibers during stretching is prone to
causing HT paper rupture, consequently impacting the strain sensing range of the composite film.
Supplementary Figure 5 indicates that fabric selection exerts minimal influence on the composite film’s
sensitivity at low strain levels.