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Zhao et al. Soft Sci. 2025, 5, 10 https://dx.doi.org/10.20517/ss.2024.61 Page 3 of 13

Nonetheless, utilizing certain raw materials in device fabrication presents potential risks to both the
environment and human health. The solvent-based preparation process offers a cost-effective approach to
sensor manufacturing; however, it involves the use of organic solvents that can be detrimental to human
health and the environment [40,41] .

Herein, we present an environmentally friendly and biocompatible carbon-based strain sensor that is
simple, cost-effective, and highly performant. By utilizing the green solvent dihydrolevoglucosenone
(Cyrene), the thermoplastic polyurethane (TPU), graphene and CNTs were dispersed in the solvent to
formulate a printable conductive ink. Subsequently, the strain sensor was fabricated by applying the ink
onto elastic fabric via the printing process, and its exceptional sensitivity and long-term stability were
evaluated. It showcased applications in monitoring joint bending, laryngeal phonation, pulse, and
electrocardiogram (ECG). Ultimately, a wearable system for monitoring neck movement and ECG signals
while sleeping was engineered, capable of detecting neck motion and ECG signals throughout sleep.

EXPERIMENTAL
Materials
Graphene powder (Oxygen content ≤ 0.01 wt%, Carbon ≥ 99 wt%, diameter: 15-25 μm) was sourced from
DT NANOTECH Inc., China. CNTs (Whisker-CNT0076G, purity: 95%, diameter: 10-30 μm) were procured
from Jiangxi Kelaiwei Carbon Nano Materials Co. Ltd. TPU powder [density: 1.20 g/cm³, granular size
distribution (GSD): 0-80 μm] was sourced from Bayer Co., Ltd. Cyrene (C H O ) was sourced from Sigma-
3
8
6
Aldrich Trading Co., Ltd., Shanghai, China. Heat transfer (HT) paper was sourced from Transmax, USA.
All materials were utilized in their as-received state without undergoing any additional purification.
Preparation of graphene-CNTs-TPU composited ink
In summary, 2 g of TPU powder was combined with Cyrene (20 mL) and a minor quantity of
fluorosurfactant (0.2 wt%, Capstone FS-3100, Du Pont Co., Ltd), and then the TPU-Cyrene mixture was
homogenized under vacuum in a defoaming apparatus at a high rotational speed of 3,000 revolutions per
min (rpm) for 10 min by a speed mixer (SIE-MIX80, Guangzhou SIENOX Technology Co., Ltd., China).
Once cooled, 1g of CNTs was introduced, and the mixture was stirred at 3,000 rpm for 5 min. Subsequently,
an equivalent mass of graphene was added, stirred at 3,000 rpm for 3 min, and then cooled for 6 min. The
stirring and cooling cycle was repeated 3 times, and ultimately, the graphene-CNTs-TPU conductive ink
was produced.

Fabrication of graphene-CNTs-TPU/fabric composite strain sensor
The conductive ink exhibits strong adhesion to HT paper, satisfying the printing requirements for extensive
[42]
patterns . Once the elastic fabric and HT paper were trimmed to the appropriate size, the highly elastic
bottom layer of the HT paper was detached from the backing sheet. The HT paper was smoothly aligned on
the fabric and then both were positioned on the HT machine workstation, subjected to pressure at 170 °C
for 10 s with a substantial force, and ultimately, the elastically enhanced decorated fabric was achieved. The
composite ink was applied to the decorated fabric using the stencil printing method and then cured in an
oven at 80 °C for 150 min, yielding a graphene-CNTs-TPU composite film with dimensions of 25 mm² × 4
mm². Ultimately, the film was cut into strip-shaped strain sensor devices, with dimensions of approximately
27 mm² × 8 mm².

Characterization and measurement
Strain sensing tests were performed on a linear test bench that incorporates a tension control system. The
ends of the strain sensor were secured in the stretching test machine (FlexTestS-P2, Hunan NanoUp
Electronics Technology Co., Ltd., China) and connected to a source meter (Keithley-2400, Tektronix Inc.,