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Yue et al. Soft Sci 2023;3:13 https://dx.doi.org/10.20517/ss.2023.02 Page 3 of 11

stretchable, such as pulse, respiration, tremor, and limb movements [26-29] . Due to these characteristics of
flexible sensors, flexible sensors can be better used in smart tires. There are various substrates for flexible
sensors, which can be stretched by structural design [30-32] , or by using the mechanical properties of the
substrate material itself [33-35] . The material of its conductive medium is also varied, and laser-induced
graphene is mostly used for the preparation of flexible smart sensors due to its good properties [36-41] . The
substrate of laser-induced graphene (LIG) flexible sensor is carbon-based silicone with good elasticity,
which can fit well with the inner surface of the tire and can follow the deformation of the tire. LIG was first
[42]
discovered accidentally during the laser cutting of commercial polyimide (PI) films . LIG can be
successfully prepared on many types of natural and synthetic materials with sufficient carbon sources [42-51] .
The prepared LIG can be transferred from PI films to stretchable elastic substrates (e.g.,
polydimethylsiloxane or polyurethane), thus increasing the flexibility and stretchability of the sensor [52,53] .

In this work, we report the design and fabrication of a flexible LIG-based sensor and its application in smart
tires to record the ground trace and wheel speed. The tensile performance of PI-based and PDMS-based
sensors is compared systematically to select the most suitable sensor for smart tires, where the PDMS-based
sensor is obtained by transferring the prepared LIG from PI film to PDMS film. The results show that
LIG-PDMS has an excellent linear response and remarkable durability, ensuring its capability as a smart tire
sensor. Then the LIG-PDMS sensor is utilized to predict the tire grounding phase and the car speed during
driving. The results show that the LIG-based sensor has a wide range of application scenarios in smart tires.

EXPERIMENTAL
Preparation of the LIG-PI sensor
The PI film was first attached to the rectangular glass plate through the thermal release tapes, ensuring that
the PI film would remain flat after the laser scan, and the thickness of the PI film was 75 μm. Then, the
prepared PI film was placed on the commercial laser cutting platform (X-7050, G.U. Eagle Automation).
The PI film was scanned with a CO laser generator, and the scanning parameters were 12% specific power,
2
20 kHz laser frequency, 300 mm/s scanning speed, and 1000 DPI. The LIG was prepared into a
10 mm × 10 mm serpentine pattern. The conductive silver was painted as electrodes to connect the wires on
two sides of LIG. The LIG-PI sensor was successfully prepared.

Preparation of the stretchable LIG-PDMS sensor
The flexible LIG-PDMS sensor was obtained by transferring the LIG on PI film to the PDMS carrier. A
rectangular pattern of 10 mm × 10 mm was scanned on the PI film by using the same method. The PDMS
solution was spin-coated on the LIG pattern with a spin-coating speed of 150 RPM and a spin-coating time
of 60 seconds before curing at 70 °C for 1 h. The PDMS to curing agent mass ratio in PDMS prepolymer is
10:1, and the thickness of the PDMS film is 1 mm. The conductive silver was coated into electrodes to
connect the wires on two sides of the LIG region after stripping the PDMS from the PI substrate. The LIG
was coated with PDMS solution after transferring it to the PDMS carrier by transfer printing technology;
the thickness of PDMS film covered with LIG is about 30 μm. The fabrication of the stretchable LIG-PDMS
sensor was completed after curing the PDMS at 70 °C for 1 h.

RESULTS AND DISCUSSION
Figure 1A depicts the method of generating LIG on PI films. A 10 × 10 mm serpentine LIG pattern was
generated by scanning along a predesigned set path using a CO laser generator, and the conductive silver
2
was coated into electrodes to connect the wires on two sides of the LIG region. The picture is shown in
Figure 1C. However, the in-plane mechanical robustness of the PI substrate limits the stretchability of the
LIG-PI sensor. For this reason, the transfer printing technology was used to transfer the LIG from PI films