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Page 2 of 11 Yue et al. Soft Sci 2023;3:13 https://dx.doi.org/10.20517/ss.2023.02

controllability of the vehicle. Considering that the forces required for the vehicle to move on the road are
generated by the interaction between the tire and the road, the key directions in vehicle research are the
mechanical properties of the tire and the contact state of tire and road. Monitoring tire movement is crucial
because it is complex and involves considerable deformation, making it difficult to quantify. In response to
these needs, the concept of smart tires has been proposed . A smart tire is a highly intelligent tire product
[1-3]
that integrates advanced technologies, such as smart material technology, sensing technology, signal
conditioning, and communication technology, and even has various sensors embedded within the tire. The
[6]
[4,5]
various tire parameters (such as tire pressure , tire temperature , tire strain , tire force [10,11] , and road
[7-9]
adhesion information [12-14] ) measured by these sensors are transmitted to the central controller of the vehicle
for real-time monitoring. Monitoring parameters in real-time and performing corresponding
improvements can improve vehicle dynamics and fuel economy, and can prevent dangerous situations, such
as blowouts, rollovers, and skidding, by warning in time before they occur. Efforts have been exerted
globally to integrate wireless sensors within tires for measuring dynamic mechanical parameters. The in-tire
sensors currently being developed need to fit well into the tire structure without interfering with the tire’s
movement and be able to adapt to changes in the tire’s internal structure and environment under different
operating conditions, which demands flexibility and form-following properties of the sensors. Traditionally,
the most classic tire pressure monitoring system transmits tire pressure signals through an in-vehicular
wireless device, which is employed to warn a driver of any loss in tire pressure [15,16] .

At present, smart tire technology has flourished, and most methods are implemented by collecting the data
generated by the tire tread deformation during the driving and actuating process of the vehicle and then
calculating and predicting the tire force and road adhesion coefficient with the help of corresponding
algorithms to obtain the specific vehicle movement state. The classification of smart tires is based on the
type of sensors embedded in the tires, including acceleration sensors [17,18] , optical sensors , acoustic
[19]
sensors , and piezoelectric sensors [1,10,21,22] .
[20]
Kim proposed an algorithm that can determine the road surface condition of a driving vehicle using an
acceleration sensor attached to the inside of a tire . Dasol presented a new load estimation algorithm
[23]
[17]
based on an acceleration sensor. This algorithm is analyzed by using a flexible annular tire model, which is a
physical tire model and is constructed on the basis of the relationship between the load contact angle
[24]
pressures. Alfred conducted tire deformation research by using surface acoustic waves to monitor the
strain on the inner surface of the tire and then determined the state of the tire under stress employing an
algorithm. However, the sensor requires the insertion of a probe into the tire carcass, which can affect the
regular use of the tire and increase the safety risks. Generally, the tire generates a sizeable centrifugal force
when it rotates at high speed. The sensor suffers a significant acceleration when it passes through the
contact area, which affects the contact quality between the tire and the ground, further increasing the safety
risks. Optical sensors are noncontact measurement sensors that measure the lateral, longitudinal, and lateral
deformation of tires with high accuracy. Tuononen et al. used optical sensors to measure the coefficient of
friction of tires on the road , and Erdogan et al. used polyvinylidene fluoride piezoelectric sensors to
[25]
monitor the deformation of the tire carcass , which are one of the few experimental cases where flexible
[21]
sensors were used as in-tire sensing.

However, most of these sensors are rigid, require an external power supply, and are manufactured using a
time-consuming, multistep process, which increases the complexity and cost. In the development and
design process of the more mature technology of intelligent tires, the overall architecture is still highly
complex. Its implementation requires the interaction of multiple subsystems in the vehicle and a number of
sensor types. Flexible sensors can be used to capture a wide range of signals because of flexible and