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Page 2 of 27 Tian et al. Soft Sci 2023;3:30 https://dx.doi.org/10.20517/ss.2023.21

INTRODUCTION
The advancement of technology is mutually reinforcing. With the rapid development of artificial
intelligence (AI) and the Internet of Things (IoT), electronic skin (e-skin) devices have manifested more
and more potential in multifunctional sensing systems, which play a vital role in converting physical stimuli
[1-3]
into electronic signals . Specifically speaking, extensive efforts have been devoted to developing highly
integrated, flexible, stretchable, and multi-responsive sensors that can adhere to the human skin in order to
sense the surrounding environment, such as pressure, temperature, humidity, and some biochemical
[4,5]
signals, and respond quickly . Superior sensing performance has made e-skin a powerful candidate for
intelligent healthcare, soft robotics, motion detection, and virtual reality .
[6-8]
As is known to us all, human skin not only protects us from external harm as a remarkable natural barrier
but a complex and coordinated system where multiple biosensors attached to the skin surface realize
functional sensing and signal processing of the nerve center. E-skin, as an imitation of real human skin, is
not limited to mimicking its functions, such as pressure and temperature sensing, but more additional
[3,6]
features beyond human skin, such as light, sound, magnetism, velocity, and acceleration . Among the five
basic human senses (visual, auditory, tactile, gustatory, and olfactory), tactile, as the most fundamental one,
should not be ignored. It involves multiple physical signal measurements, primarily pressure, temperature,
and humidity, and is the most direct demonstration of e-skin excellence . Pressure and temperature
[7]
sensing can help identify objects, sense warm or cold temperatures, and respond quickly in emergencies,
making them highly valuable in a variety of application scenarios.

Undoubtedly, the fabrication of flexible pressure and temperature sensors towards e-skin faces numerous
challenges. The selection of suitable active and substrate materials is surely a cornerstone for outstanding
performance, while multiple combinations of sensing mechanisms can determine the types of composite
sensors and serve as a bridge between material selection and structural design [8-10] . For instance, pressure
a
d e t e c t i o n b a s e d o n piezoresistivity [11-16] , capacitance [17,18] , piezoelectricity [19-20] , n d triboelectricity [21-23]
mechanisms have been demonstrated before and researched extensively. Simultaneously, thermoresistive-
based [24,25] and thermoelectric-based [11-15] temperature sensors, namely thermistors and thermocouples, have
been widely fabricated for temperature sensing. The integration of pressure and temperature sensing
[7]
mechanisms is complicated and worthy of discussion . In addition, a novel and effective structural design
can help superior specifications, such as high sensitivity, high response time, and wide detection range. And
last but not least, a cost-effective manufacturing process can combine all of the previous work to complete
the whole system design. Accordingly, thorough consideration, such as those mentioned, should be
prepared by sensor researchers.

Herein, this review article summarizes flexible pressure and temperature sensors towards e-skin from four
perspectives, material selection, mechanism integration, structural design, and manufacturing methods, as
illustrated in Figure 1. Meaningfully, this review presents flexible pressure-temperature hybrid sensors,
especially from the perspective of manufacturing and processing methods, concluding the popular printing
technology, comparing in detail the advantages, disadvantages, and applicability of different printing
methods, and providing a route for pressure-temperature sensing arrays based on printing technology. The
main contents include the following aspects: (1) Choices of materials for pressure and temperature sensors;
(2) Basic sensing mechanisms and evaluation parameters for pressure and temperature sensors; (3) Multiple
mechanism integration and structural design of pressure and temperature sensors; (4) Manufacturing
methods for pressure and temperature sensors.