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Page 2 of 16 Fan et al. Soft Sci 2024;4:11 https://dx.doi.org/10.20517/ss.2023.47

[1-3]
biocompatibility, good breathability, strong stability, wear resistance, biodegradability, etc. . The
preparation process for the leather preserves the flexibility and strength of animal skin; thus, it has been
[4,5]
used as a popular clothing and armor material throughout history . In the early days, people’s demand for
leather products was practical, beautiful, sturdy, and stab-resistant. Compared to traditional weaving fabrics,
the natural 3D weaving structure and intertwined fiber bundles of leather can better reduce acupuncture
damage. However, with the rapid development of soft functional composites and flexible electronic devices,
the single function and application of leather composites cannot satisfy the requirement . In comparison
[6,7]
to flexible substrates with a single structure, such as polydimethylsiloxane (PDMS), polyethylene
terephthalate (PET), etc., the leather is more easily modified and highly breathable due to the natural porous
[8]
structures . Furthermore, the mechanical strength of leather is reliable compared to other fiber-based
substrates, such as cotton-based textiles and polyimide (PI) . Therefore, leather will be expected to have
[9]
more potential applications in wearable electronic devices, electromagnetic interference shielding, flame
[10]
retardant protection, and intelligent thermal management .

When processed into leather, the original skin of animals requires many steps, such as removing impurities,
tanning, fat-liquoring, and drying [11,12] . After a series of treatments, the nanoscale collagen fibers, unique 3D
porous network structure, and multilevel hierarchical structure were ultimately retained . Based on this
[13]
special microstructure and biocompatible collagen fibers, a large amount of research has emerged on
multifunctional leather composites in recent years . On the one hand, various functional materials were
[14]
selected to combine with collagen fibers to form physical adhesion or chemical crosslinking bonds [3,15] ,
which were composed of conductive materials including graphene oxides (GO) [16,17] , carbon nanotubes
(CNTs) [18,19] , silver nanowires (AgNWs) , MXene , poly(3,4-ethylenedioxythiophene)-poly(styrene
[20]
[21]
[22]
sulfonate) (PEDOT:PSS) , etc., and insulating materials include silicon dioxide particles , boron
[23]
nitride , montmorillonite , etc. Then, the structural design was carried out on the leather at the macro- or
[25]
[24]
micro-scale by utilizing the above functional materials to obtain leather composites with editable
properties . These methods are similar to dyeing in the leather-making process. On the other hand, the
[26]
natural microstructure of leather fiber networks could be used for in-situ growing hydrogels or other
composites through the induction of catalytic materials. At last, the obtained hydrogels or other composites
could be further designed by the above functional materials, which enabled leather composites to get other
functions [27,28] .

By combining various functional materials, the smart leather with tunable functionalities, such as
[31]
conductive leather [29,30] , electromagnetic shielding leather , flame retardant leather [32,33] , antibacterial
leather [34-36] , thermal camouflage leather , waterproof leather [11,38] , and so on, has been developed. From this
[37]
perspective, we mainly introduce several common and efficient preparation methods for constructing
intelligent leather composites and then focus on their applications in flexible sensors, electromagnetic
interference shielding, flame retardant, body safeguarding, thermoregulatory clothing, and other aspects.
Finally, the potential difficulties and future development trends of leather composites in practical
applications are discussed. This discussion will help intelligent leather composites enter people’s daily lives
[39]
and promote the industrialization development stage of functional leather .

DESIGN AND PREPARATION OF LEATHER COMPOSITES
Currently, there have been many studies on the preparation of leather composites. In this section, five
common and effective methods are introduced in detail, including vacuum-assisted filtration, spraying, laser
direct writing (LDW), in-situ growth, and multilayer assembly.