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Fan et al. Soft Sci 2024;4:11 https://dx.doi.org/10.20517/ss.2023.47 Page 5 of 16

[50]
polymers . By configuring different precursor solutions, they can easily flow in the porous structure of the
[28]
leather and conduct gelation . Fan et al. used natural goat skin as the substrate and dipped the acrylic
4+
monomer, Zr ions, carbon quantum dots@nanosilver particles (CQDs@AgNPs), and 1,3-propanediol
(PDO) into the fiber skeleton of leather. Then, the AA monomer was in-situ polymerized to obtain a
multifunctional hydrogel containing the leather skeleton [Figure 2Aa] . The 3D network of leather in ionic
[27]
gel became an effective flow channel for loading CQDs@AgNPs and PDO, which endowed the hydrogels
with excellent mechanical properties, self-adhesiveness, transparency, UV shielding, antibacterial,
biocompatibility, and conductivity. Furthermore, the Zr(SO ) could form a strong interfacial bonding with
4 2
collagen fibers to enhance the network structure; therefore, the mechanical properties of leather composites
were strengthened [Figure 2Ab]. This method provides effective design ideas for the development of
intelligent, flexible electronic skins.

Multilayer assembly
Sandwich structures can enhance the overall mechanical properties of composites through the synergistic
coordination of different components [51,52] . Fan et al. proposed a flexibility-toughness coupling design
strategy to develop intelligent anti-impact leather. By assembling flexible shear stiffening gel (SSG), tough
leather, and nonwoven fabric (NWF) into a Leather/SSG/NWF sandwich structure, the mechanical
[53]
properties of the resulting leather composite were greatly improved [Figure 2B] . At the same time, the
leather layer could also be designed with special functions. For example, the MXene nanosheets could be
combined with leather fibers through vacuum-assisted filtration, and then the wearable Leather/MXene/
SSG/NWF safeguarding leather composite with excellent sensing, thermal management, and
electromagnetic interference shielding was obtained. Obviously, this idea can be further expanded for the
multifunctional design and application of intelligent leather [Figure 2C] .
[54]

MULTIFUNCTIONAL APPLICATIONS OF LEATHER COMPOSITES
By combining various functional materials with leather, a variety of leather composites, including
conductive leather, electromagnetic shielding leather, flame retardant leather, thermal management leather,
etc., have been successfully developed [6,26,55] . Then, intelligent leather composites are further obtained by
structural design and assembly based on single-function leather, which can be widely used for flexible
sensors, electromagnetic shielding devices, safety protection, flame retardant, intelligent displays, and
intelligent thermal management. As shown in Table 1, the preparation methods and functionalities of
different leather composites based on various materials are summarized to understand intuitively.
Obviously, intelligent leather composites provide an important research direction for the development of
wearable electronic devices.

Flexible sensors
Flexible sensors have broad application prospects in human motion monitoring, human-machine
interactions, and the intelligent wearable field [56,57] . Natural leather materials have a hierarchical structure
and elemental composition similar to human skin; thus, they can be used as an excellent substrate material
for flexible sensors. To date, various leather composites have been widely used in flexible sensors, which can
be divided into different working mechanisms, including piezoresistive sensing [13,16] , strain sensing [58,59] ,
triboelectricity [17,21] , and so on. Ma et al. prepared AgNW/leather composites and assembled them with
interdigitated copper electrodes to form a piezoresistive sensor [Figure 3Aa] . The piezoresistive sensor
[41]
showed different sensitivities in three distinct pressure stages, indicating excellent piezoresistive sensing
ability. When the pressure is lower than 2.5 kPa (stage I), the piezoresistive sensor shows a low sensitivity
due to the slight compression deformation of AgNW/leather composites. As the pressure increases to
10 kPa (stage II), the collagen fiber bundles undergo densification, resulting in a more efficient conductive
network under larger compression deformation, so the piezoresistive sensor shows a high sensitivity. If the