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Fan et al. Soft Sci 2024;4:11 https://dx.doi.org/10.20517/ss.2023.47 Page 3 of 16

Vacuum-assisted filtration
The method of vacuum-assisted filtration utilizes a vacuum environment to provide a pressure difference
between the upper and lower sides of leather, which drives functional materials to enter the interior of the
leather and combine with collagen fibers. Then, the samples were dried to obtain leather composites. Due to
its unique layered structure, there are significant differences between the two sides of leather. The grain side
is composed of collagen fibers and elastic fibers, and the fiber bundles are finer and more tightly woven,
presenting an uneven shape . The fiber side is composed of interwoven bundles of collagen fibers of
[40]
different thicknesses, forming a three-dimensional network, and its tightness is positively correlated with
the mechanical properties of leather. Ma et al. utilized this characteristic to put the fiber side of the leather
upwards, and AgNWs were dissolved and infiltrated into the porous structure of the leather through
vacuum-assisted filtration [Figure 1A] . Strong interactions between AgNWs and collagen fiber bundles
[41]
could be generated through hydrogen bonding, forming an efficient three-dimensional conductive network
in leather. The pores of leather range from tens to hundreds of nanometers, so nanoscale functional
materials, such as MXene, CNTs, GO, poly(3,4-ethylenedioxythiophene) nanofibers (PEDOT NFs), and
[42]
ionic liquids, within this scale can be used to prepare leather composites by vacuum-assisted filtration .

Spraying and soaking
The methods of spraying and soaking are attaching functional materials to leather through physical
processes, relying solely on the binding ability of functional materials and leather collagen fibers, which is
relatively simple and convenient in terms of process. In general, some polymer solutions, non-metal, and
metal particles are used to spray onto the leather substrate to form functional coatings, which can
strengthen the interfacial bonding between the material and leather due to soaking during the spraying
process [24,43] . Wilson et al. used bimetallic copper-iron oxide nanoparticles to spray on the surface of the
[15]
leather as an electromagnetic coating . The formed electrically conductive and magnetically active
bifunctional leather demonstrated the application possibilities in operating intelligent screens and magnetic
switches. Li et al. soaked La O and Bi O nanoparticles in sheep leather and sprayed them on the upper and
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lower surfaces of leather as coatings, greatly increasing the particle load and enhancing its X-ray protection
performance . Mo et al. also combined spraying and vacuum-assisted filtration to filter acidified CNTs on
[44]
the fiber side and sprayed porous cellulose acetate on the corium side to obtain multifunctional double-layer
leather composites, greatly utilizing the layered structure of leather [Figure 1Ba] . It is obvious that one
[45]
side of the leather becomes black after filling acidified multiwalled carbon nanotubes (a-MWCNTs), and the
other side turns white after spraying porous cellulose acetate. The resulting double-layer leather composites
maintain good mechanical performance and breathability [Figure 1Bb-f].
Laser direct writing
In recent years, LDW has gradually become a high-precision and efficient processing technology [46,47] . Many
fabric-based flexible electronics were developed by the LDW technique, and the various carbon precursors
were converted into graphene during the laser scanning in the textile. Based on maskless, design flexibility,
and pattern editable characteristics of LDW, Yang et al. used a femtosecond laser on Kevlar fabric to induce
graphene for various electronic textile applications . It is simple to construct wearable sensors in various
[48]
textile structures by LDW. Leather, as an emerging biomaterial, contains a large amount of carbon elements.
Various flexible electronic devices also can be manufactured without the need for other functional materials
by combining computer control with micro-processing technology. Local high temperatures are generated
by laser irradiation on the leather substrate to achieve carbonization. Wang et al. used the LDW method to
induce the carbonization of collagen fiber on the surface of the leather for fabricating wearable sensors. The
high control accuracy could directly characterize complex structures, such as arrays, on the surface
[Figure 1Ca-d] . In this case, the collagen fibers transformed from insulation to conductive materials after
[49]
carbonization and could directly serve as strain sensors to detect tensile and compressive strains. Zhang