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Page 6 of 27 Tian et al. Soft Sci 2023;3:30 https://dx.doi.org/10.20517/ss.2023.21

aforementioned polymetric strategies. “Other materials” of Figure 3 list some typical textiles, including
[39]
[40]
silk , cotton , and wool , which can enable isotropic deformation and realize a more suitable contour
[41]
for human skin. Apart from textiles, paper-based devices have been put into research and production due to
multiple virtues, such as universality, eco-friendliness, low cost, and easy fabrication [14,42] . Porous cellulose
nanofibers packed in the paper promote convenience for liquid penetration without an external pump when
[14]
fabricating and also provide the possibility of sweat-based biosensing . Other unconventional materials,
such as leather, can also be used .
[43]
Conductive interconnects
Significantly, conductive interconnects undertake the function of sensing data transmission between active
elements and external interfaces, which relates to the whole electronic performance. For stretchable
electronic devices, a tradeoff between mechanical flexibility and charge transmission is obviously not
[44]
negligible . Compared with electrodes on the rigid substrate, interconnects require stable high-
conductivity independent of flexibility and stretchability from the substrate. The deformational robustness
of materials ensures a high signal-to-noise ratio and efficient information transmission for the overall
sensing system. In this section, interconnects are classified into solid-state, gel-state, and liquid-state and
then discussed separately.

Solid-state interconnects, divided into metallic, non-metallic, and hybrid materials, are mainly used due to
their high conductivity and inertness. Different from traditional electrodes in large-scale applications,
micro/nano-scale interconnects tend to be designed with unique structures, such as serpentine structures [45]
shown in Figure 4A and wavy structures. Two main strategies are adopted, namely chemical material
[46]
[30]
modification and deterministic hierarchical architectures . Mostly, reducing the dimensionality is widely
adopted since non-3D structures can adapt better to flexible and stretchable substrates. Figure 4B lists some
metallic and non-metallic interconnect materials that comprise nanoparticles (NPs), nanowires (NWs),
[47]
[48]
[49]
[50]
nanosheets for metal and carbon nanotubes (CNTs), graphene , and conductive polymers (CPs) for
[52]
[51]
non-metal. Metallic nanostructures not singly form an excellent matrix with electrical conductivity but
absorb mechanical strain to ensure high flexibility as well . Contrastively, non-metallic materials tend to
[44]
possess relatively poor conductivity, whereas some carbon-based materials with enhanced electrical
conductivity, represented by CNTs and graphene, become suitable and promising interconnect
candidates [50,51] . Polymers mostly exploited owing to mechanical properties can be granted the conductive
capacity using a chemical doping method to create charge mobile pathways [52,53] . Interestingly, the hybrids of
metallic and non-metallic materials can be researched for the purpose of reinforcing the properties of a
single material. For instance, hybrid fillers comprising graphene and silver NWs (AgNWs) are incorporated
into extremely stretchable spandex to fabricate strain sensors by Vo et al., as implied in Figure 4C .
[54]

Gel-state materials, aforementioned as decent substrate materials, can be introduced to form interconnects.
For example, Liu et al. reported electrically conductive hydrogel-based elastic microelectronics with Young’s
modulus values in the kilopascal range for localized low-voltage neuromodulation, as shown in Figure 4D
[55] . Lastly, liquid-state materials can be integrated into substrates as microfluidic interconnects. For example,
a microfluidic tactile diaphragm pressure sensor based on embedded Galinstan microchannels is
demonstrated for ultra-low pressure detection by Gao et al., as summarized in Figure 4E .
[56]

Sensing elements
After discussing a wide range of material approaches on the substrates and interconnects, strategies on
sensing materials are considered prudently. In essence, high-performance sensing materials have been
fabricated and applied all the time, which can look back to the Eastern Han Dynasty when Zhang Heng
made the famous seismoscope . With the development of the electronic industry, sensors have ranged
[57]